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Department of Computer ScienceSouthern Illinois University Carbondale

CS441 ndash Mobile amp Wireless ComputingOverview of Computer NetworkingData Link Layer of TCPIP Model

Computer Networking A Top Down Approach Featuring the

Internet 3rd edition

Jim Kurose Keith RossAddison-Wesley July 2004

Thanks to

Computer Networks 4th edition

Andrew S Tanenbaum

Prentice-Hall PTR

Data andComputerCommunications7th edition William Stallings

Prentice-Hall PTR

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Link Layer Introduction What is Data Link Layer in

TCPIP Reference Model Also called Point-to-Point Protocol amp

Medium Access Layer (MAC) Some terminology

Hosts and routers are nodes Communication channels that connect

adjacent nodes along communication path are links

Wired links Wireless links LANs

Layer-2 packet is a frame encapsulates datagram

ldquolinkrdquo

Data-link layer has responsibility of transferring datagram from one node to adjacent node over a link

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Link layer context Datagram transferred by different link protocols over

different links eg Ethernet on first link frame relay on intermediate links 80211

on last link Each link protocol provides different services

eg may or may not provide rdt over link

Transportation analogy Trip from Princeton to Lausanne

limo Princeton to JFK plane JFK to Geneva train Geneva to Lausanne

Tourist = datagram Transport segment = communication link Transportation mode = link layer protocol Travel agent = routing algorithm

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Link Layer Services Framing link access

Encapsulate datagram into frame adding header trailer Channel access if shared medium ldquoMACrdquo addresses used in frame headers to identify source dest

Different from IP address

Reliable delivery between adjacent nodes Seldom used on low bit error link (fiber some twisted pair) Wireless links high error rates

Q why both link-level and end-end reliability

Flow Control Pacing between adjacent sending and receiving nodes

Error Detection Errors caused by signal attenuation noise Receiver detects presence of errors

Signals sender for retransmission or drops frame

Error Correction Receiver identifies and corrects bit error(s) without resorting to retransmission

Half-duplex and full-duplex With half duplex nodes at both ends of link can transmit but not at same time

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Adaptors Communicating

Link layer implemented in ldquoadaptorrdquo (aka NIC) Ethernet card PCMCI card

80211 card

Sending side Encapsulates datagram in a

frame Adds error checking bits rdt

flow control etc

Receiving side Looks for errors rdt flow

control etc Extracts datagram passes to

receiving node

Adapter is semi-autonomous

Link amp physical layers

sendingnode

frame

receivingnode

datagram

frame

adapter adapter

link layer protocol

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Link Layer - Outline

Flow Control

Error detection

Error correction

Multiple access protocols

Link-Layer Addressing

Ethernet

HDLC amp PPP

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Flow Control The Data Link Layer also provides Flow Control What is Flow Control

Ensuring the sending entity does not overwhelm the receiving entity Preventing buffer overflow

One Solution Stop and Wait Flow Control Source transmits frame Destination receives frame and replies with acknowledgement Source waits for ACK before sending next frame Destination can stop flow by not sending ACK Works well for a few large frames

BUT Large block of data may be split into small frames Limited buffer size Errors detected sooner (when whole frame received) On error retransmission of smaller frames is needed Prevents one station occupying medium for long periods

Stop and wait becomes inadequate Link Utilization

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Stop and Wait Link Utilization

Transmission time =1 Propagation time = a

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Sliding Windows Flow Control Allow multiple frames to be in transit Receiver has buffer W long Transmitter can send up to W frames without ACK Each frame is numbered ACK includes number of next frame expected Sequence number bounded by size of field (k)

Frames are numbered modulo 2k

Enhancements Receiver can acknowledge frames without permitting

further transmission (Receive Not Ready) Must send a normal acknowledge to resume If duplex use piggybacking

If no data to send use acknowledgement frame If data but no acknowledgement to send send last acknowledgement

number again or have ACK valid flag (TCP)

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Sliding Windows Diagram

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Example

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Error DetectionEDC = Error Detection and Correction bits (redundancy)D = Data protected by error checking may include header fields

Error detection not 100 reliable Protocol may miss some errors but rarely Larger EDC field yields better detection and correction

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Parity Checking Single Bit Parity

Detect single bit errors

Two Dimensional Bit ParityDetect and correct single bit errors

Internet Checksum Detect ldquoerrorsrdquo (eg flipped bits)

in transmitted segment (note used at transport layer only) 00

Sender Treat segment contents as sequence of 16-bit integers Checksum addition (1rsquos complement sum) of segment contents Sender puts checksum value into UDP checksum field

Receiver Compute checksum of received segment Check if computed checksum equals checksum field value NO - error detected YES - no error

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Cyclic Redundancy Check View data bits D as a binary number Choose r+1 bit pattern (generator) G Goal choose r CRC bits R such that

ltDRgt exactly divisible by G (modulo 2) Receiver knows G divides ltDRgt by G If non-zero remainder error

detected Can detect all burst errors less than r+1 bits

Widely used in practice (ATM HDCL)

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CRC Example

WantD2r XOR R = nG

EquivalentlyD2r = nG XOR R

Equivalently

If we divide D2r by G want remainder R

R = remainder[ ]D2r

G

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Error Correction

Detection and correction of errors due to Lost frames Damaged frames Underlying channel can lose the frames or can be garbled

Solution at the Link Layer Automatic repeat request Error detection Positive acknowledgment Retransmission after timeout Negative acknowledgement and retransmission

Automatic Repeat Request (ARQ) methods Stop and wait Go back N Selective reject (selective retransmission)

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Stop and Wait ARQ Source transmits single

frame Wait for ACK If received frame damaged

discard it Transmitter has timeout If no ACK within timeout

retransmit

If ACK damaged transmitter will not recognize it Transmitter will retransmit Receive gets two copies of frame Use ACK0 and ACK1

Simple but inefficient

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More efficient method Go-back-N Based on sliding window If no error ACK as usual with next frame expected Use window to control number of outstanding frames If error reply with rejection

Discard that frame and all future frames until error frame received correctly

Transmitter must go back and retransmit that frame and all subsequent frames

Handling Damaged Frame Receiver detects error in frame i Receiver sends rejection-i Transmitter gets rejection-i Transmitter retransmits frame i and all subsequent

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Go-back-N Handling Lost Frame First Approach

Frame i lost Transmitter sends i+1 Receiver gets frame i+1 out of sequence Receiver send reject i Transmitter goes back to frame i and retransmits

Second Approach Frame i lost and no additional frame sent Receiver gets nothing and returns neither acknowledgement nor

rejection Transmitter times out and sends acknowledgement frame with P bit

set to 1 Receiver interprets this as command which it acknowledges with the

number of the next frame it expects (frame i ) Transmitter then retransmits frame i

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Go-back-N Damaged ACK

Receiver gets frame i and send acknowledgement (i+1) which is lost

Acknowledgements are cumulative so next acknowledgement (i+n) may arrive before transmitter times out on frame i

If transmitter times out it sends acknowledgement with P bit set as before

This can be repeated a number of times before a reset procedure is initiated

Damaged Rejection Same as Lost Frame (2)

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Selective Reject

Also called selective retransmission

Only rejected frames are retransmitted

Subsequent frames are accepted by the receiver and buffered

Minimizes retransmission Receiver must maintain

large enough buffer More complex login in

transmitter

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Multiple Access Links and ProtocolsTwo types of ldquolinksrdquo Point-to-point

PPP for dial-up access Point-to-point link between Ethernet switch and host

Broadcast (shared wire or medium) Traditional Ethernet Upstream HFC 80211 wireless LAN

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Multiple Access Protocols Single shared broadcast channel Two or more simultaneous transmissions by nodes interference

Collision if node receives two or more signals at the same time

Multiple access protocol Distributed algorithm that determines how nodes share channel ie

determine when node can transmit Communication about channel sharing must use channel itself

No out-of-band channel for coordination

Ideal Multiple Access Protocol When one node wants to transmit it can send at rate R When M nodes want to transmit each can send at average rate RM Fully decentralized

No special node to coordinate transmissions No synchronization of clocks slots

Simple

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MAC Protocols A taxonomy

Three broad classes

Channel Partitioning Divide channel into smaller ldquopiecesrdquo (time slots frequency code) Allocate piece to node for exclusive use

Random Access Channel not divided allow collisions ldquoRecoverrdquo from collisions

ldquoTaking turnsrdquo Nodes take turns Nodes with more to send can take longer turns

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Channel Partitioning MAC protocols TDMA

TDMA Time division multiple access Access to channel in rounds Each station gets fixed length slot

Length = packet transmission time in each round

Unused slots go idle Example 6-station LAN

134 have packet Slots 256 idle

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Channel Partitioning MAC protocols FDMA

FDMA Frequency division multiple access Channel spectrum divided into frequency bands Each station assigned fixed frequency band Unused transmission time in frequency bands go idle Example 6-station LAN 134 have packet frequency

bands 256 idle

frequ

ency

bands time

1

2

3

4

5

6

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Random Access Protocols

When node has packet to send Transmit at full channel data rate R No a priori coordination among nodes

Two or more transmitting nodes ldquocollisionrdquo Random access MAC protocol specifies

How to detect collisions How to recover from collisions (eg via delayed retransmissions)

Examples of random access MAC protocols Slotted ALOHA ALOHA Carrier Sense Multiple Access (CSMA) CSMACollision Detection (CD) CSMACollision Avoidance (CA)

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Slotted ALOHA

Pros Single active node can continuously

transmit at full rate of channel Highly decentralized only slots in

nodes need to be in sync Simple

Cons Collisions wasting slots Idle slots Nodes may be able to detect collision in

less than time to transmit packet Clock synchronization

Assumptions All frames same size Time is divided into equal size

slots time to transmit 1 frame Nodes start to transmit frames

only at beginning of slots Nodes are synchronized If 2 or more nodes transmit in

slot all nodes detect collision

Operation When node obtains fresh frame it

transmits in next slot No collision node can send new frame in

next slot If collision node retransmits frame in

each subsequent slot with prob p until success

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Pure (unslotted) ALOHA Unslotted Aloha simpler no synchronization When frame first arrives

Transmit immediately

Collision probability increases Frame sent at t0 collides with other frames sent in [t0-1t0+1]

Even worse than slotted ALOHA

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Collisions can still occur Propagation delay means two nodes may not hear each otherrsquos transmission Entire packet transmission time wasted Role of distance amp propagation delay in determining collision probability spatial layout of nodes

Idea listen before transmit If channel sensed idle

transmit entire frame If channel sensed busy defer

transmission Human analogy donrsquot

interrupt others

CSMA (Carrier Sense Multiple Access)

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CSMACD (Collision Detection)

CSMACD carrier sensing deferral as in CSMA Collisions detected within short

time Send jam sequence Colliding transmissions aborted

reducing channel wastage

Collision detection

Easy in wired LANs measure signal strengths compare transmitted received signals

Difficult in wireless LANs receiver shut off while transmitting

Human analogy the polite conversationalist

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ldquoTaking Turnsrdquo MAC protocols Channel partitioning MAC protocols

Share channel efficiently and fairly at high load Inefficient at low load

Delay in channel access 1N bandwidth allocated even if only 1 active node

Random access MAC protocols Efficient at low load

Single node can fully utilize channel

high load Collision overhead

ldquoTaking turnsrdquo protocols Look for best of both worlds

Two implementations Polling Token

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ldquoTaking Turnsrdquo MAC protocolsPolling Master node ldquoinvitesrdquo slave

nodes to transmit in turn Concerns

Polling overhead Latency Single point of failure (master)

Token passing control token passed from

one node to next sequentially

token message concerns

token overhead latency single point of failure (token)

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Summary of MAC protocols

What do you do with a shared media Channel Partitioning by time frequency or code

Time Division Frequency Division

Random partitioning (dynamic) ALOHA S-ALOHA CSMA CSMACD Carrier sensing easy in some technologies (wire) hard in others

(wireless) CSMACD used in Ethernet CSMACA used in 80211

Taking Turns Polling from a central site token passing

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LAN technologies

Data link layer so farServicesError detectioncorrectionMultiple access

Next LAN technologiesAddressingEthernetHubs switchesPPP

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MAC Addresses and ARP 32-bit IP address

Network-layer address Used to get datagram to

destination IP subnet

MAC (or LAN or physical or Ethernet) address Used to get datagram

from one interface to another physically-connected interface (same network)

48 bit MAC address (for most LANs) burned in the adapter ROM

Find your PCrsquos MAC address

1A-2F-BB-76-09-AD

58-23-D7-FA-20-B0

0C-C4-11-6F-E3-98

71-65-F7-2B-08-53

LAN(wired orwireless)

Each adapter on LAN has unique LAN address

Broadcast address =FF-FF-FF-FF-FF-FF = adapter

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MAC Address (more)

MAC address allocation administered by IEEE Manufacturer buys portion of MAC address space (to

assure uniqueness) Analogy

(a) MAC address like Social Security Number

(b) IP address like postal address MAC flat address portability

Can move LAN card from one LAN to another

IP hierarchical address NOT portable Depends on IP subnet to which node is attached

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ARP Address Resolution Protocol

Each IP node (Host Router) on LAN has ARP table

ARP Table IPMAC address mappings for some LAN nodes

lt IP address MAC address TTLgt TTL (Time To Live)

time after which address mapping will be forgotten (typically 20 min)

Question how to determineMAC address of Bknowing Brsquos IP address

1A-2F-BB-76-09-AD

58-23-D7-FA-20-B0

0C-C4-11-6F-E3-98

71-65-F7-2B-08-53

LAN

237196723

237196778

237196714

237196788

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ARP protocol Same LAN (network) A wants to send datagram

to B and Brsquos MAC address not in Arsquos ARP table

A broadcasts ARP query packet containing Bs IP address Dest MAC address = FF-FF-

FF-FF-FF-FF All machines on LAN receive

ARP query

B receives ARP packet replies to A with its (Bs) MAC address Frame sent to Arsquos MAC

address (unicast)

A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) Soft state information that

times out (goes away) unless refreshed

ARP is ldquoplug-and-playrdquo Nodes create their ARP

tables without intervention from net administrator

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Routing to another LAN Walkthrough send datagram from A to B via R

Assume A knows B IP address

Two ARP tables in router R one for each IP network (LAN) In routing table at source Host find router 111111111110 In ARP table at source find MAC address E6-E9-00-17-BB-4B etc A creates datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110

A creates link-layer frame with Rs MAC address as dest frame contains A-to-B IP datagram

Arsquos adapter sends frame Rrsquos adapter receives frame R removes IP datagram from Ethernet frame sees its destined to B R uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B

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A

R

B

Routing to another LAN

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EthernetldquoDominantrdquo wired LAN technology Cheap $20 for 100Mbs First widely used LAN technology Simpler cheaper than token LANs and ATM Kept up with speed race 10 Mbps ndash 10

Gbps

Metcalfersquos Ethernet sketch

Star Topology Bus topology popular

through mid 90s Now star topology prevails Connection choices hub or

switch (more later)

hub orswitch

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Ethernet Frame Structure

Preamble 7 bytes with pattern 10101010 followed by one byte with pattern 10101011 Used to synchronize receiver sender clock rates

Addresses 6 bytes If adapter receives frame with matching destination address or with broadcast

address (eg ARP packet) it passes data in frame to net-layer protocol Otherwise adapter discards frame

Type Indicates the higher layer protocol (mostly IP but others may be supported

such as Novell IPX and AppleTalk) CRC

Checked at receiver if error is detected the frame is simply dropped

Sending adapter encapsulates IP datagram in Ethernet frame

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Ethernet uses CSMACD Connectionless No handshaking between sending and receiving

adapter Unreliable receiving adapter doesnrsquot send acks or nacks to

sending adapter Stream of datagrams passed to network layer can have gaps Gaps will be filled if app is using TCP Otherwise app will see the gaps

No slots Adapter doesnrsquot transmit if it senses that some other adapter is

transmitting that is carrier sense Transmitting adapter aborts when it senses that another adapter is

transmitting that is collision detection Before attempting a retransmission adapter waits a random time

that is random access

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Ethernet CSMACD algorithm1 Adaptor receives datagram from

net layer amp creates frame2 If adapter senses channel idle it

starts to transmit frame If it senses channel busy waits until channel idle and then transmits

3 If adapter transmits entire frame without detecting another transmission the adapter is done with frame

4 If adapter detects another transmission while transmitting aborts and sends jam signal

5 After aborting adapter enters exponential backoff after the mth collision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2

Jam Signal make sure all other transmitters are aware of collision 48 bits

Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec

Exponential Backoff Goal adapt retransmission

attempts to estimated current load Heavy load random wait will be

longer

First collision choose K from 01 delay is K 512 bit transmission times

After second collision choose K from 0123hellip

After ten collisions choose K from 01234hellip1023

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CSMACD efficiency Tprop = max prop between 2 nodes in LAN ttrans = time to transmit max-size frame

Efficiency goes to 1 as tprop goes to 0 Goes to 1 as ttrans goes to infinity Much better than ALOHA but still decentralized

simple and cheap

Other Ethernets Switched Ethernet Fast Ethernet Gigabit Ethernet

transprop tt 51

1efficiency

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Point to Point Data Link Control

One sender one receiver one link easier than broadcast link No Media Access Control No need for explicit MAC addressing eg dialup link ISDN line

Popular point-to-point DLC protocols PPP (point-to-point protocol) HDLC High level data link control (Data link used to be considered

ldquohigh layerrdquo in protocol stack

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HDLC

Frame format for bit-oriented protocol HDLC

Control field of

(a) An information frame

(b) A supervisory frame

(c) An unnumbered frame

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HDLC Exchange of information supervisory and unnumbered

frames Information - data to be transmitted to user (next layer up)

Flow and error control piggybacked on information frames Supervisory - ARQ when piggyback not used Unnumbered - supplementary link control

Three phases Initialization Data transfer Disconnect

Uses bit stuffing Flag delimit frame at both ends 01111110 is the Flag May close one frame and open another Bit stuffing to find flag data

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Internet Datalink Protocol PPP Design Requirements [RFC 1557]

Packet framing encapsulation of network-layer datagram in data link frame Carry network layer data of any network layer protocol (not just IP) at

same time Ability to demultiplex upwards

Bit transparency must carry any bit pattern in the data field Error detection (no correction) Connection liveness detect signal link failure to network layer Network layer address negotiation endpoint can learnconfigure

each otherrsquos network address

PPP non-design requirements No error correctionrecovery No flow control Out of order delivery OK No need to support multipoint links (eg polling)

Error recovery flow control data re-ordering all relegated to higher layers

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PPP Data Frame

Flag delimiter (framing) Address does nothing (only one option) Control does nothing in the future possible multiple

control fields Protocol upper layer protocol to which frame delivered

(eg PPP-LCP IP IPCP etc) Info upper layer data being carried Check cyclic redundancy check for error detection

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Byte Stuffing ldquoData transparencyrdquo requirement data field must be allowed to include

flag pattern lt01111110gt Q is received lt01111110gt data or flag

Sender adds (ldquostuffsrdquo) extra lt 01111110gt byte after each lt 01111110gt data byte How about bit stuffing

Receiver Two 01111110 bytes in a row Discard first byte continue data reception Single 01111110 flag byte

flag bytepatternin datato send

flag byte pattern plusstuffed byte in transmitted data

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PPP Data Control Protocol Before exchanging

network-layer data data link peers must configure PPP link

(max frame length authentication)

learnconfigure network layer information

For IP carry IP Control Protocol (IPCP) messages (protocol field 8021) to configurelearn IP address

Diagram for bring a line up and down

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Link Layer Introduction What is Data Link Layer in

TCPIP Reference Model Also called Point-to-Point Protocol amp

Medium Access Layer (MAC) Some terminology

Hosts and routers are nodes Communication channels that connect

adjacent nodes along communication path are links

Wired links Wireless links LANs

Layer-2 packet is a frame encapsulates datagram

ldquolinkrdquo

Data-link layer has responsibility of transferring datagram from one node to adjacent node over a link

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Link layer context Datagram transferred by different link protocols over

different links eg Ethernet on first link frame relay on intermediate links 80211

on last link Each link protocol provides different services

eg may or may not provide rdt over link

Transportation analogy Trip from Princeton to Lausanne

limo Princeton to JFK plane JFK to Geneva train Geneva to Lausanne

Tourist = datagram Transport segment = communication link Transportation mode = link layer protocol Travel agent = routing algorithm

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Link Layer Services Framing link access

Encapsulate datagram into frame adding header trailer Channel access if shared medium ldquoMACrdquo addresses used in frame headers to identify source dest

Different from IP address

Reliable delivery between adjacent nodes Seldom used on low bit error link (fiber some twisted pair) Wireless links high error rates

Q why both link-level and end-end reliability

Flow Control Pacing between adjacent sending and receiving nodes

Error Detection Errors caused by signal attenuation noise Receiver detects presence of errors

Signals sender for retransmission or drops frame

Error Correction Receiver identifies and corrects bit error(s) without resorting to retransmission

Half-duplex and full-duplex With half duplex nodes at both ends of link can transmit but not at same time

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Adaptors Communicating

Link layer implemented in ldquoadaptorrdquo (aka NIC) Ethernet card PCMCI card

80211 card

Sending side Encapsulates datagram in a

frame Adds error checking bits rdt

flow control etc

Receiving side Looks for errors rdt flow

control etc Extracts datagram passes to

receiving node

Adapter is semi-autonomous

Link amp physical layers

sendingnode

frame

receivingnode

datagram

frame

adapter adapter

link layer protocol

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Link Layer - Outline

Flow Control

Error detection

Error correction

Multiple access protocols

Link-Layer Addressing

Ethernet

HDLC amp PPP

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Flow Control The Data Link Layer also provides Flow Control What is Flow Control

Ensuring the sending entity does not overwhelm the receiving entity Preventing buffer overflow

One Solution Stop and Wait Flow Control Source transmits frame Destination receives frame and replies with acknowledgement Source waits for ACK before sending next frame Destination can stop flow by not sending ACK Works well for a few large frames

BUT Large block of data may be split into small frames Limited buffer size Errors detected sooner (when whole frame received) On error retransmission of smaller frames is needed Prevents one station occupying medium for long periods

Stop and wait becomes inadequate Link Utilization

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Stop and Wait Link Utilization

Transmission time =1 Propagation time = a

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Sliding Windows Flow Control Allow multiple frames to be in transit Receiver has buffer W long Transmitter can send up to W frames without ACK Each frame is numbered ACK includes number of next frame expected Sequence number bounded by size of field (k)

Frames are numbered modulo 2k

Enhancements Receiver can acknowledge frames without permitting

further transmission (Receive Not Ready) Must send a normal acknowledge to resume If duplex use piggybacking

If no data to send use acknowledgement frame If data but no acknowledgement to send send last acknowledgement

number again or have ACK valid flag (TCP)

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Sliding Windows Diagram

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Example

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Error DetectionEDC = Error Detection and Correction bits (redundancy)D = Data protected by error checking may include header fields

Error detection not 100 reliable Protocol may miss some errors but rarely Larger EDC field yields better detection and correction

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Parity Checking Single Bit Parity

Detect single bit errors

Two Dimensional Bit ParityDetect and correct single bit errors

Internet Checksum Detect ldquoerrorsrdquo (eg flipped bits)

in transmitted segment (note used at transport layer only) 00

Sender Treat segment contents as sequence of 16-bit integers Checksum addition (1rsquos complement sum) of segment contents Sender puts checksum value into UDP checksum field

Receiver Compute checksum of received segment Check if computed checksum equals checksum field value NO - error detected YES - no error

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Cyclic Redundancy Check View data bits D as a binary number Choose r+1 bit pattern (generator) G Goal choose r CRC bits R such that

ltDRgt exactly divisible by G (modulo 2) Receiver knows G divides ltDRgt by G If non-zero remainder error

detected Can detect all burst errors less than r+1 bits

Widely used in practice (ATM HDCL)

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CRC Example

WantD2r XOR R = nG

EquivalentlyD2r = nG XOR R

Equivalently

If we divide D2r by G want remainder R

R = remainder[ ]D2r

G

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Error Correction

Detection and correction of errors due to Lost frames Damaged frames Underlying channel can lose the frames or can be garbled

Solution at the Link Layer Automatic repeat request Error detection Positive acknowledgment Retransmission after timeout Negative acknowledgement and retransmission

Automatic Repeat Request (ARQ) methods Stop and wait Go back N Selective reject (selective retransmission)

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Stop and Wait ARQ Source transmits single

frame Wait for ACK If received frame damaged

discard it Transmitter has timeout If no ACK within timeout

retransmit

If ACK damaged transmitter will not recognize it Transmitter will retransmit Receive gets two copies of frame Use ACK0 and ACK1

Simple but inefficient

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More efficient method Go-back-N Based on sliding window If no error ACK as usual with next frame expected Use window to control number of outstanding frames If error reply with rejection

Discard that frame and all future frames until error frame received correctly

Transmitter must go back and retransmit that frame and all subsequent frames

Handling Damaged Frame Receiver detects error in frame i Receiver sends rejection-i Transmitter gets rejection-i Transmitter retransmits frame i and all subsequent

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Go-back-N Handling Lost Frame First Approach

Frame i lost Transmitter sends i+1 Receiver gets frame i+1 out of sequence Receiver send reject i Transmitter goes back to frame i and retransmits

Second Approach Frame i lost and no additional frame sent Receiver gets nothing and returns neither acknowledgement nor

rejection Transmitter times out and sends acknowledgement frame with P bit

set to 1 Receiver interprets this as command which it acknowledges with the

number of the next frame it expects (frame i ) Transmitter then retransmits frame i

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Go-back-N Damaged ACK

Receiver gets frame i and send acknowledgement (i+1) which is lost

Acknowledgements are cumulative so next acknowledgement (i+n) may arrive before transmitter times out on frame i

If transmitter times out it sends acknowledgement with P bit set as before

This can be repeated a number of times before a reset procedure is initiated

Damaged Rejection Same as Lost Frame (2)

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Selective Reject

Also called selective retransmission

Only rejected frames are retransmitted

Subsequent frames are accepted by the receiver and buffered

Minimizes retransmission Receiver must maintain

large enough buffer More complex login in

transmitter

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Multiple Access Links and ProtocolsTwo types of ldquolinksrdquo Point-to-point

PPP for dial-up access Point-to-point link between Ethernet switch and host

Broadcast (shared wire or medium) Traditional Ethernet Upstream HFC 80211 wireless LAN

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Multiple Access Protocols Single shared broadcast channel Two or more simultaneous transmissions by nodes interference

Collision if node receives two or more signals at the same time

Multiple access protocol Distributed algorithm that determines how nodes share channel ie

determine when node can transmit Communication about channel sharing must use channel itself

No out-of-band channel for coordination

Ideal Multiple Access Protocol When one node wants to transmit it can send at rate R When M nodes want to transmit each can send at average rate RM Fully decentralized

No special node to coordinate transmissions No synchronization of clocks slots

Simple

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MAC Protocols A taxonomy

Three broad classes

Channel Partitioning Divide channel into smaller ldquopiecesrdquo (time slots frequency code) Allocate piece to node for exclusive use

Random Access Channel not divided allow collisions ldquoRecoverrdquo from collisions

ldquoTaking turnsrdquo Nodes take turns Nodes with more to send can take longer turns

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Channel Partitioning MAC protocols TDMA

TDMA Time division multiple access Access to channel in rounds Each station gets fixed length slot

Length = packet transmission time in each round

Unused slots go idle Example 6-station LAN

134 have packet Slots 256 idle

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Channel Partitioning MAC protocols FDMA

FDMA Frequency division multiple access Channel spectrum divided into frequency bands Each station assigned fixed frequency band Unused transmission time in frequency bands go idle Example 6-station LAN 134 have packet frequency

bands 256 idle

frequ

ency

bands time

1

2

3

4

5

6

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Random Access Protocols

When node has packet to send Transmit at full channel data rate R No a priori coordination among nodes

Two or more transmitting nodes ldquocollisionrdquo Random access MAC protocol specifies

How to detect collisions How to recover from collisions (eg via delayed retransmissions)

Examples of random access MAC protocols Slotted ALOHA ALOHA Carrier Sense Multiple Access (CSMA) CSMACollision Detection (CD) CSMACollision Avoidance (CA)

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Slotted ALOHA

Pros Single active node can continuously

transmit at full rate of channel Highly decentralized only slots in

nodes need to be in sync Simple

Cons Collisions wasting slots Idle slots Nodes may be able to detect collision in

less than time to transmit packet Clock synchronization

Assumptions All frames same size Time is divided into equal size

slots time to transmit 1 frame Nodes start to transmit frames

only at beginning of slots Nodes are synchronized If 2 or more nodes transmit in

slot all nodes detect collision

Operation When node obtains fresh frame it

transmits in next slot No collision node can send new frame in

next slot If collision node retransmits frame in

each subsequent slot with prob p until success

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Pure (unslotted) ALOHA Unslotted Aloha simpler no synchronization When frame first arrives

Transmit immediately

Collision probability increases Frame sent at t0 collides with other frames sent in [t0-1t0+1]

Even worse than slotted ALOHA

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Collisions can still occur Propagation delay means two nodes may not hear each otherrsquos transmission Entire packet transmission time wasted Role of distance amp propagation delay in determining collision probability spatial layout of nodes

Idea listen before transmit If channel sensed idle

transmit entire frame If channel sensed busy defer

transmission Human analogy donrsquot

interrupt others

CSMA (Carrier Sense Multiple Access)

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CSMACD (Collision Detection)

CSMACD carrier sensing deferral as in CSMA Collisions detected within short

time Send jam sequence Colliding transmissions aborted

reducing channel wastage

Collision detection

Easy in wired LANs measure signal strengths compare transmitted received signals

Difficult in wireless LANs receiver shut off while transmitting

Human analogy the polite conversationalist

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ldquoTaking Turnsrdquo MAC protocols Channel partitioning MAC protocols

Share channel efficiently and fairly at high load Inefficient at low load

Delay in channel access 1N bandwidth allocated even if only 1 active node

Random access MAC protocols Efficient at low load

Single node can fully utilize channel

high load Collision overhead

ldquoTaking turnsrdquo protocols Look for best of both worlds

Two implementations Polling Token

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ldquoTaking Turnsrdquo MAC protocolsPolling Master node ldquoinvitesrdquo slave

nodes to transmit in turn Concerns

Polling overhead Latency Single point of failure (master)

Token passing control token passed from

one node to next sequentially

token message concerns

token overhead latency single point of failure (token)

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Summary of MAC protocols

What do you do with a shared media Channel Partitioning by time frequency or code

Time Division Frequency Division

Random partitioning (dynamic) ALOHA S-ALOHA CSMA CSMACD Carrier sensing easy in some technologies (wire) hard in others

(wireless) CSMACD used in Ethernet CSMACA used in 80211

Taking Turns Polling from a central site token passing

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LAN technologies

Data link layer so farServicesError detectioncorrectionMultiple access

Next LAN technologiesAddressingEthernetHubs switchesPPP

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MAC Addresses and ARP 32-bit IP address

Network-layer address Used to get datagram to

destination IP subnet

MAC (or LAN or physical or Ethernet) address Used to get datagram

from one interface to another physically-connected interface (same network)

48 bit MAC address (for most LANs) burned in the adapter ROM

Find your PCrsquos MAC address

1A-2F-BB-76-09-AD

58-23-D7-FA-20-B0

0C-C4-11-6F-E3-98

71-65-F7-2B-08-53

LAN(wired orwireless)

Each adapter on LAN has unique LAN address

Broadcast address =FF-FF-FF-FF-FF-FF = adapter

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MAC Address (more)

MAC address allocation administered by IEEE Manufacturer buys portion of MAC address space (to

assure uniqueness) Analogy

(a) MAC address like Social Security Number

(b) IP address like postal address MAC flat address portability

Can move LAN card from one LAN to another

IP hierarchical address NOT portable Depends on IP subnet to which node is attached

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ARP Address Resolution Protocol

Each IP node (Host Router) on LAN has ARP table

ARP Table IPMAC address mappings for some LAN nodes

lt IP address MAC address TTLgt TTL (Time To Live)

time after which address mapping will be forgotten (typically 20 min)

Question how to determineMAC address of Bknowing Brsquos IP address

1A-2F-BB-76-09-AD

58-23-D7-FA-20-B0

0C-C4-11-6F-E3-98

71-65-F7-2B-08-53

LAN

237196723

237196778

237196714

237196788

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ARP protocol Same LAN (network) A wants to send datagram

to B and Brsquos MAC address not in Arsquos ARP table

A broadcasts ARP query packet containing Bs IP address Dest MAC address = FF-FF-

FF-FF-FF-FF All machines on LAN receive

ARP query

B receives ARP packet replies to A with its (Bs) MAC address Frame sent to Arsquos MAC

address (unicast)

A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) Soft state information that

times out (goes away) unless refreshed

ARP is ldquoplug-and-playrdquo Nodes create their ARP

tables without intervention from net administrator

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Routing to another LAN Walkthrough send datagram from A to B via R

Assume A knows B IP address

Two ARP tables in router R one for each IP network (LAN) In routing table at source Host find router 111111111110 In ARP table at source find MAC address E6-E9-00-17-BB-4B etc A creates datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110

A creates link-layer frame with Rs MAC address as dest frame contains A-to-B IP datagram

Arsquos adapter sends frame Rrsquos adapter receives frame R removes IP datagram from Ethernet frame sees its destined to B R uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B

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A

R

B

Routing to another LAN

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EthernetldquoDominantrdquo wired LAN technology Cheap $20 for 100Mbs First widely used LAN technology Simpler cheaper than token LANs and ATM Kept up with speed race 10 Mbps ndash 10

Gbps

Metcalfersquos Ethernet sketch

Star Topology Bus topology popular

through mid 90s Now star topology prevails Connection choices hub or

switch (more later)

hub orswitch

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Ethernet Frame Structure

Preamble 7 bytes with pattern 10101010 followed by one byte with pattern 10101011 Used to synchronize receiver sender clock rates

Addresses 6 bytes If adapter receives frame with matching destination address or with broadcast

address (eg ARP packet) it passes data in frame to net-layer protocol Otherwise adapter discards frame

Type Indicates the higher layer protocol (mostly IP but others may be supported

such as Novell IPX and AppleTalk) CRC

Checked at receiver if error is detected the frame is simply dropped

Sending adapter encapsulates IP datagram in Ethernet frame

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Ethernet uses CSMACD Connectionless No handshaking between sending and receiving

adapter Unreliable receiving adapter doesnrsquot send acks or nacks to

sending adapter Stream of datagrams passed to network layer can have gaps Gaps will be filled if app is using TCP Otherwise app will see the gaps

No slots Adapter doesnrsquot transmit if it senses that some other adapter is

transmitting that is carrier sense Transmitting adapter aborts when it senses that another adapter is

transmitting that is collision detection Before attempting a retransmission adapter waits a random time

that is random access

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Ethernet CSMACD algorithm1 Adaptor receives datagram from

net layer amp creates frame2 If adapter senses channel idle it

starts to transmit frame If it senses channel busy waits until channel idle and then transmits

3 If adapter transmits entire frame without detecting another transmission the adapter is done with frame

4 If adapter detects another transmission while transmitting aborts and sends jam signal

5 After aborting adapter enters exponential backoff after the mth collision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2

Jam Signal make sure all other transmitters are aware of collision 48 bits

Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec

Exponential Backoff Goal adapt retransmission

attempts to estimated current load Heavy load random wait will be

longer

First collision choose K from 01 delay is K 512 bit transmission times

After second collision choose K from 0123hellip

After ten collisions choose K from 01234hellip1023

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CSMACD efficiency Tprop = max prop between 2 nodes in LAN ttrans = time to transmit max-size frame

Efficiency goes to 1 as tprop goes to 0 Goes to 1 as ttrans goes to infinity Much better than ALOHA but still decentralized

simple and cheap

Other Ethernets Switched Ethernet Fast Ethernet Gigabit Ethernet

transprop tt 51

1efficiency

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Point to Point Data Link Control

One sender one receiver one link easier than broadcast link No Media Access Control No need for explicit MAC addressing eg dialup link ISDN line

Popular point-to-point DLC protocols PPP (point-to-point protocol) HDLC High level data link control (Data link used to be considered

ldquohigh layerrdquo in protocol stack

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HDLC

Frame format for bit-oriented protocol HDLC

Control field of

(a) An information frame

(b) A supervisory frame

(c) An unnumbered frame

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HDLC Exchange of information supervisory and unnumbered

frames Information - data to be transmitted to user (next layer up)

Flow and error control piggybacked on information frames Supervisory - ARQ when piggyback not used Unnumbered - supplementary link control

Three phases Initialization Data transfer Disconnect

Uses bit stuffing Flag delimit frame at both ends 01111110 is the Flag May close one frame and open another Bit stuffing to find flag data

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Internet Datalink Protocol PPP Design Requirements [RFC 1557]

Packet framing encapsulation of network-layer datagram in data link frame Carry network layer data of any network layer protocol (not just IP) at

same time Ability to demultiplex upwards

Bit transparency must carry any bit pattern in the data field Error detection (no correction) Connection liveness detect signal link failure to network layer Network layer address negotiation endpoint can learnconfigure

each otherrsquos network address

PPP non-design requirements No error correctionrecovery No flow control Out of order delivery OK No need to support multipoint links (eg polling)

Error recovery flow control data re-ordering all relegated to higher layers

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PPP Data Frame

Flag delimiter (framing) Address does nothing (only one option) Control does nothing in the future possible multiple

control fields Protocol upper layer protocol to which frame delivered

(eg PPP-LCP IP IPCP etc) Info upper layer data being carried Check cyclic redundancy check for error detection

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Byte Stuffing ldquoData transparencyrdquo requirement data field must be allowed to include

flag pattern lt01111110gt Q is received lt01111110gt data or flag

Sender adds (ldquostuffsrdquo) extra lt 01111110gt byte after each lt 01111110gt data byte How about bit stuffing

Receiver Two 01111110 bytes in a row Discard first byte continue data reception Single 01111110 flag byte

flag bytepatternin datato send

flag byte pattern plusstuffed byte in transmitted data

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PPP Data Control Protocol Before exchanging

network-layer data data link peers must configure PPP link

(max frame length authentication)

learnconfigure network layer information

For IP carry IP Control Protocol (IPCP) messages (protocol field 8021) to configurelearn IP address

Diagram for bring a line up and down

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Link layer context Datagram transferred by different link protocols over

different links eg Ethernet on first link frame relay on intermediate links 80211

on last link Each link protocol provides different services

eg may or may not provide rdt over link

Transportation analogy Trip from Princeton to Lausanne

limo Princeton to JFK plane JFK to Geneva train Geneva to Lausanne

Tourist = datagram Transport segment = communication link Transportation mode = link layer protocol Travel agent = routing algorithm

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Link Layer Services Framing link access

Encapsulate datagram into frame adding header trailer Channel access if shared medium ldquoMACrdquo addresses used in frame headers to identify source dest

Different from IP address

Reliable delivery between adjacent nodes Seldom used on low bit error link (fiber some twisted pair) Wireless links high error rates

Q why both link-level and end-end reliability

Flow Control Pacing between adjacent sending and receiving nodes

Error Detection Errors caused by signal attenuation noise Receiver detects presence of errors

Signals sender for retransmission or drops frame

Error Correction Receiver identifies and corrects bit error(s) without resorting to retransmission

Half-duplex and full-duplex With half duplex nodes at both ends of link can transmit but not at same time

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Adaptors Communicating

Link layer implemented in ldquoadaptorrdquo (aka NIC) Ethernet card PCMCI card

80211 card

Sending side Encapsulates datagram in a

frame Adds error checking bits rdt

flow control etc

Receiving side Looks for errors rdt flow

control etc Extracts datagram passes to

receiving node

Adapter is semi-autonomous

Link amp physical layers

sendingnode

frame

receivingnode

datagram

frame

adapter adapter

link layer protocol

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Link Layer - Outline

Flow Control

Error detection

Error correction

Multiple access protocols

Link-Layer Addressing

Ethernet

HDLC amp PPP

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Flow Control The Data Link Layer also provides Flow Control What is Flow Control

Ensuring the sending entity does not overwhelm the receiving entity Preventing buffer overflow

One Solution Stop and Wait Flow Control Source transmits frame Destination receives frame and replies with acknowledgement Source waits for ACK before sending next frame Destination can stop flow by not sending ACK Works well for a few large frames

BUT Large block of data may be split into small frames Limited buffer size Errors detected sooner (when whole frame received) On error retransmission of smaller frames is needed Prevents one station occupying medium for long periods

Stop and wait becomes inadequate Link Utilization

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Stop and Wait Link Utilization

Transmission time =1 Propagation time = a

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Sliding Windows Flow Control Allow multiple frames to be in transit Receiver has buffer W long Transmitter can send up to W frames without ACK Each frame is numbered ACK includes number of next frame expected Sequence number bounded by size of field (k)

Frames are numbered modulo 2k

Enhancements Receiver can acknowledge frames without permitting

further transmission (Receive Not Ready) Must send a normal acknowledge to resume If duplex use piggybacking

If no data to send use acknowledgement frame If data but no acknowledgement to send send last acknowledgement

number again or have ACK valid flag (TCP)

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Sliding Windows Diagram

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Example

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Error DetectionEDC = Error Detection and Correction bits (redundancy)D = Data protected by error checking may include header fields

Error detection not 100 reliable Protocol may miss some errors but rarely Larger EDC field yields better detection and correction

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Parity Checking Single Bit Parity

Detect single bit errors

Two Dimensional Bit ParityDetect and correct single bit errors

Internet Checksum Detect ldquoerrorsrdquo (eg flipped bits)

in transmitted segment (note used at transport layer only) 00

Sender Treat segment contents as sequence of 16-bit integers Checksum addition (1rsquos complement sum) of segment contents Sender puts checksum value into UDP checksum field

Receiver Compute checksum of received segment Check if computed checksum equals checksum field value NO - error detected YES - no error

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Cyclic Redundancy Check View data bits D as a binary number Choose r+1 bit pattern (generator) G Goal choose r CRC bits R such that

ltDRgt exactly divisible by G (modulo 2) Receiver knows G divides ltDRgt by G If non-zero remainder error

detected Can detect all burst errors less than r+1 bits

Widely used in practice (ATM HDCL)

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CRC Example

WantD2r XOR R = nG

EquivalentlyD2r = nG XOR R

Equivalently

If we divide D2r by G want remainder R

R = remainder[ ]D2r

G

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Error Correction

Detection and correction of errors due to Lost frames Damaged frames Underlying channel can lose the frames or can be garbled

Solution at the Link Layer Automatic repeat request Error detection Positive acknowledgment Retransmission after timeout Negative acknowledgement and retransmission

Automatic Repeat Request (ARQ) methods Stop and wait Go back N Selective reject (selective retransmission)

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Stop and Wait ARQ Source transmits single

frame Wait for ACK If received frame damaged

discard it Transmitter has timeout If no ACK within timeout

retransmit

If ACK damaged transmitter will not recognize it Transmitter will retransmit Receive gets two copies of frame Use ACK0 and ACK1

Simple but inefficient

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More efficient method Go-back-N Based on sliding window If no error ACK as usual with next frame expected Use window to control number of outstanding frames If error reply with rejection

Discard that frame and all future frames until error frame received correctly

Transmitter must go back and retransmit that frame and all subsequent frames

Handling Damaged Frame Receiver detects error in frame i Receiver sends rejection-i Transmitter gets rejection-i Transmitter retransmits frame i and all subsequent

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Go-back-N Handling Lost Frame First Approach

Frame i lost Transmitter sends i+1 Receiver gets frame i+1 out of sequence Receiver send reject i Transmitter goes back to frame i and retransmits

Second Approach Frame i lost and no additional frame sent Receiver gets nothing and returns neither acknowledgement nor

rejection Transmitter times out and sends acknowledgement frame with P bit

set to 1 Receiver interprets this as command which it acknowledges with the

number of the next frame it expects (frame i ) Transmitter then retransmits frame i

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Go-back-N Damaged ACK

Receiver gets frame i and send acknowledgement (i+1) which is lost

Acknowledgements are cumulative so next acknowledgement (i+n) may arrive before transmitter times out on frame i

If transmitter times out it sends acknowledgement with P bit set as before

This can be repeated a number of times before a reset procedure is initiated

Damaged Rejection Same as Lost Frame (2)

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Selective Reject

Also called selective retransmission

Only rejected frames are retransmitted

Subsequent frames are accepted by the receiver and buffered

Minimizes retransmission Receiver must maintain

large enough buffer More complex login in

transmitter

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Multiple Access Links and ProtocolsTwo types of ldquolinksrdquo Point-to-point

PPP for dial-up access Point-to-point link between Ethernet switch and host

Broadcast (shared wire or medium) Traditional Ethernet Upstream HFC 80211 wireless LAN

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Multiple Access Protocols Single shared broadcast channel Two or more simultaneous transmissions by nodes interference

Collision if node receives two or more signals at the same time

Multiple access protocol Distributed algorithm that determines how nodes share channel ie

determine when node can transmit Communication about channel sharing must use channel itself

No out-of-band channel for coordination

Ideal Multiple Access Protocol When one node wants to transmit it can send at rate R When M nodes want to transmit each can send at average rate RM Fully decentralized

No special node to coordinate transmissions No synchronization of clocks slots

Simple

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MAC Protocols A taxonomy

Three broad classes

Channel Partitioning Divide channel into smaller ldquopiecesrdquo (time slots frequency code) Allocate piece to node for exclusive use

Random Access Channel not divided allow collisions ldquoRecoverrdquo from collisions

ldquoTaking turnsrdquo Nodes take turns Nodes with more to send can take longer turns

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Channel Partitioning MAC protocols TDMA

TDMA Time division multiple access Access to channel in rounds Each station gets fixed length slot

Length = packet transmission time in each round

Unused slots go idle Example 6-station LAN

134 have packet Slots 256 idle

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Channel Partitioning MAC protocols FDMA

FDMA Frequency division multiple access Channel spectrum divided into frequency bands Each station assigned fixed frequency band Unused transmission time in frequency bands go idle Example 6-station LAN 134 have packet frequency

bands 256 idle

frequ

ency

bands time

1

2

3

4

5

6

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Random Access Protocols

When node has packet to send Transmit at full channel data rate R No a priori coordination among nodes

Two or more transmitting nodes ldquocollisionrdquo Random access MAC protocol specifies

How to detect collisions How to recover from collisions (eg via delayed retransmissions)

Examples of random access MAC protocols Slotted ALOHA ALOHA Carrier Sense Multiple Access (CSMA) CSMACollision Detection (CD) CSMACollision Avoidance (CA)

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Slotted ALOHA

Pros Single active node can continuously

transmit at full rate of channel Highly decentralized only slots in

nodes need to be in sync Simple

Cons Collisions wasting slots Idle slots Nodes may be able to detect collision in

less than time to transmit packet Clock synchronization

Assumptions All frames same size Time is divided into equal size

slots time to transmit 1 frame Nodes start to transmit frames

only at beginning of slots Nodes are synchronized If 2 or more nodes transmit in

slot all nodes detect collision

Operation When node obtains fresh frame it

transmits in next slot No collision node can send new frame in

next slot If collision node retransmits frame in

each subsequent slot with prob p until success

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Pure (unslotted) ALOHA Unslotted Aloha simpler no synchronization When frame first arrives

Transmit immediately

Collision probability increases Frame sent at t0 collides with other frames sent in [t0-1t0+1]

Even worse than slotted ALOHA

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Collisions can still occur Propagation delay means two nodes may not hear each otherrsquos transmission Entire packet transmission time wasted Role of distance amp propagation delay in determining collision probability spatial layout of nodes

Idea listen before transmit If channel sensed idle

transmit entire frame If channel sensed busy defer

transmission Human analogy donrsquot

interrupt others

CSMA (Carrier Sense Multiple Access)

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CSMACD (Collision Detection)

CSMACD carrier sensing deferral as in CSMA Collisions detected within short

time Send jam sequence Colliding transmissions aborted

reducing channel wastage

Collision detection

Easy in wired LANs measure signal strengths compare transmitted received signals

Difficult in wireless LANs receiver shut off while transmitting

Human analogy the polite conversationalist

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ldquoTaking Turnsrdquo MAC protocols Channel partitioning MAC protocols

Share channel efficiently and fairly at high load Inefficient at low load

Delay in channel access 1N bandwidth allocated even if only 1 active node

Random access MAC protocols Efficient at low load

Single node can fully utilize channel

high load Collision overhead

ldquoTaking turnsrdquo protocols Look for best of both worlds

Two implementations Polling Token

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ldquoTaking Turnsrdquo MAC protocolsPolling Master node ldquoinvitesrdquo slave

nodes to transmit in turn Concerns

Polling overhead Latency Single point of failure (master)

Token passing control token passed from

one node to next sequentially

token message concerns

token overhead latency single point of failure (token)

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Summary of MAC protocols

What do you do with a shared media Channel Partitioning by time frequency or code

Time Division Frequency Division

Random partitioning (dynamic) ALOHA S-ALOHA CSMA CSMACD Carrier sensing easy in some technologies (wire) hard in others

(wireless) CSMACD used in Ethernet CSMACA used in 80211

Taking Turns Polling from a central site token passing

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LAN technologies

Data link layer so farServicesError detectioncorrectionMultiple access

Next LAN technologiesAddressingEthernetHubs switchesPPP

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MAC Addresses and ARP 32-bit IP address

Network-layer address Used to get datagram to

destination IP subnet

MAC (or LAN or physical or Ethernet) address Used to get datagram

from one interface to another physically-connected interface (same network)

48 bit MAC address (for most LANs) burned in the adapter ROM

Find your PCrsquos MAC address

1A-2F-BB-76-09-AD

58-23-D7-FA-20-B0

0C-C4-11-6F-E3-98

71-65-F7-2B-08-53

LAN(wired orwireless)

Each adapter on LAN has unique LAN address

Broadcast address =FF-FF-FF-FF-FF-FF = adapter

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MAC Address (more)

MAC address allocation administered by IEEE Manufacturer buys portion of MAC address space (to

assure uniqueness) Analogy

(a) MAC address like Social Security Number

(b) IP address like postal address MAC flat address portability

Can move LAN card from one LAN to another

IP hierarchical address NOT portable Depends on IP subnet to which node is attached

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ARP Address Resolution Protocol

Each IP node (Host Router) on LAN has ARP table

ARP Table IPMAC address mappings for some LAN nodes

lt IP address MAC address TTLgt TTL (Time To Live)

time after which address mapping will be forgotten (typically 20 min)

Question how to determineMAC address of Bknowing Brsquos IP address

1A-2F-BB-76-09-AD

58-23-D7-FA-20-B0

0C-C4-11-6F-E3-98

71-65-F7-2B-08-53

LAN

237196723

237196778

237196714

237196788

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ARP protocol Same LAN (network) A wants to send datagram

to B and Brsquos MAC address not in Arsquos ARP table

A broadcasts ARP query packet containing Bs IP address Dest MAC address = FF-FF-

FF-FF-FF-FF All machines on LAN receive

ARP query

B receives ARP packet replies to A with its (Bs) MAC address Frame sent to Arsquos MAC

address (unicast)

A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) Soft state information that

times out (goes away) unless refreshed

ARP is ldquoplug-and-playrdquo Nodes create their ARP

tables without intervention from net administrator

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Routing to another LAN Walkthrough send datagram from A to B via R

Assume A knows B IP address

Two ARP tables in router R one for each IP network (LAN) In routing table at source Host find router 111111111110 In ARP table at source find MAC address E6-E9-00-17-BB-4B etc A creates datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110

A creates link-layer frame with Rs MAC address as dest frame contains A-to-B IP datagram

Arsquos adapter sends frame Rrsquos adapter receives frame R removes IP datagram from Ethernet frame sees its destined to B R uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B

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A

R

B

Routing to another LAN

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EthernetldquoDominantrdquo wired LAN technology Cheap $20 for 100Mbs First widely used LAN technology Simpler cheaper than token LANs and ATM Kept up with speed race 10 Mbps ndash 10

Gbps

Metcalfersquos Ethernet sketch

Star Topology Bus topology popular

through mid 90s Now star topology prevails Connection choices hub or

switch (more later)

hub orswitch

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Ethernet Frame Structure

Preamble 7 bytes with pattern 10101010 followed by one byte with pattern 10101011 Used to synchronize receiver sender clock rates

Addresses 6 bytes If adapter receives frame with matching destination address or with broadcast

address (eg ARP packet) it passes data in frame to net-layer protocol Otherwise adapter discards frame

Type Indicates the higher layer protocol (mostly IP but others may be supported

such as Novell IPX and AppleTalk) CRC

Checked at receiver if error is detected the frame is simply dropped

Sending adapter encapsulates IP datagram in Ethernet frame

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Ethernet uses CSMACD Connectionless No handshaking between sending and receiving

adapter Unreliable receiving adapter doesnrsquot send acks or nacks to

sending adapter Stream of datagrams passed to network layer can have gaps Gaps will be filled if app is using TCP Otherwise app will see the gaps

No slots Adapter doesnrsquot transmit if it senses that some other adapter is

transmitting that is carrier sense Transmitting adapter aborts when it senses that another adapter is

transmitting that is collision detection Before attempting a retransmission adapter waits a random time

that is random access

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Ethernet CSMACD algorithm1 Adaptor receives datagram from

net layer amp creates frame2 If adapter senses channel idle it

starts to transmit frame If it senses channel busy waits until channel idle and then transmits

3 If adapter transmits entire frame without detecting another transmission the adapter is done with frame

4 If adapter detects another transmission while transmitting aborts and sends jam signal

5 After aborting adapter enters exponential backoff after the mth collision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2

Jam Signal make sure all other transmitters are aware of collision 48 bits

Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec

Exponential Backoff Goal adapt retransmission

attempts to estimated current load Heavy load random wait will be

longer

First collision choose K from 01 delay is K 512 bit transmission times

After second collision choose K from 0123hellip

After ten collisions choose K from 01234hellip1023

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CSMACD efficiency Tprop = max prop between 2 nodes in LAN ttrans = time to transmit max-size frame

Efficiency goes to 1 as tprop goes to 0 Goes to 1 as ttrans goes to infinity Much better than ALOHA but still decentralized

simple and cheap

Other Ethernets Switched Ethernet Fast Ethernet Gigabit Ethernet

transprop tt 51

1efficiency

copy Kemal Akkaya

Point to Point Data Link Control

One sender one receiver one link easier than broadcast link No Media Access Control No need for explicit MAC addressing eg dialup link ISDN line

Popular point-to-point DLC protocols PPP (point-to-point protocol) HDLC High level data link control (Data link used to be considered

ldquohigh layerrdquo in protocol stack

copy Kemal Akkaya

HDLC

Frame format for bit-oriented protocol HDLC

Control field of

(a) An information frame

(b) A supervisory frame

(c) An unnumbered frame

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HDLC Exchange of information supervisory and unnumbered

frames Information - data to be transmitted to user (next layer up)

Flow and error control piggybacked on information frames Supervisory - ARQ when piggyback not used Unnumbered - supplementary link control

Three phases Initialization Data transfer Disconnect

Uses bit stuffing Flag delimit frame at both ends 01111110 is the Flag May close one frame and open another Bit stuffing to find flag data

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Internet Datalink Protocol PPP Design Requirements [RFC 1557]

Packet framing encapsulation of network-layer datagram in data link frame Carry network layer data of any network layer protocol (not just IP) at

same time Ability to demultiplex upwards

Bit transparency must carry any bit pattern in the data field Error detection (no correction) Connection liveness detect signal link failure to network layer Network layer address negotiation endpoint can learnconfigure

each otherrsquos network address

PPP non-design requirements No error correctionrecovery No flow control Out of order delivery OK No need to support multipoint links (eg polling)

Error recovery flow control data re-ordering all relegated to higher layers

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PPP Data Frame

Flag delimiter (framing) Address does nothing (only one option) Control does nothing in the future possible multiple

control fields Protocol upper layer protocol to which frame delivered

(eg PPP-LCP IP IPCP etc) Info upper layer data being carried Check cyclic redundancy check for error detection

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Byte Stuffing ldquoData transparencyrdquo requirement data field must be allowed to include

flag pattern lt01111110gt Q is received lt01111110gt data or flag

Sender adds (ldquostuffsrdquo) extra lt 01111110gt byte after each lt 01111110gt data byte How about bit stuffing

Receiver Two 01111110 bytes in a row Discard first byte continue data reception Single 01111110 flag byte

flag bytepatternin datato send

flag byte pattern plusstuffed byte in transmitted data

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PPP Data Control Protocol Before exchanging

network-layer data data link peers must configure PPP link

(max frame length authentication)

learnconfigure network layer information

For IP carry IP Control Protocol (IPCP) messages (protocol field 8021) to configurelearn IP address

Diagram for bring a line up and down

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Link Layer Services Framing link access

Encapsulate datagram into frame adding header trailer Channel access if shared medium ldquoMACrdquo addresses used in frame headers to identify source dest

Different from IP address

Reliable delivery between adjacent nodes Seldom used on low bit error link (fiber some twisted pair) Wireless links high error rates

Q why both link-level and end-end reliability

Flow Control Pacing between adjacent sending and receiving nodes

Error Detection Errors caused by signal attenuation noise Receiver detects presence of errors

Signals sender for retransmission or drops frame

Error Correction Receiver identifies and corrects bit error(s) without resorting to retransmission

Half-duplex and full-duplex With half duplex nodes at both ends of link can transmit but not at same time

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Adaptors Communicating

Link layer implemented in ldquoadaptorrdquo (aka NIC) Ethernet card PCMCI card

80211 card

Sending side Encapsulates datagram in a

frame Adds error checking bits rdt

flow control etc

Receiving side Looks for errors rdt flow

control etc Extracts datagram passes to

receiving node

Adapter is semi-autonomous

Link amp physical layers

sendingnode

frame

receivingnode

datagram

frame

adapter adapter

link layer protocol

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Link Layer - Outline

Flow Control

Error detection

Error correction

Multiple access protocols

Link-Layer Addressing

Ethernet

HDLC amp PPP

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Flow Control The Data Link Layer also provides Flow Control What is Flow Control

Ensuring the sending entity does not overwhelm the receiving entity Preventing buffer overflow

One Solution Stop and Wait Flow Control Source transmits frame Destination receives frame and replies with acknowledgement Source waits for ACK before sending next frame Destination can stop flow by not sending ACK Works well for a few large frames

BUT Large block of data may be split into small frames Limited buffer size Errors detected sooner (when whole frame received) On error retransmission of smaller frames is needed Prevents one station occupying medium for long periods

Stop and wait becomes inadequate Link Utilization

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Stop and Wait Link Utilization

Transmission time =1 Propagation time = a

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Sliding Windows Flow Control Allow multiple frames to be in transit Receiver has buffer W long Transmitter can send up to W frames without ACK Each frame is numbered ACK includes number of next frame expected Sequence number bounded by size of field (k)

Frames are numbered modulo 2k

Enhancements Receiver can acknowledge frames without permitting

further transmission (Receive Not Ready) Must send a normal acknowledge to resume If duplex use piggybacking

If no data to send use acknowledgement frame If data but no acknowledgement to send send last acknowledgement

number again or have ACK valid flag (TCP)

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Sliding Windows Diagram

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Example

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Error DetectionEDC = Error Detection and Correction bits (redundancy)D = Data protected by error checking may include header fields

Error detection not 100 reliable Protocol may miss some errors but rarely Larger EDC field yields better detection and correction

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Parity Checking Single Bit Parity

Detect single bit errors

Two Dimensional Bit ParityDetect and correct single bit errors

Internet Checksum Detect ldquoerrorsrdquo (eg flipped bits)

in transmitted segment (note used at transport layer only) 00

Sender Treat segment contents as sequence of 16-bit integers Checksum addition (1rsquos complement sum) of segment contents Sender puts checksum value into UDP checksum field

Receiver Compute checksum of received segment Check if computed checksum equals checksum field value NO - error detected YES - no error

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Cyclic Redundancy Check View data bits D as a binary number Choose r+1 bit pattern (generator) G Goal choose r CRC bits R such that

ltDRgt exactly divisible by G (modulo 2) Receiver knows G divides ltDRgt by G If non-zero remainder error

detected Can detect all burst errors less than r+1 bits

Widely used in practice (ATM HDCL)

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CRC Example

WantD2r XOR R = nG

EquivalentlyD2r = nG XOR R

Equivalently

If we divide D2r by G want remainder R

R = remainder[ ]D2r

G

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Error Correction

Detection and correction of errors due to Lost frames Damaged frames Underlying channel can lose the frames or can be garbled

Solution at the Link Layer Automatic repeat request Error detection Positive acknowledgment Retransmission after timeout Negative acknowledgement and retransmission

Automatic Repeat Request (ARQ) methods Stop and wait Go back N Selective reject (selective retransmission)

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Stop and Wait ARQ Source transmits single

frame Wait for ACK If received frame damaged

discard it Transmitter has timeout If no ACK within timeout

retransmit

If ACK damaged transmitter will not recognize it Transmitter will retransmit Receive gets two copies of frame Use ACK0 and ACK1

Simple but inefficient

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More efficient method Go-back-N Based on sliding window If no error ACK as usual with next frame expected Use window to control number of outstanding frames If error reply with rejection

Discard that frame and all future frames until error frame received correctly

Transmitter must go back and retransmit that frame and all subsequent frames

Handling Damaged Frame Receiver detects error in frame i Receiver sends rejection-i Transmitter gets rejection-i Transmitter retransmits frame i and all subsequent

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Go-back-N Handling Lost Frame First Approach

Frame i lost Transmitter sends i+1 Receiver gets frame i+1 out of sequence Receiver send reject i Transmitter goes back to frame i and retransmits

Second Approach Frame i lost and no additional frame sent Receiver gets nothing and returns neither acknowledgement nor

rejection Transmitter times out and sends acknowledgement frame with P bit

set to 1 Receiver interprets this as command which it acknowledges with the

number of the next frame it expects (frame i ) Transmitter then retransmits frame i

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Go-back-N Damaged ACK

Receiver gets frame i and send acknowledgement (i+1) which is lost

Acknowledgements are cumulative so next acknowledgement (i+n) may arrive before transmitter times out on frame i

If transmitter times out it sends acknowledgement with P bit set as before

This can be repeated a number of times before a reset procedure is initiated

Damaged Rejection Same as Lost Frame (2)

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Selective Reject

Also called selective retransmission

Only rejected frames are retransmitted

Subsequent frames are accepted by the receiver and buffered

Minimizes retransmission Receiver must maintain

large enough buffer More complex login in

transmitter

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Multiple Access Links and ProtocolsTwo types of ldquolinksrdquo Point-to-point

PPP for dial-up access Point-to-point link between Ethernet switch and host

Broadcast (shared wire or medium) Traditional Ethernet Upstream HFC 80211 wireless LAN

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Multiple Access Protocols Single shared broadcast channel Two or more simultaneous transmissions by nodes interference

Collision if node receives two or more signals at the same time

Multiple access protocol Distributed algorithm that determines how nodes share channel ie

determine when node can transmit Communication about channel sharing must use channel itself

No out-of-band channel for coordination

Ideal Multiple Access Protocol When one node wants to transmit it can send at rate R When M nodes want to transmit each can send at average rate RM Fully decentralized

No special node to coordinate transmissions No synchronization of clocks slots

Simple

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MAC Protocols A taxonomy

Three broad classes

Channel Partitioning Divide channel into smaller ldquopiecesrdquo (time slots frequency code) Allocate piece to node for exclusive use

Random Access Channel not divided allow collisions ldquoRecoverrdquo from collisions

ldquoTaking turnsrdquo Nodes take turns Nodes with more to send can take longer turns

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Channel Partitioning MAC protocols TDMA

TDMA Time division multiple access Access to channel in rounds Each station gets fixed length slot

Length = packet transmission time in each round

Unused slots go idle Example 6-station LAN

134 have packet Slots 256 idle

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Channel Partitioning MAC protocols FDMA

FDMA Frequency division multiple access Channel spectrum divided into frequency bands Each station assigned fixed frequency band Unused transmission time in frequency bands go idle Example 6-station LAN 134 have packet frequency

bands 256 idle

frequ

ency

bands time

1

2

3

4

5

6

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Random Access Protocols

When node has packet to send Transmit at full channel data rate R No a priori coordination among nodes

Two or more transmitting nodes ldquocollisionrdquo Random access MAC protocol specifies

How to detect collisions How to recover from collisions (eg via delayed retransmissions)

Examples of random access MAC protocols Slotted ALOHA ALOHA Carrier Sense Multiple Access (CSMA) CSMACollision Detection (CD) CSMACollision Avoidance (CA)

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Slotted ALOHA

Pros Single active node can continuously

transmit at full rate of channel Highly decentralized only slots in

nodes need to be in sync Simple

Cons Collisions wasting slots Idle slots Nodes may be able to detect collision in

less than time to transmit packet Clock synchronization

Assumptions All frames same size Time is divided into equal size

slots time to transmit 1 frame Nodes start to transmit frames

only at beginning of slots Nodes are synchronized If 2 or more nodes transmit in

slot all nodes detect collision

Operation When node obtains fresh frame it

transmits in next slot No collision node can send new frame in

next slot If collision node retransmits frame in

each subsequent slot with prob p until success

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Pure (unslotted) ALOHA Unslotted Aloha simpler no synchronization When frame first arrives

Transmit immediately

Collision probability increases Frame sent at t0 collides with other frames sent in [t0-1t0+1]

Even worse than slotted ALOHA

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Collisions can still occur Propagation delay means two nodes may not hear each otherrsquos transmission Entire packet transmission time wasted Role of distance amp propagation delay in determining collision probability spatial layout of nodes

Idea listen before transmit If channel sensed idle

transmit entire frame If channel sensed busy defer

transmission Human analogy donrsquot

interrupt others

CSMA (Carrier Sense Multiple Access)

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CSMACD (Collision Detection)

CSMACD carrier sensing deferral as in CSMA Collisions detected within short

time Send jam sequence Colliding transmissions aborted

reducing channel wastage

Collision detection

Easy in wired LANs measure signal strengths compare transmitted received signals

Difficult in wireless LANs receiver shut off while transmitting

Human analogy the polite conversationalist

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ldquoTaking Turnsrdquo MAC protocols Channel partitioning MAC protocols

Share channel efficiently and fairly at high load Inefficient at low load

Delay in channel access 1N bandwidth allocated even if only 1 active node

Random access MAC protocols Efficient at low load

Single node can fully utilize channel

high load Collision overhead

ldquoTaking turnsrdquo protocols Look for best of both worlds

Two implementations Polling Token

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ldquoTaking Turnsrdquo MAC protocolsPolling Master node ldquoinvitesrdquo slave

nodes to transmit in turn Concerns

Polling overhead Latency Single point of failure (master)

Token passing control token passed from

one node to next sequentially

token message concerns

token overhead latency single point of failure (token)

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Summary of MAC protocols

What do you do with a shared media Channel Partitioning by time frequency or code

Time Division Frequency Division

Random partitioning (dynamic) ALOHA S-ALOHA CSMA CSMACD Carrier sensing easy in some technologies (wire) hard in others

(wireless) CSMACD used in Ethernet CSMACA used in 80211

Taking Turns Polling from a central site token passing

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LAN technologies

Data link layer so farServicesError detectioncorrectionMultiple access

Next LAN technologiesAddressingEthernetHubs switchesPPP

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MAC Addresses and ARP 32-bit IP address

Network-layer address Used to get datagram to

destination IP subnet

MAC (or LAN or physical or Ethernet) address Used to get datagram

from one interface to another physically-connected interface (same network)

48 bit MAC address (for most LANs) burned in the adapter ROM

Find your PCrsquos MAC address

1A-2F-BB-76-09-AD

58-23-D7-FA-20-B0

0C-C4-11-6F-E3-98

71-65-F7-2B-08-53

LAN(wired orwireless)

Each adapter on LAN has unique LAN address

Broadcast address =FF-FF-FF-FF-FF-FF = adapter

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MAC Address (more)

MAC address allocation administered by IEEE Manufacturer buys portion of MAC address space (to

assure uniqueness) Analogy

(a) MAC address like Social Security Number

(b) IP address like postal address MAC flat address portability

Can move LAN card from one LAN to another

IP hierarchical address NOT portable Depends on IP subnet to which node is attached

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ARP Address Resolution Protocol

Each IP node (Host Router) on LAN has ARP table

ARP Table IPMAC address mappings for some LAN nodes

lt IP address MAC address TTLgt TTL (Time To Live)

time after which address mapping will be forgotten (typically 20 min)

Question how to determineMAC address of Bknowing Brsquos IP address

1A-2F-BB-76-09-AD

58-23-D7-FA-20-B0

0C-C4-11-6F-E3-98

71-65-F7-2B-08-53

LAN

237196723

237196778

237196714

237196788

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ARP protocol Same LAN (network) A wants to send datagram

to B and Brsquos MAC address not in Arsquos ARP table

A broadcasts ARP query packet containing Bs IP address Dest MAC address = FF-FF-

FF-FF-FF-FF All machines on LAN receive

ARP query

B receives ARP packet replies to A with its (Bs) MAC address Frame sent to Arsquos MAC

address (unicast)

A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) Soft state information that

times out (goes away) unless refreshed

ARP is ldquoplug-and-playrdquo Nodes create their ARP

tables without intervention from net administrator

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Routing to another LAN Walkthrough send datagram from A to B via R

Assume A knows B IP address

Two ARP tables in router R one for each IP network (LAN) In routing table at source Host find router 111111111110 In ARP table at source find MAC address E6-E9-00-17-BB-4B etc A creates datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110

A creates link-layer frame with Rs MAC address as dest frame contains A-to-B IP datagram

Arsquos adapter sends frame Rrsquos adapter receives frame R removes IP datagram from Ethernet frame sees its destined to B R uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B

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A

R

B

Routing to another LAN

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EthernetldquoDominantrdquo wired LAN technology Cheap $20 for 100Mbs First widely used LAN technology Simpler cheaper than token LANs and ATM Kept up with speed race 10 Mbps ndash 10

Gbps

Metcalfersquos Ethernet sketch

Star Topology Bus topology popular

through mid 90s Now star topology prevails Connection choices hub or

switch (more later)

hub orswitch

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Ethernet Frame Structure

Preamble 7 bytes with pattern 10101010 followed by one byte with pattern 10101011 Used to synchronize receiver sender clock rates

Addresses 6 bytes If adapter receives frame with matching destination address or with broadcast

address (eg ARP packet) it passes data in frame to net-layer protocol Otherwise adapter discards frame

Type Indicates the higher layer protocol (mostly IP but others may be supported

such as Novell IPX and AppleTalk) CRC

Checked at receiver if error is detected the frame is simply dropped

Sending adapter encapsulates IP datagram in Ethernet frame

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Ethernet uses CSMACD Connectionless No handshaking between sending and receiving

adapter Unreliable receiving adapter doesnrsquot send acks or nacks to

sending adapter Stream of datagrams passed to network layer can have gaps Gaps will be filled if app is using TCP Otherwise app will see the gaps

No slots Adapter doesnrsquot transmit if it senses that some other adapter is

transmitting that is carrier sense Transmitting adapter aborts when it senses that another adapter is

transmitting that is collision detection Before attempting a retransmission adapter waits a random time

that is random access

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Ethernet CSMACD algorithm1 Adaptor receives datagram from

net layer amp creates frame2 If adapter senses channel idle it

starts to transmit frame If it senses channel busy waits until channel idle and then transmits

3 If adapter transmits entire frame without detecting another transmission the adapter is done with frame

4 If adapter detects another transmission while transmitting aborts and sends jam signal

5 After aborting adapter enters exponential backoff after the mth collision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2

Jam Signal make sure all other transmitters are aware of collision 48 bits

Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec

Exponential Backoff Goal adapt retransmission

attempts to estimated current load Heavy load random wait will be

longer

First collision choose K from 01 delay is K 512 bit transmission times

After second collision choose K from 0123hellip

After ten collisions choose K from 01234hellip1023

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CSMACD efficiency Tprop = max prop between 2 nodes in LAN ttrans = time to transmit max-size frame

Efficiency goes to 1 as tprop goes to 0 Goes to 1 as ttrans goes to infinity Much better than ALOHA but still decentralized

simple and cheap

Other Ethernets Switched Ethernet Fast Ethernet Gigabit Ethernet

transprop tt 51

1efficiency

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Point to Point Data Link Control

One sender one receiver one link easier than broadcast link No Media Access Control No need for explicit MAC addressing eg dialup link ISDN line

Popular point-to-point DLC protocols PPP (point-to-point protocol) HDLC High level data link control (Data link used to be considered

ldquohigh layerrdquo in protocol stack

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HDLC

Frame format for bit-oriented protocol HDLC

Control field of

(a) An information frame

(b) A supervisory frame

(c) An unnumbered frame

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HDLC Exchange of information supervisory and unnumbered

frames Information - data to be transmitted to user (next layer up)

Flow and error control piggybacked on information frames Supervisory - ARQ when piggyback not used Unnumbered - supplementary link control

Three phases Initialization Data transfer Disconnect

Uses bit stuffing Flag delimit frame at both ends 01111110 is the Flag May close one frame and open another Bit stuffing to find flag data

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Internet Datalink Protocol PPP Design Requirements [RFC 1557]

Packet framing encapsulation of network-layer datagram in data link frame Carry network layer data of any network layer protocol (not just IP) at

same time Ability to demultiplex upwards

Bit transparency must carry any bit pattern in the data field Error detection (no correction) Connection liveness detect signal link failure to network layer Network layer address negotiation endpoint can learnconfigure

each otherrsquos network address

PPP non-design requirements No error correctionrecovery No flow control Out of order delivery OK No need to support multipoint links (eg polling)

Error recovery flow control data re-ordering all relegated to higher layers

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PPP Data Frame

Flag delimiter (framing) Address does nothing (only one option) Control does nothing in the future possible multiple

control fields Protocol upper layer protocol to which frame delivered

(eg PPP-LCP IP IPCP etc) Info upper layer data being carried Check cyclic redundancy check for error detection

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Byte Stuffing ldquoData transparencyrdquo requirement data field must be allowed to include

flag pattern lt01111110gt Q is received lt01111110gt data or flag

Sender adds (ldquostuffsrdquo) extra lt 01111110gt byte after each lt 01111110gt data byte How about bit stuffing

Receiver Two 01111110 bytes in a row Discard first byte continue data reception Single 01111110 flag byte

flag bytepatternin datato send

flag byte pattern plusstuffed byte in transmitted data

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PPP Data Control Protocol Before exchanging

network-layer data data link peers must configure PPP link

(max frame length authentication)

learnconfigure network layer information

For IP carry IP Control Protocol (IPCP) messages (protocol field 8021) to configurelearn IP address

Diagram for bring a line up and down

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Adaptors Communicating

Link layer implemented in ldquoadaptorrdquo (aka NIC) Ethernet card PCMCI card

80211 card

Sending side Encapsulates datagram in a

frame Adds error checking bits rdt

flow control etc

Receiving side Looks for errors rdt flow

control etc Extracts datagram passes to

receiving node

Adapter is semi-autonomous

Link amp physical layers

sendingnode

frame

receivingnode

datagram

frame

adapter adapter

link layer protocol

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Link Layer - Outline

Flow Control

Error detection

Error correction

Multiple access protocols

Link-Layer Addressing

Ethernet

HDLC amp PPP

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Flow Control The Data Link Layer also provides Flow Control What is Flow Control

Ensuring the sending entity does not overwhelm the receiving entity Preventing buffer overflow

One Solution Stop and Wait Flow Control Source transmits frame Destination receives frame and replies with acknowledgement Source waits for ACK before sending next frame Destination can stop flow by not sending ACK Works well for a few large frames

BUT Large block of data may be split into small frames Limited buffer size Errors detected sooner (when whole frame received) On error retransmission of smaller frames is needed Prevents one station occupying medium for long periods

Stop and wait becomes inadequate Link Utilization

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Stop and Wait Link Utilization

Transmission time =1 Propagation time = a

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Sliding Windows Flow Control Allow multiple frames to be in transit Receiver has buffer W long Transmitter can send up to W frames without ACK Each frame is numbered ACK includes number of next frame expected Sequence number bounded by size of field (k)

Frames are numbered modulo 2k

Enhancements Receiver can acknowledge frames without permitting

further transmission (Receive Not Ready) Must send a normal acknowledge to resume If duplex use piggybacking

If no data to send use acknowledgement frame If data but no acknowledgement to send send last acknowledgement

number again or have ACK valid flag (TCP)

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Sliding Windows Diagram

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Example

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Error DetectionEDC = Error Detection and Correction bits (redundancy)D = Data protected by error checking may include header fields

Error detection not 100 reliable Protocol may miss some errors but rarely Larger EDC field yields better detection and correction

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Parity Checking Single Bit Parity

Detect single bit errors

Two Dimensional Bit ParityDetect and correct single bit errors

Internet Checksum Detect ldquoerrorsrdquo (eg flipped bits)

in transmitted segment (note used at transport layer only) 00

Sender Treat segment contents as sequence of 16-bit integers Checksum addition (1rsquos complement sum) of segment contents Sender puts checksum value into UDP checksum field

Receiver Compute checksum of received segment Check if computed checksum equals checksum field value NO - error detected YES - no error

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Cyclic Redundancy Check View data bits D as a binary number Choose r+1 bit pattern (generator) G Goal choose r CRC bits R such that

ltDRgt exactly divisible by G (modulo 2) Receiver knows G divides ltDRgt by G If non-zero remainder error

detected Can detect all burst errors less than r+1 bits

Widely used in practice (ATM HDCL)

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CRC Example

WantD2r XOR R = nG

EquivalentlyD2r = nG XOR R

Equivalently

If we divide D2r by G want remainder R

R = remainder[ ]D2r

G

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Error Correction

Detection and correction of errors due to Lost frames Damaged frames Underlying channel can lose the frames or can be garbled

Solution at the Link Layer Automatic repeat request Error detection Positive acknowledgment Retransmission after timeout Negative acknowledgement and retransmission

Automatic Repeat Request (ARQ) methods Stop and wait Go back N Selective reject (selective retransmission)

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Stop and Wait ARQ Source transmits single

frame Wait for ACK If received frame damaged

discard it Transmitter has timeout If no ACK within timeout

retransmit

If ACK damaged transmitter will not recognize it Transmitter will retransmit Receive gets two copies of frame Use ACK0 and ACK1

Simple but inefficient

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More efficient method Go-back-N Based on sliding window If no error ACK as usual with next frame expected Use window to control number of outstanding frames If error reply with rejection

Discard that frame and all future frames until error frame received correctly

Transmitter must go back and retransmit that frame and all subsequent frames

Handling Damaged Frame Receiver detects error in frame i Receiver sends rejection-i Transmitter gets rejection-i Transmitter retransmits frame i and all subsequent

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Go-back-N Handling Lost Frame First Approach

Frame i lost Transmitter sends i+1 Receiver gets frame i+1 out of sequence Receiver send reject i Transmitter goes back to frame i and retransmits

Second Approach Frame i lost and no additional frame sent Receiver gets nothing and returns neither acknowledgement nor

rejection Transmitter times out and sends acknowledgement frame with P bit

set to 1 Receiver interprets this as command which it acknowledges with the

number of the next frame it expects (frame i ) Transmitter then retransmits frame i

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Go-back-N Damaged ACK

Receiver gets frame i and send acknowledgement (i+1) which is lost

Acknowledgements are cumulative so next acknowledgement (i+n) may arrive before transmitter times out on frame i

If transmitter times out it sends acknowledgement with P bit set as before

This can be repeated a number of times before a reset procedure is initiated

Damaged Rejection Same as Lost Frame (2)

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Selective Reject

Also called selective retransmission

Only rejected frames are retransmitted

Subsequent frames are accepted by the receiver and buffered

Minimizes retransmission Receiver must maintain

large enough buffer More complex login in

transmitter

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Multiple Access Links and ProtocolsTwo types of ldquolinksrdquo Point-to-point

PPP for dial-up access Point-to-point link between Ethernet switch and host

Broadcast (shared wire or medium) Traditional Ethernet Upstream HFC 80211 wireless LAN

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Multiple Access Protocols Single shared broadcast channel Two or more simultaneous transmissions by nodes interference

Collision if node receives two or more signals at the same time

Multiple access protocol Distributed algorithm that determines how nodes share channel ie

determine when node can transmit Communication about channel sharing must use channel itself

No out-of-band channel for coordination

Ideal Multiple Access Protocol When one node wants to transmit it can send at rate R When M nodes want to transmit each can send at average rate RM Fully decentralized

No special node to coordinate transmissions No synchronization of clocks slots

Simple

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MAC Protocols A taxonomy

Three broad classes

Channel Partitioning Divide channel into smaller ldquopiecesrdquo (time slots frequency code) Allocate piece to node for exclusive use

Random Access Channel not divided allow collisions ldquoRecoverrdquo from collisions

ldquoTaking turnsrdquo Nodes take turns Nodes with more to send can take longer turns

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Channel Partitioning MAC protocols TDMA

TDMA Time division multiple access Access to channel in rounds Each station gets fixed length slot

Length = packet transmission time in each round

Unused slots go idle Example 6-station LAN

134 have packet Slots 256 idle

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Channel Partitioning MAC protocols FDMA

FDMA Frequency division multiple access Channel spectrum divided into frequency bands Each station assigned fixed frequency band Unused transmission time in frequency bands go idle Example 6-station LAN 134 have packet frequency

bands 256 idle

frequ

ency

bands time

1

2

3

4

5

6

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Random Access Protocols

When node has packet to send Transmit at full channel data rate R No a priori coordination among nodes

Two or more transmitting nodes ldquocollisionrdquo Random access MAC protocol specifies

How to detect collisions How to recover from collisions (eg via delayed retransmissions)

Examples of random access MAC protocols Slotted ALOHA ALOHA Carrier Sense Multiple Access (CSMA) CSMACollision Detection (CD) CSMACollision Avoidance (CA)

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Slotted ALOHA

Pros Single active node can continuously

transmit at full rate of channel Highly decentralized only slots in

nodes need to be in sync Simple

Cons Collisions wasting slots Idle slots Nodes may be able to detect collision in

less than time to transmit packet Clock synchronization

Assumptions All frames same size Time is divided into equal size

slots time to transmit 1 frame Nodes start to transmit frames

only at beginning of slots Nodes are synchronized If 2 or more nodes transmit in

slot all nodes detect collision

Operation When node obtains fresh frame it

transmits in next slot No collision node can send new frame in

next slot If collision node retransmits frame in

each subsequent slot with prob p until success

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Pure (unslotted) ALOHA Unslotted Aloha simpler no synchronization When frame first arrives

Transmit immediately

Collision probability increases Frame sent at t0 collides with other frames sent in [t0-1t0+1]

Even worse than slotted ALOHA

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Collisions can still occur Propagation delay means two nodes may not hear each otherrsquos transmission Entire packet transmission time wasted Role of distance amp propagation delay in determining collision probability spatial layout of nodes

Idea listen before transmit If channel sensed idle

transmit entire frame If channel sensed busy defer

transmission Human analogy donrsquot

interrupt others

CSMA (Carrier Sense Multiple Access)

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CSMACD (Collision Detection)

CSMACD carrier sensing deferral as in CSMA Collisions detected within short

time Send jam sequence Colliding transmissions aborted

reducing channel wastage

Collision detection

Easy in wired LANs measure signal strengths compare transmitted received signals

Difficult in wireless LANs receiver shut off while transmitting

Human analogy the polite conversationalist

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ldquoTaking Turnsrdquo MAC protocols Channel partitioning MAC protocols

Share channel efficiently and fairly at high load Inefficient at low load

Delay in channel access 1N bandwidth allocated even if only 1 active node

Random access MAC protocols Efficient at low load

Single node can fully utilize channel

high load Collision overhead

ldquoTaking turnsrdquo protocols Look for best of both worlds

Two implementations Polling Token

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ldquoTaking Turnsrdquo MAC protocolsPolling Master node ldquoinvitesrdquo slave

nodes to transmit in turn Concerns

Polling overhead Latency Single point of failure (master)

Token passing control token passed from

one node to next sequentially

token message concerns

token overhead latency single point of failure (token)

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Summary of MAC protocols

What do you do with a shared media Channel Partitioning by time frequency or code

Time Division Frequency Division

Random partitioning (dynamic) ALOHA S-ALOHA CSMA CSMACD Carrier sensing easy in some technologies (wire) hard in others

(wireless) CSMACD used in Ethernet CSMACA used in 80211

Taking Turns Polling from a central site token passing

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LAN technologies

Data link layer so farServicesError detectioncorrectionMultiple access

Next LAN technologiesAddressingEthernetHubs switchesPPP

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MAC Addresses and ARP 32-bit IP address

Network-layer address Used to get datagram to

destination IP subnet

MAC (or LAN or physical or Ethernet) address Used to get datagram

from one interface to another physically-connected interface (same network)

48 bit MAC address (for most LANs) burned in the adapter ROM

Find your PCrsquos MAC address

1A-2F-BB-76-09-AD

58-23-D7-FA-20-B0

0C-C4-11-6F-E3-98

71-65-F7-2B-08-53

LAN(wired orwireless)

Each adapter on LAN has unique LAN address

Broadcast address =FF-FF-FF-FF-FF-FF = adapter

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MAC Address (more)

MAC address allocation administered by IEEE Manufacturer buys portion of MAC address space (to

assure uniqueness) Analogy

(a) MAC address like Social Security Number

(b) IP address like postal address MAC flat address portability

Can move LAN card from one LAN to another

IP hierarchical address NOT portable Depends on IP subnet to which node is attached

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ARP Address Resolution Protocol

Each IP node (Host Router) on LAN has ARP table

ARP Table IPMAC address mappings for some LAN nodes

lt IP address MAC address TTLgt TTL (Time To Live)

time after which address mapping will be forgotten (typically 20 min)

Question how to determineMAC address of Bknowing Brsquos IP address

1A-2F-BB-76-09-AD

58-23-D7-FA-20-B0

0C-C4-11-6F-E3-98

71-65-F7-2B-08-53

LAN

237196723

237196778

237196714

237196788

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ARP protocol Same LAN (network) A wants to send datagram

to B and Brsquos MAC address not in Arsquos ARP table

A broadcasts ARP query packet containing Bs IP address Dest MAC address = FF-FF-

FF-FF-FF-FF All machines on LAN receive

ARP query

B receives ARP packet replies to A with its (Bs) MAC address Frame sent to Arsquos MAC

address (unicast)

A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) Soft state information that

times out (goes away) unless refreshed

ARP is ldquoplug-and-playrdquo Nodes create their ARP

tables without intervention from net administrator

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Routing to another LAN Walkthrough send datagram from A to B via R

Assume A knows B IP address

Two ARP tables in router R one for each IP network (LAN) In routing table at source Host find router 111111111110 In ARP table at source find MAC address E6-E9-00-17-BB-4B etc A creates datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110

A creates link-layer frame with Rs MAC address as dest frame contains A-to-B IP datagram

Arsquos adapter sends frame Rrsquos adapter receives frame R removes IP datagram from Ethernet frame sees its destined to B R uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B

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A

R

B

Routing to another LAN

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EthernetldquoDominantrdquo wired LAN technology Cheap $20 for 100Mbs First widely used LAN technology Simpler cheaper than token LANs and ATM Kept up with speed race 10 Mbps ndash 10

Gbps

Metcalfersquos Ethernet sketch

Star Topology Bus topology popular

through mid 90s Now star topology prevails Connection choices hub or

switch (more later)

hub orswitch

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Ethernet Frame Structure

Preamble 7 bytes with pattern 10101010 followed by one byte with pattern 10101011 Used to synchronize receiver sender clock rates

Addresses 6 bytes If adapter receives frame with matching destination address or with broadcast

address (eg ARP packet) it passes data in frame to net-layer protocol Otherwise adapter discards frame

Type Indicates the higher layer protocol (mostly IP but others may be supported

such as Novell IPX and AppleTalk) CRC

Checked at receiver if error is detected the frame is simply dropped

Sending adapter encapsulates IP datagram in Ethernet frame

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Ethernet uses CSMACD Connectionless No handshaking between sending and receiving

adapter Unreliable receiving adapter doesnrsquot send acks or nacks to

sending adapter Stream of datagrams passed to network layer can have gaps Gaps will be filled if app is using TCP Otherwise app will see the gaps

No slots Adapter doesnrsquot transmit if it senses that some other adapter is

transmitting that is carrier sense Transmitting adapter aborts when it senses that another adapter is

transmitting that is collision detection Before attempting a retransmission adapter waits a random time

that is random access

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Ethernet CSMACD algorithm1 Adaptor receives datagram from

net layer amp creates frame2 If adapter senses channel idle it

starts to transmit frame If it senses channel busy waits until channel idle and then transmits

3 If adapter transmits entire frame without detecting another transmission the adapter is done with frame

4 If adapter detects another transmission while transmitting aborts and sends jam signal

5 After aborting adapter enters exponential backoff after the mth collision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2

Jam Signal make sure all other transmitters are aware of collision 48 bits

Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec

Exponential Backoff Goal adapt retransmission

attempts to estimated current load Heavy load random wait will be

longer

First collision choose K from 01 delay is K 512 bit transmission times

After second collision choose K from 0123hellip

After ten collisions choose K from 01234hellip1023

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CSMACD efficiency Tprop = max prop between 2 nodes in LAN ttrans = time to transmit max-size frame

Efficiency goes to 1 as tprop goes to 0 Goes to 1 as ttrans goes to infinity Much better than ALOHA but still decentralized

simple and cheap

Other Ethernets Switched Ethernet Fast Ethernet Gigabit Ethernet

transprop tt 51

1efficiency

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Point to Point Data Link Control

One sender one receiver one link easier than broadcast link No Media Access Control No need for explicit MAC addressing eg dialup link ISDN line

Popular point-to-point DLC protocols PPP (point-to-point protocol) HDLC High level data link control (Data link used to be considered

ldquohigh layerrdquo in protocol stack

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HDLC

Frame format for bit-oriented protocol HDLC

Control field of

(a) An information frame

(b) A supervisory frame

(c) An unnumbered frame

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HDLC Exchange of information supervisory and unnumbered

frames Information - data to be transmitted to user (next layer up)

Flow and error control piggybacked on information frames Supervisory - ARQ when piggyback not used Unnumbered - supplementary link control

Three phases Initialization Data transfer Disconnect

Uses bit stuffing Flag delimit frame at both ends 01111110 is the Flag May close one frame and open another Bit stuffing to find flag data

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Internet Datalink Protocol PPP Design Requirements [RFC 1557]

Packet framing encapsulation of network-layer datagram in data link frame Carry network layer data of any network layer protocol (not just IP) at

same time Ability to demultiplex upwards

Bit transparency must carry any bit pattern in the data field Error detection (no correction) Connection liveness detect signal link failure to network layer Network layer address negotiation endpoint can learnconfigure

each otherrsquos network address

PPP non-design requirements No error correctionrecovery No flow control Out of order delivery OK No need to support multipoint links (eg polling)

Error recovery flow control data re-ordering all relegated to higher layers

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PPP Data Frame

Flag delimiter (framing) Address does nothing (only one option) Control does nothing in the future possible multiple

control fields Protocol upper layer protocol to which frame delivered

(eg PPP-LCP IP IPCP etc) Info upper layer data being carried Check cyclic redundancy check for error detection

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Byte Stuffing ldquoData transparencyrdquo requirement data field must be allowed to include

flag pattern lt01111110gt Q is received lt01111110gt data or flag

Sender adds (ldquostuffsrdquo) extra lt 01111110gt byte after each lt 01111110gt data byte How about bit stuffing

Receiver Two 01111110 bytes in a row Discard first byte continue data reception Single 01111110 flag byte

flag bytepatternin datato send

flag byte pattern plusstuffed byte in transmitted data

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PPP Data Control Protocol Before exchanging

network-layer data data link peers must configure PPP link

(max frame length authentication)

learnconfigure network layer information

For IP carry IP Control Protocol (IPCP) messages (protocol field 8021) to configurelearn IP address

Diagram for bring a line up and down

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Link Layer - Outline

Flow Control

Error detection

Error correction

Multiple access protocols

Link-Layer Addressing

Ethernet

HDLC amp PPP

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Flow Control The Data Link Layer also provides Flow Control What is Flow Control

Ensuring the sending entity does not overwhelm the receiving entity Preventing buffer overflow

One Solution Stop and Wait Flow Control Source transmits frame Destination receives frame and replies with acknowledgement Source waits for ACK before sending next frame Destination can stop flow by not sending ACK Works well for a few large frames

BUT Large block of data may be split into small frames Limited buffer size Errors detected sooner (when whole frame received) On error retransmission of smaller frames is needed Prevents one station occupying medium for long periods

Stop and wait becomes inadequate Link Utilization

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Stop and Wait Link Utilization

Transmission time =1 Propagation time = a

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Sliding Windows Flow Control Allow multiple frames to be in transit Receiver has buffer W long Transmitter can send up to W frames without ACK Each frame is numbered ACK includes number of next frame expected Sequence number bounded by size of field (k)

Frames are numbered modulo 2k

Enhancements Receiver can acknowledge frames without permitting

further transmission (Receive Not Ready) Must send a normal acknowledge to resume If duplex use piggybacking

If no data to send use acknowledgement frame If data but no acknowledgement to send send last acknowledgement

number again or have ACK valid flag (TCP)

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Sliding Windows Diagram

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Example

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Error DetectionEDC = Error Detection and Correction bits (redundancy)D = Data protected by error checking may include header fields

Error detection not 100 reliable Protocol may miss some errors but rarely Larger EDC field yields better detection and correction

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Parity Checking Single Bit Parity

Detect single bit errors

Two Dimensional Bit ParityDetect and correct single bit errors

Internet Checksum Detect ldquoerrorsrdquo (eg flipped bits)

in transmitted segment (note used at transport layer only) 00

Sender Treat segment contents as sequence of 16-bit integers Checksum addition (1rsquos complement sum) of segment contents Sender puts checksum value into UDP checksum field

Receiver Compute checksum of received segment Check if computed checksum equals checksum field value NO - error detected YES - no error

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Cyclic Redundancy Check View data bits D as a binary number Choose r+1 bit pattern (generator) G Goal choose r CRC bits R such that

ltDRgt exactly divisible by G (modulo 2) Receiver knows G divides ltDRgt by G If non-zero remainder error

detected Can detect all burst errors less than r+1 bits

Widely used in practice (ATM HDCL)

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CRC Example

WantD2r XOR R = nG

EquivalentlyD2r = nG XOR R

Equivalently

If we divide D2r by G want remainder R

R = remainder[ ]D2r

G

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Error Correction

Detection and correction of errors due to Lost frames Damaged frames Underlying channel can lose the frames or can be garbled

Solution at the Link Layer Automatic repeat request Error detection Positive acknowledgment Retransmission after timeout Negative acknowledgement and retransmission

Automatic Repeat Request (ARQ) methods Stop and wait Go back N Selective reject (selective retransmission)

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Stop and Wait ARQ Source transmits single

frame Wait for ACK If received frame damaged

discard it Transmitter has timeout If no ACK within timeout

retransmit

If ACK damaged transmitter will not recognize it Transmitter will retransmit Receive gets two copies of frame Use ACK0 and ACK1

Simple but inefficient

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More efficient method Go-back-N Based on sliding window If no error ACK as usual with next frame expected Use window to control number of outstanding frames If error reply with rejection

Discard that frame and all future frames until error frame received correctly

Transmitter must go back and retransmit that frame and all subsequent frames

Handling Damaged Frame Receiver detects error in frame i Receiver sends rejection-i Transmitter gets rejection-i Transmitter retransmits frame i and all subsequent

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Go-back-N Handling Lost Frame First Approach

Frame i lost Transmitter sends i+1 Receiver gets frame i+1 out of sequence Receiver send reject i Transmitter goes back to frame i and retransmits

Second Approach Frame i lost and no additional frame sent Receiver gets nothing and returns neither acknowledgement nor

rejection Transmitter times out and sends acknowledgement frame with P bit

set to 1 Receiver interprets this as command which it acknowledges with the

number of the next frame it expects (frame i ) Transmitter then retransmits frame i

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Go-back-N Damaged ACK

Receiver gets frame i and send acknowledgement (i+1) which is lost

Acknowledgements are cumulative so next acknowledgement (i+n) may arrive before transmitter times out on frame i

If transmitter times out it sends acknowledgement with P bit set as before

This can be repeated a number of times before a reset procedure is initiated

Damaged Rejection Same as Lost Frame (2)

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Selective Reject

Also called selective retransmission

Only rejected frames are retransmitted

Subsequent frames are accepted by the receiver and buffered

Minimizes retransmission Receiver must maintain

large enough buffer More complex login in

transmitter

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Multiple Access Links and ProtocolsTwo types of ldquolinksrdquo Point-to-point

PPP for dial-up access Point-to-point link between Ethernet switch and host

Broadcast (shared wire or medium) Traditional Ethernet Upstream HFC 80211 wireless LAN

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Multiple Access Protocols Single shared broadcast channel Two or more simultaneous transmissions by nodes interference

Collision if node receives two or more signals at the same time

Multiple access protocol Distributed algorithm that determines how nodes share channel ie

determine when node can transmit Communication about channel sharing must use channel itself

No out-of-band channel for coordination

Ideal Multiple Access Protocol When one node wants to transmit it can send at rate R When M nodes want to transmit each can send at average rate RM Fully decentralized

No special node to coordinate transmissions No synchronization of clocks slots

Simple

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MAC Protocols A taxonomy

Three broad classes

Channel Partitioning Divide channel into smaller ldquopiecesrdquo (time slots frequency code) Allocate piece to node for exclusive use

Random Access Channel not divided allow collisions ldquoRecoverrdquo from collisions

ldquoTaking turnsrdquo Nodes take turns Nodes with more to send can take longer turns

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Channel Partitioning MAC protocols TDMA

TDMA Time division multiple access Access to channel in rounds Each station gets fixed length slot

Length = packet transmission time in each round

Unused slots go idle Example 6-station LAN

134 have packet Slots 256 idle

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Channel Partitioning MAC protocols FDMA

FDMA Frequency division multiple access Channel spectrum divided into frequency bands Each station assigned fixed frequency band Unused transmission time in frequency bands go idle Example 6-station LAN 134 have packet frequency

bands 256 idle

frequ

ency

bands time

1

2

3

4

5

6

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Random Access Protocols

When node has packet to send Transmit at full channel data rate R No a priori coordination among nodes

Two or more transmitting nodes ldquocollisionrdquo Random access MAC protocol specifies

How to detect collisions How to recover from collisions (eg via delayed retransmissions)

Examples of random access MAC protocols Slotted ALOHA ALOHA Carrier Sense Multiple Access (CSMA) CSMACollision Detection (CD) CSMACollision Avoidance (CA)

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Slotted ALOHA

Pros Single active node can continuously

transmit at full rate of channel Highly decentralized only slots in

nodes need to be in sync Simple

Cons Collisions wasting slots Idle slots Nodes may be able to detect collision in

less than time to transmit packet Clock synchronization

Assumptions All frames same size Time is divided into equal size

slots time to transmit 1 frame Nodes start to transmit frames

only at beginning of slots Nodes are synchronized If 2 or more nodes transmit in

slot all nodes detect collision

Operation When node obtains fresh frame it

transmits in next slot No collision node can send new frame in

next slot If collision node retransmits frame in

each subsequent slot with prob p until success

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Pure (unslotted) ALOHA Unslotted Aloha simpler no synchronization When frame first arrives

Transmit immediately

Collision probability increases Frame sent at t0 collides with other frames sent in [t0-1t0+1]

Even worse than slotted ALOHA

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Collisions can still occur Propagation delay means two nodes may not hear each otherrsquos transmission Entire packet transmission time wasted Role of distance amp propagation delay in determining collision probability spatial layout of nodes

Idea listen before transmit If channel sensed idle

transmit entire frame If channel sensed busy defer

transmission Human analogy donrsquot

interrupt others

CSMA (Carrier Sense Multiple Access)

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CSMACD (Collision Detection)

CSMACD carrier sensing deferral as in CSMA Collisions detected within short

time Send jam sequence Colliding transmissions aborted

reducing channel wastage

Collision detection

Easy in wired LANs measure signal strengths compare transmitted received signals

Difficult in wireless LANs receiver shut off while transmitting

Human analogy the polite conversationalist

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ldquoTaking Turnsrdquo MAC protocols Channel partitioning MAC protocols

Share channel efficiently and fairly at high load Inefficient at low load

Delay in channel access 1N bandwidth allocated even if only 1 active node

Random access MAC protocols Efficient at low load

Single node can fully utilize channel

high load Collision overhead

ldquoTaking turnsrdquo protocols Look for best of both worlds

Two implementations Polling Token

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ldquoTaking Turnsrdquo MAC protocolsPolling Master node ldquoinvitesrdquo slave

nodes to transmit in turn Concerns

Polling overhead Latency Single point of failure (master)

Token passing control token passed from

one node to next sequentially

token message concerns

token overhead latency single point of failure (token)

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Summary of MAC protocols

What do you do with a shared media Channel Partitioning by time frequency or code

Time Division Frequency Division

Random partitioning (dynamic) ALOHA S-ALOHA CSMA CSMACD Carrier sensing easy in some technologies (wire) hard in others

(wireless) CSMACD used in Ethernet CSMACA used in 80211

Taking Turns Polling from a central site token passing

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LAN technologies

Data link layer so farServicesError detectioncorrectionMultiple access

Next LAN technologiesAddressingEthernetHubs switchesPPP

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MAC Addresses and ARP 32-bit IP address

Network-layer address Used to get datagram to

destination IP subnet

MAC (or LAN or physical or Ethernet) address Used to get datagram

from one interface to another physically-connected interface (same network)

48 bit MAC address (for most LANs) burned in the adapter ROM

Find your PCrsquos MAC address

1A-2F-BB-76-09-AD

58-23-D7-FA-20-B0

0C-C4-11-6F-E3-98

71-65-F7-2B-08-53

LAN(wired orwireless)

Each adapter on LAN has unique LAN address

Broadcast address =FF-FF-FF-FF-FF-FF = adapter

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MAC Address (more)

MAC address allocation administered by IEEE Manufacturer buys portion of MAC address space (to

assure uniqueness) Analogy

(a) MAC address like Social Security Number

(b) IP address like postal address MAC flat address portability

Can move LAN card from one LAN to another

IP hierarchical address NOT portable Depends on IP subnet to which node is attached

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ARP Address Resolution Protocol

Each IP node (Host Router) on LAN has ARP table

ARP Table IPMAC address mappings for some LAN nodes

lt IP address MAC address TTLgt TTL (Time To Live)

time after which address mapping will be forgotten (typically 20 min)

Question how to determineMAC address of Bknowing Brsquos IP address

1A-2F-BB-76-09-AD

58-23-D7-FA-20-B0

0C-C4-11-6F-E3-98

71-65-F7-2B-08-53

LAN

237196723

237196778

237196714

237196788

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ARP protocol Same LAN (network) A wants to send datagram

to B and Brsquos MAC address not in Arsquos ARP table

A broadcasts ARP query packet containing Bs IP address Dest MAC address = FF-FF-

FF-FF-FF-FF All machines on LAN receive

ARP query

B receives ARP packet replies to A with its (Bs) MAC address Frame sent to Arsquos MAC

address (unicast)

A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) Soft state information that

times out (goes away) unless refreshed

ARP is ldquoplug-and-playrdquo Nodes create their ARP

tables without intervention from net administrator

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Routing to another LAN Walkthrough send datagram from A to B via R

Assume A knows B IP address

Two ARP tables in router R one for each IP network (LAN) In routing table at source Host find router 111111111110 In ARP table at source find MAC address E6-E9-00-17-BB-4B etc A creates datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110

A creates link-layer frame with Rs MAC address as dest frame contains A-to-B IP datagram

Arsquos adapter sends frame Rrsquos adapter receives frame R removes IP datagram from Ethernet frame sees its destined to B R uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B

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A

R

B

Routing to another LAN

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EthernetldquoDominantrdquo wired LAN technology Cheap $20 for 100Mbs First widely used LAN technology Simpler cheaper than token LANs and ATM Kept up with speed race 10 Mbps ndash 10

Gbps

Metcalfersquos Ethernet sketch

Star Topology Bus topology popular

through mid 90s Now star topology prevails Connection choices hub or

switch (more later)

hub orswitch

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Ethernet Frame Structure

Preamble 7 bytes with pattern 10101010 followed by one byte with pattern 10101011 Used to synchronize receiver sender clock rates

Addresses 6 bytes If adapter receives frame with matching destination address or with broadcast

address (eg ARP packet) it passes data in frame to net-layer protocol Otherwise adapter discards frame

Type Indicates the higher layer protocol (mostly IP but others may be supported

such as Novell IPX and AppleTalk) CRC

Checked at receiver if error is detected the frame is simply dropped

Sending adapter encapsulates IP datagram in Ethernet frame

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Ethernet uses CSMACD Connectionless No handshaking between sending and receiving

adapter Unreliable receiving adapter doesnrsquot send acks or nacks to

sending adapter Stream of datagrams passed to network layer can have gaps Gaps will be filled if app is using TCP Otherwise app will see the gaps

No slots Adapter doesnrsquot transmit if it senses that some other adapter is

transmitting that is carrier sense Transmitting adapter aborts when it senses that another adapter is

transmitting that is collision detection Before attempting a retransmission adapter waits a random time

that is random access

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Ethernet CSMACD algorithm1 Adaptor receives datagram from

net layer amp creates frame2 If adapter senses channel idle it

starts to transmit frame If it senses channel busy waits until channel idle and then transmits

3 If adapter transmits entire frame without detecting another transmission the adapter is done with frame

4 If adapter detects another transmission while transmitting aborts and sends jam signal

5 After aborting adapter enters exponential backoff after the mth collision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2

Jam Signal make sure all other transmitters are aware of collision 48 bits

Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec

Exponential Backoff Goal adapt retransmission

attempts to estimated current load Heavy load random wait will be

longer

First collision choose K from 01 delay is K 512 bit transmission times

After second collision choose K from 0123hellip

After ten collisions choose K from 01234hellip1023

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CSMACD efficiency Tprop = max prop between 2 nodes in LAN ttrans = time to transmit max-size frame

Efficiency goes to 1 as tprop goes to 0 Goes to 1 as ttrans goes to infinity Much better than ALOHA but still decentralized

simple and cheap

Other Ethernets Switched Ethernet Fast Ethernet Gigabit Ethernet

transprop tt 51

1efficiency

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Point to Point Data Link Control

One sender one receiver one link easier than broadcast link No Media Access Control No need for explicit MAC addressing eg dialup link ISDN line

Popular point-to-point DLC protocols PPP (point-to-point protocol) HDLC High level data link control (Data link used to be considered

ldquohigh layerrdquo in protocol stack

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HDLC

Frame format for bit-oriented protocol HDLC

Control field of

(a) An information frame

(b) A supervisory frame

(c) An unnumbered frame

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HDLC Exchange of information supervisory and unnumbered

frames Information - data to be transmitted to user (next layer up)

Flow and error control piggybacked on information frames Supervisory - ARQ when piggyback not used Unnumbered - supplementary link control

Three phases Initialization Data transfer Disconnect

Uses bit stuffing Flag delimit frame at both ends 01111110 is the Flag May close one frame and open another Bit stuffing to find flag data

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Internet Datalink Protocol PPP Design Requirements [RFC 1557]

Packet framing encapsulation of network-layer datagram in data link frame Carry network layer data of any network layer protocol (not just IP) at

same time Ability to demultiplex upwards

Bit transparency must carry any bit pattern in the data field Error detection (no correction) Connection liveness detect signal link failure to network layer Network layer address negotiation endpoint can learnconfigure

each otherrsquos network address

PPP non-design requirements No error correctionrecovery No flow control Out of order delivery OK No need to support multipoint links (eg polling)

Error recovery flow control data re-ordering all relegated to higher layers

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PPP Data Frame

Flag delimiter (framing) Address does nothing (only one option) Control does nothing in the future possible multiple

control fields Protocol upper layer protocol to which frame delivered

(eg PPP-LCP IP IPCP etc) Info upper layer data being carried Check cyclic redundancy check for error detection

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Byte Stuffing ldquoData transparencyrdquo requirement data field must be allowed to include

flag pattern lt01111110gt Q is received lt01111110gt data or flag

Sender adds (ldquostuffsrdquo) extra lt 01111110gt byte after each lt 01111110gt data byte How about bit stuffing

Receiver Two 01111110 bytes in a row Discard first byte continue data reception Single 01111110 flag byte

flag bytepatternin datato send

flag byte pattern plusstuffed byte in transmitted data

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PPP Data Control Protocol Before exchanging

network-layer data data link peers must configure PPP link

(max frame length authentication)

learnconfigure network layer information

For IP carry IP Control Protocol (IPCP) messages (protocol field 8021) to configurelearn IP address

Diagram for bring a line up and down

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Flow Control The Data Link Layer also provides Flow Control What is Flow Control

Ensuring the sending entity does not overwhelm the receiving entity Preventing buffer overflow

One Solution Stop and Wait Flow Control Source transmits frame Destination receives frame and replies with acknowledgement Source waits for ACK before sending next frame Destination can stop flow by not sending ACK Works well for a few large frames

BUT Large block of data may be split into small frames Limited buffer size Errors detected sooner (when whole frame received) On error retransmission of smaller frames is needed Prevents one station occupying medium for long periods

Stop and wait becomes inadequate Link Utilization

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Stop and Wait Link Utilization

Transmission time =1 Propagation time = a

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Sliding Windows Flow Control Allow multiple frames to be in transit Receiver has buffer W long Transmitter can send up to W frames without ACK Each frame is numbered ACK includes number of next frame expected Sequence number bounded by size of field (k)

Frames are numbered modulo 2k

Enhancements Receiver can acknowledge frames without permitting

further transmission (Receive Not Ready) Must send a normal acknowledge to resume If duplex use piggybacking

If no data to send use acknowledgement frame If data but no acknowledgement to send send last acknowledgement

number again or have ACK valid flag (TCP)

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Sliding Windows Diagram

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Example

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Error DetectionEDC = Error Detection and Correction bits (redundancy)D = Data protected by error checking may include header fields

Error detection not 100 reliable Protocol may miss some errors but rarely Larger EDC field yields better detection and correction

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Parity Checking Single Bit Parity

Detect single bit errors

Two Dimensional Bit ParityDetect and correct single bit errors

Internet Checksum Detect ldquoerrorsrdquo (eg flipped bits)

in transmitted segment (note used at transport layer only) 00

Sender Treat segment contents as sequence of 16-bit integers Checksum addition (1rsquos complement sum) of segment contents Sender puts checksum value into UDP checksum field

Receiver Compute checksum of received segment Check if computed checksum equals checksum field value NO - error detected YES - no error

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Cyclic Redundancy Check View data bits D as a binary number Choose r+1 bit pattern (generator) G Goal choose r CRC bits R such that

ltDRgt exactly divisible by G (modulo 2) Receiver knows G divides ltDRgt by G If non-zero remainder error

detected Can detect all burst errors less than r+1 bits

Widely used in practice (ATM HDCL)

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CRC Example

WantD2r XOR R = nG

EquivalentlyD2r = nG XOR R

Equivalently

If we divide D2r by G want remainder R

R = remainder[ ]D2r

G

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Error Correction

Detection and correction of errors due to Lost frames Damaged frames Underlying channel can lose the frames or can be garbled

Solution at the Link Layer Automatic repeat request Error detection Positive acknowledgment Retransmission after timeout Negative acknowledgement and retransmission

Automatic Repeat Request (ARQ) methods Stop and wait Go back N Selective reject (selective retransmission)

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Stop and Wait ARQ Source transmits single

frame Wait for ACK If received frame damaged

discard it Transmitter has timeout If no ACK within timeout

retransmit

If ACK damaged transmitter will not recognize it Transmitter will retransmit Receive gets two copies of frame Use ACK0 and ACK1

Simple but inefficient

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More efficient method Go-back-N Based on sliding window If no error ACK as usual with next frame expected Use window to control number of outstanding frames If error reply with rejection

Discard that frame and all future frames until error frame received correctly

Transmitter must go back and retransmit that frame and all subsequent frames

Handling Damaged Frame Receiver detects error in frame i Receiver sends rejection-i Transmitter gets rejection-i Transmitter retransmits frame i and all subsequent

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Go-back-N Handling Lost Frame First Approach

Frame i lost Transmitter sends i+1 Receiver gets frame i+1 out of sequence Receiver send reject i Transmitter goes back to frame i and retransmits

Second Approach Frame i lost and no additional frame sent Receiver gets nothing and returns neither acknowledgement nor

rejection Transmitter times out and sends acknowledgement frame with P bit

set to 1 Receiver interprets this as command which it acknowledges with the

number of the next frame it expects (frame i ) Transmitter then retransmits frame i

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Go-back-N Damaged ACK

Receiver gets frame i and send acknowledgement (i+1) which is lost

Acknowledgements are cumulative so next acknowledgement (i+n) may arrive before transmitter times out on frame i

If transmitter times out it sends acknowledgement with P bit set as before

This can be repeated a number of times before a reset procedure is initiated

Damaged Rejection Same as Lost Frame (2)

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Selective Reject

Also called selective retransmission

Only rejected frames are retransmitted

Subsequent frames are accepted by the receiver and buffered

Minimizes retransmission Receiver must maintain

large enough buffer More complex login in

transmitter

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Multiple Access Links and ProtocolsTwo types of ldquolinksrdquo Point-to-point

PPP for dial-up access Point-to-point link between Ethernet switch and host

Broadcast (shared wire or medium) Traditional Ethernet Upstream HFC 80211 wireless LAN

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Multiple Access Protocols Single shared broadcast channel Two or more simultaneous transmissions by nodes interference

Collision if node receives two or more signals at the same time

Multiple access protocol Distributed algorithm that determines how nodes share channel ie

determine when node can transmit Communication about channel sharing must use channel itself

No out-of-band channel for coordination

Ideal Multiple Access Protocol When one node wants to transmit it can send at rate R When M nodes want to transmit each can send at average rate RM Fully decentralized

No special node to coordinate transmissions No synchronization of clocks slots

Simple

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MAC Protocols A taxonomy

Three broad classes

Channel Partitioning Divide channel into smaller ldquopiecesrdquo (time slots frequency code) Allocate piece to node for exclusive use

Random Access Channel not divided allow collisions ldquoRecoverrdquo from collisions

ldquoTaking turnsrdquo Nodes take turns Nodes with more to send can take longer turns

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Channel Partitioning MAC protocols TDMA

TDMA Time division multiple access Access to channel in rounds Each station gets fixed length slot

Length = packet transmission time in each round

Unused slots go idle Example 6-station LAN

134 have packet Slots 256 idle

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Channel Partitioning MAC protocols FDMA

FDMA Frequency division multiple access Channel spectrum divided into frequency bands Each station assigned fixed frequency band Unused transmission time in frequency bands go idle Example 6-station LAN 134 have packet frequency

bands 256 idle

frequ

ency

bands time

1

2

3

4

5

6

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Random Access Protocols

When node has packet to send Transmit at full channel data rate R No a priori coordination among nodes

Two or more transmitting nodes ldquocollisionrdquo Random access MAC protocol specifies

How to detect collisions How to recover from collisions (eg via delayed retransmissions)

Examples of random access MAC protocols Slotted ALOHA ALOHA Carrier Sense Multiple Access (CSMA) CSMACollision Detection (CD) CSMACollision Avoidance (CA)

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Slotted ALOHA

Pros Single active node can continuously

transmit at full rate of channel Highly decentralized only slots in

nodes need to be in sync Simple

Cons Collisions wasting slots Idle slots Nodes may be able to detect collision in

less than time to transmit packet Clock synchronization

Assumptions All frames same size Time is divided into equal size

slots time to transmit 1 frame Nodes start to transmit frames

only at beginning of slots Nodes are synchronized If 2 or more nodes transmit in

slot all nodes detect collision

Operation When node obtains fresh frame it

transmits in next slot No collision node can send new frame in

next slot If collision node retransmits frame in

each subsequent slot with prob p until success

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Pure (unslotted) ALOHA Unslotted Aloha simpler no synchronization When frame first arrives

Transmit immediately

Collision probability increases Frame sent at t0 collides with other frames sent in [t0-1t0+1]

Even worse than slotted ALOHA

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Collisions can still occur Propagation delay means two nodes may not hear each otherrsquos transmission Entire packet transmission time wasted Role of distance amp propagation delay in determining collision probability spatial layout of nodes

Idea listen before transmit If channel sensed idle

transmit entire frame If channel sensed busy defer

transmission Human analogy donrsquot

interrupt others

CSMA (Carrier Sense Multiple Access)

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CSMACD (Collision Detection)

CSMACD carrier sensing deferral as in CSMA Collisions detected within short

time Send jam sequence Colliding transmissions aborted

reducing channel wastage

Collision detection

Easy in wired LANs measure signal strengths compare transmitted received signals

Difficult in wireless LANs receiver shut off while transmitting

Human analogy the polite conversationalist

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ldquoTaking Turnsrdquo MAC protocols Channel partitioning MAC protocols

Share channel efficiently and fairly at high load Inefficient at low load

Delay in channel access 1N bandwidth allocated even if only 1 active node

Random access MAC protocols Efficient at low load

Single node can fully utilize channel

high load Collision overhead

ldquoTaking turnsrdquo protocols Look for best of both worlds

Two implementations Polling Token

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ldquoTaking Turnsrdquo MAC protocolsPolling Master node ldquoinvitesrdquo slave

nodes to transmit in turn Concerns

Polling overhead Latency Single point of failure (master)

Token passing control token passed from

one node to next sequentially

token message concerns

token overhead latency single point of failure (token)

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Summary of MAC protocols

What do you do with a shared media Channel Partitioning by time frequency or code

Time Division Frequency Division

Random partitioning (dynamic) ALOHA S-ALOHA CSMA CSMACD Carrier sensing easy in some technologies (wire) hard in others

(wireless) CSMACD used in Ethernet CSMACA used in 80211

Taking Turns Polling from a central site token passing

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LAN technologies

Data link layer so farServicesError detectioncorrectionMultiple access

Next LAN technologiesAddressingEthernetHubs switchesPPP

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MAC Addresses and ARP 32-bit IP address

Network-layer address Used to get datagram to

destination IP subnet

MAC (or LAN or physical or Ethernet) address Used to get datagram

from one interface to another physically-connected interface (same network)

48 bit MAC address (for most LANs) burned in the adapter ROM

Find your PCrsquos MAC address

1A-2F-BB-76-09-AD

58-23-D7-FA-20-B0

0C-C4-11-6F-E3-98

71-65-F7-2B-08-53

LAN(wired orwireless)

Each adapter on LAN has unique LAN address

Broadcast address =FF-FF-FF-FF-FF-FF = adapter

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MAC Address (more)

MAC address allocation administered by IEEE Manufacturer buys portion of MAC address space (to

assure uniqueness) Analogy

(a) MAC address like Social Security Number

(b) IP address like postal address MAC flat address portability

Can move LAN card from one LAN to another

IP hierarchical address NOT portable Depends on IP subnet to which node is attached

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ARP Address Resolution Protocol

Each IP node (Host Router) on LAN has ARP table

ARP Table IPMAC address mappings for some LAN nodes

lt IP address MAC address TTLgt TTL (Time To Live)

time after which address mapping will be forgotten (typically 20 min)

Question how to determineMAC address of Bknowing Brsquos IP address

1A-2F-BB-76-09-AD

58-23-D7-FA-20-B0

0C-C4-11-6F-E3-98

71-65-F7-2B-08-53

LAN

237196723

237196778

237196714

237196788

copy Kemal Akkaya

ARP protocol Same LAN (network) A wants to send datagram

to B and Brsquos MAC address not in Arsquos ARP table

A broadcasts ARP query packet containing Bs IP address Dest MAC address = FF-FF-

FF-FF-FF-FF All machines on LAN receive

ARP query

B receives ARP packet replies to A with its (Bs) MAC address Frame sent to Arsquos MAC

address (unicast)

A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) Soft state information that

times out (goes away) unless refreshed

ARP is ldquoplug-and-playrdquo Nodes create their ARP

tables without intervention from net administrator

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Routing to another LAN Walkthrough send datagram from A to B via R

Assume A knows B IP address

Two ARP tables in router R one for each IP network (LAN) In routing table at source Host find router 111111111110 In ARP table at source find MAC address E6-E9-00-17-BB-4B etc A creates datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110

A creates link-layer frame with Rs MAC address as dest frame contains A-to-B IP datagram

Arsquos adapter sends frame Rrsquos adapter receives frame R removes IP datagram from Ethernet frame sees its destined to B R uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B

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A

R

B

Routing to another LAN

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EthernetldquoDominantrdquo wired LAN technology Cheap $20 for 100Mbs First widely used LAN technology Simpler cheaper than token LANs and ATM Kept up with speed race 10 Mbps ndash 10

Gbps

Metcalfersquos Ethernet sketch

Star Topology Bus topology popular

through mid 90s Now star topology prevails Connection choices hub or

switch (more later)

hub orswitch

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Ethernet Frame Structure

Preamble 7 bytes with pattern 10101010 followed by one byte with pattern 10101011 Used to synchronize receiver sender clock rates

Addresses 6 bytes If adapter receives frame with matching destination address or with broadcast

address (eg ARP packet) it passes data in frame to net-layer protocol Otherwise adapter discards frame

Type Indicates the higher layer protocol (mostly IP but others may be supported

such as Novell IPX and AppleTalk) CRC

Checked at receiver if error is detected the frame is simply dropped

Sending adapter encapsulates IP datagram in Ethernet frame

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Ethernet uses CSMACD Connectionless No handshaking between sending and receiving

adapter Unreliable receiving adapter doesnrsquot send acks or nacks to

sending adapter Stream of datagrams passed to network layer can have gaps Gaps will be filled if app is using TCP Otherwise app will see the gaps

No slots Adapter doesnrsquot transmit if it senses that some other adapter is

transmitting that is carrier sense Transmitting adapter aborts when it senses that another adapter is

transmitting that is collision detection Before attempting a retransmission adapter waits a random time

that is random access

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Ethernet CSMACD algorithm1 Adaptor receives datagram from

net layer amp creates frame2 If adapter senses channel idle it

starts to transmit frame If it senses channel busy waits until channel idle and then transmits

3 If adapter transmits entire frame without detecting another transmission the adapter is done with frame

4 If adapter detects another transmission while transmitting aborts and sends jam signal

5 After aborting adapter enters exponential backoff after the mth collision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2

Jam Signal make sure all other transmitters are aware of collision 48 bits

Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec

Exponential Backoff Goal adapt retransmission

attempts to estimated current load Heavy load random wait will be

longer

First collision choose K from 01 delay is K 512 bit transmission times

After second collision choose K from 0123hellip

After ten collisions choose K from 01234hellip1023

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CSMACD efficiency Tprop = max prop between 2 nodes in LAN ttrans = time to transmit max-size frame

Efficiency goes to 1 as tprop goes to 0 Goes to 1 as ttrans goes to infinity Much better than ALOHA but still decentralized

simple and cheap

Other Ethernets Switched Ethernet Fast Ethernet Gigabit Ethernet

transprop tt 51

1efficiency

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Point to Point Data Link Control

One sender one receiver one link easier than broadcast link No Media Access Control No need for explicit MAC addressing eg dialup link ISDN line

Popular point-to-point DLC protocols PPP (point-to-point protocol) HDLC High level data link control (Data link used to be considered

ldquohigh layerrdquo in protocol stack

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HDLC

Frame format for bit-oriented protocol HDLC

Control field of

(a) An information frame

(b) A supervisory frame

(c) An unnumbered frame

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HDLC Exchange of information supervisory and unnumbered

frames Information - data to be transmitted to user (next layer up)

Flow and error control piggybacked on information frames Supervisory - ARQ when piggyback not used Unnumbered - supplementary link control

Three phases Initialization Data transfer Disconnect

Uses bit stuffing Flag delimit frame at both ends 01111110 is the Flag May close one frame and open another Bit stuffing to find flag data

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Internet Datalink Protocol PPP Design Requirements [RFC 1557]

Packet framing encapsulation of network-layer datagram in data link frame Carry network layer data of any network layer protocol (not just IP) at

same time Ability to demultiplex upwards

Bit transparency must carry any bit pattern in the data field Error detection (no correction) Connection liveness detect signal link failure to network layer Network layer address negotiation endpoint can learnconfigure

each otherrsquos network address

PPP non-design requirements No error correctionrecovery No flow control Out of order delivery OK No need to support multipoint links (eg polling)

Error recovery flow control data re-ordering all relegated to higher layers

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PPP Data Frame

Flag delimiter (framing) Address does nothing (only one option) Control does nothing in the future possible multiple

control fields Protocol upper layer protocol to which frame delivered

(eg PPP-LCP IP IPCP etc) Info upper layer data being carried Check cyclic redundancy check for error detection

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Byte Stuffing ldquoData transparencyrdquo requirement data field must be allowed to include

flag pattern lt01111110gt Q is received lt01111110gt data or flag

Sender adds (ldquostuffsrdquo) extra lt 01111110gt byte after each lt 01111110gt data byte How about bit stuffing

Receiver Two 01111110 bytes in a row Discard first byte continue data reception Single 01111110 flag byte

flag bytepatternin datato send

flag byte pattern plusstuffed byte in transmitted data

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PPP Data Control Protocol Before exchanging

network-layer data data link peers must configure PPP link

(max frame length authentication)

learnconfigure network layer information

For IP carry IP Control Protocol (IPCP) messages (protocol field 8021) to configurelearn IP address

Diagram for bring a line up and down

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Stop and Wait Link Utilization

Transmission time =1 Propagation time = a

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Sliding Windows Flow Control Allow multiple frames to be in transit Receiver has buffer W long Transmitter can send up to W frames without ACK Each frame is numbered ACK includes number of next frame expected Sequence number bounded by size of field (k)

Frames are numbered modulo 2k

Enhancements Receiver can acknowledge frames without permitting

further transmission (Receive Not Ready) Must send a normal acknowledge to resume If duplex use piggybacking

If no data to send use acknowledgement frame If data but no acknowledgement to send send last acknowledgement

number again or have ACK valid flag (TCP)

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Sliding Windows Diagram

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Example

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Error DetectionEDC = Error Detection and Correction bits (redundancy)D = Data protected by error checking may include header fields

Error detection not 100 reliable Protocol may miss some errors but rarely Larger EDC field yields better detection and correction

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Parity Checking Single Bit Parity

Detect single bit errors

Two Dimensional Bit ParityDetect and correct single bit errors

Internet Checksum Detect ldquoerrorsrdquo (eg flipped bits)

in transmitted segment (note used at transport layer only) 00

Sender Treat segment contents as sequence of 16-bit integers Checksum addition (1rsquos complement sum) of segment contents Sender puts checksum value into UDP checksum field

Receiver Compute checksum of received segment Check if computed checksum equals checksum field value NO - error detected YES - no error

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Cyclic Redundancy Check View data bits D as a binary number Choose r+1 bit pattern (generator) G Goal choose r CRC bits R such that

ltDRgt exactly divisible by G (modulo 2) Receiver knows G divides ltDRgt by G If non-zero remainder error

detected Can detect all burst errors less than r+1 bits

Widely used in practice (ATM HDCL)

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CRC Example

WantD2r XOR R = nG

EquivalentlyD2r = nG XOR R

Equivalently

If we divide D2r by G want remainder R

R = remainder[ ]D2r

G

copy Kemal Akkaya

Error Correction

Detection and correction of errors due to Lost frames Damaged frames Underlying channel can lose the frames or can be garbled

Solution at the Link Layer Automatic repeat request Error detection Positive acknowledgment Retransmission after timeout Negative acknowledgement and retransmission

Automatic Repeat Request (ARQ) methods Stop and wait Go back N Selective reject (selective retransmission)

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Stop and Wait ARQ Source transmits single

frame Wait for ACK If received frame damaged

discard it Transmitter has timeout If no ACK within timeout

retransmit

If ACK damaged transmitter will not recognize it Transmitter will retransmit Receive gets two copies of frame Use ACK0 and ACK1

Simple but inefficient

copy Kemal Akkaya

More efficient method Go-back-N Based on sliding window If no error ACK as usual with next frame expected Use window to control number of outstanding frames If error reply with rejection

Discard that frame and all future frames until error frame received correctly

Transmitter must go back and retransmit that frame and all subsequent frames

Handling Damaged Frame Receiver detects error in frame i Receiver sends rejection-i Transmitter gets rejection-i Transmitter retransmits frame i and all subsequent

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Go-back-N Handling Lost Frame First Approach

Frame i lost Transmitter sends i+1 Receiver gets frame i+1 out of sequence Receiver send reject i Transmitter goes back to frame i and retransmits

Second Approach Frame i lost and no additional frame sent Receiver gets nothing and returns neither acknowledgement nor

rejection Transmitter times out and sends acknowledgement frame with P bit

set to 1 Receiver interprets this as command which it acknowledges with the

number of the next frame it expects (frame i ) Transmitter then retransmits frame i

copy Kemal Akkaya

Go-back-N Damaged ACK

Receiver gets frame i and send acknowledgement (i+1) which is lost

Acknowledgements are cumulative so next acknowledgement (i+n) may arrive before transmitter times out on frame i

If transmitter times out it sends acknowledgement with P bit set as before

This can be repeated a number of times before a reset procedure is initiated

Damaged Rejection Same as Lost Frame (2)

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Selective Reject

Also called selective retransmission

Only rejected frames are retransmitted

Subsequent frames are accepted by the receiver and buffered

Minimizes retransmission Receiver must maintain

large enough buffer More complex login in

transmitter

copy Kemal Akkaya

Multiple Access Links and ProtocolsTwo types of ldquolinksrdquo Point-to-point

PPP for dial-up access Point-to-point link between Ethernet switch and host

Broadcast (shared wire or medium) Traditional Ethernet Upstream HFC 80211 wireless LAN

copy Kemal Akkaya

Multiple Access Protocols Single shared broadcast channel Two or more simultaneous transmissions by nodes interference

Collision if node receives two or more signals at the same time

Multiple access protocol Distributed algorithm that determines how nodes share channel ie

determine when node can transmit Communication about channel sharing must use channel itself

No out-of-band channel for coordination

Ideal Multiple Access Protocol When one node wants to transmit it can send at rate R When M nodes want to transmit each can send at average rate RM Fully decentralized

No special node to coordinate transmissions No synchronization of clocks slots

Simple

copy Kemal Akkaya

MAC Protocols A taxonomy

Three broad classes

Channel Partitioning Divide channel into smaller ldquopiecesrdquo (time slots frequency code) Allocate piece to node for exclusive use

Random Access Channel not divided allow collisions ldquoRecoverrdquo from collisions

ldquoTaking turnsrdquo Nodes take turns Nodes with more to send can take longer turns

copy Kemal Akkaya

Channel Partitioning MAC protocols TDMA

TDMA Time division multiple access Access to channel in rounds Each station gets fixed length slot

Length = packet transmission time in each round

Unused slots go idle Example 6-station LAN

134 have packet Slots 256 idle

copy Kemal Akkaya

Channel Partitioning MAC protocols FDMA

FDMA Frequency division multiple access Channel spectrum divided into frequency bands Each station assigned fixed frequency band Unused transmission time in frequency bands go idle Example 6-station LAN 134 have packet frequency

bands 256 idle

frequ

ency

bands time

1

2

3

4

5

6

copy Kemal Akkaya

Random Access Protocols

When node has packet to send Transmit at full channel data rate R No a priori coordination among nodes

Two or more transmitting nodes ldquocollisionrdquo Random access MAC protocol specifies

How to detect collisions How to recover from collisions (eg via delayed retransmissions)

Examples of random access MAC protocols Slotted ALOHA ALOHA Carrier Sense Multiple Access (CSMA) CSMACollision Detection (CD) CSMACollision Avoidance (CA)

copy Kemal Akkaya

Slotted ALOHA

Pros Single active node can continuously

transmit at full rate of channel Highly decentralized only slots in

nodes need to be in sync Simple

Cons Collisions wasting slots Idle slots Nodes may be able to detect collision in

less than time to transmit packet Clock synchronization

Assumptions All frames same size Time is divided into equal size

slots time to transmit 1 frame Nodes start to transmit frames

only at beginning of slots Nodes are synchronized If 2 or more nodes transmit in

slot all nodes detect collision

Operation When node obtains fresh frame it

transmits in next slot No collision node can send new frame in

next slot If collision node retransmits frame in

each subsequent slot with prob p until success

copy Kemal Akkaya

Pure (unslotted) ALOHA Unslotted Aloha simpler no synchronization When frame first arrives

Transmit immediately

Collision probability increases Frame sent at t0 collides with other frames sent in [t0-1t0+1]

Even worse than slotted ALOHA

copy Kemal Akkaya

Collisions can still occur Propagation delay means two nodes may not hear each otherrsquos transmission Entire packet transmission time wasted Role of distance amp propagation delay in determining collision probability spatial layout of nodes

Idea listen before transmit If channel sensed idle

transmit entire frame If channel sensed busy defer

transmission Human analogy donrsquot

interrupt others

CSMA (Carrier Sense Multiple Access)

copy Kemal Akkaya

CSMACD (Collision Detection)

CSMACD carrier sensing deferral as in CSMA Collisions detected within short

time Send jam sequence Colliding transmissions aborted

reducing channel wastage

Collision detection

Easy in wired LANs measure signal strengths compare transmitted received signals

Difficult in wireless LANs receiver shut off while transmitting

Human analogy the polite conversationalist

copy Kemal Akkaya

ldquoTaking Turnsrdquo MAC protocols Channel partitioning MAC protocols

Share channel efficiently and fairly at high load Inefficient at low load

Delay in channel access 1N bandwidth allocated even if only 1 active node

Random access MAC protocols Efficient at low load

Single node can fully utilize channel

high load Collision overhead

ldquoTaking turnsrdquo protocols Look for best of both worlds

Two implementations Polling Token

copy Kemal Akkaya

ldquoTaking Turnsrdquo MAC protocolsPolling Master node ldquoinvitesrdquo slave

nodes to transmit in turn Concerns

Polling overhead Latency Single point of failure (master)

Token passing control token passed from

one node to next sequentially

token message concerns

token overhead latency single point of failure (token)

copy Kemal Akkaya

Summary of MAC protocols

What do you do with a shared media Channel Partitioning by time frequency or code

Time Division Frequency Division

Random partitioning (dynamic) ALOHA S-ALOHA CSMA CSMACD Carrier sensing easy in some technologies (wire) hard in others

(wireless) CSMACD used in Ethernet CSMACA used in 80211

Taking Turns Polling from a central site token passing

copy Kemal Akkaya

LAN technologies

Data link layer so farServicesError detectioncorrectionMultiple access

Next LAN technologiesAddressingEthernetHubs switchesPPP

copy Kemal Akkaya

MAC Addresses and ARP 32-bit IP address

Network-layer address Used to get datagram to

destination IP subnet

MAC (or LAN or physical or Ethernet) address Used to get datagram

from one interface to another physically-connected interface (same network)

48 bit MAC address (for most LANs) burned in the adapter ROM

Find your PCrsquos MAC address

1A-2F-BB-76-09-AD

58-23-D7-FA-20-B0

0C-C4-11-6F-E3-98

71-65-F7-2B-08-53

LAN(wired orwireless)

Each adapter on LAN has unique LAN address

Broadcast address =FF-FF-FF-FF-FF-FF = adapter

copy Kemal Akkaya

MAC Address (more)

MAC address allocation administered by IEEE Manufacturer buys portion of MAC address space (to

assure uniqueness) Analogy

(a) MAC address like Social Security Number

(b) IP address like postal address MAC flat address portability

Can move LAN card from one LAN to another

IP hierarchical address NOT portable Depends on IP subnet to which node is attached

copy Kemal Akkaya

ARP Address Resolution Protocol

Each IP node (Host Router) on LAN has ARP table

ARP Table IPMAC address mappings for some LAN nodes

lt IP address MAC address TTLgt TTL (Time To Live)

time after which address mapping will be forgotten (typically 20 min)

Question how to determineMAC address of Bknowing Brsquos IP address

1A-2F-BB-76-09-AD

58-23-D7-FA-20-B0

0C-C4-11-6F-E3-98

71-65-F7-2B-08-53

LAN

237196723

237196778

237196714

237196788

copy Kemal Akkaya

ARP protocol Same LAN (network) A wants to send datagram

to B and Brsquos MAC address not in Arsquos ARP table

A broadcasts ARP query packet containing Bs IP address Dest MAC address = FF-FF-

FF-FF-FF-FF All machines on LAN receive

ARP query

B receives ARP packet replies to A with its (Bs) MAC address Frame sent to Arsquos MAC

address (unicast)

A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) Soft state information that

times out (goes away) unless refreshed

ARP is ldquoplug-and-playrdquo Nodes create their ARP

tables without intervention from net administrator

copy Kemal Akkaya

Routing to another LAN Walkthrough send datagram from A to B via R

Assume A knows B IP address

Two ARP tables in router R one for each IP network (LAN) In routing table at source Host find router 111111111110 In ARP table at source find MAC address E6-E9-00-17-BB-4B etc A creates datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110

A creates link-layer frame with Rs MAC address as dest frame contains A-to-B IP datagram

Arsquos adapter sends frame Rrsquos adapter receives frame R removes IP datagram from Ethernet frame sees its destined to B R uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B

copy Kemal Akkaya

A

R

B

Routing to another LAN

copy Kemal Akkaya

EthernetldquoDominantrdquo wired LAN technology Cheap $20 for 100Mbs First widely used LAN technology Simpler cheaper than token LANs and ATM Kept up with speed race 10 Mbps ndash 10

Gbps

Metcalfersquos Ethernet sketch

Star Topology Bus topology popular

through mid 90s Now star topology prevails Connection choices hub or

switch (more later)

hub orswitch

copy Kemal Akkaya

Ethernet Frame Structure

Preamble 7 bytes with pattern 10101010 followed by one byte with pattern 10101011 Used to synchronize receiver sender clock rates

Addresses 6 bytes If adapter receives frame with matching destination address or with broadcast

address (eg ARP packet) it passes data in frame to net-layer protocol Otherwise adapter discards frame

Type Indicates the higher layer protocol (mostly IP but others may be supported

such as Novell IPX and AppleTalk) CRC

Checked at receiver if error is detected the frame is simply dropped

Sending adapter encapsulates IP datagram in Ethernet frame

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Ethernet uses CSMACD Connectionless No handshaking between sending and receiving

adapter Unreliable receiving adapter doesnrsquot send acks or nacks to

sending adapter Stream of datagrams passed to network layer can have gaps Gaps will be filled if app is using TCP Otherwise app will see the gaps

No slots Adapter doesnrsquot transmit if it senses that some other adapter is

transmitting that is carrier sense Transmitting adapter aborts when it senses that another adapter is

transmitting that is collision detection Before attempting a retransmission adapter waits a random time

that is random access

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Ethernet CSMACD algorithm1 Adaptor receives datagram from

net layer amp creates frame2 If adapter senses channel idle it

starts to transmit frame If it senses channel busy waits until channel idle and then transmits

3 If adapter transmits entire frame without detecting another transmission the adapter is done with frame

4 If adapter detects another transmission while transmitting aborts and sends jam signal

5 After aborting adapter enters exponential backoff after the mth collision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2

Jam Signal make sure all other transmitters are aware of collision 48 bits

Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec

Exponential Backoff Goal adapt retransmission

attempts to estimated current load Heavy load random wait will be

longer

First collision choose K from 01 delay is K 512 bit transmission times

After second collision choose K from 0123hellip

After ten collisions choose K from 01234hellip1023

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CSMACD efficiency Tprop = max prop between 2 nodes in LAN ttrans = time to transmit max-size frame

Efficiency goes to 1 as tprop goes to 0 Goes to 1 as ttrans goes to infinity Much better than ALOHA but still decentralized

simple and cheap

Other Ethernets Switched Ethernet Fast Ethernet Gigabit Ethernet

transprop tt 51

1efficiency

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Point to Point Data Link Control

One sender one receiver one link easier than broadcast link No Media Access Control No need for explicit MAC addressing eg dialup link ISDN line

Popular point-to-point DLC protocols PPP (point-to-point protocol) HDLC High level data link control (Data link used to be considered

ldquohigh layerrdquo in protocol stack

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HDLC

Frame format for bit-oriented protocol HDLC

Control field of

(a) An information frame

(b) A supervisory frame

(c) An unnumbered frame

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HDLC Exchange of information supervisory and unnumbered

frames Information - data to be transmitted to user (next layer up)

Flow and error control piggybacked on information frames Supervisory - ARQ when piggyback not used Unnumbered - supplementary link control

Three phases Initialization Data transfer Disconnect

Uses bit stuffing Flag delimit frame at both ends 01111110 is the Flag May close one frame and open another Bit stuffing to find flag data

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Internet Datalink Protocol PPP Design Requirements [RFC 1557]

Packet framing encapsulation of network-layer datagram in data link frame Carry network layer data of any network layer protocol (not just IP) at

same time Ability to demultiplex upwards

Bit transparency must carry any bit pattern in the data field Error detection (no correction) Connection liveness detect signal link failure to network layer Network layer address negotiation endpoint can learnconfigure

each otherrsquos network address

PPP non-design requirements No error correctionrecovery No flow control Out of order delivery OK No need to support multipoint links (eg polling)

Error recovery flow control data re-ordering all relegated to higher layers

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PPP Data Frame

Flag delimiter (framing) Address does nothing (only one option) Control does nothing in the future possible multiple

control fields Protocol upper layer protocol to which frame delivered

(eg PPP-LCP IP IPCP etc) Info upper layer data being carried Check cyclic redundancy check for error detection

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Byte Stuffing ldquoData transparencyrdquo requirement data field must be allowed to include

flag pattern lt01111110gt Q is received lt01111110gt data or flag

Sender adds (ldquostuffsrdquo) extra lt 01111110gt byte after each lt 01111110gt data byte How about bit stuffing

Receiver Two 01111110 bytes in a row Discard first byte continue data reception Single 01111110 flag byte

flag bytepatternin datato send

flag byte pattern plusstuffed byte in transmitted data

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PPP Data Control Protocol Before exchanging

network-layer data data link peers must configure PPP link

(max frame length authentication)

learnconfigure network layer information

For IP carry IP Control Protocol (IPCP) messages (protocol field 8021) to configurelearn IP address

Diagram for bring a line up and down

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Sliding Windows Flow Control Allow multiple frames to be in transit Receiver has buffer W long Transmitter can send up to W frames without ACK Each frame is numbered ACK includes number of next frame expected Sequence number bounded by size of field (k)

Frames are numbered modulo 2k

Enhancements Receiver can acknowledge frames without permitting

further transmission (Receive Not Ready) Must send a normal acknowledge to resume If duplex use piggybacking

If no data to send use acknowledgement frame If data but no acknowledgement to send send last acknowledgement

number again or have ACK valid flag (TCP)

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Sliding Windows Diagram

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Example

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Error DetectionEDC = Error Detection and Correction bits (redundancy)D = Data protected by error checking may include header fields

Error detection not 100 reliable Protocol may miss some errors but rarely Larger EDC field yields better detection and correction

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Parity Checking Single Bit Parity

Detect single bit errors

Two Dimensional Bit ParityDetect and correct single bit errors

Internet Checksum Detect ldquoerrorsrdquo (eg flipped bits)

in transmitted segment (note used at transport layer only) 00

Sender Treat segment contents as sequence of 16-bit integers Checksum addition (1rsquos complement sum) of segment contents Sender puts checksum value into UDP checksum field

Receiver Compute checksum of received segment Check if computed checksum equals checksum field value NO - error detected YES - no error

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Cyclic Redundancy Check View data bits D as a binary number Choose r+1 bit pattern (generator) G Goal choose r CRC bits R such that

ltDRgt exactly divisible by G (modulo 2) Receiver knows G divides ltDRgt by G If non-zero remainder error

detected Can detect all burst errors less than r+1 bits

Widely used in practice (ATM HDCL)

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CRC Example

WantD2r XOR R = nG

EquivalentlyD2r = nG XOR R

Equivalently

If we divide D2r by G want remainder R

R = remainder[ ]D2r

G

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Error Correction

Detection and correction of errors due to Lost frames Damaged frames Underlying channel can lose the frames or can be garbled

Solution at the Link Layer Automatic repeat request Error detection Positive acknowledgment Retransmission after timeout Negative acknowledgement and retransmission

Automatic Repeat Request (ARQ) methods Stop and wait Go back N Selective reject (selective retransmission)

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Stop and Wait ARQ Source transmits single

frame Wait for ACK If received frame damaged

discard it Transmitter has timeout If no ACK within timeout

retransmit

If ACK damaged transmitter will not recognize it Transmitter will retransmit Receive gets two copies of frame Use ACK0 and ACK1

Simple but inefficient

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More efficient method Go-back-N Based on sliding window If no error ACK as usual with next frame expected Use window to control number of outstanding frames If error reply with rejection

Discard that frame and all future frames until error frame received correctly

Transmitter must go back and retransmit that frame and all subsequent frames

Handling Damaged Frame Receiver detects error in frame i Receiver sends rejection-i Transmitter gets rejection-i Transmitter retransmits frame i and all subsequent

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Go-back-N Handling Lost Frame First Approach

Frame i lost Transmitter sends i+1 Receiver gets frame i+1 out of sequence Receiver send reject i Transmitter goes back to frame i and retransmits

Second Approach Frame i lost and no additional frame sent Receiver gets nothing and returns neither acknowledgement nor

rejection Transmitter times out and sends acknowledgement frame with P bit

set to 1 Receiver interprets this as command which it acknowledges with the

number of the next frame it expects (frame i ) Transmitter then retransmits frame i

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Go-back-N Damaged ACK

Receiver gets frame i and send acknowledgement (i+1) which is lost

Acknowledgements are cumulative so next acknowledgement (i+n) may arrive before transmitter times out on frame i

If transmitter times out it sends acknowledgement with P bit set as before

This can be repeated a number of times before a reset procedure is initiated

Damaged Rejection Same as Lost Frame (2)

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Selective Reject

Also called selective retransmission

Only rejected frames are retransmitted

Subsequent frames are accepted by the receiver and buffered

Minimizes retransmission Receiver must maintain

large enough buffer More complex login in

transmitter

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Multiple Access Links and ProtocolsTwo types of ldquolinksrdquo Point-to-point

PPP for dial-up access Point-to-point link between Ethernet switch and host

Broadcast (shared wire or medium) Traditional Ethernet Upstream HFC 80211 wireless LAN

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Multiple Access Protocols Single shared broadcast channel Two or more simultaneous transmissions by nodes interference

Collision if node receives two or more signals at the same time

Multiple access protocol Distributed algorithm that determines how nodes share channel ie

determine when node can transmit Communication about channel sharing must use channel itself

No out-of-band channel for coordination

Ideal Multiple Access Protocol When one node wants to transmit it can send at rate R When M nodes want to transmit each can send at average rate RM Fully decentralized

No special node to coordinate transmissions No synchronization of clocks slots

Simple

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MAC Protocols A taxonomy

Three broad classes

Channel Partitioning Divide channel into smaller ldquopiecesrdquo (time slots frequency code) Allocate piece to node for exclusive use

Random Access Channel not divided allow collisions ldquoRecoverrdquo from collisions

ldquoTaking turnsrdquo Nodes take turns Nodes with more to send can take longer turns

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Channel Partitioning MAC protocols TDMA

TDMA Time division multiple access Access to channel in rounds Each station gets fixed length slot

Length = packet transmission time in each round

Unused slots go idle Example 6-station LAN

134 have packet Slots 256 idle

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Channel Partitioning MAC protocols FDMA

FDMA Frequency division multiple access Channel spectrum divided into frequency bands Each station assigned fixed frequency band Unused transmission time in frequency bands go idle Example 6-station LAN 134 have packet frequency

bands 256 idle

frequ

ency

bands time

1

2

3

4

5

6

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Random Access Protocols

When node has packet to send Transmit at full channel data rate R No a priori coordination among nodes

Two or more transmitting nodes ldquocollisionrdquo Random access MAC protocol specifies

How to detect collisions How to recover from collisions (eg via delayed retransmissions)

Examples of random access MAC protocols Slotted ALOHA ALOHA Carrier Sense Multiple Access (CSMA) CSMACollision Detection (CD) CSMACollision Avoidance (CA)

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Slotted ALOHA

Pros Single active node can continuously

transmit at full rate of channel Highly decentralized only slots in

nodes need to be in sync Simple

Cons Collisions wasting slots Idle slots Nodes may be able to detect collision in

less than time to transmit packet Clock synchronization

Assumptions All frames same size Time is divided into equal size

slots time to transmit 1 frame Nodes start to transmit frames

only at beginning of slots Nodes are synchronized If 2 or more nodes transmit in

slot all nodes detect collision

Operation When node obtains fresh frame it

transmits in next slot No collision node can send new frame in

next slot If collision node retransmits frame in

each subsequent slot with prob p until success

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Pure (unslotted) ALOHA Unslotted Aloha simpler no synchronization When frame first arrives

Transmit immediately

Collision probability increases Frame sent at t0 collides with other frames sent in [t0-1t0+1]

Even worse than slotted ALOHA

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Collisions can still occur Propagation delay means two nodes may not hear each otherrsquos transmission Entire packet transmission time wasted Role of distance amp propagation delay in determining collision probability spatial layout of nodes

Idea listen before transmit If channel sensed idle

transmit entire frame If channel sensed busy defer

transmission Human analogy donrsquot

interrupt others

CSMA (Carrier Sense Multiple Access)

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CSMACD (Collision Detection)

CSMACD carrier sensing deferral as in CSMA Collisions detected within short

time Send jam sequence Colliding transmissions aborted

reducing channel wastage

Collision detection

Easy in wired LANs measure signal strengths compare transmitted received signals

Difficult in wireless LANs receiver shut off while transmitting

Human analogy the polite conversationalist

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ldquoTaking Turnsrdquo MAC protocols Channel partitioning MAC protocols

Share channel efficiently and fairly at high load Inefficient at low load

Delay in channel access 1N bandwidth allocated even if only 1 active node

Random access MAC protocols Efficient at low load

Single node can fully utilize channel

high load Collision overhead

ldquoTaking turnsrdquo protocols Look for best of both worlds

Two implementations Polling Token

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ldquoTaking Turnsrdquo MAC protocolsPolling Master node ldquoinvitesrdquo slave

nodes to transmit in turn Concerns

Polling overhead Latency Single point of failure (master)

Token passing control token passed from

one node to next sequentially

token message concerns

token overhead latency single point of failure (token)

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Summary of MAC protocols

What do you do with a shared media Channel Partitioning by time frequency or code

Time Division Frequency Division

Random partitioning (dynamic) ALOHA S-ALOHA CSMA CSMACD Carrier sensing easy in some technologies (wire) hard in others

(wireless) CSMACD used in Ethernet CSMACA used in 80211

Taking Turns Polling from a central site token passing

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LAN technologies

Data link layer so farServicesError detectioncorrectionMultiple access

Next LAN technologiesAddressingEthernetHubs switchesPPP

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MAC Addresses and ARP 32-bit IP address

Network-layer address Used to get datagram to

destination IP subnet

MAC (or LAN or physical or Ethernet) address Used to get datagram

from one interface to another physically-connected interface (same network)

48 bit MAC address (for most LANs) burned in the adapter ROM

Find your PCrsquos MAC address

1A-2F-BB-76-09-AD

58-23-D7-FA-20-B0

0C-C4-11-6F-E3-98

71-65-F7-2B-08-53

LAN(wired orwireless)

Each adapter on LAN has unique LAN address

Broadcast address =FF-FF-FF-FF-FF-FF = adapter

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MAC Address (more)

MAC address allocation administered by IEEE Manufacturer buys portion of MAC address space (to

assure uniqueness) Analogy

(a) MAC address like Social Security Number

(b) IP address like postal address MAC flat address portability

Can move LAN card from one LAN to another

IP hierarchical address NOT portable Depends on IP subnet to which node is attached

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ARP Address Resolution Protocol

Each IP node (Host Router) on LAN has ARP table

ARP Table IPMAC address mappings for some LAN nodes

lt IP address MAC address TTLgt TTL (Time To Live)

time after which address mapping will be forgotten (typically 20 min)

Question how to determineMAC address of Bknowing Brsquos IP address

1A-2F-BB-76-09-AD

58-23-D7-FA-20-B0

0C-C4-11-6F-E3-98

71-65-F7-2B-08-53

LAN

237196723

237196778

237196714

237196788

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ARP protocol Same LAN (network) A wants to send datagram

to B and Brsquos MAC address not in Arsquos ARP table

A broadcasts ARP query packet containing Bs IP address Dest MAC address = FF-FF-

FF-FF-FF-FF All machines on LAN receive

ARP query

B receives ARP packet replies to A with its (Bs) MAC address Frame sent to Arsquos MAC

address (unicast)

A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) Soft state information that

times out (goes away) unless refreshed

ARP is ldquoplug-and-playrdquo Nodes create their ARP

tables without intervention from net administrator

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Routing to another LAN Walkthrough send datagram from A to B via R

Assume A knows B IP address

Two ARP tables in router R one for each IP network (LAN) In routing table at source Host find router 111111111110 In ARP table at source find MAC address E6-E9-00-17-BB-4B etc A creates datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110

A creates link-layer frame with Rs MAC address as dest frame contains A-to-B IP datagram

Arsquos adapter sends frame Rrsquos adapter receives frame R removes IP datagram from Ethernet frame sees its destined to B R uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B

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A

R

B

Routing to another LAN

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EthernetldquoDominantrdquo wired LAN technology Cheap $20 for 100Mbs First widely used LAN technology Simpler cheaper than token LANs and ATM Kept up with speed race 10 Mbps ndash 10

Gbps

Metcalfersquos Ethernet sketch

Star Topology Bus topology popular

through mid 90s Now star topology prevails Connection choices hub or

switch (more later)

hub orswitch

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Ethernet Frame Structure

Preamble 7 bytes with pattern 10101010 followed by one byte with pattern 10101011 Used to synchronize receiver sender clock rates

Addresses 6 bytes If adapter receives frame with matching destination address or with broadcast

address (eg ARP packet) it passes data in frame to net-layer protocol Otherwise adapter discards frame

Type Indicates the higher layer protocol (mostly IP but others may be supported

such as Novell IPX and AppleTalk) CRC

Checked at receiver if error is detected the frame is simply dropped

Sending adapter encapsulates IP datagram in Ethernet frame

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Ethernet uses CSMACD Connectionless No handshaking between sending and receiving

adapter Unreliable receiving adapter doesnrsquot send acks or nacks to

sending adapter Stream of datagrams passed to network layer can have gaps Gaps will be filled if app is using TCP Otherwise app will see the gaps

No slots Adapter doesnrsquot transmit if it senses that some other adapter is

transmitting that is carrier sense Transmitting adapter aborts when it senses that another adapter is

transmitting that is collision detection Before attempting a retransmission adapter waits a random time

that is random access

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Ethernet CSMACD algorithm1 Adaptor receives datagram from

net layer amp creates frame2 If adapter senses channel idle it

starts to transmit frame If it senses channel busy waits until channel idle and then transmits

3 If adapter transmits entire frame without detecting another transmission the adapter is done with frame

4 If adapter detects another transmission while transmitting aborts and sends jam signal

5 After aborting adapter enters exponential backoff after the mth collision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2

Jam Signal make sure all other transmitters are aware of collision 48 bits

Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec

Exponential Backoff Goal adapt retransmission

attempts to estimated current load Heavy load random wait will be

longer

First collision choose K from 01 delay is K 512 bit transmission times

After second collision choose K from 0123hellip

After ten collisions choose K from 01234hellip1023

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CSMACD efficiency Tprop = max prop between 2 nodes in LAN ttrans = time to transmit max-size frame

Efficiency goes to 1 as tprop goes to 0 Goes to 1 as ttrans goes to infinity Much better than ALOHA but still decentralized

simple and cheap

Other Ethernets Switched Ethernet Fast Ethernet Gigabit Ethernet

transprop tt 51

1efficiency

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Point to Point Data Link Control

One sender one receiver one link easier than broadcast link No Media Access Control No need for explicit MAC addressing eg dialup link ISDN line

Popular point-to-point DLC protocols PPP (point-to-point protocol) HDLC High level data link control (Data link used to be considered

ldquohigh layerrdquo in protocol stack

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HDLC

Frame format for bit-oriented protocol HDLC

Control field of

(a) An information frame

(b) A supervisory frame

(c) An unnumbered frame

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HDLC Exchange of information supervisory and unnumbered

frames Information - data to be transmitted to user (next layer up)

Flow and error control piggybacked on information frames Supervisory - ARQ when piggyback not used Unnumbered - supplementary link control

Three phases Initialization Data transfer Disconnect

Uses bit stuffing Flag delimit frame at both ends 01111110 is the Flag May close one frame and open another Bit stuffing to find flag data

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Internet Datalink Protocol PPP Design Requirements [RFC 1557]

Packet framing encapsulation of network-layer datagram in data link frame Carry network layer data of any network layer protocol (not just IP) at

same time Ability to demultiplex upwards

Bit transparency must carry any bit pattern in the data field Error detection (no correction) Connection liveness detect signal link failure to network layer Network layer address negotiation endpoint can learnconfigure

each otherrsquos network address

PPP non-design requirements No error correctionrecovery No flow control Out of order delivery OK No need to support multipoint links (eg polling)

Error recovery flow control data re-ordering all relegated to higher layers

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PPP Data Frame

Flag delimiter (framing) Address does nothing (only one option) Control does nothing in the future possible multiple

control fields Protocol upper layer protocol to which frame delivered

(eg PPP-LCP IP IPCP etc) Info upper layer data being carried Check cyclic redundancy check for error detection

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Byte Stuffing ldquoData transparencyrdquo requirement data field must be allowed to include

flag pattern lt01111110gt Q is received lt01111110gt data or flag

Sender adds (ldquostuffsrdquo) extra lt 01111110gt byte after each lt 01111110gt data byte How about bit stuffing

Receiver Two 01111110 bytes in a row Discard first byte continue data reception Single 01111110 flag byte

flag bytepatternin datato send

flag byte pattern plusstuffed byte in transmitted data

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PPP Data Control Protocol Before exchanging

network-layer data data link peers must configure PPP link

(max frame length authentication)

learnconfigure network layer information

For IP carry IP Control Protocol (IPCP) messages (protocol field 8021) to configurelearn IP address

Diagram for bring a line up and down

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Sliding Windows Diagram

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Example

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Error DetectionEDC = Error Detection and Correction bits (redundancy)D = Data protected by error checking may include header fields

Error detection not 100 reliable Protocol may miss some errors but rarely Larger EDC field yields better detection and correction

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Parity Checking Single Bit Parity

Detect single bit errors

Two Dimensional Bit ParityDetect and correct single bit errors

Internet Checksum Detect ldquoerrorsrdquo (eg flipped bits)

in transmitted segment (note used at transport layer only) 00

Sender Treat segment contents as sequence of 16-bit integers Checksum addition (1rsquos complement sum) of segment contents Sender puts checksum value into UDP checksum field

Receiver Compute checksum of received segment Check if computed checksum equals checksum field value NO - error detected YES - no error

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Cyclic Redundancy Check View data bits D as a binary number Choose r+1 bit pattern (generator) G Goal choose r CRC bits R such that

ltDRgt exactly divisible by G (modulo 2) Receiver knows G divides ltDRgt by G If non-zero remainder error

detected Can detect all burst errors less than r+1 bits

Widely used in practice (ATM HDCL)

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CRC Example

WantD2r XOR R = nG

EquivalentlyD2r = nG XOR R

Equivalently

If we divide D2r by G want remainder R

R = remainder[ ]D2r

G

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Error Correction

Detection and correction of errors due to Lost frames Damaged frames Underlying channel can lose the frames or can be garbled

Solution at the Link Layer Automatic repeat request Error detection Positive acknowledgment Retransmission after timeout Negative acknowledgement and retransmission

Automatic Repeat Request (ARQ) methods Stop and wait Go back N Selective reject (selective retransmission)

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Stop and Wait ARQ Source transmits single

frame Wait for ACK If received frame damaged

discard it Transmitter has timeout If no ACK within timeout

retransmit

If ACK damaged transmitter will not recognize it Transmitter will retransmit Receive gets two copies of frame Use ACK0 and ACK1

Simple but inefficient

copy Kemal Akkaya

More efficient method Go-back-N Based on sliding window If no error ACK as usual with next frame expected Use window to control number of outstanding frames If error reply with rejection

Discard that frame and all future frames until error frame received correctly

Transmitter must go back and retransmit that frame and all subsequent frames

Handling Damaged Frame Receiver detects error in frame i Receiver sends rejection-i Transmitter gets rejection-i Transmitter retransmits frame i and all subsequent

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Go-back-N Handling Lost Frame First Approach

Frame i lost Transmitter sends i+1 Receiver gets frame i+1 out of sequence Receiver send reject i Transmitter goes back to frame i and retransmits

Second Approach Frame i lost and no additional frame sent Receiver gets nothing and returns neither acknowledgement nor

rejection Transmitter times out and sends acknowledgement frame with P bit

set to 1 Receiver interprets this as command which it acknowledges with the

number of the next frame it expects (frame i ) Transmitter then retransmits frame i

copy Kemal Akkaya

Go-back-N Damaged ACK

Receiver gets frame i and send acknowledgement (i+1) which is lost

Acknowledgements are cumulative so next acknowledgement (i+n) may arrive before transmitter times out on frame i

If transmitter times out it sends acknowledgement with P bit set as before

This can be repeated a number of times before a reset procedure is initiated

Damaged Rejection Same as Lost Frame (2)

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Selective Reject

Also called selective retransmission

Only rejected frames are retransmitted

Subsequent frames are accepted by the receiver and buffered

Minimizes retransmission Receiver must maintain

large enough buffer More complex login in

transmitter

copy Kemal Akkaya

Multiple Access Links and ProtocolsTwo types of ldquolinksrdquo Point-to-point

PPP for dial-up access Point-to-point link between Ethernet switch and host

Broadcast (shared wire or medium) Traditional Ethernet Upstream HFC 80211 wireless LAN

copy Kemal Akkaya

Multiple Access Protocols Single shared broadcast channel Two or more simultaneous transmissions by nodes interference

Collision if node receives two or more signals at the same time

Multiple access protocol Distributed algorithm that determines how nodes share channel ie

determine when node can transmit Communication about channel sharing must use channel itself

No out-of-band channel for coordination

Ideal Multiple Access Protocol When one node wants to transmit it can send at rate R When M nodes want to transmit each can send at average rate RM Fully decentralized

No special node to coordinate transmissions No synchronization of clocks slots

Simple

copy Kemal Akkaya

MAC Protocols A taxonomy

Three broad classes

Channel Partitioning Divide channel into smaller ldquopiecesrdquo (time slots frequency code) Allocate piece to node for exclusive use

Random Access Channel not divided allow collisions ldquoRecoverrdquo from collisions

ldquoTaking turnsrdquo Nodes take turns Nodes with more to send can take longer turns

copy Kemal Akkaya

Channel Partitioning MAC protocols TDMA

TDMA Time division multiple access Access to channel in rounds Each station gets fixed length slot

Length = packet transmission time in each round

Unused slots go idle Example 6-station LAN

134 have packet Slots 256 idle

copy Kemal Akkaya

Channel Partitioning MAC protocols FDMA

FDMA Frequency division multiple access Channel spectrum divided into frequency bands Each station assigned fixed frequency band Unused transmission time in frequency bands go idle Example 6-station LAN 134 have packet frequency

bands 256 idle

frequ

ency

bands time

1

2

3

4

5

6

copy Kemal Akkaya

Random Access Protocols

When node has packet to send Transmit at full channel data rate R No a priori coordination among nodes

Two or more transmitting nodes ldquocollisionrdquo Random access MAC protocol specifies

How to detect collisions How to recover from collisions (eg via delayed retransmissions)

Examples of random access MAC protocols Slotted ALOHA ALOHA Carrier Sense Multiple Access (CSMA) CSMACollision Detection (CD) CSMACollision Avoidance (CA)

copy Kemal Akkaya

Slotted ALOHA

Pros Single active node can continuously

transmit at full rate of channel Highly decentralized only slots in

nodes need to be in sync Simple

Cons Collisions wasting slots Idle slots Nodes may be able to detect collision in

less than time to transmit packet Clock synchronization

Assumptions All frames same size Time is divided into equal size

slots time to transmit 1 frame Nodes start to transmit frames

only at beginning of slots Nodes are synchronized If 2 or more nodes transmit in

slot all nodes detect collision

Operation When node obtains fresh frame it

transmits in next slot No collision node can send new frame in

next slot If collision node retransmits frame in

each subsequent slot with prob p until success

copy Kemal Akkaya

Pure (unslotted) ALOHA Unslotted Aloha simpler no synchronization When frame first arrives

Transmit immediately

Collision probability increases Frame sent at t0 collides with other frames sent in [t0-1t0+1]

Even worse than slotted ALOHA

copy Kemal Akkaya

Collisions can still occur Propagation delay means two nodes may not hear each otherrsquos transmission Entire packet transmission time wasted Role of distance amp propagation delay in determining collision probability spatial layout of nodes

Idea listen before transmit If channel sensed idle

transmit entire frame If channel sensed busy defer

transmission Human analogy donrsquot

interrupt others

CSMA (Carrier Sense Multiple Access)

copy Kemal Akkaya

CSMACD (Collision Detection)

CSMACD carrier sensing deferral as in CSMA Collisions detected within short

time Send jam sequence Colliding transmissions aborted

reducing channel wastage

Collision detection

Easy in wired LANs measure signal strengths compare transmitted received signals

Difficult in wireless LANs receiver shut off while transmitting

Human analogy the polite conversationalist

copy Kemal Akkaya

ldquoTaking Turnsrdquo MAC protocols Channel partitioning MAC protocols

Share channel efficiently and fairly at high load Inefficient at low load

Delay in channel access 1N bandwidth allocated even if only 1 active node

Random access MAC protocols Efficient at low load

Single node can fully utilize channel

high load Collision overhead

ldquoTaking turnsrdquo protocols Look for best of both worlds

Two implementations Polling Token

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ldquoTaking Turnsrdquo MAC protocolsPolling Master node ldquoinvitesrdquo slave

nodes to transmit in turn Concerns

Polling overhead Latency Single point of failure (master)

Token passing control token passed from

one node to next sequentially

token message concerns

token overhead latency single point of failure (token)

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Summary of MAC protocols

What do you do with a shared media Channel Partitioning by time frequency or code

Time Division Frequency Division

Random partitioning (dynamic) ALOHA S-ALOHA CSMA CSMACD Carrier sensing easy in some technologies (wire) hard in others

(wireless) CSMACD used in Ethernet CSMACA used in 80211

Taking Turns Polling from a central site token passing

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LAN technologies

Data link layer so farServicesError detectioncorrectionMultiple access

Next LAN technologiesAddressingEthernetHubs switchesPPP

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MAC Addresses and ARP 32-bit IP address

Network-layer address Used to get datagram to

destination IP subnet

MAC (or LAN or physical or Ethernet) address Used to get datagram

from one interface to another physically-connected interface (same network)

48 bit MAC address (for most LANs) burned in the adapter ROM

Find your PCrsquos MAC address

1A-2F-BB-76-09-AD

58-23-D7-FA-20-B0

0C-C4-11-6F-E3-98

71-65-F7-2B-08-53

LAN(wired orwireless)

Each adapter on LAN has unique LAN address

Broadcast address =FF-FF-FF-FF-FF-FF = adapter

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MAC Address (more)

MAC address allocation administered by IEEE Manufacturer buys portion of MAC address space (to

assure uniqueness) Analogy

(a) MAC address like Social Security Number

(b) IP address like postal address MAC flat address portability

Can move LAN card from one LAN to another

IP hierarchical address NOT portable Depends on IP subnet to which node is attached

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ARP Address Resolution Protocol

Each IP node (Host Router) on LAN has ARP table

ARP Table IPMAC address mappings for some LAN nodes

lt IP address MAC address TTLgt TTL (Time To Live)

time after which address mapping will be forgotten (typically 20 min)

Question how to determineMAC address of Bknowing Brsquos IP address

1A-2F-BB-76-09-AD

58-23-D7-FA-20-B0

0C-C4-11-6F-E3-98

71-65-F7-2B-08-53

LAN

237196723

237196778

237196714

237196788

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ARP protocol Same LAN (network) A wants to send datagram

to B and Brsquos MAC address not in Arsquos ARP table

A broadcasts ARP query packet containing Bs IP address Dest MAC address = FF-FF-

FF-FF-FF-FF All machines on LAN receive

ARP query

B receives ARP packet replies to A with its (Bs) MAC address Frame sent to Arsquos MAC

address (unicast)

A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) Soft state information that

times out (goes away) unless refreshed

ARP is ldquoplug-and-playrdquo Nodes create their ARP

tables without intervention from net administrator

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Routing to another LAN Walkthrough send datagram from A to B via R

Assume A knows B IP address

Two ARP tables in router R one for each IP network (LAN) In routing table at source Host find router 111111111110 In ARP table at source find MAC address E6-E9-00-17-BB-4B etc A creates datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110

A creates link-layer frame with Rs MAC address as dest frame contains A-to-B IP datagram

Arsquos adapter sends frame Rrsquos adapter receives frame R removes IP datagram from Ethernet frame sees its destined to B R uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B

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A

R

B

Routing to another LAN

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EthernetldquoDominantrdquo wired LAN technology Cheap $20 for 100Mbs First widely used LAN technology Simpler cheaper than token LANs and ATM Kept up with speed race 10 Mbps ndash 10

Gbps

Metcalfersquos Ethernet sketch

Star Topology Bus topology popular

through mid 90s Now star topology prevails Connection choices hub or

switch (more later)

hub orswitch

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Ethernet Frame Structure

Preamble 7 bytes with pattern 10101010 followed by one byte with pattern 10101011 Used to synchronize receiver sender clock rates

Addresses 6 bytes If adapter receives frame with matching destination address or with broadcast

address (eg ARP packet) it passes data in frame to net-layer protocol Otherwise adapter discards frame

Type Indicates the higher layer protocol (mostly IP but others may be supported

such as Novell IPX and AppleTalk) CRC

Checked at receiver if error is detected the frame is simply dropped

Sending adapter encapsulates IP datagram in Ethernet frame

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Ethernet uses CSMACD Connectionless No handshaking between sending and receiving

adapter Unreliable receiving adapter doesnrsquot send acks or nacks to

sending adapter Stream of datagrams passed to network layer can have gaps Gaps will be filled if app is using TCP Otherwise app will see the gaps

No slots Adapter doesnrsquot transmit if it senses that some other adapter is

transmitting that is carrier sense Transmitting adapter aborts when it senses that another adapter is

transmitting that is collision detection Before attempting a retransmission adapter waits a random time

that is random access

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Ethernet CSMACD algorithm1 Adaptor receives datagram from

net layer amp creates frame2 If adapter senses channel idle it

starts to transmit frame If it senses channel busy waits until channel idle and then transmits

3 If adapter transmits entire frame without detecting another transmission the adapter is done with frame

4 If adapter detects another transmission while transmitting aborts and sends jam signal

5 After aborting adapter enters exponential backoff after the mth collision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2

Jam Signal make sure all other transmitters are aware of collision 48 bits

Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec

Exponential Backoff Goal adapt retransmission

attempts to estimated current load Heavy load random wait will be

longer

First collision choose K from 01 delay is K 512 bit transmission times

After second collision choose K from 0123hellip

After ten collisions choose K from 01234hellip1023

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CSMACD efficiency Tprop = max prop between 2 nodes in LAN ttrans = time to transmit max-size frame

Efficiency goes to 1 as tprop goes to 0 Goes to 1 as ttrans goes to infinity Much better than ALOHA but still decentralized

simple and cheap

Other Ethernets Switched Ethernet Fast Ethernet Gigabit Ethernet

transprop tt 51

1efficiency

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Point to Point Data Link Control

One sender one receiver one link easier than broadcast link No Media Access Control No need for explicit MAC addressing eg dialup link ISDN line

Popular point-to-point DLC protocols PPP (point-to-point protocol) HDLC High level data link control (Data link used to be considered

ldquohigh layerrdquo in protocol stack

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HDLC

Frame format for bit-oriented protocol HDLC

Control field of

(a) An information frame

(b) A supervisory frame

(c) An unnumbered frame

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HDLC Exchange of information supervisory and unnumbered

frames Information - data to be transmitted to user (next layer up)

Flow and error control piggybacked on information frames Supervisory - ARQ when piggyback not used Unnumbered - supplementary link control

Three phases Initialization Data transfer Disconnect

Uses bit stuffing Flag delimit frame at both ends 01111110 is the Flag May close one frame and open another Bit stuffing to find flag data

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Internet Datalink Protocol PPP Design Requirements [RFC 1557]

Packet framing encapsulation of network-layer datagram in data link frame Carry network layer data of any network layer protocol (not just IP) at

same time Ability to demultiplex upwards

Bit transparency must carry any bit pattern in the data field Error detection (no correction) Connection liveness detect signal link failure to network layer Network layer address negotiation endpoint can learnconfigure

each otherrsquos network address

PPP non-design requirements No error correctionrecovery No flow control Out of order delivery OK No need to support multipoint links (eg polling)

Error recovery flow control data re-ordering all relegated to higher layers

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PPP Data Frame

Flag delimiter (framing) Address does nothing (only one option) Control does nothing in the future possible multiple

control fields Protocol upper layer protocol to which frame delivered

(eg PPP-LCP IP IPCP etc) Info upper layer data being carried Check cyclic redundancy check for error detection

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Byte Stuffing ldquoData transparencyrdquo requirement data field must be allowed to include

flag pattern lt01111110gt Q is received lt01111110gt data or flag

Sender adds (ldquostuffsrdquo) extra lt 01111110gt byte after each lt 01111110gt data byte How about bit stuffing

Receiver Two 01111110 bytes in a row Discard first byte continue data reception Single 01111110 flag byte

flag bytepatternin datato send

flag byte pattern plusstuffed byte in transmitted data

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PPP Data Control Protocol Before exchanging

network-layer data data link peers must configure PPP link

(max frame length authentication)

learnconfigure network layer information

For IP carry IP Control Protocol (IPCP) messages (protocol field 8021) to configurelearn IP address

Diagram for bring a line up and down

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Example

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Error DetectionEDC = Error Detection and Correction bits (redundancy)D = Data protected by error checking may include header fields

Error detection not 100 reliable Protocol may miss some errors but rarely Larger EDC field yields better detection and correction

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Parity Checking Single Bit Parity

Detect single bit errors

Two Dimensional Bit ParityDetect and correct single bit errors

Internet Checksum Detect ldquoerrorsrdquo (eg flipped bits)

in transmitted segment (note used at transport layer only) 00

Sender Treat segment contents as sequence of 16-bit integers Checksum addition (1rsquos complement sum) of segment contents Sender puts checksum value into UDP checksum field

Receiver Compute checksum of received segment Check if computed checksum equals checksum field value NO - error detected YES - no error

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Cyclic Redundancy Check View data bits D as a binary number Choose r+1 bit pattern (generator) G Goal choose r CRC bits R such that

ltDRgt exactly divisible by G (modulo 2) Receiver knows G divides ltDRgt by G If non-zero remainder error

detected Can detect all burst errors less than r+1 bits

Widely used in practice (ATM HDCL)

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CRC Example

WantD2r XOR R = nG

EquivalentlyD2r = nG XOR R

Equivalently

If we divide D2r by G want remainder R

R = remainder[ ]D2r

G

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Error Correction

Detection and correction of errors due to Lost frames Damaged frames Underlying channel can lose the frames or can be garbled

Solution at the Link Layer Automatic repeat request Error detection Positive acknowledgment Retransmission after timeout Negative acknowledgement and retransmission

Automatic Repeat Request (ARQ) methods Stop and wait Go back N Selective reject (selective retransmission)

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Stop and Wait ARQ Source transmits single

frame Wait for ACK If received frame damaged

discard it Transmitter has timeout If no ACK within timeout

retransmit

If ACK damaged transmitter will not recognize it Transmitter will retransmit Receive gets two copies of frame Use ACK0 and ACK1

Simple but inefficient

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More efficient method Go-back-N Based on sliding window If no error ACK as usual with next frame expected Use window to control number of outstanding frames If error reply with rejection

Discard that frame and all future frames until error frame received correctly

Transmitter must go back and retransmit that frame and all subsequent frames

Handling Damaged Frame Receiver detects error in frame i Receiver sends rejection-i Transmitter gets rejection-i Transmitter retransmits frame i and all subsequent

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Go-back-N Handling Lost Frame First Approach

Frame i lost Transmitter sends i+1 Receiver gets frame i+1 out of sequence Receiver send reject i Transmitter goes back to frame i and retransmits

Second Approach Frame i lost and no additional frame sent Receiver gets nothing and returns neither acknowledgement nor

rejection Transmitter times out and sends acknowledgement frame with P bit

set to 1 Receiver interprets this as command which it acknowledges with the

number of the next frame it expects (frame i ) Transmitter then retransmits frame i

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Go-back-N Damaged ACK

Receiver gets frame i and send acknowledgement (i+1) which is lost

Acknowledgements are cumulative so next acknowledgement (i+n) may arrive before transmitter times out on frame i

If transmitter times out it sends acknowledgement with P bit set as before

This can be repeated a number of times before a reset procedure is initiated

Damaged Rejection Same as Lost Frame (2)

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Selective Reject

Also called selective retransmission

Only rejected frames are retransmitted

Subsequent frames are accepted by the receiver and buffered

Minimizes retransmission Receiver must maintain

large enough buffer More complex login in

transmitter

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Multiple Access Links and ProtocolsTwo types of ldquolinksrdquo Point-to-point

PPP for dial-up access Point-to-point link between Ethernet switch and host

Broadcast (shared wire or medium) Traditional Ethernet Upstream HFC 80211 wireless LAN

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Multiple Access Protocols Single shared broadcast channel Two or more simultaneous transmissions by nodes interference

Collision if node receives two or more signals at the same time

Multiple access protocol Distributed algorithm that determines how nodes share channel ie

determine when node can transmit Communication about channel sharing must use channel itself

No out-of-band channel for coordination

Ideal Multiple Access Protocol When one node wants to transmit it can send at rate R When M nodes want to transmit each can send at average rate RM Fully decentralized

No special node to coordinate transmissions No synchronization of clocks slots

Simple

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MAC Protocols A taxonomy

Three broad classes

Channel Partitioning Divide channel into smaller ldquopiecesrdquo (time slots frequency code) Allocate piece to node for exclusive use

Random Access Channel not divided allow collisions ldquoRecoverrdquo from collisions

ldquoTaking turnsrdquo Nodes take turns Nodes with more to send can take longer turns

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Channel Partitioning MAC protocols TDMA

TDMA Time division multiple access Access to channel in rounds Each station gets fixed length slot

Length = packet transmission time in each round

Unused slots go idle Example 6-station LAN

134 have packet Slots 256 idle

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Channel Partitioning MAC protocols FDMA

FDMA Frequency division multiple access Channel spectrum divided into frequency bands Each station assigned fixed frequency band Unused transmission time in frequency bands go idle Example 6-station LAN 134 have packet frequency

bands 256 idle

frequ

ency

bands time

1

2

3

4

5

6

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Random Access Protocols

When node has packet to send Transmit at full channel data rate R No a priori coordination among nodes

Two or more transmitting nodes ldquocollisionrdquo Random access MAC protocol specifies

How to detect collisions How to recover from collisions (eg via delayed retransmissions)

Examples of random access MAC protocols Slotted ALOHA ALOHA Carrier Sense Multiple Access (CSMA) CSMACollision Detection (CD) CSMACollision Avoidance (CA)

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Slotted ALOHA

Pros Single active node can continuously

transmit at full rate of channel Highly decentralized only slots in

nodes need to be in sync Simple

Cons Collisions wasting slots Idle slots Nodes may be able to detect collision in

less than time to transmit packet Clock synchronization

Assumptions All frames same size Time is divided into equal size

slots time to transmit 1 frame Nodes start to transmit frames

only at beginning of slots Nodes are synchronized If 2 or more nodes transmit in

slot all nodes detect collision

Operation When node obtains fresh frame it

transmits in next slot No collision node can send new frame in

next slot If collision node retransmits frame in

each subsequent slot with prob p until success

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Pure (unslotted) ALOHA Unslotted Aloha simpler no synchronization When frame first arrives

Transmit immediately

Collision probability increases Frame sent at t0 collides with other frames sent in [t0-1t0+1]

Even worse than slotted ALOHA

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Collisions can still occur Propagation delay means two nodes may not hear each otherrsquos transmission Entire packet transmission time wasted Role of distance amp propagation delay in determining collision probability spatial layout of nodes

Idea listen before transmit If channel sensed idle

transmit entire frame If channel sensed busy defer

transmission Human analogy donrsquot

interrupt others

CSMA (Carrier Sense Multiple Access)

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CSMACD (Collision Detection)

CSMACD carrier sensing deferral as in CSMA Collisions detected within short

time Send jam sequence Colliding transmissions aborted

reducing channel wastage

Collision detection

Easy in wired LANs measure signal strengths compare transmitted received signals

Difficult in wireless LANs receiver shut off while transmitting

Human analogy the polite conversationalist

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ldquoTaking Turnsrdquo MAC protocols Channel partitioning MAC protocols

Share channel efficiently and fairly at high load Inefficient at low load

Delay in channel access 1N bandwidth allocated even if only 1 active node

Random access MAC protocols Efficient at low load

Single node can fully utilize channel

high load Collision overhead

ldquoTaking turnsrdquo protocols Look for best of both worlds

Two implementations Polling Token

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ldquoTaking Turnsrdquo MAC protocolsPolling Master node ldquoinvitesrdquo slave

nodes to transmit in turn Concerns

Polling overhead Latency Single point of failure (master)

Token passing control token passed from

one node to next sequentially

token message concerns

token overhead latency single point of failure (token)

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Summary of MAC protocols

What do you do with a shared media Channel Partitioning by time frequency or code

Time Division Frequency Division

Random partitioning (dynamic) ALOHA S-ALOHA CSMA CSMACD Carrier sensing easy in some technologies (wire) hard in others

(wireless) CSMACD used in Ethernet CSMACA used in 80211

Taking Turns Polling from a central site token passing

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LAN technologies

Data link layer so farServicesError detectioncorrectionMultiple access

Next LAN technologiesAddressingEthernetHubs switchesPPP

copy Kemal Akkaya

MAC Addresses and ARP 32-bit IP address

Network-layer address Used to get datagram to

destination IP subnet

MAC (or LAN or physical or Ethernet) address Used to get datagram

from one interface to another physically-connected interface (same network)

48 bit MAC address (for most LANs) burned in the adapter ROM

Find your PCrsquos MAC address

1A-2F-BB-76-09-AD

58-23-D7-FA-20-B0

0C-C4-11-6F-E3-98

71-65-F7-2B-08-53

LAN(wired orwireless)

Each adapter on LAN has unique LAN address

Broadcast address =FF-FF-FF-FF-FF-FF = adapter

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MAC Address (more)

MAC address allocation administered by IEEE Manufacturer buys portion of MAC address space (to

assure uniqueness) Analogy

(a) MAC address like Social Security Number

(b) IP address like postal address MAC flat address portability

Can move LAN card from one LAN to another

IP hierarchical address NOT portable Depends on IP subnet to which node is attached

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ARP Address Resolution Protocol

Each IP node (Host Router) on LAN has ARP table

ARP Table IPMAC address mappings for some LAN nodes

lt IP address MAC address TTLgt TTL (Time To Live)

time after which address mapping will be forgotten (typically 20 min)

Question how to determineMAC address of Bknowing Brsquos IP address

1A-2F-BB-76-09-AD

58-23-D7-FA-20-B0

0C-C4-11-6F-E3-98

71-65-F7-2B-08-53

LAN

237196723

237196778

237196714

237196788

copy Kemal Akkaya

ARP protocol Same LAN (network) A wants to send datagram

to B and Brsquos MAC address not in Arsquos ARP table

A broadcasts ARP query packet containing Bs IP address Dest MAC address = FF-FF-

FF-FF-FF-FF All machines on LAN receive

ARP query

B receives ARP packet replies to A with its (Bs) MAC address Frame sent to Arsquos MAC

address (unicast)

A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) Soft state information that

times out (goes away) unless refreshed

ARP is ldquoplug-and-playrdquo Nodes create their ARP

tables without intervention from net administrator

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Routing to another LAN Walkthrough send datagram from A to B via R

Assume A knows B IP address

Two ARP tables in router R one for each IP network (LAN) In routing table at source Host find router 111111111110 In ARP table at source find MAC address E6-E9-00-17-BB-4B etc A creates datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110

A creates link-layer frame with Rs MAC address as dest frame contains A-to-B IP datagram

Arsquos adapter sends frame Rrsquos adapter receives frame R removes IP datagram from Ethernet frame sees its destined to B R uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B

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A

R

B

Routing to another LAN

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EthernetldquoDominantrdquo wired LAN technology Cheap $20 for 100Mbs First widely used LAN technology Simpler cheaper than token LANs and ATM Kept up with speed race 10 Mbps ndash 10

Gbps

Metcalfersquos Ethernet sketch

Star Topology Bus topology popular

through mid 90s Now star topology prevails Connection choices hub or

switch (more later)

hub orswitch

copy Kemal Akkaya

Ethernet Frame Structure

Preamble 7 bytes with pattern 10101010 followed by one byte with pattern 10101011 Used to synchronize receiver sender clock rates

Addresses 6 bytes If adapter receives frame with matching destination address or with broadcast

address (eg ARP packet) it passes data in frame to net-layer protocol Otherwise adapter discards frame

Type Indicates the higher layer protocol (mostly IP but others may be supported

such as Novell IPX and AppleTalk) CRC

Checked at receiver if error is detected the frame is simply dropped

Sending adapter encapsulates IP datagram in Ethernet frame

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Ethernet uses CSMACD Connectionless No handshaking between sending and receiving

adapter Unreliable receiving adapter doesnrsquot send acks or nacks to

sending adapter Stream of datagrams passed to network layer can have gaps Gaps will be filled if app is using TCP Otherwise app will see the gaps

No slots Adapter doesnrsquot transmit if it senses that some other adapter is

transmitting that is carrier sense Transmitting adapter aborts when it senses that another adapter is

transmitting that is collision detection Before attempting a retransmission adapter waits a random time

that is random access

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Ethernet CSMACD algorithm1 Adaptor receives datagram from

net layer amp creates frame2 If adapter senses channel idle it

starts to transmit frame If it senses channel busy waits until channel idle and then transmits

3 If adapter transmits entire frame without detecting another transmission the adapter is done with frame

4 If adapter detects another transmission while transmitting aborts and sends jam signal

5 After aborting adapter enters exponential backoff after the mth collision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2

Jam Signal make sure all other transmitters are aware of collision 48 bits

Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec

Exponential Backoff Goal adapt retransmission

attempts to estimated current load Heavy load random wait will be

longer

First collision choose K from 01 delay is K 512 bit transmission times

After second collision choose K from 0123hellip

After ten collisions choose K from 01234hellip1023

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CSMACD efficiency Tprop = max prop between 2 nodes in LAN ttrans = time to transmit max-size frame

Efficiency goes to 1 as tprop goes to 0 Goes to 1 as ttrans goes to infinity Much better than ALOHA but still decentralized

simple and cheap

Other Ethernets Switched Ethernet Fast Ethernet Gigabit Ethernet

transprop tt 51

1efficiency

copy Kemal Akkaya

Point to Point Data Link Control

One sender one receiver one link easier than broadcast link No Media Access Control No need for explicit MAC addressing eg dialup link ISDN line

Popular point-to-point DLC protocols PPP (point-to-point protocol) HDLC High level data link control (Data link used to be considered

ldquohigh layerrdquo in protocol stack

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HDLC

Frame format for bit-oriented protocol HDLC

Control field of

(a) An information frame

(b) A supervisory frame

(c) An unnumbered frame

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HDLC Exchange of information supervisory and unnumbered

frames Information - data to be transmitted to user (next layer up)

Flow and error control piggybacked on information frames Supervisory - ARQ when piggyback not used Unnumbered - supplementary link control

Three phases Initialization Data transfer Disconnect

Uses bit stuffing Flag delimit frame at both ends 01111110 is the Flag May close one frame and open another Bit stuffing to find flag data

copy Kemal Akkaya

Internet Datalink Protocol PPP Design Requirements [RFC 1557]

Packet framing encapsulation of network-layer datagram in data link frame Carry network layer data of any network layer protocol (not just IP) at

same time Ability to demultiplex upwards

Bit transparency must carry any bit pattern in the data field Error detection (no correction) Connection liveness detect signal link failure to network layer Network layer address negotiation endpoint can learnconfigure

each otherrsquos network address

PPP non-design requirements No error correctionrecovery No flow control Out of order delivery OK No need to support multipoint links (eg polling)

Error recovery flow control data re-ordering all relegated to higher layers

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PPP Data Frame

Flag delimiter (framing) Address does nothing (only one option) Control does nothing in the future possible multiple

control fields Protocol upper layer protocol to which frame delivered

(eg PPP-LCP IP IPCP etc) Info upper layer data being carried Check cyclic redundancy check for error detection

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Byte Stuffing ldquoData transparencyrdquo requirement data field must be allowed to include

flag pattern lt01111110gt Q is received lt01111110gt data or flag

Sender adds (ldquostuffsrdquo) extra lt 01111110gt byte after each lt 01111110gt data byte How about bit stuffing

Receiver Two 01111110 bytes in a row Discard first byte continue data reception Single 01111110 flag byte

flag bytepatternin datato send

flag byte pattern plusstuffed byte in transmitted data

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PPP Data Control Protocol Before exchanging

network-layer data data link peers must configure PPP link

(max frame length authentication)

learnconfigure network layer information

For IP carry IP Control Protocol (IPCP) messages (protocol field 8021) to configurelearn IP address

Diagram for bring a line up and down

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Error DetectionEDC = Error Detection and Correction bits (redundancy)D = Data protected by error checking may include header fields

Error detection not 100 reliable Protocol may miss some errors but rarely Larger EDC field yields better detection and correction

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Parity Checking Single Bit Parity

Detect single bit errors

Two Dimensional Bit ParityDetect and correct single bit errors

Internet Checksum Detect ldquoerrorsrdquo (eg flipped bits)

in transmitted segment (note used at transport layer only) 00

Sender Treat segment contents as sequence of 16-bit integers Checksum addition (1rsquos complement sum) of segment contents Sender puts checksum value into UDP checksum field

Receiver Compute checksum of received segment Check if computed checksum equals checksum field value NO - error detected YES - no error

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Cyclic Redundancy Check View data bits D as a binary number Choose r+1 bit pattern (generator) G Goal choose r CRC bits R such that

ltDRgt exactly divisible by G (modulo 2) Receiver knows G divides ltDRgt by G If non-zero remainder error

detected Can detect all burst errors less than r+1 bits

Widely used in practice (ATM HDCL)

copy Kemal Akkaya

CRC Example

WantD2r XOR R = nG

EquivalentlyD2r = nG XOR R

Equivalently

If we divide D2r by G want remainder R

R = remainder[ ]D2r

G

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Error Correction

Detection and correction of errors due to Lost frames Damaged frames Underlying channel can lose the frames or can be garbled

Solution at the Link Layer Automatic repeat request Error detection Positive acknowledgment Retransmission after timeout Negative acknowledgement and retransmission

Automatic Repeat Request (ARQ) methods Stop and wait Go back N Selective reject (selective retransmission)

copy Kemal Akkaya

Stop and Wait ARQ Source transmits single

frame Wait for ACK If received frame damaged

discard it Transmitter has timeout If no ACK within timeout

retransmit

If ACK damaged transmitter will not recognize it Transmitter will retransmit Receive gets two copies of frame Use ACK0 and ACK1

Simple but inefficient

copy Kemal Akkaya

More efficient method Go-back-N Based on sliding window If no error ACK as usual with next frame expected Use window to control number of outstanding frames If error reply with rejection

Discard that frame and all future frames until error frame received correctly

Transmitter must go back and retransmit that frame and all subsequent frames

Handling Damaged Frame Receiver detects error in frame i Receiver sends rejection-i Transmitter gets rejection-i Transmitter retransmits frame i and all subsequent

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Go-back-N Handling Lost Frame First Approach

Frame i lost Transmitter sends i+1 Receiver gets frame i+1 out of sequence Receiver send reject i Transmitter goes back to frame i and retransmits

Second Approach Frame i lost and no additional frame sent Receiver gets nothing and returns neither acknowledgement nor

rejection Transmitter times out and sends acknowledgement frame with P bit

set to 1 Receiver interprets this as command which it acknowledges with the

number of the next frame it expects (frame i ) Transmitter then retransmits frame i

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Go-back-N Damaged ACK

Receiver gets frame i and send acknowledgement (i+1) which is lost

Acknowledgements are cumulative so next acknowledgement (i+n) may arrive before transmitter times out on frame i

If transmitter times out it sends acknowledgement with P bit set as before

This can be repeated a number of times before a reset procedure is initiated

Damaged Rejection Same as Lost Frame (2)

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Selective Reject

Also called selective retransmission

Only rejected frames are retransmitted

Subsequent frames are accepted by the receiver and buffered

Minimizes retransmission Receiver must maintain

large enough buffer More complex login in

transmitter

copy Kemal Akkaya

Multiple Access Links and ProtocolsTwo types of ldquolinksrdquo Point-to-point

PPP for dial-up access Point-to-point link between Ethernet switch and host

Broadcast (shared wire or medium) Traditional Ethernet Upstream HFC 80211 wireless LAN

copy Kemal Akkaya

Multiple Access Protocols Single shared broadcast channel Two or more simultaneous transmissions by nodes interference

Collision if node receives two or more signals at the same time

Multiple access protocol Distributed algorithm that determines how nodes share channel ie

determine when node can transmit Communication about channel sharing must use channel itself

No out-of-band channel for coordination

Ideal Multiple Access Protocol When one node wants to transmit it can send at rate R When M nodes want to transmit each can send at average rate RM Fully decentralized

No special node to coordinate transmissions No synchronization of clocks slots

Simple

copy Kemal Akkaya

MAC Protocols A taxonomy

Three broad classes

Channel Partitioning Divide channel into smaller ldquopiecesrdquo (time slots frequency code) Allocate piece to node for exclusive use

Random Access Channel not divided allow collisions ldquoRecoverrdquo from collisions

ldquoTaking turnsrdquo Nodes take turns Nodes with more to send can take longer turns

copy Kemal Akkaya

Channel Partitioning MAC protocols TDMA

TDMA Time division multiple access Access to channel in rounds Each station gets fixed length slot

Length = packet transmission time in each round

Unused slots go idle Example 6-station LAN

134 have packet Slots 256 idle

copy Kemal Akkaya

Channel Partitioning MAC protocols FDMA

FDMA Frequency division multiple access Channel spectrum divided into frequency bands Each station assigned fixed frequency band Unused transmission time in frequency bands go idle Example 6-station LAN 134 have packet frequency

bands 256 idle

frequ

ency

bands time

1

2

3

4

5

6

copy Kemal Akkaya

Random Access Protocols

When node has packet to send Transmit at full channel data rate R No a priori coordination among nodes

Two or more transmitting nodes ldquocollisionrdquo Random access MAC protocol specifies

How to detect collisions How to recover from collisions (eg via delayed retransmissions)

Examples of random access MAC protocols Slotted ALOHA ALOHA Carrier Sense Multiple Access (CSMA) CSMACollision Detection (CD) CSMACollision Avoidance (CA)

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Slotted ALOHA

Pros Single active node can continuously

transmit at full rate of channel Highly decentralized only slots in

nodes need to be in sync Simple

Cons Collisions wasting slots Idle slots Nodes may be able to detect collision in

less than time to transmit packet Clock synchronization

Assumptions All frames same size Time is divided into equal size

slots time to transmit 1 frame Nodes start to transmit frames

only at beginning of slots Nodes are synchronized If 2 or more nodes transmit in

slot all nodes detect collision

Operation When node obtains fresh frame it

transmits in next slot No collision node can send new frame in

next slot If collision node retransmits frame in

each subsequent slot with prob p until success

copy Kemal Akkaya

Pure (unslotted) ALOHA Unslotted Aloha simpler no synchronization When frame first arrives

Transmit immediately

Collision probability increases Frame sent at t0 collides with other frames sent in [t0-1t0+1]

Even worse than slotted ALOHA

copy Kemal Akkaya

Collisions can still occur Propagation delay means two nodes may not hear each otherrsquos transmission Entire packet transmission time wasted Role of distance amp propagation delay in determining collision probability spatial layout of nodes

Idea listen before transmit If channel sensed idle

transmit entire frame If channel sensed busy defer

transmission Human analogy donrsquot

interrupt others

CSMA (Carrier Sense Multiple Access)

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CSMACD (Collision Detection)

CSMACD carrier sensing deferral as in CSMA Collisions detected within short

time Send jam sequence Colliding transmissions aborted

reducing channel wastage

Collision detection

Easy in wired LANs measure signal strengths compare transmitted received signals

Difficult in wireless LANs receiver shut off while transmitting

Human analogy the polite conversationalist

copy Kemal Akkaya

ldquoTaking Turnsrdquo MAC protocols Channel partitioning MAC protocols

Share channel efficiently and fairly at high load Inefficient at low load

Delay in channel access 1N bandwidth allocated even if only 1 active node

Random access MAC protocols Efficient at low load

Single node can fully utilize channel

high load Collision overhead

ldquoTaking turnsrdquo protocols Look for best of both worlds

Two implementations Polling Token

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ldquoTaking Turnsrdquo MAC protocolsPolling Master node ldquoinvitesrdquo slave

nodes to transmit in turn Concerns

Polling overhead Latency Single point of failure (master)

Token passing control token passed from

one node to next sequentially

token message concerns

token overhead latency single point of failure (token)

copy Kemal Akkaya

Summary of MAC protocols

What do you do with a shared media Channel Partitioning by time frequency or code

Time Division Frequency Division

Random partitioning (dynamic) ALOHA S-ALOHA CSMA CSMACD Carrier sensing easy in some technologies (wire) hard in others

(wireless) CSMACD used in Ethernet CSMACA used in 80211

Taking Turns Polling from a central site token passing

copy Kemal Akkaya

LAN technologies

Data link layer so farServicesError detectioncorrectionMultiple access

Next LAN technologiesAddressingEthernetHubs switchesPPP

copy Kemal Akkaya

MAC Addresses and ARP 32-bit IP address

Network-layer address Used to get datagram to

destination IP subnet

MAC (or LAN or physical or Ethernet) address Used to get datagram

from one interface to another physically-connected interface (same network)

48 bit MAC address (for most LANs) burned in the adapter ROM

Find your PCrsquos MAC address

1A-2F-BB-76-09-AD

58-23-D7-FA-20-B0

0C-C4-11-6F-E3-98

71-65-F7-2B-08-53

LAN(wired orwireless)

Each adapter on LAN has unique LAN address

Broadcast address =FF-FF-FF-FF-FF-FF = adapter

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MAC Address (more)

MAC address allocation administered by IEEE Manufacturer buys portion of MAC address space (to

assure uniqueness) Analogy

(a) MAC address like Social Security Number

(b) IP address like postal address MAC flat address portability

Can move LAN card from one LAN to another

IP hierarchical address NOT portable Depends on IP subnet to which node is attached

copy Kemal Akkaya

ARP Address Resolution Protocol

Each IP node (Host Router) on LAN has ARP table

ARP Table IPMAC address mappings for some LAN nodes

lt IP address MAC address TTLgt TTL (Time To Live)

time after which address mapping will be forgotten (typically 20 min)

Question how to determineMAC address of Bknowing Brsquos IP address

1A-2F-BB-76-09-AD

58-23-D7-FA-20-B0

0C-C4-11-6F-E3-98

71-65-F7-2B-08-53

LAN

237196723

237196778

237196714

237196788

copy Kemal Akkaya

ARP protocol Same LAN (network) A wants to send datagram

to B and Brsquos MAC address not in Arsquos ARP table

A broadcasts ARP query packet containing Bs IP address Dest MAC address = FF-FF-

FF-FF-FF-FF All machines on LAN receive

ARP query

B receives ARP packet replies to A with its (Bs) MAC address Frame sent to Arsquos MAC

address (unicast)

A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) Soft state information that

times out (goes away) unless refreshed

ARP is ldquoplug-and-playrdquo Nodes create their ARP

tables without intervention from net administrator

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Routing to another LAN Walkthrough send datagram from A to B via R

Assume A knows B IP address

Two ARP tables in router R one for each IP network (LAN) In routing table at source Host find router 111111111110 In ARP table at source find MAC address E6-E9-00-17-BB-4B etc A creates datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110

A creates link-layer frame with Rs MAC address as dest frame contains A-to-B IP datagram

Arsquos adapter sends frame Rrsquos adapter receives frame R removes IP datagram from Ethernet frame sees its destined to B R uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B

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A

R

B

Routing to another LAN

copy Kemal Akkaya

EthernetldquoDominantrdquo wired LAN technology Cheap $20 for 100Mbs First widely used LAN technology Simpler cheaper than token LANs and ATM Kept up with speed race 10 Mbps ndash 10

Gbps

Metcalfersquos Ethernet sketch

Star Topology Bus topology popular

through mid 90s Now star topology prevails Connection choices hub or

switch (more later)

hub orswitch

copy Kemal Akkaya

Ethernet Frame Structure

Preamble 7 bytes with pattern 10101010 followed by one byte with pattern 10101011 Used to synchronize receiver sender clock rates

Addresses 6 bytes If adapter receives frame with matching destination address or with broadcast

address (eg ARP packet) it passes data in frame to net-layer protocol Otherwise adapter discards frame

Type Indicates the higher layer protocol (mostly IP but others may be supported

such as Novell IPX and AppleTalk) CRC

Checked at receiver if error is detected the frame is simply dropped

Sending adapter encapsulates IP datagram in Ethernet frame

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Ethernet uses CSMACD Connectionless No handshaking between sending and receiving

adapter Unreliable receiving adapter doesnrsquot send acks or nacks to

sending adapter Stream of datagrams passed to network layer can have gaps Gaps will be filled if app is using TCP Otherwise app will see the gaps

No slots Adapter doesnrsquot transmit if it senses that some other adapter is

transmitting that is carrier sense Transmitting adapter aborts when it senses that another adapter is

transmitting that is collision detection Before attempting a retransmission adapter waits a random time

that is random access

copy Kemal Akkaya

Ethernet CSMACD algorithm1 Adaptor receives datagram from

net layer amp creates frame2 If adapter senses channel idle it

starts to transmit frame If it senses channel busy waits until channel idle and then transmits

3 If adapter transmits entire frame without detecting another transmission the adapter is done with frame

4 If adapter detects another transmission while transmitting aborts and sends jam signal

5 After aborting adapter enters exponential backoff after the mth collision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2

Jam Signal make sure all other transmitters are aware of collision 48 bits

Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec

Exponential Backoff Goal adapt retransmission

attempts to estimated current load Heavy load random wait will be

longer

First collision choose K from 01 delay is K 512 bit transmission times

After second collision choose K from 0123hellip

After ten collisions choose K from 01234hellip1023

copy Kemal Akkaya

CSMACD efficiency Tprop = max prop between 2 nodes in LAN ttrans = time to transmit max-size frame

Efficiency goes to 1 as tprop goes to 0 Goes to 1 as ttrans goes to infinity Much better than ALOHA but still decentralized

simple and cheap

Other Ethernets Switched Ethernet Fast Ethernet Gigabit Ethernet

transprop tt 51

1efficiency

copy Kemal Akkaya

Point to Point Data Link Control

One sender one receiver one link easier than broadcast link No Media Access Control No need for explicit MAC addressing eg dialup link ISDN line

Popular point-to-point DLC protocols PPP (point-to-point protocol) HDLC High level data link control (Data link used to be considered

ldquohigh layerrdquo in protocol stack

copy Kemal Akkaya

HDLC

Frame format for bit-oriented protocol HDLC

Control field of

(a) An information frame

(b) A supervisory frame

(c) An unnumbered frame

copy Kemal Akkaya

HDLC Exchange of information supervisory and unnumbered

frames Information - data to be transmitted to user (next layer up)

Flow and error control piggybacked on information frames Supervisory - ARQ when piggyback not used Unnumbered - supplementary link control

Three phases Initialization Data transfer Disconnect

Uses bit stuffing Flag delimit frame at both ends 01111110 is the Flag May close one frame and open another Bit stuffing to find flag data

copy Kemal Akkaya

Internet Datalink Protocol PPP Design Requirements [RFC 1557]

Packet framing encapsulation of network-layer datagram in data link frame Carry network layer data of any network layer protocol (not just IP) at

same time Ability to demultiplex upwards

Bit transparency must carry any bit pattern in the data field Error detection (no correction) Connection liveness detect signal link failure to network layer Network layer address negotiation endpoint can learnconfigure

each otherrsquos network address

PPP non-design requirements No error correctionrecovery No flow control Out of order delivery OK No need to support multipoint links (eg polling)

Error recovery flow control data re-ordering all relegated to higher layers

copy Kemal Akkaya

PPP Data Frame

Flag delimiter (framing) Address does nothing (only one option) Control does nothing in the future possible multiple

control fields Protocol upper layer protocol to which frame delivered

(eg PPP-LCP IP IPCP etc) Info upper layer data being carried Check cyclic redundancy check for error detection

copy Kemal Akkaya

Byte Stuffing ldquoData transparencyrdquo requirement data field must be allowed to include

flag pattern lt01111110gt Q is received lt01111110gt data or flag

Sender adds (ldquostuffsrdquo) extra lt 01111110gt byte after each lt 01111110gt data byte How about bit stuffing

Receiver Two 01111110 bytes in a row Discard first byte continue data reception Single 01111110 flag byte

flag bytepatternin datato send

flag byte pattern plusstuffed byte in transmitted data

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PPP Data Control Protocol Before exchanging

network-layer data data link peers must configure PPP link

(max frame length authentication)

learnconfigure network layer information

For IP carry IP Control Protocol (IPCP) messages (protocol field 8021) to configurelearn IP address

Diagram for bring a line up and down

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Parity Checking Single Bit Parity

Detect single bit errors

Two Dimensional Bit ParityDetect and correct single bit errors

Internet Checksum Detect ldquoerrorsrdquo (eg flipped bits)

in transmitted segment (note used at transport layer only) 00

Sender Treat segment contents as sequence of 16-bit integers Checksum addition (1rsquos complement sum) of segment contents Sender puts checksum value into UDP checksum field

Receiver Compute checksum of received segment Check if computed checksum equals checksum field value NO - error detected YES - no error

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Cyclic Redundancy Check View data bits D as a binary number Choose r+1 bit pattern (generator) G Goal choose r CRC bits R such that

ltDRgt exactly divisible by G (modulo 2) Receiver knows G divides ltDRgt by G If non-zero remainder error

detected Can detect all burst errors less than r+1 bits

Widely used in practice (ATM HDCL)

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CRC Example

WantD2r XOR R = nG

EquivalentlyD2r = nG XOR R

Equivalently

If we divide D2r by G want remainder R

R = remainder[ ]D2r

G

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Error Correction

Detection and correction of errors due to Lost frames Damaged frames Underlying channel can lose the frames or can be garbled

Solution at the Link Layer Automatic repeat request Error detection Positive acknowledgment Retransmission after timeout Negative acknowledgement and retransmission

Automatic Repeat Request (ARQ) methods Stop and wait Go back N Selective reject (selective retransmission)

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Stop and Wait ARQ Source transmits single

frame Wait for ACK If received frame damaged

discard it Transmitter has timeout If no ACK within timeout

retransmit

If ACK damaged transmitter will not recognize it Transmitter will retransmit Receive gets two copies of frame Use ACK0 and ACK1

Simple but inefficient

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More efficient method Go-back-N Based on sliding window If no error ACK as usual with next frame expected Use window to control number of outstanding frames If error reply with rejection

Discard that frame and all future frames until error frame received correctly

Transmitter must go back and retransmit that frame and all subsequent frames

Handling Damaged Frame Receiver detects error in frame i Receiver sends rejection-i Transmitter gets rejection-i Transmitter retransmits frame i and all subsequent

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Go-back-N Handling Lost Frame First Approach

Frame i lost Transmitter sends i+1 Receiver gets frame i+1 out of sequence Receiver send reject i Transmitter goes back to frame i and retransmits

Second Approach Frame i lost and no additional frame sent Receiver gets nothing and returns neither acknowledgement nor

rejection Transmitter times out and sends acknowledgement frame with P bit

set to 1 Receiver interprets this as command which it acknowledges with the

number of the next frame it expects (frame i ) Transmitter then retransmits frame i

copy Kemal Akkaya

Go-back-N Damaged ACK

Receiver gets frame i and send acknowledgement (i+1) which is lost

Acknowledgements are cumulative so next acknowledgement (i+n) may arrive before transmitter times out on frame i

If transmitter times out it sends acknowledgement with P bit set as before

This can be repeated a number of times before a reset procedure is initiated

Damaged Rejection Same as Lost Frame (2)

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Selective Reject

Also called selective retransmission

Only rejected frames are retransmitted

Subsequent frames are accepted by the receiver and buffered

Minimizes retransmission Receiver must maintain

large enough buffer More complex login in

transmitter

copy Kemal Akkaya

Multiple Access Links and ProtocolsTwo types of ldquolinksrdquo Point-to-point

PPP for dial-up access Point-to-point link between Ethernet switch and host

Broadcast (shared wire or medium) Traditional Ethernet Upstream HFC 80211 wireless LAN

copy Kemal Akkaya

Multiple Access Protocols Single shared broadcast channel Two or more simultaneous transmissions by nodes interference

Collision if node receives two or more signals at the same time

Multiple access protocol Distributed algorithm that determines how nodes share channel ie

determine when node can transmit Communication about channel sharing must use channel itself

No out-of-band channel for coordination

Ideal Multiple Access Protocol When one node wants to transmit it can send at rate R When M nodes want to transmit each can send at average rate RM Fully decentralized

No special node to coordinate transmissions No synchronization of clocks slots

Simple

copy Kemal Akkaya

MAC Protocols A taxonomy

Three broad classes

Channel Partitioning Divide channel into smaller ldquopiecesrdquo (time slots frequency code) Allocate piece to node for exclusive use

Random Access Channel not divided allow collisions ldquoRecoverrdquo from collisions

ldquoTaking turnsrdquo Nodes take turns Nodes with more to send can take longer turns

copy Kemal Akkaya

Channel Partitioning MAC protocols TDMA

TDMA Time division multiple access Access to channel in rounds Each station gets fixed length slot

Length = packet transmission time in each round

Unused slots go idle Example 6-station LAN

134 have packet Slots 256 idle

copy Kemal Akkaya

Channel Partitioning MAC protocols FDMA

FDMA Frequency division multiple access Channel spectrum divided into frequency bands Each station assigned fixed frequency band Unused transmission time in frequency bands go idle Example 6-station LAN 134 have packet frequency

bands 256 idle

frequ

ency

bands time

1

2

3

4

5

6

copy Kemal Akkaya

Random Access Protocols

When node has packet to send Transmit at full channel data rate R No a priori coordination among nodes

Two or more transmitting nodes ldquocollisionrdquo Random access MAC protocol specifies

How to detect collisions How to recover from collisions (eg via delayed retransmissions)

Examples of random access MAC protocols Slotted ALOHA ALOHA Carrier Sense Multiple Access (CSMA) CSMACollision Detection (CD) CSMACollision Avoidance (CA)

copy Kemal Akkaya

Slotted ALOHA

Pros Single active node can continuously

transmit at full rate of channel Highly decentralized only slots in

nodes need to be in sync Simple

Cons Collisions wasting slots Idle slots Nodes may be able to detect collision in

less than time to transmit packet Clock synchronization

Assumptions All frames same size Time is divided into equal size

slots time to transmit 1 frame Nodes start to transmit frames

only at beginning of slots Nodes are synchronized If 2 or more nodes transmit in

slot all nodes detect collision

Operation When node obtains fresh frame it

transmits in next slot No collision node can send new frame in

next slot If collision node retransmits frame in

each subsequent slot with prob p until success

copy Kemal Akkaya

Pure (unslotted) ALOHA Unslotted Aloha simpler no synchronization When frame first arrives

Transmit immediately

Collision probability increases Frame sent at t0 collides with other frames sent in [t0-1t0+1]

Even worse than slotted ALOHA

copy Kemal Akkaya

Collisions can still occur Propagation delay means two nodes may not hear each otherrsquos transmission Entire packet transmission time wasted Role of distance amp propagation delay in determining collision probability spatial layout of nodes

Idea listen before transmit If channel sensed idle

transmit entire frame If channel sensed busy defer

transmission Human analogy donrsquot

interrupt others

CSMA (Carrier Sense Multiple Access)

copy Kemal Akkaya

CSMACD (Collision Detection)

CSMACD carrier sensing deferral as in CSMA Collisions detected within short

time Send jam sequence Colliding transmissions aborted

reducing channel wastage

Collision detection

Easy in wired LANs measure signal strengths compare transmitted received signals

Difficult in wireless LANs receiver shut off while transmitting

Human analogy the polite conversationalist

copy Kemal Akkaya

ldquoTaking Turnsrdquo MAC protocols Channel partitioning MAC protocols

Share channel efficiently and fairly at high load Inefficient at low load

Delay in channel access 1N bandwidth allocated even if only 1 active node

Random access MAC protocols Efficient at low load

Single node can fully utilize channel

high load Collision overhead

ldquoTaking turnsrdquo protocols Look for best of both worlds

Two implementations Polling Token

copy Kemal Akkaya

ldquoTaking Turnsrdquo MAC protocolsPolling Master node ldquoinvitesrdquo slave

nodes to transmit in turn Concerns

Polling overhead Latency Single point of failure (master)

Token passing control token passed from

one node to next sequentially

token message concerns

token overhead latency single point of failure (token)

copy Kemal Akkaya

Summary of MAC protocols

What do you do with a shared media Channel Partitioning by time frequency or code

Time Division Frequency Division

Random partitioning (dynamic) ALOHA S-ALOHA CSMA CSMACD Carrier sensing easy in some technologies (wire) hard in others

(wireless) CSMACD used in Ethernet CSMACA used in 80211

Taking Turns Polling from a central site token passing

copy Kemal Akkaya

LAN technologies

Data link layer so farServicesError detectioncorrectionMultiple access

Next LAN technologiesAddressingEthernetHubs switchesPPP

copy Kemal Akkaya

MAC Addresses and ARP 32-bit IP address

Network-layer address Used to get datagram to

destination IP subnet

MAC (or LAN or physical or Ethernet) address Used to get datagram

from one interface to another physically-connected interface (same network)

48 bit MAC address (for most LANs) burned in the adapter ROM

Find your PCrsquos MAC address

1A-2F-BB-76-09-AD

58-23-D7-FA-20-B0

0C-C4-11-6F-E3-98

71-65-F7-2B-08-53

LAN(wired orwireless)

Each adapter on LAN has unique LAN address

Broadcast address =FF-FF-FF-FF-FF-FF = adapter

copy Kemal Akkaya

MAC Address (more)

MAC address allocation administered by IEEE Manufacturer buys portion of MAC address space (to

assure uniqueness) Analogy

(a) MAC address like Social Security Number

(b) IP address like postal address MAC flat address portability

Can move LAN card from one LAN to another

IP hierarchical address NOT portable Depends on IP subnet to which node is attached

copy Kemal Akkaya

ARP Address Resolution Protocol

Each IP node (Host Router) on LAN has ARP table

ARP Table IPMAC address mappings for some LAN nodes

lt IP address MAC address TTLgt TTL (Time To Live)

time after which address mapping will be forgotten (typically 20 min)

Question how to determineMAC address of Bknowing Brsquos IP address

1A-2F-BB-76-09-AD

58-23-D7-FA-20-B0

0C-C4-11-6F-E3-98

71-65-F7-2B-08-53

LAN

237196723

237196778

237196714

237196788

copy Kemal Akkaya

ARP protocol Same LAN (network) A wants to send datagram

to B and Brsquos MAC address not in Arsquos ARP table

A broadcasts ARP query packet containing Bs IP address Dest MAC address = FF-FF-

FF-FF-FF-FF All machines on LAN receive

ARP query

B receives ARP packet replies to A with its (Bs) MAC address Frame sent to Arsquos MAC

address (unicast)

A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) Soft state information that

times out (goes away) unless refreshed

ARP is ldquoplug-and-playrdquo Nodes create their ARP

tables without intervention from net administrator

copy Kemal Akkaya

Routing to another LAN Walkthrough send datagram from A to B via R

Assume A knows B IP address

Two ARP tables in router R one for each IP network (LAN) In routing table at source Host find router 111111111110 In ARP table at source find MAC address E6-E9-00-17-BB-4B etc A creates datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110

A creates link-layer frame with Rs MAC address as dest frame contains A-to-B IP datagram

Arsquos adapter sends frame Rrsquos adapter receives frame R removes IP datagram from Ethernet frame sees its destined to B R uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B

copy Kemal Akkaya

A

R

B

Routing to another LAN

copy Kemal Akkaya

EthernetldquoDominantrdquo wired LAN technology Cheap $20 for 100Mbs First widely used LAN technology Simpler cheaper than token LANs and ATM Kept up with speed race 10 Mbps ndash 10

Gbps

Metcalfersquos Ethernet sketch

Star Topology Bus topology popular

through mid 90s Now star topology prevails Connection choices hub or

switch (more later)

hub orswitch

copy Kemal Akkaya

Ethernet Frame Structure

Preamble 7 bytes with pattern 10101010 followed by one byte with pattern 10101011 Used to synchronize receiver sender clock rates

Addresses 6 bytes If adapter receives frame with matching destination address or with broadcast

address (eg ARP packet) it passes data in frame to net-layer protocol Otherwise adapter discards frame

Type Indicates the higher layer protocol (mostly IP but others may be supported

such as Novell IPX and AppleTalk) CRC

Checked at receiver if error is detected the frame is simply dropped

Sending adapter encapsulates IP datagram in Ethernet frame

copy Kemal Akkaya

Ethernet uses CSMACD Connectionless No handshaking between sending and receiving

adapter Unreliable receiving adapter doesnrsquot send acks or nacks to

sending adapter Stream of datagrams passed to network layer can have gaps Gaps will be filled if app is using TCP Otherwise app will see the gaps

No slots Adapter doesnrsquot transmit if it senses that some other adapter is

transmitting that is carrier sense Transmitting adapter aborts when it senses that another adapter is

transmitting that is collision detection Before attempting a retransmission adapter waits a random time

that is random access

copy Kemal Akkaya

Ethernet CSMACD algorithm1 Adaptor receives datagram from

net layer amp creates frame2 If adapter senses channel idle it

starts to transmit frame If it senses channel busy waits until channel idle and then transmits

3 If adapter transmits entire frame without detecting another transmission the adapter is done with frame

4 If adapter detects another transmission while transmitting aborts and sends jam signal

5 After aborting adapter enters exponential backoff after the mth collision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2

Jam Signal make sure all other transmitters are aware of collision 48 bits

Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec

Exponential Backoff Goal adapt retransmission

attempts to estimated current load Heavy load random wait will be

longer

First collision choose K from 01 delay is K 512 bit transmission times

After second collision choose K from 0123hellip

After ten collisions choose K from 01234hellip1023

copy Kemal Akkaya

CSMACD efficiency Tprop = max prop between 2 nodes in LAN ttrans = time to transmit max-size frame

Efficiency goes to 1 as tprop goes to 0 Goes to 1 as ttrans goes to infinity Much better than ALOHA but still decentralized

simple and cheap

Other Ethernets Switched Ethernet Fast Ethernet Gigabit Ethernet

transprop tt 51

1efficiency

copy Kemal Akkaya

Point to Point Data Link Control

One sender one receiver one link easier than broadcast link No Media Access Control No need for explicit MAC addressing eg dialup link ISDN line

Popular point-to-point DLC protocols PPP (point-to-point protocol) HDLC High level data link control (Data link used to be considered

ldquohigh layerrdquo in protocol stack

copy Kemal Akkaya

HDLC

Frame format for bit-oriented protocol HDLC

Control field of

(a) An information frame

(b) A supervisory frame

(c) An unnumbered frame

copy Kemal Akkaya

HDLC Exchange of information supervisory and unnumbered

frames Information - data to be transmitted to user (next layer up)

Flow and error control piggybacked on information frames Supervisory - ARQ when piggyback not used Unnumbered - supplementary link control

Three phases Initialization Data transfer Disconnect

Uses bit stuffing Flag delimit frame at both ends 01111110 is the Flag May close one frame and open another Bit stuffing to find flag data

copy Kemal Akkaya

Internet Datalink Protocol PPP Design Requirements [RFC 1557]

Packet framing encapsulation of network-layer datagram in data link frame Carry network layer data of any network layer protocol (not just IP) at

same time Ability to demultiplex upwards

Bit transparency must carry any bit pattern in the data field Error detection (no correction) Connection liveness detect signal link failure to network layer Network layer address negotiation endpoint can learnconfigure

each otherrsquos network address

PPP non-design requirements No error correctionrecovery No flow control Out of order delivery OK No need to support multipoint links (eg polling)

Error recovery flow control data re-ordering all relegated to higher layers

copy Kemal Akkaya

PPP Data Frame

Flag delimiter (framing) Address does nothing (only one option) Control does nothing in the future possible multiple

control fields Protocol upper layer protocol to which frame delivered

(eg PPP-LCP IP IPCP etc) Info upper layer data being carried Check cyclic redundancy check for error detection

copy Kemal Akkaya

Byte Stuffing ldquoData transparencyrdquo requirement data field must be allowed to include

flag pattern lt01111110gt Q is received lt01111110gt data or flag

Sender adds (ldquostuffsrdquo) extra lt 01111110gt byte after each lt 01111110gt data byte How about bit stuffing

Receiver Two 01111110 bytes in a row Discard first byte continue data reception Single 01111110 flag byte

flag bytepatternin datato send

flag byte pattern plusstuffed byte in transmitted data

copy Kemal Akkaya

PPP Data Control Protocol Before exchanging

network-layer data data link peers must configure PPP link

(max frame length authentication)

learnconfigure network layer information

For IP carry IP Control Protocol (IPCP) messages (protocol field 8021) to configurelearn IP address

Diagram for bring a line up and down

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Cyclic Redundancy Check View data bits D as a binary number Choose r+1 bit pattern (generator) G Goal choose r CRC bits R such that

ltDRgt exactly divisible by G (modulo 2) Receiver knows G divides ltDRgt by G If non-zero remainder error

detected Can detect all burst errors less than r+1 bits

Widely used in practice (ATM HDCL)

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CRC Example

WantD2r XOR R = nG

EquivalentlyD2r = nG XOR R

Equivalently

If we divide D2r by G want remainder R

R = remainder[ ]D2r

G

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Error Correction

Detection and correction of errors due to Lost frames Damaged frames Underlying channel can lose the frames or can be garbled

Solution at the Link Layer Automatic repeat request Error detection Positive acknowledgment Retransmission after timeout Negative acknowledgement and retransmission

Automatic Repeat Request (ARQ) methods Stop and wait Go back N Selective reject (selective retransmission)

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Stop and Wait ARQ Source transmits single

frame Wait for ACK If received frame damaged

discard it Transmitter has timeout If no ACK within timeout

retransmit

If ACK damaged transmitter will not recognize it Transmitter will retransmit Receive gets two copies of frame Use ACK0 and ACK1

Simple but inefficient

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More efficient method Go-back-N Based on sliding window If no error ACK as usual with next frame expected Use window to control number of outstanding frames If error reply with rejection

Discard that frame and all future frames until error frame received correctly

Transmitter must go back and retransmit that frame and all subsequent frames

Handling Damaged Frame Receiver detects error in frame i Receiver sends rejection-i Transmitter gets rejection-i Transmitter retransmits frame i and all subsequent

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Go-back-N Handling Lost Frame First Approach

Frame i lost Transmitter sends i+1 Receiver gets frame i+1 out of sequence Receiver send reject i Transmitter goes back to frame i and retransmits

Second Approach Frame i lost and no additional frame sent Receiver gets nothing and returns neither acknowledgement nor

rejection Transmitter times out and sends acknowledgement frame with P bit

set to 1 Receiver interprets this as command which it acknowledges with the

number of the next frame it expects (frame i ) Transmitter then retransmits frame i

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Go-back-N Damaged ACK

Receiver gets frame i and send acknowledgement (i+1) which is lost

Acknowledgements are cumulative so next acknowledgement (i+n) may arrive before transmitter times out on frame i

If transmitter times out it sends acknowledgement with P bit set as before

This can be repeated a number of times before a reset procedure is initiated

Damaged Rejection Same as Lost Frame (2)

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Selective Reject

Also called selective retransmission

Only rejected frames are retransmitted

Subsequent frames are accepted by the receiver and buffered

Minimizes retransmission Receiver must maintain

large enough buffer More complex login in

transmitter

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Multiple Access Links and ProtocolsTwo types of ldquolinksrdquo Point-to-point

PPP for dial-up access Point-to-point link between Ethernet switch and host

Broadcast (shared wire or medium) Traditional Ethernet Upstream HFC 80211 wireless LAN

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Multiple Access Protocols Single shared broadcast channel Two or more simultaneous transmissions by nodes interference

Collision if node receives two or more signals at the same time

Multiple access protocol Distributed algorithm that determines how nodes share channel ie

determine when node can transmit Communication about channel sharing must use channel itself

No out-of-band channel for coordination

Ideal Multiple Access Protocol When one node wants to transmit it can send at rate R When M nodes want to transmit each can send at average rate RM Fully decentralized

No special node to coordinate transmissions No synchronization of clocks slots

Simple

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MAC Protocols A taxonomy

Three broad classes

Channel Partitioning Divide channel into smaller ldquopiecesrdquo (time slots frequency code) Allocate piece to node for exclusive use

Random Access Channel not divided allow collisions ldquoRecoverrdquo from collisions

ldquoTaking turnsrdquo Nodes take turns Nodes with more to send can take longer turns

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Channel Partitioning MAC protocols TDMA

TDMA Time division multiple access Access to channel in rounds Each station gets fixed length slot

Length = packet transmission time in each round

Unused slots go idle Example 6-station LAN

134 have packet Slots 256 idle

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Channel Partitioning MAC protocols FDMA

FDMA Frequency division multiple access Channel spectrum divided into frequency bands Each station assigned fixed frequency band Unused transmission time in frequency bands go idle Example 6-station LAN 134 have packet frequency

bands 256 idle

frequ

ency

bands time

1

2

3

4

5

6

copy Kemal Akkaya

Random Access Protocols

When node has packet to send Transmit at full channel data rate R No a priori coordination among nodes

Two or more transmitting nodes ldquocollisionrdquo Random access MAC protocol specifies

How to detect collisions How to recover from collisions (eg via delayed retransmissions)

Examples of random access MAC protocols Slotted ALOHA ALOHA Carrier Sense Multiple Access (CSMA) CSMACollision Detection (CD) CSMACollision Avoidance (CA)

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Slotted ALOHA

Pros Single active node can continuously

transmit at full rate of channel Highly decentralized only slots in

nodes need to be in sync Simple

Cons Collisions wasting slots Idle slots Nodes may be able to detect collision in

less than time to transmit packet Clock synchronization

Assumptions All frames same size Time is divided into equal size

slots time to transmit 1 frame Nodes start to transmit frames

only at beginning of slots Nodes are synchronized If 2 or more nodes transmit in

slot all nodes detect collision

Operation When node obtains fresh frame it

transmits in next slot No collision node can send new frame in

next slot If collision node retransmits frame in

each subsequent slot with prob p until success

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Pure (unslotted) ALOHA Unslotted Aloha simpler no synchronization When frame first arrives

Transmit immediately

Collision probability increases Frame sent at t0 collides with other frames sent in [t0-1t0+1]

Even worse than slotted ALOHA

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Collisions can still occur Propagation delay means two nodes may not hear each otherrsquos transmission Entire packet transmission time wasted Role of distance amp propagation delay in determining collision probability spatial layout of nodes

Idea listen before transmit If channel sensed idle

transmit entire frame If channel sensed busy defer

transmission Human analogy donrsquot

interrupt others

CSMA (Carrier Sense Multiple Access)

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CSMACD (Collision Detection)

CSMACD carrier sensing deferral as in CSMA Collisions detected within short

time Send jam sequence Colliding transmissions aborted

reducing channel wastage

Collision detection

Easy in wired LANs measure signal strengths compare transmitted received signals

Difficult in wireless LANs receiver shut off while transmitting

Human analogy the polite conversationalist

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ldquoTaking Turnsrdquo MAC protocols Channel partitioning MAC protocols

Share channel efficiently and fairly at high load Inefficient at low load

Delay in channel access 1N bandwidth allocated even if only 1 active node

Random access MAC protocols Efficient at low load

Single node can fully utilize channel

high load Collision overhead

ldquoTaking turnsrdquo protocols Look for best of both worlds

Two implementations Polling Token

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ldquoTaking Turnsrdquo MAC protocolsPolling Master node ldquoinvitesrdquo slave

nodes to transmit in turn Concerns

Polling overhead Latency Single point of failure (master)

Token passing control token passed from

one node to next sequentially

token message concerns

token overhead latency single point of failure (token)

copy Kemal Akkaya

Summary of MAC protocols

What do you do with a shared media Channel Partitioning by time frequency or code

Time Division Frequency Division

Random partitioning (dynamic) ALOHA S-ALOHA CSMA CSMACD Carrier sensing easy in some technologies (wire) hard in others

(wireless) CSMACD used in Ethernet CSMACA used in 80211

Taking Turns Polling from a central site token passing

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LAN technologies

Data link layer so farServicesError detectioncorrectionMultiple access

Next LAN technologiesAddressingEthernetHubs switchesPPP

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MAC Addresses and ARP 32-bit IP address

Network-layer address Used to get datagram to

destination IP subnet

MAC (or LAN or physical or Ethernet) address Used to get datagram

from one interface to another physically-connected interface (same network)

48 bit MAC address (for most LANs) burned in the adapter ROM

Find your PCrsquos MAC address

1A-2F-BB-76-09-AD

58-23-D7-FA-20-B0

0C-C4-11-6F-E3-98

71-65-F7-2B-08-53

LAN(wired orwireless)

Each adapter on LAN has unique LAN address

Broadcast address =FF-FF-FF-FF-FF-FF = adapter

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MAC Address (more)

MAC address allocation administered by IEEE Manufacturer buys portion of MAC address space (to

assure uniqueness) Analogy

(a) MAC address like Social Security Number

(b) IP address like postal address MAC flat address portability

Can move LAN card from one LAN to another

IP hierarchical address NOT portable Depends on IP subnet to which node is attached

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ARP Address Resolution Protocol

Each IP node (Host Router) on LAN has ARP table

ARP Table IPMAC address mappings for some LAN nodes

lt IP address MAC address TTLgt TTL (Time To Live)

time after which address mapping will be forgotten (typically 20 min)

Question how to determineMAC address of Bknowing Brsquos IP address

1A-2F-BB-76-09-AD

58-23-D7-FA-20-B0

0C-C4-11-6F-E3-98

71-65-F7-2B-08-53

LAN

237196723

237196778

237196714

237196788

copy Kemal Akkaya

ARP protocol Same LAN (network) A wants to send datagram

to B and Brsquos MAC address not in Arsquos ARP table

A broadcasts ARP query packet containing Bs IP address Dest MAC address = FF-FF-

FF-FF-FF-FF All machines on LAN receive

ARP query

B receives ARP packet replies to A with its (Bs) MAC address Frame sent to Arsquos MAC

address (unicast)

A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) Soft state information that

times out (goes away) unless refreshed

ARP is ldquoplug-and-playrdquo Nodes create their ARP

tables without intervention from net administrator

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Routing to another LAN Walkthrough send datagram from A to B via R

Assume A knows B IP address

Two ARP tables in router R one for each IP network (LAN) In routing table at source Host find router 111111111110 In ARP table at source find MAC address E6-E9-00-17-BB-4B etc A creates datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110

A creates link-layer frame with Rs MAC address as dest frame contains A-to-B IP datagram

Arsquos adapter sends frame Rrsquos adapter receives frame R removes IP datagram from Ethernet frame sees its destined to B R uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B

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A

R

B

Routing to another LAN

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EthernetldquoDominantrdquo wired LAN technology Cheap $20 for 100Mbs First widely used LAN technology Simpler cheaper than token LANs and ATM Kept up with speed race 10 Mbps ndash 10

Gbps

Metcalfersquos Ethernet sketch

Star Topology Bus topology popular

through mid 90s Now star topology prevails Connection choices hub or

switch (more later)

hub orswitch

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Ethernet Frame Structure

Preamble 7 bytes with pattern 10101010 followed by one byte with pattern 10101011 Used to synchronize receiver sender clock rates

Addresses 6 bytes If adapter receives frame with matching destination address or with broadcast

address (eg ARP packet) it passes data in frame to net-layer protocol Otherwise adapter discards frame

Type Indicates the higher layer protocol (mostly IP but others may be supported

such as Novell IPX and AppleTalk) CRC

Checked at receiver if error is detected the frame is simply dropped

Sending adapter encapsulates IP datagram in Ethernet frame

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Ethernet uses CSMACD Connectionless No handshaking between sending and receiving

adapter Unreliable receiving adapter doesnrsquot send acks or nacks to

sending adapter Stream of datagrams passed to network layer can have gaps Gaps will be filled if app is using TCP Otherwise app will see the gaps

No slots Adapter doesnrsquot transmit if it senses that some other adapter is

transmitting that is carrier sense Transmitting adapter aborts when it senses that another adapter is

transmitting that is collision detection Before attempting a retransmission adapter waits a random time

that is random access

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Ethernet CSMACD algorithm1 Adaptor receives datagram from

net layer amp creates frame2 If adapter senses channel idle it

starts to transmit frame If it senses channel busy waits until channel idle and then transmits

3 If adapter transmits entire frame without detecting another transmission the adapter is done with frame

4 If adapter detects another transmission while transmitting aborts and sends jam signal

5 After aborting adapter enters exponential backoff after the mth collision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2

Jam Signal make sure all other transmitters are aware of collision 48 bits

Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec

Exponential Backoff Goal adapt retransmission

attempts to estimated current load Heavy load random wait will be

longer

First collision choose K from 01 delay is K 512 bit transmission times

After second collision choose K from 0123hellip

After ten collisions choose K from 01234hellip1023

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CSMACD efficiency Tprop = max prop between 2 nodes in LAN ttrans = time to transmit max-size frame

Efficiency goes to 1 as tprop goes to 0 Goes to 1 as ttrans goes to infinity Much better than ALOHA but still decentralized

simple and cheap

Other Ethernets Switched Ethernet Fast Ethernet Gigabit Ethernet

transprop tt 51

1efficiency

copy Kemal Akkaya

Point to Point Data Link Control

One sender one receiver one link easier than broadcast link No Media Access Control No need for explicit MAC addressing eg dialup link ISDN line

Popular point-to-point DLC protocols PPP (point-to-point protocol) HDLC High level data link control (Data link used to be considered

ldquohigh layerrdquo in protocol stack

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HDLC

Frame format for bit-oriented protocol HDLC

Control field of

(a) An information frame

(b) A supervisory frame

(c) An unnumbered frame

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HDLC Exchange of information supervisory and unnumbered

frames Information - data to be transmitted to user (next layer up)

Flow and error control piggybacked on information frames Supervisory - ARQ when piggyback not used Unnumbered - supplementary link control

Three phases Initialization Data transfer Disconnect

Uses bit stuffing Flag delimit frame at both ends 01111110 is the Flag May close one frame and open another Bit stuffing to find flag data

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Internet Datalink Protocol PPP Design Requirements [RFC 1557]

Packet framing encapsulation of network-layer datagram in data link frame Carry network layer data of any network layer protocol (not just IP) at

same time Ability to demultiplex upwards

Bit transparency must carry any bit pattern in the data field Error detection (no correction) Connection liveness detect signal link failure to network layer Network layer address negotiation endpoint can learnconfigure

each otherrsquos network address

PPP non-design requirements No error correctionrecovery No flow control Out of order delivery OK No need to support multipoint links (eg polling)

Error recovery flow control data re-ordering all relegated to higher layers

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PPP Data Frame

Flag delimiter (framing) Address does nothing (only one option) Control does nothing in the future possible multiple

control fields Protocol upper layer protocol to which frame delivered

(eg PPP-LCP IP IPCP etc) Info upper layer data being carried Check cyclic redundancy check for error detection

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Byte Stuffing ldquoData transparencyrdquo requirement data field must be allowed to include

flag pattern lt01111110gt Q is received lt01111110gt data or flag

Sender adds (ldquostuffsrdquo) extra lt 01111110gt byte after each lt 01111110gt data byte How about bit stuffing

Receiver Two 01111110 bytes in a row Discard first byte continue data reception Single 01111110 flag byte

flag bytepatternin datato send

flag byte pattern plusstuffed byte in transmitted data

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PPP Data Control Protocol Before exchanging

network-layer data data link peers must configure PPP link

(max frame length authentication)

learnconfigure network layer information

For IP carry IP Control Protocol (IPCP) messages (protocol field 8021) to configurelearn IP address

Diagram for bring a line up and down

copy Kemal Akkaya

CRC Example

WantD2r XOR R = nG

EquivalentlyD2r = nG XOR R

Equivalently

If we divide D2r by G want remainder R

R = remainder[ ]D2r

G

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Error Correction

Detection and correction of errors due to Lost frames Damaged frames Underlying channel can lose the frames or can be garbled

Solution at the Link Layer Automatic repeat request Error detection Positive acknowledgment Retransmission after timeout Negative acknowledgement and retransmission

Automatic Repeat Request (ARQ) methods Stop and wait Go back N Selective reject (selective retransmission)

copy Kemal Akkaya

Stop and Wait ARQ Source transmits single

frame Wait for ACK If received frame damaged

discard it Transmitter has timeout If no ACK within timeout

retransmit

If ACK damaged transmitter will not recognize it Transmitter will retransmit Receive gets two copies of frame Use ACK0 and ACK1

Simple but inefficient

copy Kemal Akkaya

More efficient method Go-back-N Based on sliding window If no error ACK as usual with next frame expected Use window to control number of outstanding frames If error reply with rejection

Discard that frame and all future frames until error frame received correctly

Transmitter must go back and retransmit that frame and all subsequent frames

Handling Damaged Frame Receiver detects error in frame i Receiver sends rejection-i Transmitter gets rejection-i Transmitter retransmits frame i and all subsequent

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Go-back-N Handling Lost Frame First Approach

Frame i lost Transmitter sends i+1 Receiver gets frame i+1 out of sequence Receiver send reject i Transmitter goes back to frame i and retransmits

Second Approach Frame i lost and no additional frame sent Receiver gets nothing and returns neither acknowledgement nor

rejection Transmitter times out and sends acknowledgement frame with P bit

set to 1 Receiver interprets this as command which it acknowledges with the

number of the next frame it expects (frame i ) Transmitter then retransmits frame i

copy Kemal Akkaya

Go-back-N Damaged ACK

Receiver gets frame i and send acknowledgement (i+1) which is lost

Acknowledgements are cumulative so next acknowledgement (i+n) may arrive before transmitter times out on frame i

If transmitter times out it sends acknowledgement with P bit set as before

This can be repeated a number of times before a reset procedure is initiated

Damaged Rejection Same as Lost Frame (2)

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Selective Reject

Also called selective retransmission

Only rejected frames are retransmitted

Subsequent frames are accepted by the receiver and buffered

Minimizes retransmission Receiver must maintain

large enough buffer More complex login in

transmitter

copy Kemal Akkaya

Multiple Access Links and ProtocolsTwo types of ldquolinksrdquo Point-to-point

PPP for dial-up access Point-to-point link between Ethernet switch and host

Broadcast (shared wire or medium) Traditional Ethernet Upstream HFC 80211 wireless LAN

copy Kemal Akkaya

Multiple Access Protocols Single shared broadcast channel Two or more simultaneous transmissions by nodes interference

Collision if node receives two or more signals at the same time

Multiple access protocol Distributed algorithm that determines how nodes share channel ie

determine when node can transmit Communication about channel sharing must use channel itself

No out-of-band channel for coordination

Ideal Multiple Access Protocol When one node wants to transmit it can send at rate R When M nodes want to transmit each can send at average rate RM Fully decentralized

No special node to coordinate transmissions No synchronization of clocks slots

Simple

copy Kemal Akkaya

MAC Protocols A taxonomy

Three broad classes

Channel Partitioning Divide channel into smaller ldquopiecesrdquo (time slots frequency code) Allocate piece to node for exclusive use

Random Access Channel not divided allow collisions ldquoRecoverrdquo from collisions

ldquoTaking turnsrdquo Nodes take turns Nodes with more to send can take longer turns

copy Kemal Akkaya

Channel Partitioning MAC protocols TDMA

TDMA Time division multiple access Access to channel in rounds Each station gets fixed length slot

Length = packet transmission time in each round

Unused slots go idle Example 6-station LAN

134 have packet Slots 256 idle

copy Kemal Akkaya

Channel Partitioning MAC protocols FDMA

FDMA Frequency division multiple access Channel spectrum divided into frequency bands Each station assigned fixed frequency band Unused transmission time in frequency bands go idle Example 6-station LAN 134 have packet frequency

bands 256 idle

frequ

ency

bands time

1

2

3

4

5

6

copy Kemal Akkaya

Random Access Protocols

When node has packet to send Transmit at full channel data rate R No a priori coordination among nodes

Two or more transmitting nodes ldquocollisionrdquo Random access MAC protocol specifies

How to detect collisions How to recover from collisions (eg via delayed retransmissions)

Examples of random access MAC protocols Slotted ALOHA ALOHA Carrier Sense Multiple Access (CSMA) CSMACollision Detection (CD) CSMACollision Avoidance (CA)

copy Kemal Akkaya

Slotted ALOHA

Pros Single active node can continuously

transmit at full rate of channel Highly decentralized only slots in

nodes need to be in sync Simple

Cons Collisions wasting slots Idle slots Nodes may be able to detect collision in

less than time to transmit packet Clock synchronization

Assumptions All frames same size Time is divided into equal size

slots time to transmit 1 frame Nodes start to transmit frames

only at beginning of slots Nodes are synchronized If 2 or more nodes transmit in

slot all nodes detect collision

Operation When node obtains fresh frame it

transmits in next slot No collision node can send new frame in

next slot If collision node retransmits frame in

each subsequent slot with prob p until success

copy Kemal Akkaya

Pure (unslotted) ALOHA Unslotted Aloha simpler no synchronization When frame first arrives

Transmit immediately

Collision probability increases Frame sent at t0 collides with other frames sent in [t0-1t0+1]

Even worse than slotted ALOHA

copy Kemal Akkaya

Collisions can still occur Propagation delay means two nodes may not hear each otherrsquos transmission Entire packet transmission time wasted Role of distance amp propagation delay in determining collision probability spatial layout of nodes

Idea listen before transmit If channel sensed idle

transmit entire frame If channel sensed busy defer

transmission Human analogy donrsquot

interrupt others

CSMA (Carrier Sense Multiple Access)

copy Kemal Akkaya

CSMACD (Collision Detection)

CSMACD carrier sensing deferral as in CSMA Collisions detected within short

time Send jam sequence Colliding transmissions aborted

reducing channel wastage

Collision detection

Easy in wired LANs measure signal strengths compare transmitted received signals

Difficult in wireless LANs receiver shut off while transmitting

Human analogy the polite conversationalist

copy Kemal Akkaya

ldquoTaking Turnsrdquo MAC protocols Channel partitioning MAC protocols

Share channel efficiently and fairly at high load Inefficient at low load

Delay in channel access 1N bandwidth allocated even if only 1 active node

Random access MAC protocols Efficient at low load

Single node can fully utilize channel

high load Collision overhead

ldquoTaking turnsrdquo protocols Look for best of both worlds

Two implementations Polling Token

copy Kemal Akkaya

ldquoTaking Turnsrdquo MAC protocolsPolling Master node ldquoinvitesrdquo slave

nodes to transmit in turn Concerns

Polling overhead Latency Single point of failure (master)

Token passing control token passed from

one node to next sequentially

token message concerns

token overhead latency single point of failure (token)

copy Kemal Akkaya

Summary of MAC protocols

What do you do with a shared media Channel Partitioning by time frequency or code

Time Division Frequency Division

Random partitioning (dynamic) ALOHA S-ALOHA CSMA CSMACD Carrier sensing easy in some technologies (wire) hard in others

(wireless) CSMACD used in Ethernet CSMACA used in 80211

Taking Turns Polling from a central site token passing

copy Kemal Akkaya

LAN technologies

Data link layer so farServicesError detectioncorrectionMultiple access

Next LAN technologiesAddressingEthernetHubs switchesPPP

copy Kemal Akkaya

MAC Addresses and ARP 32-bit IP address

Network-layer address Used to get datagram to

destination IP subnet

MAC (or LAN or physical or Ethernet) address Used to get datagram

from one interface to another physically-connected interface (same network)

48 bit MAC address (for most LANs) burned in the adapter ROM

Find your PCrsquos MAC address

1A-2F-BB-76-09-AD

58-23-D7-FA-20-B0

0C-C4-11-6F-E3-98

71-65-F7-2B-08-53

LAN(wired orwireless)

Each adapter on LAN has unique LAN address

Broadcast address =FF-FF-FF-FF-FF-FF = adapter

copy Kemal Akkaya

MAC Address (more)

MAC address allocation administered by IEEE Manufacturer buys portion of MAC address space (to

assure uniqueness) Analogy

(a) MAC address like Social Security Number

(b) IP address like postal address MAC flat address portability

Can move LAN card from one LAN to another

IP hierarchical address NOT portable Depends on IP subnet to which node is attached

copy Kemal Akkaya

ARP Address Resolution Protocol

Each IP node (Host Router) on LAN has ARP table

ARP Table IPMAC address mappings for some LAN nodes

lt IP address MAC address TTLgt TTL (Time To Live)

time after which address mapping will be forgotten (typically 20 min)

Question how to determineMAC address of Bknowing Brsquos IP address

1A-2F-BB-76-09-AD

58-23-D7-FA-20-B0

0C-C4-11-6F-E3-98

71-65-F7-2B-08-53

LAN

237196723

237196778

237196714

237196788

copy Kemal Akkaya

ARP protocol Same LAN (network) A wants to send datagram

to B and Brsquos MAC address not in Arsquos ARP table

A broadcasts ARP query packet containing Bs IP address Dest MAC address = FF-FF-

FF-FF-FF-FF All machines on LAN receive

ARP query

B receives ARP packet replies to A with its (Bs) MAC address Frame sent to Arsquos MAC

address (unicast)

A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) Soft state information that

times out (goes away) unless refreshed

ARP is ldquoplug-and-playrdquo Nodes create their ARP

tables without intervention from net administrator

copy Kemal Akkaya

Routing to another LAN Walkthrough send datagram from A to B via R

Assume A knows B IP address

Two ARP tables in router R one for each IP network (LAN) In routing table at source Host find router 111111111110 In ARP table at source find MAC address E6-E9-00-17-BB-4B etc A creates datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110

A creates link-layer frame with Rs MAC address as dest frame contains A-to-B IP datagram

Arsquos adapter sends frame Rrsquos adapter receives frame R removes IP datagram from Ethernet frame sees its destined to B R uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B

copy Kemal Akkaya

A

R

B

Routing to another LAN

copy Kemal Akkaya

EthernetldquoDominantrdquo wired LAN technology Cheap $20 for 100Mbs First widely used LAN technology Simpler cheaper than token LANs and ATM Kept up with speed race 10 Mbps ndash 10

Gbps

Metcalfersquos Ethernet sketch

Star Topology Bus topology popular

through mid 90s Now star topology prevails Connection choices hub or

switch (more later)

hub orswitch

copy Kemal Akkaya

Ethernet Frame Structure

Preamble 7 bytes with pattern 10101010 followed by one byte with pattern 10101011 Used to synchronize receiver sender clock rates

Addresses 6 bytes If adapter receives frame with matching destination address or with broadcast

address (eg ARP packet) it passes data in frame to net-layer protocol Otherwise adapter discards frame

Type Indicates the higher layer protocol (mostly IP but others may be supported

such as Novell IPX and AppleTalk) CRC

Checked at receiver if error is detected the frame is simply dropped

Sending adapter encapsulates IP datagram in Ethernet frame

copy Kemal Akkaya

Ethernet uses CSMACD Connectionless No handshaking between sending and receiving

adapter Unreliable receiving adapter doesnrsquot send acks or nacks to

sending adapter Stream of datagrams passed to network layer can have gaps Gaps will be filled if app is using TCP Otherwise app will see the gaps

No slots Adapter doesnrsquot transmit if it senses that some other adapter is

transmitting that is carrier sense Transmitting adapter aborts when it senses that another adapter is

transmitting that is collision detection Before attempting a retransmission adapter waits a random time

that is random access

copy Kemal Akkaya

Ethernet CSMACD algorithm1 Adaptor receives datagram from

net layer amp creates frame2 If adapter senses channel idle it

starts to transmit frame If it senses channel busy waits until channel idle and then transmits

3 If adapter transmits entire frame without detecting another transmission the adapter is done with frame

4 If adapter detects another transmission while transmitting aborts and sends jam signal

5 After aborting adapter enters exponential backoff after the mth collision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2

Jam Signal make sure all other transmitters are aware of collision 48 bits

Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec

Exponential Backoff Goal adapt retransmission

attempts to estimated current load Heavy load random wait will be

longer

First collision choose K from 01 delay is K 512 bit transmission times

After second collision choose K from 0123hellip

After ten collisions choose K from 01234hellip1023

copy Kemal Akkaya

CSMACD efficiency Tprop = max prop between 2 nodes in LAN ttrans = time to transmit max-size frame

Efficiency goes to 1 as tprop goes to 0 Goes to 1 as ttrans goes to infinity Much better than ALOHA but still decentralized

simple and cheap

Other Ethernets Switched Ethernet Fast Ethernet Gigabit Ethernet

transprop tt 51

1efficiency

copy Kemal Akkaya

Point to Point Data Link Control

One sender one receiver one link easier than broadcast link No Media Access Control No need for explicit MAC addressing eg dialup link ISDN line

Popular point-to-point DLC protocols PPP (point-to-point protocol) HDLC High level data link control (Data link used to be considered

ldquohigh layerrdquo in protocol stack

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HDLC

Frame format for bit-oriented protocol HDLC

Control field of

(a) An information frame

(b) A supervisory frame

(c) An unnumbered frame

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HDLC Exchange of information supervisory and unnumbered

frames Information - data to be transmitted to user (next layer up)

Flow and error control piggybacked on information frames Supervisory - ARQ when piggyback not used Unnumbered - supplementary link control

Three phases Initialization Data transfer Disconnect

Uses bit stuffing Flag delimit frame at both ends 01111110 is the Flag May close one frame and open another Bit stuffing to find flag data

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Internet Datalink Protocol PPP Design Requirements [RFC 1557]

Packet framing encapsulation of network-layer datagram in data link frame Carry network layer data of any network layer protocol (not just IP) at

same time Ability to demultiplex upwards

Bit transparency must carry any bit pattern in the data field Error detection (no correction) Connection liveness detect signal link failure to network layer Network layer address negotiation endpoint can learnconfigure

each otherrsquos network address

PPP non-design requirements No error correctionrecovery No flow control Out of order delivery OK No need to support multipoint links (eg polling)

Error recovery flow control data re-ordering all relegated to higher layers

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PPP Data Frame

Flag delimiter (framing) Address does nothing (only one option) Control does nothing in the future possible multiple

control fields Protocol upper layer protocol to which frame delivered

(eg PPP-LCP IP IPCP etc) Info upper layer data being carried Check cyclic redundancy check for error detection

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Byte Stuffing ldquoData transparencyrdquo requirement data field must be allowed to include

flag pattern lt01111110gt Q is received lt01111110gt data or flag

Sender adds (ldquostuffsrdquo) extra lt 01111110gt byte after each lt 01111110gt data byte How about bit stuffing

Receiver Two 01111110 bytes in a row Discard first byte continue data reception Single 01111110 flag byte

flag bytepatternin datato send

flag byte pattern plusstuffed byte in transmitted data

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PPP Data Control Protocol Before exchanging

network-layer data data link peers must configure PPP link

(max frame length authentication)

learnconfigure network layer information

For IP carry IP Control Protocol (IPCP) messages (protocol field 8021) to configurelearn IP address

Diagram for bring a line up and down

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Error Correction

Detection and correction of errors due to Lost frames Damaged frames Underlying channel can lose the frames or can be garbled

Solution at the Link Layer Automatic repeat request Error detection Positive acknowledgment Retransmission after timeout Negative acknowledgement and retransmission

Automatic Repeat Request (ARQ) methods Stop and wait Go back N Selective reject (selective retransmission)

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Stop and Wait ARQ Source transmits single

frame Wait for ACK If received frame damaged

discard it Transmitter has timeout If no ACK within timeout

retransmit

If ACK damaged transmitter will not recognize it Transmitter will retransmit Receive gets two copies of frame Use ACK0 and ACK1

Simple but inefficient

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More efficient method Go-back-N Based on sliding window If no error ACK as usual with next frame expected Use window to control number of outstanding frames If error reply with rejection

Discard that frame and all future frames until error frame received correctly

Transmitter must go back and retransmit that frame and all subsequent frames

Handling Damaged Frame Receiver detects error in frame i Receiver sends rejection-i Transmitter gets rejection-i Transmitter retransmits frame i and all subsequent

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Go-back-N Handling Lost Frame First Approach

Frame i lost Transmitter sends i+1 Receiver gets frame i+1 out of sequence Receiver send reject i Transmitter goes back to frame i and retransmits

Second Approach Frame i lost and no additional frame sent Receiver gets nothing and returns neither acknowledgement nor

rejection Transmitter times out and sends acknowledgement frame with P bit

set to 1 Receiver interprets this as command which it acknowledges with the

number of the next frame it expects (frame i ) Transmitter then retransmits frame i

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Go-back-N Damaged ACK

Receiver gets frame i and send acknowledgement (i+1) which is lost

Acknowledgements are cumulative so next acknowledgement (i+n) may arrive before transmitter times out on frame i

If transmitter times out it sends acknowledgement with P bit set as before

This can be repeated a number of times before a reset procedure is initiated

Damaged Rejection Same as Lost Frame (2)

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Selective Reject

Also called selective retransmission

Only rejected frames are retransmitted

Subsequent frames are accepted by the receiver and buffered

Minimizes retransmission Receiver must maintain

large enough buffer More complex login in

transmitter

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Multiple Access Links and ProtocolsTwo types of ldquolinksrdquo Point-to-point

PPP for dial-up access Point-to-point link between Ethernet switch and host

Broadcast (shared wire or medium) Traditional Ethernet Upstream HFC 80211 wireless LAN

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Multiple Access Protocols Single shared broadcast channel Two or more simultaneous transmissions by nodes interference

Collision if node receives two or more signals at the same time

Multiple access protocol Distributed algorithm that determines how nodes share channel ie

determine when node can transmit Communication about channel sharing must use channel itself

No out-of-band channel for coordination

Ideal Multiple Access Protocol When one node wants to transmit it can send at rate R When M nodes want to transmit each can send at average rate RM Fully decentralized

No special node to coordinate transmissions No synchronization of clocks slots

Simple

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MAC Protocols A taxonomy

Three broad classes

Channel Partitioning Divide channel into smaller ldquopiecesrdquo (time slots frequency code) Allocate piece to node for exclusive use

Random Access Channel not divided allow collisions ldquoRecoverrdquo from collisions

ldquoTaking turnsrdquo Nodes take turns Nodes with more to send can take longer turns

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Channel Partitioning MAC protocols TDMA

TDMA Time division multiple access Access to channel in rounds Each station gets fixed length slot

Length = packet transmission time in each round

Unused slots go idle Example 6-station LAN

134 have packet Slots 256 idle

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Channel Partitioning MAC protocols FDMA

FDMA Frequency division multiple access Channel spectrum divided into frequency bands Each station assigned fixed frequency band Unused transmission time in frequency bands go idle Example 6-station LAN 134 have packet frequency

bands 256 idle

frequ

ency

bands time

1

2

3

4

5

6

copy Kemal Akkaya

Random Access Protocols

When node has packet to send Transmit at full channel data rate R No a priori coordination among nodes

Two or more transmitting nodes ldquocollisionrdquo Random access MAC protocol specifies

How to detect collisions How to recover from collisions (eg via delayed retransmissions)

Examples of random access MAC protocols Slotted ALOHA ALOHA Carrier Sense Multiple Access (CSMA) CSMACollision Detection (CD) CSMACollision Avoidance (CA)

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Slotted ALOHA

Pros Single active node can continuously

transmit at full rate of channel Highly decentralized only slots in

nodes need to be in sync Simple

Cons Collisions wasting slots Idle slots Nodes may be able to detect collision in

less than time to transmit packet Clock synchronization

Assumptions All frames same size Time is divided into equal size

slots time to transmit 1 frame Nodes start to transmit frames

only at beginning of slots Nodes are synchronized If 2 or more nodes transmit in

slot all nodes detect collision

Operation When node obtains fresh frame it

transmits in next slot No collision node can send new frame in

next slot If collision node retransmits frame in

each subsequent slot with prob p until success

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Pure (unslotted) ALOHA Unslotted Aloha simpler no synchronization When frame first arrives

Transmit immediately

Collision probability increases Frame sent at t0 collides with other frames sent in [t0-1t0+1]

Even worse than slotted ALOHA

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Collisions can still occur Propagation delay means two nodes may not hear each otherrsquos transmission Entire packet transmission time wasted Role of distance amp propagation delay in determining collision probability spatial layout of nodes

Idea listen before transmit If channel sensed idle

transmit entire frame If channel sensed busy defer

transmission Human analogy donrsquot

interrupt others

CSMA (Carrier Sense Multiple Access)

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CSMACD (Collision Detection)

CSMACD carrier sensing deferral as in CSMA Collisions detected within short

time Send jam sequence Colliding transmissions aborted

reducing channel wastage

Collision detection

Easy in wired LANs measure signal strengths compare transmitted received signals

Difficult in wireless LANs receiver shut off while transmitting

Human analogy the polite conversationalist

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ldquoTaking Turnsrdquo MAC protocols Channel partitioning MAC protocols

Share channel efficiently and fairly at high load Inefficient at low load

Delay in channel access 1N bandwidth allocated even if only 1 active node

Random access MAC protocols Efficient at low load

Single node can fully utilize channel

high load Collision overhead

ldquoTaking turnsrdquo protocols Look for best of both worlds

Two implementations Polling Token

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ldquoTaking Turnsrdquo MAC protocolsPolling Master node ldquoinvitesrdquo slave

nodes to transmit in turn Concerns

Polling overhead Latency Single point of failure (master)

Token passing control token passed from

one node to next sequentially

token message concerns

token overhead latency single point of failure (token)

copy Kemal Akkaya

Summary of MAC protocols

What do you do with a shared media Channel Partitioning by time frequency or code

Time Division Frequency Division

Random partitioning (dynamic) ALOHA S-ALOHA CSMA CSMACD Carrier sensing easy in some technologies (wire) hard in others

(wireless) CSMACD used in Ethernet CSMACA used in 80211

Taking Turns Polling from a central site token passing

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LAN technologies

Data link layer so farServicesError detectioncorrectionMultiple access

Next LAN technologiesAddressingEthernetHubs switchesPPP

copy Kemal Akkaya

MAC Addresses and ARP 32-bit IP address

Network-layer address Used to get datagram to

destination IP subnet

MAC (or LAN or physical or Ethernet) address Used to get datagram

from one interface to another physically-connected interface (same network)

48 bit MAC address (for most LANs) burned in the adapter ROM

Find your PCrsquos MAC address

1A-2F-BB-76-09-AD

58-23-D7-FA-20-B0

0C-C4-11-6F-E3-98

71-65-F7-2B-08-53

LAN(wired orwireless)

Each adapter on LAN has unique LAN address

Broadcast address =FF-FF-FF-FF-FF-FF = adapter

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MAC Address (more)

MAC address allocation administered by IEEE Manufacturer buys portion of MAC address space (to

assure uniqueness) Analogy

(a) MAC address like Social Security Number

(b) IP address like postal address MAC flat address portability

Can move LAN card from one LAN to another

IP hierarchical address NOT portable Depends on IP subnet to which node is attached

copy Kemal Akkaya

ARP Address Resolution Protocol

Each IP node (Host Router) on LAN has ARP table

ARP Table IPMAC address mappings for some LAN nodes

lt IP address MAC address TTLgt TTL (Time To Live)

time after which address mapping will be forgotten (typically 20 min)

Question how to determineMAC address of Bknowing Brsquos IP address

1A-2F-BB-76-09-AD

58-23-D7-FA-20-B0

0C-C4-11-6F-E3-98

71-65-F7-2B-08-53

LAN

237196723

237196778

237196714

237196788

copy Kemal Akkaya

ARP protocol Same LAN (network) A wants to send datagram

to B and Brsquos MAC address not in Arsquos ARP table

A broadcasts ARP query packet containing Bs IP address Dest MAC address = FF-FF-

FF-FF-FF-FF All machines on LAN receive

ARP query

B receives ARP packet replies to A with its (Bs) MAC address Frame sent to Arsquos MAC

address (unicast)

A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) Soft state information that

times out (goes away) unless refreshed

ARP is ldquoplug-and-playrdquo Nodes create their ARP

tables without intervention from net administrator

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Routing to another LAN Walkthrough send datagram from A to B via R

Assume A knows B IP address

Two ARP tables in router R one for each IP network (LAN) In routing table at source Host find router 111111111110 In ARP table at source find MAC address E6-E9-00-17-BB-4B etc A creates datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110

A creates link-layer frame with Rs MAC address as dest frame contains A-to-B IP datagram

Arsquos adapter sends frame Rrsquos adapter receives frame R removes IP datagram from Ethernet frame sees its destined to B R uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B

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A

R

B

Routing to another LAN

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EthernetldquoDominantrdquo wired LAN technology Cheap $20 for 100Mbs First widely used LAN technology Simpler cheaper than token LANs and ATM Kept up with speed race 10 Mbps ndash 10

Gbps

Metcalfersquos Ethernet sketch

Star Topology Bus topology popular

through mid 90s Now star topology prevails Connection choices hub or

switch (more later)

hub orswitch

copy Kemal Akkaya

Ethernet Frame Structure

Preamble 7 bytes with pattern 10101010 followed by one byte with pattern 10101011 Used to synchronize receiver sender clock rates

Addresses 6 bytes If adapter receives frame with matching destination address or with broadcast

address (eg ARP packet) it passes data in frame to net-layer protocol Otherwise adapter discards frame

Type Indicates the higher layer protocol (mostly IP but others may be supported

such as Novell IPX and AppleTalk) CRC

Checked at receiver if error is detected the frame is simply dropped

Sending adapter encapsulates IP datagram in Ethernet frame

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Ethernet uses CSMACD Connectionless No handshaking between sending and receiving

adapter Unreliable receiving adapter doesnrsquot send acks or nacks to

sending adapter Stream of datagrams passed to network layer can have gaps Gaps will be filled if app is using TCP Otherwise app will see the gaps

No slots Adapter doesnrsquot transmit if it senses that some other adapter is

transmitting that is carrier sense Transmitting adapter aborts when it senses that another adapter is

transmitting that is collision detection Before attempting a retransmission adapter waits a random time

that is random access

copy Kemal Akkaya

Ethernet CSMACD algorithm1 Adaptor receives datagram from

net layer amp creates frame2 If adapter senses channel idle it

starts to transmit frame If it senses channel busy waits until channel idle and then transmits

3 If adapter transmits entire frame without detecting another transmission the adapter is done with frame

4 If adapter detects another transmission while transmitting aborts and sends jam signal

5 After aborting adapter enters exponential backoff after the mth collision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2

Jam Signal make sure all other transmitters are aware of collision 48 bits

Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec

Exponential Backoff Goal adapt retransmission

attempts to estimated current load Heavy load random wait will be

longer

First collision choose K from 01 delay is K 512 bit transmission times

After second collision choose K from 0123hellip

After ten collisions choose K from 01234hellip1023

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CSMACD efficiency Tprop = max prop between 2 nodes in LAN ttrans = time to transmit max-size frame

Efficiency goes to 1 as tprop goes to 0 Goes to 1 as ttrans goes to infinity Much better than ALOHA but still decentralized

simple and cheap

Other Ethernets Switched Ethernet Fast Ethernet Gigabit Ethernet

transprop tt 51

1efficiency

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Point to Point Data Link Control

One sender one receiver one link easier than broadcast link No Media Access Control No need for explicit MAC addressing eg dialup link ISDN line

Popular point-to-point DLC protocols PPP (point-to-point protocol) HDLC High level data link control (Data link used to be considered

ldquohigh layerrdquo in protocol stack

copy Kemal Akkaya

HDLC

Frame format for bit-oriented protocol HDLC

Control field of

(a) An information frame

(b) A supervisory frame

(c) An unnumbered frame

copy Kemal Akkaya

HDLC Exchange of information supervisory and unnumbered

frames Information - data to be transmitted to user (next layer up)

Flow and error control piggybacked on information frames Supervisory - ARQ when piggyback not used Unnumbered - supplementary link control

Three phases Initialization Data transfer Disconnect

Uses bit stuffing Flag delimit frame at both ends 01111110 is the Flag May close one frame and open another Bit stuffing to find flag data

copy Kemal Akkaya

Internet Datalink Protocol PPP Design Requirements [RFC 1557]

Packet framing encapsulation of network-layer datagram in data link frame Carry network layer data of any network layer protocol (not just IP) at

same time Ability to demultiplex upwards

Bit transparency must carry any bit pattern in the data field Error detection (no correction) Connection liveness detect signal link failure to network layer Network layer address negotiation endpoint can learnconfigure

each otherrsquos network address

PPP non-design requirements No error correctionrecovery No flow control Out of order delivery OK No need to support multipoint links (eg polling)

Error recovery flow control data re-ordering all relegated to higher layers

copy Kemal Akkaya

PPP Data Frame

Flag delimiter (framing) Address does nothing (only one option) Control does nothing in the future possible multiple

control fields Protocol upper layer protocol to which frame delivered

(eg PPP-LCP IP IPCP etc) Info upper layer data being carried Check cyclic redundancy check for error detection

copy Kemal Akkaya

Byte Stuffing ldquoData transparencyrdquo requirement data field must be allowed to include

flag pattern lt01111110gt Q is received lt01111110gt data or flag

Sender adds (ldquostuffsrdquo) extra lt 01111110gt byte after each lt 01111110gt data byte How about bit stuffing

Receiver Two 01111110 bytes in a row Discard first byte continue data reception Single 01111110 flag byte

flag bytepatternin datato send

flag byte pattern plusstuffed byte in transmitted data

copy Kemal Akkaya

PPP Data Control Protocol Before exchanging

network-layer data data link peers must configure PPP link

(max frame length authentication)

learnconfigure network layer information

For IP carry IP Control Protocol (IPCP) messages (protocol field 8021) to configurelearn IP address

Diagram for bring a line up and down

copy Kemal Akkaya

Stop and Wait ARQ Source transmits single

frame Wait for ACK If received frame damaged

discard it Transmitter has timeout If no ACK within timeout

retransmit

If ACK damaged transmitter will not recognize it Transmitter will retransmit Receive gets two copies of frame Use ACK0 and ACK1

Simple but inefficient

copy Kemal Akkaya

More efficient method Go-back-N Based on sliding window If no error ACK as usual with next frame expected Use window to control number of outstanding frames If error reply with rejection

Discard that frame and all future frames until error frame received correctly

Transmitter must go back and retransmit that frame and all subsequent frames

Handling Damaged Frame Receiver detects error in frame i Receiver sends rejection-i Transmitter gets rejection-i Transmitter retransmits frame i and all subsequent

copy Kemal Akkaya

Go-back-N Handling Lost Frame First Approach

Frame i lost Transmitter sends i+1 Receiver gets frame i+1 out of sequence Receiver send reject i Transmitter goes back to frame i and retransmits

Second Approach Frame i lost and no additional frame sent Receiver gets nothing and returns neither acknowledgement nor

rejection Transmitter times out and sends acknowledgement frame with P bit

set to 1 Receiver interprets this as command which it acknowledges with the

number of the next frame it expects (frame i ) Transmitter then retransmits frame i

copy Kemal Akkaya

Go-back-N Damaged ACK

Receiver gets frame i and send acknowledgement (i+1) which is lost

Acknowledgements are cumulative so next acknowledgement (i+n) may arrive before transmitter times out on frame i

If transmitter times out it sends acknowledgement with P bit set as before

This can be repeated a number of times before a reset procedure is initiated

Damaged Rejection Same as Lost Frame (2)

copy Kemal Akkaya

Selective Reject

Also called selective retransmission

Only rejected frames are retransmitted

Subsequent frames are accepted by the receiver and buffered

Minimizes retransmission Receiver must maintain

large enough buffer More complex login in

transmitter

copy Kemal Akkaya

Multiple Access Links and ProtocolsTwo types of ldquolinksrdquo Point-to-point

PPP for dial-up access Point-to-point link between Ethernet switch and host

Broadcast (shared wire or medium) Traditional Ethernet Upstream HFC 80211 wireless LAN

copy Kemal Akkaya

Multiple Access Protocols Single shared broadcast channel Two or more simultaneous transmissions by nodes interference

Collision if node receives two or more signals at the same time

Multiple access protocol Distributed algorithm that determines how nodes share channel ie

determine when node can transmit Communication about channel sharing must use channel itself

No out-of-band channel for coordination

Ideal Multiple Access Protocol When one node wants to transmit it can send at rate R When M nodes want to transmit each can send at average rate RM Fully decentralized

No special node to coordinate transmissions No synchronization of clocks slots

Simple

copy Kemal Akkaya

MAC Protocols A taxonomy

Three broad classes

Channel Partitioning Divide channel into smaller ldquopiecesrdquo (time slots frequency code) Allocate piece to node for exclusive use

Random Access Channel not divided allow collisions ldquoRecoverrdquo from collisions

ldquoTaking turnsrdquo Nodes take turns Nodes with more to send can take longer turns

copy Kemal Akkaya

Channel Partitioning MAC protocols TDMA

TDMA Time division multiple access Access to channel in rounds Each station gets fixed length slot

Length = packet transmission time in each round

Unused slots go idle Example 6-station LAN

134 have packet Slots 256 idle

copy Kemal Akkaya

Channel Partitioning MAC protocols FDMA

FDMA Frequency division multiple access Channel spectrum divided into frequency bands Each station assigned fixed frequency band Unused transmission time in frequency bands go idle Example 6-station LAN 134 have packet frequency

bands 256 idle

frequ

ency

bands time

1

2

3

4

5

6

copy Kemal Akkaya

Random Access Protocols

When node has packet to send Transmit at full channel data rate R No a priori coordination among nodes

Two or more transmitting nodes ldquocollisionrdquo Random access MAC protocol specifies

How to detect collisions How to recover from collisions (eg via delayed retransmissions)

Examples of random access MAC protocols Slotted ALOHA ALOHA Carrier Sense Multiple Access (CSMA) CSMACollision Detection (CD) CSMACollision Avoidance (CA)

copy Kemal Akkaya

Slotted ALOHA

Pros Single active node can continuously

transmit at full rate of channel Highly decentralized only slots in

nodes need to be in sync Simple

Cons Collisions wasting slots Idle slots Nodes may be able to detect collision in

less than time to transmit packet Clock synchronization

Assumptions All frames same size Time is divided into equal size

slots time to transmit 1 frame Nodes start to transmit frames

only at beginning of slots Nodes are synchronized If 2 or more nodes transmit in

slot all nodes detect collision

Operation When node obtains fresh frame it

transmits in next slot No collision node can send new frame in

next slot If collision node retransmits frame in

each subsequent slot with prob p until success

copy Kemal Akkaya

Pure (unslotted) ALOHA Unslotted Aloha simpler no synchronization When frame first arrives

Transmit immediately

Collision probability increases Frame sent at t0 collides with other frames sent in [t0-1t0+1]

Even worse than slotted ALOHA

copy Kemal Akkaya

Collisions can still occur Propagation delay means two nodes may not hear each otherrsquos transmission Entire packet transmission time wasted Role of distance amp propagation delay in determining collision probability spatial layout of nodes

Idea listen before transmit If channel sensed idle

transmit entire frame If channel sensed busy defer

transmission Human analogy donrsquot

interrupt others

CSMA (Carrier Sense Multiple Access)

copy Kemal Akkaya

CSMACD (Collision Detection)

CSMACD carrier sensing deferral as in CSMA Collisions detected within short

time Send jam sequence Colliding transmissions aborted

reducing channel wastage

Collision detection

Easy in wired LANs measure signal strengths compare transmitted received signals

Difficult in wireless LANs receiver shut off while transmitting

Human analogy the polite conversationalist

copy Kemal Akkaya

ldquoTaking Turnsrdquo MAC protocols Channel partitioning MAC protocols

Share channel efficiently and fairly at high load Inefficient at low load

Delay in channel access 1N bandwidth allocated even if only 1 active node

Random access MAC protocols Efficient at low load

Single node can fully utilize channel

high load Collision overhead

ldquoTaking turnsrdquo protocols Look for best of both worlds

Two implementations Polling Token

copy Kemal Akkaya

ldquoTaking Turnsrdquo MAC protocolsPolling Master node ldquoinvitesrdquo slave

nodes to transmit in turn Concerns

Polling overhead Latency Single point of failure (master)

Token passing control token passed from

one node to next sequentially

token message concerns

token overhead latency single point of failure (token)

copy Kemal Akkaya

Summary of MAC protocols

What do you do with a shared media Channel Partitioning by time frequency or code

Time Division Frequency Division

Random partitioning (dynamic) ALOHA S-ALOHA CSMA CSMACD Carrier sensing easy in some technologies (wire) hard in others

(wireless) CSMACD used in Ethernet CSMACA used in 80211

Taking Turns Polling from a central site token passing

copy Kemal Akkaya

LAN technologies

Data link layer so farServicesError detectioncorrectionMultiple access

Next LAN technologiesAddressingEthernetHubs switchesPPP

copy Kemal Akkaya

MAC Addresses and ARP 32-bit IP address

Network-layer address Used to get datagram to

destination IP subnet

MAC (or LAN or physical or Ethernet) address Used to get datagram

from one interface to another physically-connected interface (same network)

48 bit MAC address (for most LANs) burned in the adapter ROM

Find your PCrsquos MAC address

1A-2F-BB-76-09-AD

58-23-D7-FA-20-B0

0C-C4-11-6F-E3-98

71-65-F7-2B-08-53

LAN(wired orwireless)

Each adapter on LAN has unique LAN address

Broadcast address =FF-FF-FF-FF-FF-FF = adapter

copy Kemal Akkaya

MAC Address (more)

MAC address allocation administered by IEEE Manufacturer buys portion of MAC address space (to

assure uniqueness) Analogy

(a) MAC address like Social Security Number

(b) IP address like postal address MAC flat address portability

Can move LAN card from one LAN to another

IP hierarchical address NOT portable Depends on IP subnet to which node is attached

copy Kemal Akkaya

ARP Address Resolution Protocol

Each IP node (Host Router) on LAN has ARP table

ARP Table IPMAC address mappings for some LAN nodes

lt IP address MAC address TTLgt TTL (Time To Live)

time after which address mapping will be forgotten (typically 20 min)

Question how to determineMAC address of Bknowing Brsquos IP address

1A-2F-BB-76-09-AD

58-23-D7-FA-20-B0

0C-C4-11-6F-E3-98

71-65-F7-2B-08-53

LAN

237196723

237196778

237196714

237196788

copy Kemal Akkaya

ARP protocol Same LAN (network) A wants to send datagram

to B and Brsquos MAC address not in Arsquos ARP table

A broadcasts ARP query packet containing Bs IP address Dest MAC address = FF-FF-

FF-FF-FF-FF All machines on LAN receive

ARP query

B receives ARP packet replies to A with its (Bs) MAC address Frame sent to Arsquos MAC

address (unicast)

A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) Soft state information that

times out (goes away) unless refreshed

ARP is ldquoplug-and-playrdquo Nodes create their ARP

tables without intervention from net administrator

copy Kemal Akkaya

Routing to another LAN Walkthrough send datagram from A to B via R

Assume A knows B IP address

Two ARP tables in router R one for each IP network (LAN) In routing table at source Host find router 111111111110 In ARP table at source find MAC address E6-E9-00-17-BB-4B etc A creates datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110

A creates link-layer frame with Rs MAC address as dest frame contains A-to-B IP datagram

Arsquos adapter sends frame Rrsquos adapter receives frame R removes IP datagram from Ethernet frame sees its destined to B R uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B

copy Kemal Akkaya

A

R

B

Routing to another LAN

copy Kemal Akkaya

EthernetldquoDominantrdquo wired LAN technology Cheap $20 for 100Mbs First widely used LAN technology Simpler cheaper than token LANs and ATM Kept up with speed race 10 Mbps ndash 10

Gbps

Metcalfersquos Ethernet sketch

Star Topology Bus topology popular

through mid 90s Now star topology prevails Connection choices hub or

switch (more later)

hub orswitch

copy Kemal Akkaya

Ethernet Frame Structure

Preamble 7 bytes with pattern 10101010 followed by one byte with pattern 10101011 Used to synchronize receiver sender clock rates

Addresses 6 bytes If adapter receives frame with matching destination address or with broadcast

address (eg ARP packet) it passes data in frame to net-layer protocol Otherwise adapter discards frame

Type Indicates the higher layer protocol (mostly IP but others may be supported

such as Novell IPX and AppleTalk) CRC

Checked at receiver if error is detected the frame is simply dropped

Sending adapter encapsulates IP datagram in Ethernet frame

copy Kemal Akkaya

Ethernet uses CSMACD Connectionless No handshaking between sending and receiving

adapter Unreliable receiving adapter doesnrsquot send acks or nacks to

sending adapter Stream of datagrams passed to network layer can have gaps Gaps will be filled if app is using TCP Otherwise app will see the gaps

No slots Adapter doesnrsquot transmit if it senses that some other adapter is

transmitting that is carrier sense Transmitting adapter aborts when it senses that another adapter is

transmitting that is collision detection Before attempting a retransmission adapter waits a random time

that is random access

copy Kemal Akkaya

Ethernet CSMACD algorithm1 Adaptor receives datagram from

net layer amp creates frame2 If adapter senses channel idle it

starts to transmit frame If it senses channel busy waits until channel idle and then transmits

3 If adapter transmits entire frame without detecting another transmission the adapter is done with frame

4 If adapter detects another transmission while transmitting aborts and sends jam signal

5 After aborting adapter enters exponential backoff after the mth collision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2

Jam Signal make sure all other transmitters are aware of collision 48 bits

Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec

Exponential Backoff Goal adapt retransmission

attempts to estimated current load Heavy load random wait will be

longer

First collision choose K from 01 delay is K 512 bit transmission times

After second collision choose K from 0123hellip

After ten collisions choose K from 01234hellip1023

copy Kemal Akkaya

CSMACD efficiency Tprop = max prop between 2 nodes in LAN ttrans = time to transmit max-size frame

Efficiency goes to 1 as tprop goes to 0 Goes to 1 as ttrans goes to infinity Much better than ALOHA but still decentralized

simple and cheap

Other Ethernets Switched Ethernet Fast Ethernet Gigabit Ethernet

transprop tt 51

1efficiency

copy Kemal Akkaya

Point to Point Data Link Control

One sender one receiver one link easier than broadcast link No Media Access Control No need for explicit MAC addressing eg dialup link ISDN line

Popular point-to-point DLC protocols PPP (point-to-point protocol) HDLC High level data link control (Data link used to be considered

ldquohigh layerrdquo in protocol stack

copy Kemal Akkaya

HDLC

Frame format for bit-oriented protocol HDLC

Control field of

(a) An information frame

(b) A supervisory frame

(c) An unnumbered frame

copy Kemal Akkaya

HDLC Exchange of information supervisory and unnumbered

frames Information - data to be transmitted to user (next layer up)

Flow and error control piggybacked on information frames Supervisory - ARQ when piggyback not used Unnumbered - supplementary link control

Three phases Initialization Data transfer Disconnect

Uses bit stuffing Flag delimit frame at both ends 01111110 is the Flag May close one frame and open another Bit stuffing to find flag data

copy Kemal Akkaya

Internet Datalink Protocol PPP Design Requirements [RFC 1557]

Packet framing encapsulation of network-layer datagram in data link frame Carry network layer data of any network layer protocol (not just IP) at

same time Ability to demultiplex upwards

Bit transparency must carry any bit pattern in the data field Error detection (no correction) Connection liveness detect signal link failure to network layer Network layer address negotiation endpoint can learnconfigure

each otherrsquos network address

PPP non-design requirements No error correctionrecovery No flow control Out of order delivery OK No need to support multipoint links (eg polling)

Error recovery flow control data re-ordering all relegated to higher layers

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PPP Data Frame

Flag delimiter (framing) Address does nothing (only one option) Control does nothing in the future possible multiple

control fields Protocol upper layer protocol to which frame delivered

(eg PPP-LCP IP IPCP etc) Info upper layer data being carried Check cyclic redundancy check for error detection

copy Kemal Akkaya

Byte Stuffing ldquoData transparencyrdquo requirement data field must be allowed to include

flag pattern lt01111110gt Q is received lt01111110gt data or flag

Sender adds (ldquostuffsrdquo) extra lt 01111110gt byte after each lt 01111110gt data byte How about bit stuffing

Receiver Two 01111110 bytes in a row Discard first byte continue data reception Single 01111110 flag byte

flag bytepatternin datato send

flag byte pattern plusstuffed byte in transmitted data

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PPP Data Control Protocol Before exchanging

network-layer data data link peers must configure PPP link

(max frame length authentication)

learnconfigure network layer information

For IP carry IP Control Protocol (IPCP) messages (protocol field 8021) to configurelearn IP address

Diagram for bring a line up and down

copy Kemal Akkaya

More efficient method Go-back-N Based on sliding window If no error ACK as usual with next frame expected Use window to control number of outstanding frames If error reply with rejection

Discard that frame and all future frames until error frame received correctly

Transmitter must go back and retransmit that frame and all subsequent frames

Handling Damaged Frame Receiver detects error in frame i Receiver sends rejection-i Transmitter gets rejection-i Transmitter retransmits frame i and all subsequent

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Go-back-N Handling Lost Frame First Approach

Frame i lost Transmitter sends i+1 Receiver gets frame i+1 out of sequence Receiver send reject i Transmitter goes back to frame i and retransmits

Second Approach Frame i lost and no additional frame sent Receiver gets nothing and returns neither acknowledgement nor

rejection Transmitter times out and sends acknowledgement frame with P bit

set to 1 Receiver interprets this as command which it acknowledges with the

number of the next frame it expects (frame i ) Transmitter then retransmits frame i

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Go-back-N Damaged ACK

Receiver gets frame i and send acknowledgement (i+1) which is lost

Acknowledgements are cumulative so next acknowledgement (i+n) may arrive before transmitter times out on frame i

If transmitter times out it sends acknowledgement with P bit set as before

This can be repeated a number of times before a reset procedure is initiated

Damaged Rejection Same as Lost Frame (2)

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Selective Reject

Also called selective retransmission

Only rejected frames are retransmitted

Subsequent frames are accepted by the receiver and buffered

Minimizes retransmission Receiver must maintain

large enough buffer More complex login in

transmitter

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Multiple Access Links and ProtocolsTwo types of ldquolinksrdquo Point-to-point

PPP for dial-up access Point-to-point link between Ethernet switch and host

Broadcast (shared wire or medium) Traditional Ethernet Upstream HFC 80211 wireless LAN

copy Kemal Akkaya

Multiple Access Protocols Single shared broadcast channel Two or more simultaneous transmissions by nodes interference

Collision if node receives two or more signals at the same time

Multiple access protocol Distributed algorithm that determines how nodes share channel ie

determine when node can transmit Communication about channel sharing must use channel itself

No out-of-band channel for coordination

Ideal Multiple Access Protocol When one node wants to transmit it can send at rate R When M nodes want to transmit each can send at average rate RM Fully decentralized

No special node to coordinate transmissions No synchronization of clocks slots

Simple

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MAC Protocols A taxonomy

Three broad classes

Channel Partitioning Divide channel into smaller ldquopiecesrdquo (time slots frequency code) Allocate piece to node for exclusive use

Random Access Channel not divided allow collisions ldquoRecoverrdquo from collisions

ldquoTaking turnsrdquo Nodes take turns Nodes with more to send can take longer turns

copy Kemal Akkaya

Channel Partitioning MAC protocols TDMA

TDMA Time division multiple access Access to channel in rounds Each station gets fixed length slot

Length = packet transmission time in each round

Unused slots go idle Example 6-station LAN

134 have packet Slots 256 idle

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Channel Partitioning MAC protocols FDMA

FDMA Frequency division multiple access Channel spectrum divided into frequency bands Each station assigned fixed frequency band Unused transmission time in frequency bands go idle Example 6-station LAN 134 have packet frequency

bands 256 idle

frequ

ency

bands time

1

2

3

4

5

6

copy Kemal Akkaya

Random Access Protocols

When node has packet to send Transmit at full channel data rate R No a priori coordination among nodes

Two or more transmitting nodes ldquocollisionrdquo Random access MAC protocol specifies

How to detect collisions How to recover from collisions (eg via delayed retransmissions)

Examples of random access MAC protocols Slotted ALOHA ALOHA Carrier Sense Multiple Access (CSMA) CSMACollision Detection (CD) CSMACollision Avoidance (CA)

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Slotted ALOHA

Pros Single active node can continuously

transmit at full rate of channel Highly decentralized only slots in

nodes need to be in sync Simple

Cons Collisions wasting slots Idle slots Nodes may be able to detect collision in

less than time to transmit packet Clock synchronization

Assumptions All frames same size Time is divided into equal size

slots time to transmit 1 frame Nodes start to transmit frames

only at beginning of slots Nodes are synchronized If 2 or more nodes transmit in

slot all nodes detect collision

Operation When node obtains fresh frame it

transmits in next slot No collision node can send new frame in

next slot If collision node retransmits frame in

each subsequent slot with prob p until success

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Pure (unslotted) ALOHA Unslotted Aloha simpler no synchronization When frame first arrives

Transmit immediately

Collision probability increases Frame sent at t0 collides with other frames sent in [t0-1t0+1]

Even worse than slotted ALOHA

copy Kemal Akkaya

Collisions can still occur Propagation delay means two nodes may not hear each otherrsquos transmission Entire packet transmission time wasted Role of distance amp propagation delay in determining collision probability spatial layout of nodes

Idea listen before transmit If channel sensed idle

transmit entire frame If channel sensed busy defer

transmission Human analogy donrsquot

interrupt others

CSMA (Carrier Sense Multiple Access)

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CSMACD (Collision Detection)

CSMACD carrier sensing deferral as in CSMA Collisions detected within short

time Send jam sequence Colliding transmissions aborted

reducing channel wastage

Collision detection

Easy in wired LANs measure signal strengths compare transmitted received signals

Difficult in wireless LANs receiver shut off while transmitting

Human analogy the polite conversationalist

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ldquoTaking Turnsrdquo MAC protocols Channel partitioning MAC protocols

Share channel efficiently and fairly at high load Inefficient at low load

Delay in channel access 1N bandwidth allocated even if only 1 active node

Random access MAC protocols Efficient at low load

Single node can fully utilize channel

high load Collision overhead

ldquoTaking turnsrdquo protocols Look for best of both worlds

Two implementations Polling Token

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ldquoTaking Turnsrdquo MAC protocolsPolling Master node ldquoinvitesrdquo slave

nodes to transmit in turn Concerns

Polling overhead Latency Single point of failure (master)

Token passing control token passed from

one node to next sequentially

token message concerns

token overhead latency single point of failure (token)

copy Kemal Akkaya

Summary of MAC protocols

What do you do with a shared media Channel Partitioning by time frequency or code

Time Division Frequency Division

Random partitioning (dynamic) ALOHA S-ALOHA CSMA CSMACD Carrier sensing easy in some technologies (wire) hard in others

(wireless) CSMACD used in Ethernet CSMACA used in 80211

Taking Turns Polling from a central site token passing

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LAN technologies

Data link layer so farServicesError detectioncorrectionMultiple access

Next LAN technologiesAddressingEthernetHubs switchesPPP

copy Kemal Akkaya

MAC Addresses and ARP 32-bit IP address

Network-layer address Used to get datagram to

destination IP subnet

MAC (or LAN or physical or Ethernet) address Used to get datagram

from one interface to another physically-connected interface (same network)

48 bit MAC address (for most LANs) burned in the adapter ROM

Find your PCrsquos MAC address

1A-2F-BB-76-09-AD

58-23-D7-FA-20-B0

0C-C4-11-6F-E3-98

71-65-F7-2B-08-53

LAN(wired orwireless)

Each adapter on LAN has unique LAN address

Broadcast address =FF-FF-FF-FF-FF-FF = adapter

copy Kemal Akkaya

MAC Address (more)

MAC address allocation administered by IEEE Manufacturer buys portion of MAC address space (to

assure uniqueness) Analogy

(a) MAC address like Social Security Number

(b) IP address like postal address MAC flat address portability

Can move LAN card from one LAN to another

IP hierarchical address NOT portable Depends on IP subnet to which node is attached

copy Kemal Akkaya

ARP Address Resolution Protocol

Each IP node (Host Router) on LAN has ARP table

ARP Table IPMAC address mappings for some LAN nodes

lt IP address MAC address TTLgt TTL (Time To Live)

time after which address mapping will be forgotten (typically 20 min)

Question how to determineMAC address of Bknowing Brsquos IP address

1A-2F-BB-76-09-AD

58-23-D7-FA-20-B0

0C-C4-11-6F-E3-98

71-65-F7-2B-08-53

LAN

237196723

237196778

237196714

237196788

copy Kemal Akkaya

ARP protocol Same LAN (network) A wants to send datagram

to B and Brsquos MAC address not in Arsquos ARP table

A broadcasts ARP query packet containing Bs IP address Dest MAC address = FF-FF-

FF-FF-FF-FF All machines on LAN receive

ARP query

B receives ARP packet replies to A with its (Bs) MAC address Frame sent to Arsquos MAC

address (unicast)

A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) Soft state information that

times out (goes away) unless refreshed

ARP is ldquoplug-and-playrdquo Nodes create their ARP

tables without intervention from net administrator

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Routing to another LAN Walkthrough send datagram from A to B via R

Assume A knows B IP address

Two ARP tables in router R one for each IP network (LAN) In routing table at source Host find router 111111111110 In ARP table at source find MAC address E6-E9-00-17-BB-4B etc A creates datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110

A creates link-layer frame with Rs MAC address as dest frame contains A-to-B IP datagram

Arsquos adapter sends frame Rrsquos adapter receives frame R removes IP datagram from Ethernet frame sees its destined to B R uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B

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A

R

B

Routing to another LAN

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EthernetldquoDominantrdquo wired LAN technology Cheap $20 for 100Mbs First widely used LAN technology Simpler cheaper than token LANs and ATM Kept up with speed race 10 Mbps ndash 10

Gbps

Metcalfersquos Ethernet sketch

Star Topology Bus topology popular

through mid 90s Now star topology prevails Connection choices hub or

switch (more later)

hub orswitch

copy Kemal Akkaya

Ethernet Frame Structure

Preamble 7 bytes with pattern 10101010 followed by one byte with pattern 10101011 Used to synchronize receiver sender clock rates

Addresses 6 bytes If adapter receives frame with matching destination address or with broadcast

address (eg ARP packet) it passes data in frame to net-layer protocol Otherwise adapter discards frame

Type Indicates the higher layer protocol (mostly IP but others may be supported

such as Novell IPX and AppleTalk) CRC

Checked at receiver if error is detected the frame is simply dropped

Sending adapter encapsulates IP datagram in Ethernet frame

copy Kemal Akkaya

Ethernet uses CSMACD Connectionless No handshaking between sending and receiving

adapter Unreliable receiving adapter doesnrsquot send acks or nacks to

sending adapter Stream of datagrams passed to network layer can have gaps Gaps will be filled if app is using TCP Otherwise app will see the gaps

No slots Adapter doesnrsquot transmit if it senses that some other adapter is

transmitting that is carrier sense Transmitting adapter aborts when it senses that another adapter is

transmitting that is collision detection Before attempting a retransmission adapter waits a random time

that is random access

copy Kemal Akkaya

Ethernet CSMACD algorithm1 Adaptor receives datagram from

net layer amp creates frame2 If adapter senses channel idle it

starts to transmit frame If it senses channel busy waits until channel idle and then transmits

3 If adapter transmits entire frame without detecting another transmission the adapter is done with frame

4 If adapter detects another transmission while transmitting aborts and sends jam signal

5 After aborting adapter enters exponential backoff after the mth collision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2

Jam Signal make sure all other transmitters are aware of collision 48 bits

Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec

Exponential Backoff Goal adapt retransmission

attempts to estimated current load Heavy load random wait will be

longer

First collision choose K from 01 delay is K 512 bit transmission times

After second collision choose K from 0123hellip

After ten collisions choose K from 01234hellip1023

copy Kemal Akkaya

CSMACD efficiency Tprop = max prop between 2 nodes in LAN ttrans = time to transmit max-size frame

Efficiency goes to 1 as tprop goes to 0 Goes to 1 as ttrans goes to infinity Much better than ALOHA but still decentralized

simple and cheap

Other Ethernets Switched Ethernet Fast Ethernet Gigabit Ethernet

transprop tt 51

1efficiency

copy Kemal Akkaya

Point to Point Data Link Control

One sender one receiver one link easier than broadcast link No Media Access Control No need for explicit MAC addressing eg dialup link ISDN line

Popular point-to-point DLC protocols PPP (point-to-point protocol) HDLC High level data link control (Data link used to be considered

ldquohigh layerrdquo in protocol stack

copy Kemal Akkaya

HDLC

Frame format for bit-oriented protocol HDLC

Control field of

(a) An information frame

(b) A supervisory frame

(c) An unnumbered frame

copy Kemal Akkaya

HDLC Exchange of information supervisory and unnumbered

frames Information - data to be transmitted to user (next layer up)

Flow and error control piggybacked on information frames Supervisory - ARQ when piggyback not used Unnumbered - supplementary link control

Three phases Initialization Data transfer Disconnect

Uses bit stuffing Flag delimit frame at both ends 01111110 is the Flag May close one frame and open another Bit stuffing to find flag data

copy Kemal Akkaya

Internet Datalink Protocol PPP Design Requirements [RFC 1557]

Packet framing encapsulation of network-layer datagram in data link frame Carry network layer data of any network layer protocol (not just IP) at

same time Ability to demultiplex upwards

Bit transparency must carry any bit pattern in the data field Error detection (no correction) Connection liveness detect signal link failure to network layer Network layer address negotiation endpoint can learnconfigure

each otherrsquos network address

PPP non-design requirements No error correctionrecovery No flow control Out of order delivery OK No need to support multipoint links (eg polling)

Error recovery flow control data re-ordering all relegated to higher layers

copy Kemal Akkaya

PPP Data Frame

Flag delimiter (framing) Address does nothing (only one option) Control does nothing in the future possible multiple

control fields Protocol upper layer protocol to which frame delivered

(eg PPP-LCP IP IPCP etc) Info upper layer data being carried Check cyclic redundancy check for error detection

copy Kemal Akkaya

Byte Stuffing ldquoData transparencyrdquo requirement data field must be allowed to include

flag pattern lt01111110gt Q is received lt01111110gt data or flag

Sender adds (ldquostuffsrdquo) extra lt 01111110gt byte after each lt 01111110gt data byte How about bit stuffing

Receiver Two 01111110 bytes in a row Discard first byte continue data reception Single 01111110 flag byte

flag bytepatternin datato send

flag byte pattern plusstuffed byte in transmitted data

copy Kemal Akkaya

PPP Data Control Protocol Before exchanging

network-layer data data link peers must configure PPP link

(max frame length authentication)

learnconfigure network layer information

For IP carry IP Control Protocol (IPCP) messages (protocol field 8021) to configurelearn IP address

Diagram for bring a line up and down

copy Kemal Akkaya

Go-back-N Handling Lost Frame First Approach

Frame i lost Transmitter sends i+1 Receiver gets frame i+1 out of sequence Receiver send reject i Transmitter goes back to frame i and retransmits

Second Approach Frame i lost and no additional frame sent Receiver gets nothing and returns neither acknowledgement nor

rejection Transmitter times out and sends acknowledgement frame with P bit

set to 1 Receiver interprets this as command which it acknowledges with the

number of the next frame it expects (frame i ) Transmitter then retransmits frame i

copy Kemal Akkaya

Go-back-N Damaged ACK

Receiver gets frame i and send acknowledgement (i+1) which is lost

Acknowledgements are cumulative so next acknowledgement (i+n) may arrive before transmitter times out on frame i

If transmitter times out it sends acknowledgement with P bit set as before

This can be repeated a number of times before a reset procedure is initiated

Damaged Rejection Same as Lost Frame (2)

copy Kemal Akkaya

Selective Reject

Also called selective retransmission

Only rejected frames are retransmitted

Subsequent frames are accepted by the receiver and buffered

Minimizes retransmission Receiver must maintain

large enough buffer More complex login in

transmitter

copy Kemal Akkaya

Multiple Access Links and ProtocolsTwo types of ldquolinksrdquo Point-to-point

PPP for dial-up access Point-to-point link between Ethernet switch and host

Broadcast (shared wire or medium) Traditional Ethernet Upstream HFC 80211 wireless LAN

copy Kemal Akkaya

Multiple Access Protocols Single shared broadcast channel Two or more simultaneous transmissions by nodes interference

Collision if node receives two or more signals at the same time

Multiple access protocol Distributed algorithm that determines how nodes share channel ie

determine when node can transmit Communication about channel sharing must use channel itself

No out-of-band channel for coordination

Ideal Multiple Access Protocol When one node wants to transmit it can send at rate R When M nodes want to transmit each can send at average rate RM Fully decentralized

No special node to coordinate transmissions No synchronization of clocks slots

Simple

copy Kemal Akkaya

MAC Protocols A taxonomy

Three broad classes

Channel Partitioning Divide channel into smaller ldquopiecesrdquo (time slots frequency code) Allocate piece to node for exclusive use

Random Access Channel not divided allow collisions ldquoRecoverrdquo from collisions

ldquoTaking turnsrdquo Nodes take turns Nodes with more to send can take longer turns

copy Kemal Akkaya

Channel Partitioning MAC protocols TDMA

TDMA Time division multiple access Access to channel in rounds Each station gets fixed length slot

Length = packet transmission time in each round

Unused slots go idle Example 6-station LAN

134 have packet Slots 256 idle

copy Kemal Akkaya

Channel Partitioning MAC protocols FDMA

FDMA Frequency division multiple access Channel spectrum divided into frequency bands Each station assigned fixed frequency band Unused transmission time in frequency bands go idle Example 6-station LAN 134 have packet frequency

bands 256 idle

frequ

ency

bands time

1

2

3

4

5

6

copy Kemal Akkaya

Random Access Protocols

When node has packet to send Transmit at full channel data rate R No a priori coordination among nodes

Two or more transmitting nodes ldquocollisionrdquo Random access MAC protocol specifies

How to detect collisions How to recover from collisions (eg via delayed retransmissions)

Examples of random access MAC protocols Slotted ALOHA ALOHA Carrier Sense Multiple Access (CSMA) CSMACollision Detection (CD) CSMACollision Avoidance (CA)

copy Kemal Akkaya

Slotted ALOHA

Pros Single active node can continuously

transmit at full rate of channel Highly decentralized only slots in

nodes need to be in sync Simple

Cons Collisions wasting slots Idle slots Nodes may be able to detect collision in

less than time to transmit packet Clock synchronization

Assumptions All frames same size Time is divided into equal size

slots time to transmit 1 frame Nodes start to transmit frames

only at beginning of slots Nodes are synchronized If 2 or more nodes transmit in

slot all nodes detect collision

Operation When node obtains fresh frame it

transmits in next slot No collision node can send new frame in

next slot If collision node retransmits frame in

each subsequent slot with prob p until success

copy Kemal Akkaya

Pure (unslotted) ALOHA Unslotted Aloha simpler no synchronization When frame first arrives

Transmit immediately

Collision probability increases Frame sent at t0 collides with other frames sent in [t0-1t0+1]

Even worse than slotted ALOHA

copy Kemal Akkaya

Collisions can still occur Propagation delay means two nodes may not hear each otherrsquos transmission Entire packet transmission time wasted Role of distance amp propagation delay in determining collision probability spatial layout of nodes

Idea listen before transmit If channel sensed idle

transmit entire frame If channel sensed busy defer

transmission Human analogy donrsquot

interrupt others

CSMA (Carrier Sense Multiple Access)

copy Kemal Akkaya

CSMACD (Collision Detection)

CSMACD carrier sensing deferral as in CSMA Collisions detected within short

time Send jam sequence Colliding transmissions aborted

reducing channel wastage

Collision detection

Easy in wired LANs measure signal strengths compare transmitted received signals

Difficult in wireless LANs receiver shut off while transmitting

Human analogy the polite conversationalist

copy Kemal Akkaya

ldquoTaking Turnsrdquo MAC protocols Channel partitioning MAC protocols

Share channel efficiently and fairly at high load Inefficient at low load

Delay in channel access 1N bandwidth allocated even if only 1 active node

Random access MAC protocols Efficient at low load

Single node can fully utilize channel

high load Collision overhead

ldquoTaking turnsrdquo protocols Look for best of both worlds

Two implementations Polling Token

copy Kemal Akkaya

ldquoTaking Turnsrdquo MAC protocolsPolling Master node ldquoinvitesrdquo slave

nodes to transmit in turn Concerns

Polling overhead Latency Single point of failure (master)

Token passing control token passed from

one node to next sequentially

token message concerns

token overhead latency single point of failure (token)

copy Kemal Akkaya

Summary of MAC protocols

What do you do with a shared media Channel Partitioning by time frequency or code

Time Division Frequency Division

Random partitioning (dynamic) ALOHA S-ALOHA CSMA CSMACD Carrier sensing easy in some technologies (wire) hard in others

(wireless) CSMACD used in Ethernet CSMACA used in 80211

Taking Turns Polling from a central site token passing

copy Kemal Akkaya

LAN technologies

Data link layer so farServicesError detectioncorrectionMultiple access

Next LAN technologiesAddressingEthernetHubs switchesPPP

copy Kemal Akkaya

MAC Addresses and ARP 32-bit IP address

Network-layer address Used to get datagram to

destination IP subnet

MAC (or LAN or physical or Ethernet) address Used to get datagram

from one interface to another physically-connected interface (same network)

48 bit MAC address (for most LANs) burned in the adapter ROM

Find your PCrsquos MAC address

1A-2F-BB-76-09-AD

58-23-D7-FA-20-B0

0C-C4-11-6F-E3-98

71-65-F7-2B-08-53

LAN(wired orwireless)

Each adapter on LAN has unique LAN address

Broadcast address =FF-FF-FF-FF-FF-FF = adapter

copy Kemal Akkaya

MAC Address (more)

MAC address allocation administered by IEEE Manufacturer buys portion of MAC address space (to

assure uniqueness) Analogy

(a) MAC address like Social Security Number

(b) IP address like postal address MAC flat address portability

Can move LAN card from one LAN to another

IP hierarchical address NOT portable Depends on IP subnet to which node is attached

copy Kemal Akkaya

ARP Address Resolution Protocol

Each IP node (Host Router) on LAN has ARP table

ARP Table IPMAC address mappings for some LAN nodes

lt IP address MAC address TTLgt TTL (Time To Live)

time after which address mapping will be forgotten (typically 20 min)

Question how to determineMAC address of Bknowing Brsquos IP address

1A-2F-BB-76-09-AD

58-23-D7-FA-20-B0

0C-C4-11-6F-E3-98

71-65-F7-2B-08-53

LAN

237196723

237196778

237196714

237196788

copy Kemal Akkaya

ARP protocol Same LAN (network) A wants to send datagram

to B and Brsquos MAC address not in Arsquos ARP table

A broadcasts ARP query packet containing Bs IP address Dest MAC address = FF-FF-

FF-FF-FF-FF All machines on LAN receive

ARP query

B receives ARP packet replies to A with its (Bs) MAC address Frame sent to Arsquos MAC

address (unicast)

A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) Soft state information that

times out (goes away) unless refreshed

ARP is ldquoplug-and-playrdquo Nodes create their ARP

tables without intervention from net administrator

copy Kemal Akkaya

Routing to another LAN Walkthrough send datagram from A to B via R

Assume A knows B IP address

Two ARP tables in router R one for each IP network (LAN) In routing table at source Host find router 111111111110 In ARP table at source find MAC address E6-E9-00-17-BB-4B etc A creates datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110

A creates link-layer frame with Rs MAC address as dest frame contains A-to-B IP datagram

Arsquos adapter sends frame Rrsquos adapter receives frame R removes IP datagram from Ethernet frame sees its destined to B R uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B

copy Kemal Akkaya

A

R

B

Routing to another LAN

copy Kemal Akkaya

EthernetldquoDominantrdquo wired LAN technology Cheap $20 for 100Mbs First widely used LAN technology Simpler cheaper than token LANs and ATM Kept up with speed race 10 Mbps ndash 10

Gbps

Metcalfersquos Ethernet sketch

Star Topology Bus topology popular

through mid 90s Now star topology prevails Connection choices hub or

switch (more later)

hub orswitch

copy Kemal Akkaya

Ethernet Frame Structure

Preamble 7 bytes with pattern 10101010 followed by one byte with pattern 10101011 Used to synchronize receiver sender clock rates

Addresses 6 bytes If adapter receives frame with matching destination address or with broadcast

address (eg ARP packet) it passes data in frame to net-layer protocol Otherwise adapter discards frame

Type Indicates the higher layer protocol (mostly IP but others may be supported

such as Novell IPX and AppleTalk) CRC

Checked at receiver if error is detected the frame is simply dropped

Sending adapter encapsulates IP datagram in Ethernet frame

copy Kemal Akkaya

Ethernet uses CSMACD Connectionless No handshaking between sending and receiving

adapter Unreliable receiving adapter doesnrsquot send acks or nacks to

sending adapter Stream of datagrams passed to network layer can have gaps Gaps will be filled if app is using TCP Otherwise app will see the gaps

No slots Adapter doesnrsquot transmit if it senses that some other adapter is

transmitting that is carrier sense Transmitting adapter aborts when it senses that another adapter is

transmitting that is collision detection Before attempting a retransmission adapter waits a random time

that is random access

copy Kemal Akkaya

Ethernet CSMACD algorithm1 Adaptor receives datagram from

net layer amp creates frame2 If adapter senses channel idle it

starts to transmit frame If it senses channel busy waits until channel idle and then transmits

3 If adapter transmits entire frame without detecting another transmission the adapter is done with frame

4 If adapter detects another transmission while transmitting aborts and sends jam signal

5 After aborting adapter enters exponential backoff after the mth collision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2

Jam Signal make sure all other transmitters are aware of collision 48 bits

Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec

Exponential Backoff Goal adapt retransmission

attempts to estimated current load Heavy load random wait will be

longer

First collision choose K from 01 delay is K 512 bit transmission times

After second collision choose K from 0123hellip

After ten collisions choose K from 01234hellip1023

copy Kemal Akkaya

CSMACD efficiency Tprop = max prop between 2 nodes in LAN ttrans = time to transmit max-size frame

Efficiency goes to 1 as tprop goes to 0 Goes to 1 as ttrans goes to infinity Much better than ALOHA but still decentralized

simple and cheap

Other Ethernets Switched Ethernet Fast Ethernet Gigabit Ethernet

transprop tt 51

1efficiency

copy Kemal Akkaya

Point to Point Data Link Control

One sender one receiver one link easier than broadcast link No Media Access Control No need for explicit MAC addressing eg dialup link ISDN line

Popular point-to-point DLC protocols PPP (point-to-point protocol) HDLC High level data link control (Data link used to be considered

ldquohigh layerrdquo in protocol stack

copy Kemal Akkaya

HDLC

Frame format for bit-oriented protocol HDLC

Control field of

(a) An information frame

(b) A supervisory frame

(c) An unnumbered frame

copy Kemal Akkaya

HDLC Exchange of information supervisory and unnumbered

frames Information - data to be transmitted to user (next layer up)

Flow and error control piggybacked on information frames Supervisory - ARQ when piggyback not used Unnumbered - supplementary link control

Three phases Initialization Data transfer Disconnect

Uses bit stuffing Flag delimit frame at both ends 01111110 is the Flag May close one frame and open another Bit stuffing to find flag data

copy Kemal Akkaya

Internet Datalink Protocol PPP Design Requirements [RFC 1557]

Packet framing encapsulation of network-layer datagram in data link frame Carry network layer data of any network layer protocol (not just IP) at

same time Ability to demultiplex upwards

Bit transparency must carry any bit pattern in the data field Error detection (no correction) Connection liveness detect signal link failure to network layer Network layer address negotiation endpoint can learnconfigure

each otherrsquos network address

PPP non-design requirements No error correctionrecovery No flow control Out of order delivery OK No need to support multipoint links (eg polling)

Error recovery flow control data re-ordering all relegated to higher layers

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PPP Data Frame

Flag delimiter (framing) Address does nothing (only one option) Control does nothing in the future possible multiple

control fields Protocol upper layer protocol to which frame delivered

(eg PPP-LCP IP IPCP etc) Info upper layer data being carried Check cyclic redundancy check for error detection

copy Kemal Akkaya

Byte Stuffing ldquoData transparencyrdquo requirement data field must be allowed to include

flag pattern lt01111110gt Q is received lt01111110gt data or flag

Sender adds (ldquostuffsrdquo) extra lt 01111110gt byte after each lt 01111110gt data byte How about bit stuffing

Receiver Two 01111110 bytes in a row Discard first byte continue data reception Single 01111110 flag byte

flag bytepatternin datato send

flag byte pattern plusstuffed byte in transmitted data

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PPP Data Control Protocol Before exchanging

network-layer data data link peers must configure PPP link

(max frame length authentication)

learnconfigure network layer information

For IP carry IP Control Protocol (IPCP) messages (protocol field 8021) to configurelearn IP address

Diagram for bring a line up and down

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Go-back-N Damaged ACK

Receiver gets frame i and send acknowledgement (i+1) which is lost

Acknowledgements are cumulative so next acknowledgement (i+n) may arrive before transmitter times out on frame i

If transmitter times out it sends acknowledgement with P bit set as before

This can be repeated a number of times before a reset procedure is initiated

Damaged Rejection Same as Lost Frame (2)

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Selective Reject

Also called selective retransmission

Only rejected frames are retransmitted

Subsequent frames are accepted by the receiver and buffered

Minimizes retransmission Receiver must maintain

large enough buffer More complex login in

transmitter

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Multiple Access Links and ProtocolsTwo types of ldquolinksrdquo Point-to-point

PPP for dial-up access Point-to-point link between Ethernet switch and host

Broadcast (shared wire or medium) Traditional Ethernet Upstream HFC 80211 wireless LAN

copy Kemal Akkaya

Multiple Access Protocols Single shared broadcast channel Two or more simultaneous transmissions by nodes interference

Collision if node receives two or more signals at the same time

Multiple access protocol Distributed algorithm that determines how nodes share channel ie

determine when node can transmit Communication about channel sharing must use channel itself

No out-of-band channel for coordination

Ideal Multiple Access Protocol When one node wants to transmit it can send at rate R When M nodes want to transmit each can send at average rate RM Fully decentralized

No special node to coordinate transmissions No synchronization of clocks slots

Simple

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MAC Protocols A taxonomy

Three broad classes

Channel Partitioning Divide channel into smaller ldquopiecesrdquo (time slots frequency code) Allocate piece to node for exclusive use

Random Access Channel not divided allow collisions ldquoRecoverrdquo from collisions

ldquoTaking turnsrdquo Nodes take turns Nodes with more to send can take longer turns

copy Kemal Akkaya

Channel Partitioning MAC protocols TDMA

TDMA Time division multiple access Access to channel in rounds Each station gets fixed length slot

Length = packet transmission time in each round

Unused slots go idle Example 6-station LAN

134 have packet Slots 256 idle

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Channel Partitioning MAC protocols FDMA

FDMA Frequency division multiple access Channel spectrum divided into frequency bands Each station assigned fixed frequency band Unused transmission time in frequency bands go idle Example 6-station LAN 134 have packet frequency

bands 256 idle

frequ

ency

bands time

1

2

3

4

5

6

copy Kemal Akkaya

Random Access Protocols

When node has packet to send Transmit at full channel data rate R No a priori coordination among nodes

Two or more transmitting nodes ldquocollisionrdquo Random access MAC protocol specifies

How to detect collisions How to recover from collisions (eg via delayed retransmissions)

Examples of random access MAC protocols Slotted ALOHA ALOHA Carrier Sense Multiple Access (CSMA) CSMACollision Detection (CD) CSMACollision Avoidance (CA)

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Slotted ALOHA

Pros Single active node can continuously

transmit at full rate of channel Highly decentralized only slots in

nodes need to be in sync Simple

Cons Collisions wasting slots Idle slots Nodes may be able to detect collision in

less than time to transmit packet Clock synchronization

Assumptions All frames same size Time is divided into equal size

slots time to transmit 1 frame Nodes start to transmit frames

only at beginning of slots Nodes are synchronized If 2 or more nodes transmit in

slot all nodes detect collision

Operation When node obtains fresh frame it

transmits in next slot No collision node can send new frame in

next slot If collision node retransmits frame in

each subsequent slot with prob p until success

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Pure (unslotted) ALOHA Unslotted Aloha simpler no synchronization When frame first arrives

Transmit immediately

Collision probability increases Frame sent at t0 collides with other frames sent in [t0-1t0+1]

Even worse than slotted ALOHA

copy Kemal Akkaya

Collisions can still occur Propagation delay means two nodes may not hear each otherrsquos transmission Entire packet transmission time wasted Role of distance amp propagation delay in determining collision probability spatial layout of nodes

Idea listen before transmit If channel sensed idle

transmit entire frame If channel sensed busy defer

transmission Human analogy donrsquot

interrupt others

CSMA (Carrier Sense Multiple Access)

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CSMACD (Collision Detection)

CSMACD carrier sensing deferral as in CSMA Collisions detected within short

time Send jam sequence Colliding transmissions aborted

reducing channel wastage

Collision detection

Easy in wired LANs measure signal strengths compare transmitted received signals

Difficult in wireless LANs receiver shut off while transmitting

Human analogy the polite conversationalist

copy Kemal Akkaya

ldquoTaking Turnsrdquo MAC protocols Channel partitioning MAC protocols

Share channel efficiently and fairly at high load Inefficient at low load

Delay in channel access 1N bandwidth allocated even if only 1 active node

Random access MAC protocols Efficient at low load

Single node can fully utilize channel

high load Collision overhead

ldquoTaking turnsrdquo protocols Look for best of both worlds

Two implementations Polling Token

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ldquoTaking Turnsrdquo MAC protocolsPolling Master node ldquoinvitesrdquo slave

nodes to transmit in turn Concerns

Polling overhead Latency Single point of failure (master)

Token passing control token passed from

one node to next sequentially

token message concerns

token overhead latency single point of failure (token)

copy Kemal Akkaya

Summary of MAC protocols

What do you do with a shared media Channel Partitioning by time frequency or code

Time Division Frequency Division

Random partitioning (dynamic) ALOHA S-ALOHA CSMA CSMACD Carrier sensing easy in some technologies (wire) hard in others

(wireless) CSMACD used in Ethernet CSMACA used in 80211

Taking Turns Polling from a central site token passing

copy Kemal Akkaya

LAN technologies

Data link layer so farServicesError detectioncorrectionMultiple access

Next LAN technologiesAddressingEthernetHubs switchesPPP

copy Kemal Akkaya

MAC Addresses and ARP 32-bit IP address

Network-layer address Used to get datagram to

destination IP subnet

MAC (or LAN or physical or Ethernet) address Used to get datagram

from one interface to another physically-connected interface (same network)

48 bit MAC address (for most LANs) burned in the adapter ROM

Find your PCrsquos MAC address

1A-2F-BB-76-09-AD

58-23-D7-FA-20-B0

0C-C4-11-6F-E3-98

71-65-F7-2B-08-53

LAN(wired orwireless)

Each adapter on LAN has unique LAN address

Broadcast address =FF-FF-FF-FF-FF-FF = adapter

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MAC Address (more)

MAC address allocation administered by IEEE Manufacturer buys portion of MAC address space (to

assure uniqueness) Analogy

(a) MAC address like Social Security Number

(b) IP address like postal address MAC flat address portability

Can move LAN card from one LAN to another

IP hierarchical address NOT portable Depends on IP subnet to which node is attached

copy Kemal Akkaya

ARP Address Resolution Protocol

Each IP node (Host Router) on LAN has ARP table

ARP Table IPMAC address mappings for some LAN nodes

lt IP address MAC address TTLgt TTL (Time To Live)

time after which address mapping will be forgotten (typically 20 min)

Question how to determineMAC address of Bknowing Brsquos IP address

1A-2F-BB-76-09-AD

58-23-D7-FA-20-B0

0C-C4-11-6F-E3-98

71-65-F7-2B-08-53

LAN

237196723

237196778

237196714

237196788

copy Kemal Akkaya

ARP protocol Same LAN (network) A wants to send datagram

to B and Brsquos MAC address not in Arsquos ARP table

A broadcasts ARP query packet containing Bs IP address Dest MAC address = FF-FF-

FF-FF-FF-FF All machines on LAN receive

ARP query

B receives ARP packet replies to A with its (Bs) MAC address Frame sent to Arsquos MAC

address (unicast)

A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) Soft state information that

times out (goes away) unless refreshed

ARP is ldquoplug-and-playrdquo Nodes create their ARP

tables without intervention from net administrator

copy Kemal Akkaya

Routing to another LAN Walkthrough send datagram from A to B via R

Assume A knows B IP address

Two ARP tables in router R one for each IP network (LAN) In routing table at source Host find router 111111111110 In ARP table at source find MAC address E6-E9-00-17-BB-4B etc A creates datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110

A creates link-layer frame with Rs MAC address as dest frame contains A-to-B IP datagram

Arsquos adapter sends frame Rrsquos adapter receives frame R removes IP datagram from Ethernet frame sees its destined to B R uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B

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A

R

B

Routing to another LAN

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EthernetldquoDominantrdquo wired LAN technology Cheap $20 for 100Mbs First widely used LAN technology Simpler cheaper than token LANs and ATM Kept up with speed race 10 Mbps ndash 10

Gbps

Metcalfersquos Ethernet sketch

Star Topology Bus topology popular

through mid 90s Now star topology prevails Connection choices hub or

switch (more later)

hub orswitch

copy Kemal Akkaya

Ethernet Frame Structure

Preamble 7 bytes with pattern 10101010 followed by one byte with pattern 10101011 Used to synchronize receiver sender clock rates

Addresses 6 bytes If adapter receives frame with matching destination address or with broadcast

address (eg ARP packet) it passes data in frame to net-layer protocol Otherwise adapter discards frame

Type Indicates the higher layer protocol (mostly IP but others may be supported

such as Novell IPX and AppleTalk) CRC

Checked at receiver if error is detected the frame is simply dropped

Sending adapter encapsulates IP datagram in Ethernet frame

copy Kemal Akkaya

Ethernet uses CSMACD Connectionless No handshaking between sending and receiving

adapter Unreliable receiving adapter doesnrsquot send acks or nacks to

sending adapter Stream of datagrams passed to network layer can have gaps Gaps will be filled if app is using TCP Otherwise app will see the gaps

No slots Adapter doesnrsquot transmit if it senses that some other adapter is

transmitting that is carrier sense Transmitting adapter aborts when it senses that another adapter is

transmitting that is collision detection Before attempting a retransmission adapter waits a random time

that is random access

copy Kemal Akkaya

Ethernet CSMACD algorithm1 Adaptor receives datagram from

net layer amp creates frame2 If adapter senses channel idle it

starts to transmit frame If it senses channel busy waits until channel idle and then transmits

3 If adapter transmits entire frame without detecting another transmission the adapter is done with frame

4 If adapter detects another transmission while transmitting aborts and sends jam signal

5 After aborting adapter enters exponential backoff after the mth collision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2

Jam Signal make sure all other transmitters are aware of collision 48 bits

Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec

Exponential Backoff Goal adapt retransmission

attempts to estimated current load Heavy load random wait will be

longer

First collision choose K from 01 delay is K 512 bit transmission times

After second collision choose K from 0123hellip

After ten collisions choose K from 01234hellip1023

copy Kemal Akkaya

CSMACD efficiency Tprop = max prop between 2 nodes in LAN ttrans = time to transmit max-size frame

Efficiency goes to 1 as tprop goes to 0 Goes to 1 as ttrans goes to infinity Much better than ALOHA but still decentralized

simple and cheap

Other Ethernets Switched Ethernet Fast Ethernet Gigabit Ethernet

transprop tt 51

1efficiency

copy Kemal Akkaya

Point to Point Data Link Control

One sender one receiver one link easier than broadcast link No Media Access Control No need for explicit MAC addressing eg dialup link ISDN line

Popular point-to-point DLC protocols PPP (point-to-point protocol) HDLC High level data link control (Data link used to be considered

ldquohigh layerrdquo in protocol stack

copy Kemal Akkaya

HDLC

Frame format for bit-oriented protocol HDLC

Control field of

(a) An information frame

(b) A supervisory frame

(c) An unnumbered frame

copy Kemal Akkaya

HDLC Exchange of information supervisory and unnumbered

frames Information - data to be transmitted to user (next layer up)

Flow and error control piggybacked on information frames Supervisory - ARQ when piggyback not used Unnumbered - supplementary link control

Three phases Initialization Data transfer Disconnect

Uses bit stuffing Flag delimit frame at both ends 01111110 is the Flag May close one frame and open another Bit stuffing to find flag data

copy Kemal Akkaya

Internet Datalink Protocol PPP Design Requirements [RFC 1557]

Packet framing encapsulation of network-layer datagram in data link frame Carry network layer data of any network layer protocol (not just IP) at

same time Ability to demultiplex upwards

Bit transparency must carry any bit pattern in the data field Error detection (no correction) Connection liveness detect signal link failure to network layer Network layer address negotiation endpoint can learnconfigure

each otherrsquos network address

PPP non-design requirements No error correctionrecovery No flow control Out of order delivery OK No need to support multipoint links (eg polling)

Error recovery flow control data re-ordering all relegated to higher layers

copy Kemal Akkaya

PPP Data Frame

Flag delimiter (framing) Address does nothing (only one option) Control does nothing in the future possible multiple

control fields Protocol upper layer protocol to which frame delivered

(eg PPP-LCP IP IPCP etc) Info upper layer data being carried Check cyclic redundancy check for error detection

copy Kemal Akkaya

Byte Stuffing ldquoData transparencyrdquo requirement data field must be allowed to include

flag pattern lt01111110gt Q is received lt01111110gt data or flag

Sender adds (ldquostuffsrdquo) extra lt 01111110gt byte after each lt 01111110gt data byte How about bit stuffing

Receiver Two 01111110 bytes in a row Discard first byte continue data reception Single 01111110 flag byte

flag bytepatternin datato send

flag byte pattern plusstuffed byte in transmitted data

copy Kemal Akkaya

PPP Data Control Protocol Before exchanging

network-layer data data link peers must configure PPP link

(max frame length authentication)

learnconfigure network layer information

For IP carry IP Control Protocol (IPCP) messages (protocol field 8021) to configurelearn IP address

Diagram for bring a line up and down

copy Kemal Akkaya

Selective Reject

Also called selective retransmission

Only rejected frames are retransmitted

Subsequent frames are accepted by the receiver and buffered

Minimizes retransmission Receiver must maintain

large enough buffer More complex login in

transmitter

copy Kemal Akkaya

Multiple Access Links and ProtocolsTwo types of ldquolinksrdquo Point-to-point

PPP for dial-up access Point-to-point link between Ethernet switch and host

Broadcast (shared wire or medium) Traditional Ethernet Upstream HFC 80211 wireless LAN

copy Kemal Akkaya

Multiple Access Protocols Single shared broadcast channel Two or more simultaneous transmissions by nodes interference

Collision if node receives two or more signals at the same time

Multiple access protocol Distributed algorithm that determines how nodes share channel ie

determine when node can transmit Communication about channel sharing must use channel itself

No out-of-band channel for coordination

Ideal Multiple Access Protocol When one node wants to transmit it can send at rate R When M nodes want to transmit each can send at average rate RM Fully decentralized

No special node to coordinate transmissions No synchronization of clocks slots

Simple

copy Kemal Akkaya

MAC Protocols A taxonomy

Three broad classes

Channel Partitioning Divide channel into smaller ldquopiecesrdquo (time slots frequency code) Allocate piece to node for exclusive use

Random Access Channel not divided allow collisions ldquoRecoverrdquo from collisions

ldquoTaking turnsrdquo Nodes take turns Nodes with more to send can take longer turns

copy Kemal Akkaya

Channel Partitioning MAC protocols TDMA

TDMA Time division multiple access Access to channel in rounds Each station gets fixed length slot

Length = packet transmission time in each round

Unused slots go idle Example 6-station LAN

134 have packet Slots 256 idle

copy Kemal Akkaya

Channel Partitioning MAC protocols FDMA

FDMA Frequency division multiple access Channel spectrum divided into frequency bands Each station assigned fixed frequency band Unused transmission time in frequency bands go idle Example 6-station LAN 134 have packet frequency

bands 256 idle

frequ

ency

bands time

1

2

3

4

5

6

copy Kemal Akkaya

Random Access Protocols

When node has packet to send Transmit at full channel data rate R No a priori coordination among nodes

Two or more transmitting nodes ldquocollisionrdquo Random access MAC protocol specifies

How to detect collisions How to recover from collisions (eg via delayed retransmissions)

Examples of random access MAC protocols Slotted ALOHA ALOHA Carrier Sense Multiple Access (CSMA) CSMACollision Detection (CD) CSMACollision Avoidance (CA)

copy Kemal Akkaya

Slotted ALOHA

Pros Single active node can continuously

transmit at full rate of channel Highly decentralized only slots in

nodes need to be in sync Simple

Cons Collisions wasting slots Idle slots Nodes may be able to detect collision in

less than time to transmit packet Clock synchronization

Assumptions All frames same size Time is divided into equal size

slots time to transmit 1 frame Nodes start to transmit frames

only at beginning of slots Nodes are synchronized If 2 or more nodes transmit in

slot all nodes detect collision

Operation When node obtains fresh frame it

transmits in next slot No collision node can send new frame in

next slot If collision node retransmits frame in

each subsequent slot with prob p until success

copy Kemal Akkaya

Pure (unslotted) ALOHA Unslotted Aloha simpler no synchronization When frame first arrives

Transmit immediately

Collision probability increases Frame sent at t0 collides with other frames sent in [t0-1t0+1]

Even worse than slotted ALOHA

copy Kemal Akkaya

Collisions can still occur Propagation delay means two nodes may not hear each otherrsquos transmission Entire packet transmission time wasted Role of distance amp propagation delay in determining collision probability spatial layout of nodes

Idea listen before transmit If channel sensed idle

transmit entire frame If channel sensed busy defer

transmission Human analogy donrsquot

interrupt others

CSMA (Carrier Sense Multiple Access)

copy Kemal Akkaya

CSMACD (Collision Detection)

CSMACD carrier sensing deferral as in CSMA Collisions detected within short

time Send jam sequence Colliding transmissions aborted

reducing channel wastage

Collision detection

Easy in wired LANs measure signal strengths compare transmitted received signals

Difficult in wireless LANs receiver shut off while transmitting

Human analogy the polite conversationalist

copy Kemal Akkaya

ldquoTaking Turnsrdquo MAC protocols Channel partitioning MAC protocols

Share channel efficiently and fairly at high load Inefficient at low load

Delay in channel access 1N bandwidth allocated even if only 1 active node

Random access MAC protocols Efficient at low load

Single node can fully utilize channel

high load Collision overhead

ldquoTaking turnsrdquo protocols Look for best of both worlds

Two implementations Polling Token

copy Kemal Akkaya

ldquoTaking Turnsrdquo MAC protocolsPolling Master node ldquoinvitesrdquo slave

nodes to transmit in turn Concerns

Polling overhead Latency Single point of failure (master)

Token passing control token passed from

one node to next sequentially

token message concerns

token overhead latency single point of failure (token)

copy Kemal Akkaya

Summary of MAC protocols

What do you do with a shared media Channel Partitioning by time frequency or code

Time Division Frequency Division

Random partitioning (dynamic) ALOHA S-ALOHA CSMA CSMACD Carrier sensing easy in some technologies (wire) hard in others

(wireless) CSMACD used in Ethernet CSMACA used in 80211

Taking Turns Polling from a central site token passing

copy Kemal Akkaya

LAN technologies

Data link layer so farServicesError detectioncorrectionMultiple access

Next LAN technologiesAddressingEthernetHubs switchesPPP

copy Kemal Akkaya

MAC Addresses and ARP 32-bit IP address

Network-layer address Used to get datagram to

destination IP subnet

MAC (or LAN or physical or Ethernet) address Used to get datagram

from one interface to another physically-connected interface (same network)

48 bit MAC address (for most LANs) burned in the adapter ROM

Find your PCrsquos MAC address

1A-2F-BB-76-09-AD

58-23-D7-FA-20-B0

0C-C4-11-6F-E3-98

71-65-F7-2B-08-53

LAN(wired orwireless)

Each adapter on LAN has unique LAN address

Broadcast address =FF-FF-FF-FF-FF-FF = adapter

copy Kemal Akkaya

MAC Address (more)

MAC address allocation administered by IEEE Manufacturer buys portion of MAC address space (to

assure uniqueness) Analogy

(a) MAC address like Social Security Number

(b) IP address like postal address MAC flat address portability

Can move LAN card from one LAN to another

IP hierarchical address NOT portable Depends on IP subnet to which node is attached

copy Kemal Akkaya

ARP Address Resolution Protocol

Each IP node (Host Router) on LAN has ARP table

ARP Table IPMAC address mappings for some LAN nodes

lt IP address MAC address TTLgt TTL (Time To Live)

time after which address mapping will be forgotten (typically 20 min)

Question how to determineMAC address of Bknowing Brsquos IP address

1A-2F-BB-76-09-AD

58-23-D7-FA-20-B0

0C-C4-11-6F-E3-98

71-65-F7-2B-08-53

LAN

237196723

237196778

237196714

237196788

copy Kemal Akkaya

ARP protocol Same LAN (network) A wants to send datagram

to B and Brsquos MAC address not in Arsquos ARP table

A broadcasts ARP query packet containing Bs IP address Dest MAC address = FF-FF-

FF-FF-FF-FF All machines on LAN receive

ARP query

B receives ARP packet replies to A with its (Bs) MAC address Frame sent to Arsquos MAC

address (unicast)

A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) Soft state information that

times out (goes away) unless refreshed

ARP is ldquoplug-and-playrdquo Nodes create their ARP

tables without intervention from net administrator

copy Kemal Akkaya

Routing to another LAN Walkthrough send datagram from A to B via R

Assume A knows B IP address

Two ARP tables in router R one for each IP network (LAN) In routing table at source Host find router 111111111110 In ARP table at source find MAC address E6-E9-00-17-BB-4B etc A creates datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110

A creates link-layer frame with Rs MAC address as dest frame contains A-to-B IP datagram

Arsquos adapter sends frame Rrsquos adapter receives frame R removes IP datagram from Ethernet frame sees its destined to B R uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B

copy Kemal Akkaya

A

R

B

Routing to another LAN

copy Kemal Akkaya

EthernetldquoDominantrdquo wired LAN technology Cheap $20 for 100Mbs First widely used LAN technology Simpler cheaper than token LANs and ATM Kept up with speed race 10 Mbps ndash 10

Gbps

Metcalfersquos Ethernet sketch

Star Topology Bus topology popular

through mid 90s Now star topology prevails Connection choices hub or

switch (more later)

hub orswitch

copy Kemal Akkaya

Ethernet Frame Structure

Preamble 7 bytes with pattern 10101010 followed by one byte with pattern 10101011 Used to synchronize receiver sender clock rates

Addresses 6 bytes If adapter receives frame with matching destination address or with broadcast

address (eg ARP packet) it passes data in frame to net-layer protocol Otherwise adapter discards frame

Type Indicates the higher layer protocol (mostly IP but others may be supported

such as Novell IPX and AppleTalk) CRC

Checked at receiver if error is detected the frame is simply dropped

Sending adapter encapsulates IP datagram in Ethernet frame

copy Kemal Akkaya

Ethernet uses CSMACD Connectionless No handshaking between sending and receiving

adapter Unreliable receiving adapter doesnrsquot send acks or nacks to

sending adapter Stream of datagrams passed to network layer can have gaps Gaps will be filled if app is using TCP Otherwise app will see the gaps

No slots Adapter doesnrsquot transmit if it senses that some other adapter is

transmitting that is carrier sense Transmitting adapter aborts when it senses that another adapter is

transmitting that is collision detection Before attempting a retransmission adapter waits a random time

that is random access

copy Kemal Akkaya

Ethernet CSMACD algorithm1 Adaptor receives datagram from

net layer amp creates frame2 If adapter senses channel idle it

starts to transmit frame If it senses channel busy waits until channel idle and then transmits

3 If adapter transmits entire frame without detecting another transmission the adapter is done with frame

4 If adapter detects another transmission while transmitting aborts and sends jam signal

5 After aborting adapter enters exponential backoff after the mth collision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2

Jam Signal make sure all other transmitters are aware of collision 48 bits

Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec

Exponential Backoff Goal adapt retransmission

attempts to estimated current load Heavy load random wait will be

longer

First collision choose K from 01 delay is K 512 bit transmission times

After second collision choose K from 0123hellip

After ten collisions choose K from 01234hellip1023

copy Kemal Akkaya

CSMACD efficiency Tprop = max prop between 2 nodes in LAN ttrans = time to transmit max-size frame

Efficiency goes to 1 as tprop goes to 0 Goes to 1 as ttrans goes to infinity Much better than ALOHA but still decentralized

simple and cheap

Other Ethernets Switched Ethernet Fast Ethernet Gigabit Ethernet

transprop tt 51

1efficiency

copy Kemal Akkaya

Point to Point Data Link Control

One sender one receiver one link easier than broadcast link No Media Access Control No need for explicit MAC addressing eg dialup link ISDN line

Popular point-to-point DLC protocols PPP (point-to-point protocol) HDLC High level data link control (Data link used to be considered

ldquohigh layerrdquo in protocol stack

copy Kemal Akkaya

HDLC

Frame format for bit-oriented protocol HDLC

Control field of

(a) An information frame

(b) A supervisory frame

(c) An unnumbered frame

copy Kemal Akkaya

HDLC Exchange of information supervisory and unnumbered

frames Information - data to be transmitted to user (next layer up)

Flow and error control piggybacked on information frames Supervisory - ARQ when piggyback not used Unnumbered - supplementary link control

Three phases Initialization Data transfer Disconnect

Uses bit stuffing Flag delimit frame at both ends 01111110 is the Flag May close one frame and open another Bit stuffing to find flag data

copy Kemal Akkaya

Internet Datalink Protocol PPP Design Requirements [RFC 1557]

Packet framing encapsulation of network-layer datagram in data link frame Carry network layer data of any network layer protocol (not just IP) at

same time Ability to demultiplex upwards

Bit transparency must carry any bit pattern in the data field Error detection (no correction) Connection liveness detect signal link failure to network layer Network layer address negotiation endpoint can learnconfigure

each otherrsquos network address

PPP non-design requirements No error correctionrecovery No flow control Out of order delivery OK No need to support multipoint links (eg polling)

Error recovery flow control data re-ordering all relegated to higher layers

copy Kemal Akkaya

PPP Data Frame

Flag delimiter (framing) Address does nothing (only one option) Control does nothing in the future possible multiple

control fields Protocol upper layer protocol to which frame delivered

(eg PPP-LCP IP IPCP etc) Info upper layer data being carried Check cyclic redundancy check for error detection

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Byte Stuffing ldquoData transparencyrdquo requirement data field must be allowed to include

flag pattern lt01111110gt Q is received lt01111110gt data or flag

Sender adds (ldquostuffsrdquo) extra lt 01111110gt byte after each lt 01111110gt data byte How about bit stuffing

Receiver Two 01111110 bytes in a row Discard first byte continue data reception Single 01111110 flag byte

flag bytepatternin datato send

flag byte pattern plusstuffed byte in transmitted data

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PPP Data Control Protocol Before exchanging

network-layer data data link peers must configure PPP link

(max frame length authentication)

learnconfigure network layer information

For IP carry IP Control Protocol (IPCP) messages (protocol field 8021) to configurelearn IP address

Diagram for bring a line up and down

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Multiple Access Links and ProtocolsTwo types of ldquolinksrdquo Point-to-point

PPP for dial-up access Point-to-point link between Ethernet switch and host

Broadcast (shared wire or medium) Traditional Ethernet Upstream HFC 80211 wireless LAN

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Multiple Access Protocols Single shared broadcast channel Two or more simultaneous transmissions by nodes interference

Collision if node receives two or more signals at the same time

Multiple access protocol Distributed algorithm that determines how nodes share channel ie

determine when node can transmit Communication about channel sharing must use channel itself

No out-of-band channel for coordination

Ideal Multiple Access Protocol When one node wants to transmit it can send at rate R When M nodes want to transmit each can send at average rate RM Fully decentralized

No special node to coordinate transmissions No synchronization of clocks slots

Simple

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MAC Protocols A taxonomy

Three broad classes

Channel Partitioning Divide channel into smaller ldquopiecesrdquo (time slots frequency code) Allocate piece to node for exclusive use

Random Access Channel not divided allow collisions ldquoRecoverrdquo from collisions

ldquoTaking turnsrdquo Nodes take turns Nodes with more to send can take longer turns

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Channel Partitioning MAC protocols TDMA

TDMA Time division multiple access Access to channel in rounds Each station gets fixed length slot

Length = packet transmission time in each round

Unused slots go idle Example 6-station LAN

134 have packet Slots 256 idle

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Channel Partitioning MAC protocols FDMA

FDMA Frequency division multiple access Channel spectrum divided into frequency bands Each station assigned fixed frequency band Unused transmission time in frequency bands go idle Example 6-station LAN 134 have packet frequency

bands 256 idle

frequ

ency

bands time

1

2

3

4

5

6

copy Kemal Akkaya

Random Access Protocols

When node has packet to send Transmit at full channel data rate R No a priori coordination among nodes

Two or more transmitting nodes ldquocollisionrdquo Random access MAC protocol specifies

How to detect collisions How to recover from collisions (eg via delayed retransmissions)

Examples of random access MAC protocols Slotted ALOHA ALOHA Carrier Sense Multiple Access (CSMA) CSMACollision Detection (CD) CSMACollision Avoidance (CA)

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Slotted ALOHA

Pros Single active node can continuously

transmit at full rate of channel Highly decentralized only slots in

nodes need to be in sync Simple

Cons Collisions wasting slots Idle slots Nodes may be able to detect collision in

less than time to transmit packet Clock synchronization

Assumptions All frames same size Time is divided into equal size

slots time to transmit 1 frame Nodes start to transmit frames

only at beginning of slots Nodes are synchronized If 2 or more nodes transmit in

slot all nodes detect collision

Operation When node obtains fresh frame it

transmits in next slot No collision node can send new frame in

next slot If collision node retransmits frame in

each subsequent slot with prob p until success

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Pure (unslotted) ALOHA Unslotted Aloha simpler no synchronization When frame first arrives

Transmit immediately

Collision probability increases Frame sent at t0 collides with other frames sent in [t0-1t0+1]

Even worse than slotted ALOHA

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Collisions can still occur Propagation delay means two nodes may not hear each otherrsquos transmission Entire packet transmission time wasted Role of distance amp propagation delay in determining collision probability spatial layout of nodes

Idea listen before transmit If channel sensed idle

transmit entire frame If channel sensed busy defer

transmission Human analogy donrsquot

interrupt others

CSMA (Carrier Sense Multiple Access)

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CSMACD (Collision Detection)

CSMACD carrier sensing deferral as in CSMA Collisions detected within short

time Send jam sequence Colliding transmissions aborted

reducing channel wastage

Collision detection

Easy in wired LANs measure signal strengths compare transmitted received signals

Difficult in wireless LANs receiver shut off while transmitting

Human analogy the polite conversationalist

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ldquoTaking Turnsrdquo MAC protocols Channel partitioning MAC protocols

Share channel efficiently and fairly at high load Inefficient at low load

Delay in channel access 1N bandwidth allocated even if only 1 active node

Random access MAC protocols Efficient at low load

Single node can fully utilize channel

high load Collision overhead

ldquoTaking turnsrdquo protocols Look for best of both worlds

Two implementations Polling Token

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ldquoTaking Turnsrdquo MAC protocolsPolling Master node ldquoinvitesrdquo slave

nodes to transmit in turn Concerns

Polling overhead Latency Single point of failure (master)

Token passing control token passed from

one node to next sequentially

token message concerns

token overhead latency single point of failure (token)

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Summary of MAC protocols

What do you do with a shared media Channel Partitioning by time frequency or code

Time Division Frequency Division

Random partitioning (dynamic) ALOHA S-ALOHA CSMA CSMACD Carrier sensing easy in some technologies (wire) hard in others

(wireless) CSMACD used in Ethernet CSMACA used in 80211

Taking Turns Polling from a central site token passing

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LAN technologies

Data link layer so farServicesError detectioncorrectionMultiple access

Next LAN technologiesAddressingEthernetHubs switchesPPP

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MAC Addresses and ARP 32-bit IP address

Network-layer address Used to get datagram to

destination IP subnet

MAC (or LAN or physical or Ethernet) address Used to get datagram

from one interface to another physically-connected interface (same network)

48 bit MAC address (for most LANs) burned in the adapter ROM

Find your PCrsquos MAC address

1A-2F-BB-76-09-AD

58-23-D7-FA-20-B0

0C-C4-11-6F-E3-98

71-65-F7-2B-08-53

LAN(wired orwireless)

Each adapter on LAN has unique LAN address

Broadcast address =FF-FF-FF-FF-FF-FF = adapter

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MAC Address (more)

MAC address allocation administered by IEEE Manufacturer buys portion of MAC address space (to

assure uniqueness) Analogy

(a) MAC address like Social Security Number

(b) IP address like postal address MAC flat address portability

Can move LAN card from one LAN to another

IP hierarchical address NOT portable Depends on IP subnet to which node is attached

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ARP Address Resolution Protocol

Each IP node (Host Router) on LAN has ARP table

ARP Table IPMAC address mappings for some LAN nodes

lt IP address MAC address TTLgt TTL (Time To Live)

time after which address mapping will be forgotten (typically 20 min)

Question how to determineMAC address of Bknowing Brsquos IP address

1A-2F-BB-76-09-AD

58-23-D7-FA-20-B0

0C-C4-11-6F-E3-98

71-65-F7-2B-08-53

LAN

237196723

237196778

237196714

237196788

copy Kemal Akkaya

ARP protocol Same LAN (network) A wants to send datagram

to B and Brsquos MAC address not in Arsquos ARP table

A broadcasts ARP query packet containing Bs IP address Dest MAC address = FF-FF-

FF-FF-FF-FF All machines on LAN receive

ARP query

B receives ARP packet replies to A with its (Bs) MAC address Frame sent to Arsquos MAC

address (unicast)

A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) Soft state information that

times out (goes away) unless refreshed

ARP is ldquoplug-and-playrdquo Nodes create their ARP

tables without intervention from net administrator

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Routing to another LAN Walkthrough send datagram from A to B via R

Assume A knows B IP address

Two ARP tables in router R one for each IP network (LAN) In routing table at source Host find router 111111111110 In ARP table at source find MAC address E6-E9-00-17-BB-4B etc A creates datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110

A creates link-layer frame with Rs MAC address as dest frame contains A-to-B IP datagram

Arsquos adapter sends frame Rrsquos adapter receives frame R removes IP datagram from Ethernet frame sees its destined to B R uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B

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A

R

B

Routing to another LAN

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EthernetldquoDominantrdquo wired LAN technology Cheap $20 for 100Mbs First widely used LAN technology Simpler cheaper than token LANs and ATM Kept up with speed race 10 Mbps ndash 10

Gbps

Metcalfersquos Ethernet sketch

Star Topology Bus topology popular

through mid 90s Now star topology prevails Connection choices hub or

switch (more later)

hub orswitch

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Ethernet Frame Structure

Preamble 7 bytes with pattern 10101010 followed by one byte with pattern 10101011 Used to synchronize receiver sender clock rates

Addresses 6 bytes If adapter receives frame with matching destination address or with broadcast

address (eg ARP packet) it passes data in frame to net-layer protocol Otherwise adapter discards frame

Type Indicates the higher layer protocol (mostly IP but others may be supported

such as Novell IPX and AppleTalk) CRC

Checked at receiver if error is detected the frame is simply dropped

Sending adapter encapsulates IP datagram in Ethernet frame

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Ethernet uses CSMACD Connectionless No handshaking between sending and receiving

adapter Unreliable receiving adapter doesnrsquot send acks or nacks to

sending adapter Stream of datagrams passed to network layer can have gaps Gaps will be filled if app is using TCP Otherwise app will see the gaps

No slots Adapter doesnrsquot transmit if it senses that some other adapter is

transmitting that is carrier sense Transmitting adapter aborts when it senses that another adapter is

transmitting that is collision detection Before attempting a retransmission adapter waits a random time

that is random access

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Ethernet CSMACD algorithm1 Adaptor receives datagram from

net layer amp creates frame2 If adapter senses channel idle it

starts to transmit frame If it senses channel busy waits until channel idle and then transmits

3 If adapter transmits entire frame without detecting another transmission the adapter is done with frame

4 If adapter detects another transmission while transmitting aborts and sends jam signal

5 After aborting adapter enters exponential backoff after the mth collision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2

Jam Signal make sure all other transmitters are aware of collision 48 bits

Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec

Exponential Backoff Goal adapt retransmission

attempts to estimated current load Heavy load random wait will be

longer

First collision choose K from 01 delay is K 512 bit transmission times

After second collision choose K from 0123hellip

After ten collisions choose K from 01234hellip1023

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CSMACD efficiency Tprop = max prop between 2 nodes in LAN ttrans = time to transmit max-size frame

Efficiency goes to 1 as tprop goes to 0 Goes to 1 as ttrans goes to infinity Much better than ALOHA but still decentralized

simple and cheap

Other Ethernets Switched Ethernet Fast Ethernet Gigabit Ethernet

transprop tt 51

1efficiency

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Point to Point Data Link Control

One sender one receiver one link easier than broadcast link No Media Access Control No need for explicit MAC addressing eg dialup link ISDN line

Popular point-to-point DLC protocols PPP (point-to-point protocol) HDLC High level data link control (Data link used to be considered

ldquohigh layerrdquo in protocol stack

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HDLC

Frame format for bit-oriented protocol HDLC

Control field of

(a) An information frame

(b) A supervisory frame

(c) An unnumbered frame

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HDLC Exchange of information supervisory and unnumbered

frames Information - data to be transmitted to user (next layer up)

Flow and error control piggybacked on information frames Supervisory - ARQ when piggyback not used Unnumbered - supplementary link control

Three phases Initialization Data transfer Disconnect

Uses bit stuffing Flag delimit frame at both ends 01111110 is the Flag May close one frame and open another Bit stuffing to find flag data

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Internet Datalink Protocol PPP Design Requirements [RFC 1557]

Packet framing encapsulation of network-layer datagram in data link frame Carry network layer data of any network layer protocol (not just IP) at

same time Ability to demultiplex upwards

Bit transparency must carry any bit pattern in the data field Error detection (no correction) Connection liveness detect signal link failure to network layer Network layer address negotiation endpoint can learnconfigure

each otherrsquos network address

PPP non-design requirements No error correctionrecovery No flow control Out of order delivery OK No need to support multipoint links (eg polling)

Error recovery flow control data re-ordering all relegated to higher layers

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PPP Data Frame

Flag delimiter (framing) Address does nothing (only one option) Control does nothing in the future possible multiple

control fields Protocol upper layer protocol to which frame delivered

(eg PPP-LCP IP IPCP etc) Info upper layer data being carried Check cyclic redundancy check for error detection

copy Kemal Akkaya

Byte Stuffing ldquoData transparencyrdquo requirement data field must be allowed to include

flag pattern lt01111110gt Q is received lt01111110gt data or flag

Sender adds (ldquostuffsrdquo) extra lt 01111110gt byte after each lt 01111110gt data byte How about bit stuffing

Receiver Two 01111110 bytes in a row Discard first byte continue data reception Single 01111110 flag byte

flag bytepatternin datato send

flag byte pattern plusstuffed byte in transmitted data

copy Kemal Akkaya

PPP Data Control Protocol Before exchanging

network-layer data data link peers must configure PPP link

(max frame length authentication)

learnconfigure network layer information

For IP carry IP Control Protocol (IPCP) messages (protocol field 8021) to configurelearn IP address

Diagram for bring a line up and down

copy Kemal Akkaya

Multiple Access Protocols Single shared broadcast channel Two or more simultaneous transmissions by nodes interference

Collision if node receives two or more signals at the same time

Multiple access protocol Distributed algorithm that determines how nodes share channel ie

determine when node can transmit Communication about channel sharing must use channel itself

No out-of-band channel for coordination

Ideal Multiple Access Protocol When one node wants to transmit it can send at rate R When M nodes want to transmit each can send at average rate RM Fully decentralized

No special node to coordinate transmissions No synchronization of clocks slots

Simple

copy Kemal Akkaya

MAC Protocols A taxonomy

Three broad classes

Channel Partitioning Divide channel into smaller ldquopiecesrdquo (time slots frequency code) Allocate piece to node for exclusive use

Random Access Channel not divided allow collisions ldquoRecoverrdquo from collisions

ldquoTaking turnsrdquo Nodes take turns Nodes with more to send can take longer turns

copy Kemal Akkaya

Channel Partitioning MAC protocols TDMA

TDMA Time division multiple access Access to channel in rounds Each station gets fixed length slot

Length = packet transmission time in each round

Unused slots go idle Example 6-station LAN

134 have packet Slots 256 idle

copy Kemal Akkaya

Channel Partitioning MAC protocols FDMA

FDMA Frequency division multiple access Channel spectrum divided into frequency bands Each station assigned fixed frequency band Unused transmission time in frequency bands go idle Example 6-station LAN 134 have packet frequency

bands 256 idle

frequ

ency

bands time

1

2

3

4

5

6

copy Kemal Akkaya

Random Access Protocols

When node has packet to send Transmit at full channel data rate R No a priori coordination among nodes

Two or more transmitting nodes ldquocollisionrdquo Random access MAC protocol specifies

How to detect collisions How to recover from collisions (eg via delayed retransmissions)

Examples of random access MAC protocols Slotted ALOHA ALOHA Carrier Sense Multiple Access (CSMA) CSMACollision Detection (CD) CSMACollision Avoidance (CA)

copy Kemal Akkaya

Slotted ALOHA

Pros Single active node can continuously

transmit at full rate of channel Highly decentralized only slots in

nodes need to be in sync Simple

Cons Collisions wasting slots Idle slots Nodes may be able to detect collision in

less than time to transmit packet Clock synchronization

Assumptions All frames same size Time is divided into equal size

slots time to transmit 1 frame Nodes start to transmit frames

only at beginning of slots Nodes are synchronized If 2 or more nodes transmit in

slot all nodes detect collision

Operation When node obtains fresh frame it

transmits in next slot No collision node can send new frame in

next slot If collision node retransmits frame in

each subsequent slot with prob p until success

copy Kemal Akkaya

Pure (unslotted) ALOHA Unslotted Aloha simpler no synchronization When frame first arrives

Transmit immediately

Collision probability increases Frame sent at t0 collides with other frames sent in [t0-1t0+1]

Even worse than slotted ALOHA

copy Kemal Akkaya

Collisions can still occur Propagation delay means two nodes may not hear each otherrsquos transmission Entire packet transmission time wasted Role of distance amp propagation delay in determining collision probability spatial layout of nodes

Idea listen before transmit If channel sensed idle

transmit entire frame If channel sensed busy defer

transmission Human analogy donrsquot

interrupt others

CSMA (Carrier Sense Multiple Access)

copy Kemal Akkaya

CSMACD (Collision Detection)

CSMACD carrier sensing deferral as in CSMA Collisions detected within short

time Send jam sequence Colliding transmissions aborted

reducing channel wastage

Collision detection

Easy in wired LANs measure signal strengths compare transmitted received signals

Difficult in wireless LANs receiver shut off while transmitting

Human analogy the polite conversationalist

copy Kemal Akkaya

ldquoTaking Turnsrdquo MAC protocols Channel partitioning MAC protocols

Share channel efficiently and fairly at high load Inefficient at low load

Delay in channel access 1N bandwidth allocated even if only 1 active node

Random access MAC protocols Efficient at low load

Single node can fully utilize channel

high load Collision overhead

ldquoTaking turnsrdquo protocols Look for best of both worlds

Two implementations Polling Token

copy Kemal Akkaya

ldquoTaking Turnsrdquo MAC protocolsPolling Master node ldquoinvitesrdquo slave

nodes to transmit in turn Concerns

Polling overhead Latency Single point of failure (master)

Token passing control token passed from

one node to next sequentially

token message concerns

token overhead latency single point of failure (token)

copy Kemal Akkaya

Summary of MAC protocols

What do you do with a shared media Channel Partitioning by time frequency or code

Time Division Frequency Division

Random partitioning (dynamic) ALOHA S-ALOHA CSMA CSMACD Carrier sensing easy in some technologies (wire) hard in others

(wireless) CSMACD used in Ethernet CSMACA used in 80211

Taking Turns Polling from a central site token passing

copy Kemal Akkaya

LAN technologies

Data link layer so farServicesError detectioncorrectionMultiple access

Next LAN technologiesAddressingEthernetHubs switchesPPP

copy Kemal Akkaya

MAC Addresses and ARP 32-bit IP address

Network-layer address Used to get datagram to

destination IP subnet

MAC (or LAN or physical or Ethernet) address Used to get datagram

from one interface to another physically-connected interface (same network)

48 bit MAC address (for most LANs) burned in the adapter ROM

Find your PCrsquos MAC address

1A-2F-BB-76-09-AD

58-23-D7-FA-20-B0

0C-C4-11-6F-E3-98

71-65-F7-2B-08-53

LAN(wired orwireless)

Each adapter on LAN has unique LAN address

Broadcast address =FF-FF-FF-FF-FF-FF = adapter

copy Kemal Akkaya

MAC Address (more)

MAC address allocation administered by IEEE Manufacturer buys portion of MAC address space (to

assure uniqueness) Analogy

(a) MAC address like Social Security Number

(b) IP address like postal address MAC flat address portability

Can move LAN card from one LAN to another

IP hierarchical address NOT portable Depends on IP subnet to which node is attached

copy Kemal Akkaya

ARP Address Resolution Protocol

Each IP node (Host Router) on LAN has ARP table

ARP Table IPMAC address mappings for some LAN nodes

lt IP address MAC address TTLgt TTL (Time To Live)

time after which address mapping will be forgotten (typically 20 min)

Question how to determineMAC address of Bknowing Brsquos IP address

1A-2F-BB-76-09-AD

58-23-D7-FA-20-B0

0C-C4-11-6F-E3-98

71-65-F7-2B-08-53

LAN

237196723

237196778

237196714

237196788

copy Kemal Akkaya

ARP protocol Same LAN (network) A wants to send datagram

to B and Brsquos MAC address not in Arsquos ARP table

A broadcasts ARP query packet containing Bs IP address Dest MAC address = FF-FF-

FF-FF-FF-FF All machines on LAN receive

ARP query

B receives ARP packet replies to A with its (Bs) MAC address Frame sent to Arsquos MAC

address (unicast)

A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) Soft state information that

times out (goes away) unless refreshed

ARP is ldquoplug-and-playrdquo Nodes create their ARP

tables without intervention from net administrator

copy Kemal Akkaya

Routing to another LAN Walkthrough send datagram from A to B via R

Assume A knows B IP address

Two ARP tables in router R one for each IP network (LAN) In routing table at source Host find router 111111111110 In ARP table at source find MAC address E6-E9-00-17-BB-4B etc A creates datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110

A creates link-layer frame with Rs MAC address as dest frame contains A-to-B IP datagram

Arsquos adapter sends frame Rrsquos adapter receives frame R removes IP datagram from Ethernet frame sees its destined to B R uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B

copy Kemal Akkaya

A

R

B

Routing to another LAN

copy Kemal Akkaya

EthernetldquoDominantrdquo wired LAN technology Cheap $20 for 100Mbs First widely used LAN technology Simpler cheaper than token LANs and ATM Kept up with speed race 10 Mbps ndash 10

Gbps

Metcalfersquos Ethernet sketch

Star Topology Bus topology popular

through mid 90s Now star topology prevails Connection choices hub or

switch (more later)

hub orswitch

copy Kemal Akkaya

Ethernet Frame Structure

Preamble 7 bytes with pattern 10101010 followed by one byte with pattern 10101011 Used to synchronize receiver sender clock rates

Addresses 6 bytes If adapter receives frame with matching destination address or with broadcast

address (eg ARP packet) it passes data in frame to net-layer protocol Otherwise adapter discards frame

Type Indicates the higher layer protocol (mostly IP but others may be supported

such as Novell IPX and AppleTalk) CRC

Checked at receiver if error is detected the frame is simply dropped

Sending adapter encapsulates IP datagram in Ethernet frame

copy Kemal Akkaya

Ethernet uses CSMACD Connectionless No handshaking between sending and receiving

adapter Unreliable receiving adapter doesnrsquot send acks or nacks to

sending adapter Stream of datagrams passed to network layer can have gaps Gaps will be filled if app is using TCP Otherwise app will see the gaps

No slots Adapter doesnrsquot transmit if it senses that some other adapter is

transmitting that is carrier sense Transmitting adapter aborts when it senses that another adapter is

transmitting that is collision detection Before attempting a retransmission adapter waits a random time

that is random access

copy Kemal Akkaya

Ethernet CSMACD algorithm1 Adaptor receives datagram from

net layer amp creates frame2 If adapter senses channel idle it

starts to transmit frame If it senses channel busy waits until channel idle and then transmits

3 If adapter transmits entire frame without detecting another transmission the adapter is done with frame

4 If adapter detects another transmission while transmitting aborts and sends jam signal

5 After aborting adapter enters exponential backoff after the mth collision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2

Jam Signal make sure all other transmitters are aware of collision 48 bits

Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec

Exponential Backoff Goal adapt retransmission

attempts to estimated current load Heavy load random wait will be

longer

First collision choose K from 01 delay is K 512 bit transmission times

After second collision choose K from 0123hellip

After ten collisions choose K from 01234hellip1023

copy Kemal Akkaya

CSMACD efficiency Tprop = max prop between 2 nodes in LAN ttrans = time to transmit max-size frame

Efficiency goes to 1 as tprop goes to 0 Goes to 1 as ttrans goes to infinity Much better than ALOHA but still decentralized

simple and cheap

Other Ethernets Switched Ethernet Fast Ethernet Gigabit Ethernet

transprop tt 51

1efficiency

copy Kemal Akkaya

Point to Point Data Link Control

One sender one receiver one link easier than broadcast link No Media Access Control No need for explicit MAC addressing eg dialup link ISDN line

Popular point-to-point DLC protocols PPP (point-to-point protocol) HDLC High level data link control (Data link used to be considered

ldquohigh layerrdquo in protocol stack

copy Kemal Akkaya

HDLC

Frame format for bit-oriented protocol HDLC

Control field of

(a) An information frame

(b) A supervisory frame

(c) An unnumbered frame

copy Kemal Akkaya

HDLC Exchange of information supervisory and unnumbered

frames Information - data to be transmitted to user (next layer up)

Flow and error control piggybacked on information frames Supervisory - ARQ when piggyback not used Unnumbered - supplementary link control

Three phases Initialization Data transfer Disconnect

Uses bit stuffing Flag delimit frame at both ends 01111110 is the Flag May close one frame and open another Bit stuffing to find flag data

copy Kemal Akkaya

Internet Datalink Protocol PPP Design Requirements [RFC 1557]

Packet framing encapsulation of network-layer datagram in data link frame Carry network layer data of any network layer protocol (not just IP) at

same time Ability to demultiplex upwards

Bit transparency must carry any bit pattern in the data field Error detection (no correction) Connection liveness detect signal link failure to network layer Network layer address negotiation endpoint can learnconfigure

each otherrsquos network address

PPP non-design requirements No error correctionrecovery No flow control Out of order delivery OK No need to support multipoint links (eg polling)

Error recovery flow control data re-ordering all relegated to higher layers

copy Kemal Akkaya

PPP Data Frame

Flag delimiter (framing) Address does nothing (only one option) Control does nothing in the future possible multiple

control fields Protocol upper layer protocol to which frame delivered

(eg PPP-LCP IP IPCP etc) Info upper layer data being carried Check cyclic redundancy check for error detection

copy Kemal Akkaya

Byte Stuffing ldquoData transparencyrdquo requirement data field must be allowed to include

flag pattern lt01111110gt Q is received lt01111110gt data or flag

Sender adds (ldquostuffsrdquo) extra lt 01111110gt byte after each lt 01111110gt data byte How about bit stuffing

Receiver Two 01111110 bytes in a row Discard first byte continue data reception Single 01111110 flag byte

flag bytepatternin datato send

flag byte pattern plusstuffed byte in transmitted data

copy Kemal Akkaya

PPP Data Control Protocol Before exchanging

network-layer data data link peers must configure PPP link

(max frame length authentication)

learnconfigure network layer information

For IP carry IP Control Protocol (IPCP) messages (protocol field 8021) to configurelearn IP address

Diagram for bring a line up and down

copy Kemal Akkaya

MAC Protocols A taxonomy

Three broad classes

Channel Partitioning Divide channel into smaller ldquopiecesrdquo (time slots frequency code) Allocate piece to node for exclusive use

Random Access Channel not divided allow collisions ldquoRecoverrdquo from collisions

ldquoTaking turnsrdquo Nodes take turns Nodes with more to send can take longer turns

copy Kemal Akkaya

Channel Partitioning MAC protocols TDMA

TDMA Time division multiple access Access to channel in rounds Each station gets fixed length slot

Length = packet transmission time in each round

Unused slots go idle Example 6-station LAN

134 have packet Slots 256 idle

copy Kemal Akkaya

Channel Partitioning MAC protocols FDMA

FDMA Frequency division multiple access Channel spectrum divided into frequency bands Each station assigned fixed frequency band Unused transmission time in frequency bands go idle Example 6-station LAN 134 have packet frequency

bands 256 idle

frequ

ency

bands time

1

2

3

4

5

6

copy Kemal Akkaya

Random Access Protocols

When node has packet to send Transmit at full channel data rate R No a priori coordination among nodes

Two or more transmitting nodes ldquocollisionrdquo Random access MAC protocol specifies

How to detect collisions How to recover from collisions (eg via delayed retransmissions)

Examples of random access MAC protocols Slotted ALOHA ALOHA Carrier Sense Multiple Access (CSMA) CSMACollision Detection (CD) CSMACollision Avoidance (CA)

copy Kemal Akkaya

Slotted ALOHA

Pros Single active node can continuously

transmit at full rate of channel Highly decentralized only slots in

nodes need to be in sync Simple

Cons Collisions wasting slots Idle slots Nodes may be able to detect collision in

less than time to transmit packet Clock synchronization

Assumptions All frames same size Time is divided into equal size

slots time to transmit 1 frame Nodes start to transmit frames

only at beginning of slots Nodes are synchronized If 2 or more nodes transmit in

slot all nodes detect collision

Operation When node obtains fresh frame it

transmits in next slot No collision node can send new frame in

next slot If collision node retransmits frame in

each subsequent slot with prob p until success

copy Kemal Akkaya

Pure (unslotted) ALOHA Unslotted Aloha simpler no synchronization When frame first arrives

Transmit immediately

Collision probability increases Frame sent at t0 collides with other frames sent in [t0-1t0+1]

Even worse than slotted ALOHA

copy Kemal Akkaya

Collisions can still occur Propagation delay means two nodes may not hear each otherrsquos transmission Entire packet transmission time wasted Role of distance amp propagation delay in determining collision probability spatial layout of nodes

Idea listen before transmit If channel sensed idle

transmit entire frame If channel sensed busy defer

transmission Human analogy donrsquot

interrupt others

CSMA (Carrier Sense Multiple Access)

copy Kemal Akkaya

CSMACD (Collision Detection)

CSMACD carrier sensing deferral as in CSMA Collisions detected within short

time Send jam sequence Colliding transmissions aborted

reducing channel wastage

Collision detection

Easy in wired LANs measure signal strengths compare transmitted received signals

Difficult in wireless LANs receiver shut off while transmitting

Human analogy the polite conversationalist

copy Kemal Akkaya

ldquoTaking Turnsrdquo MAC protocols Channel partitioning MAC protocols

Share channel efficiently and fairly at high load Inefficient at low load

Delay in channel access 1N bandwidth allocated even if only 1 active node

Random access MAC protocols Efficient at low load

Single node can fully utilize channel

high load Collision overhead

ldquoTaking turnsrdquo protocols Look for best of both worlds

Two implementations Polling Token

copy Kemal Akkaya

ldquoTaking Turnsrdquo MAC protocolsPolling Master node ldquoinvitesrdquo slave

nodes to transmit in turn Concerns

Polling overhead Latency Single point of failure (master)

Token passing control token passed from

one node to next sequentially

token message concerns

token overhead latency single point of failure (token)

copy Kemal Akkaya

Summary of MAC protocols

What do you do with a shared media Channel Partitioning by time frequency or code

Time Division Frequency Division

Random partitioning (dynamic) ALOHA S-ALOHA CSMA CSMACD Carrier sensing easy in some technologies (wire) hard in others

(wireless) CSMACD used in Ethernet CSMACA used in 80211

Taking Turns Polling from a central site token passing

copy Kemal Akkaya

LAN technologies

Data link layer so farServicesError detectioncorrectionMultiple access

Next LAN technologiesAddressingEthernetHubs switchesPPP

copy Kemal Akkaya

MAC Addresses and ARP 32-bit IP address

Network-layer address Used to get datagram to

destination IP subnet

MAC (or LAN or physical or Ethernet) address Used to get datagram

from one interface to another physically-connected interface (same network)

48 bit MAC address (for most LANs) burned in the adapter ROM

Find your PCrsquos MAC address

1A-2F-BB-76-09-AD

58-23-D7-FA-20-B0

0C-C4-11-6F-E3-98

71-65-F7-2B-08-53

LAN(wired orwireless)

Each adapter on LAN has unique LAN address

Broadcast address =FF-FF-FF-FF-FF-FF = adapter

copy Kemal Akkaya

MAC Address (more)

MAC address allocation administered by IEEE Manufacturer buys portion of MAC address space (to

assure uniqueness) Analogy

(a) MAC address like Social Security Number

(b) IP address like postal address MAC flat address portability

Can move LAN card from one LAN to another

IP hierarchical address NOT portable Depends on IP subnet to which node is attached

copy Kemal Akkaya

ARP Address Resolution Protocol

Each IP node (Host Router) on LAN has ARP table

ARP Table IPMAC address mappings for some LAN nodes

lt IP address MAC address TTLgt TTL (Time To Live)

time after which address mapping will be forgotten (typically 20 min)

Question how to determineMAC address of Bknowing Brsquos IP address

1A-2F-BB-76-09-AD

58-23-D7-FA-20-B0

0C-C4-11-6F-E3-98

71-65-F7-2B-08-53

LAN

237196723

237196778

237196714

237196788

copy Kemal Akkaya

ARP protocol Same LAN (network) A wants to send datagram

to B and Brsquos MAC address not in Arsquos ARP table

A broadcasts ARP query packet containing Bs IP address Dest MAC address = FF-FF-

FF-FF-FF-FF All machines on LAN receive

ARP query

B receives ARP packet replies to A with its (Bs) MAC address Frame sent to Arsquos MAC

address (unicast)

A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) Soft state information that

times out (goes away) unless refreshed

ARP is ldquoplug-and-playrdquo Nodes create their ARP

tables without intervention from net administrator

copy Kemal Akkaya

Routing to another LAN Walkthrough send datagram from A to B via R

Assume A knows B IP address

Two ARP tables in router R one for each IP network (LAN) In routing table at source Host find router 111111111110 In ARP table at source find MAC address E6-E9-00-17-BB-4B etc A creates datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110

A creates link-layer frame with Rs MAC address as dest frame contains A-to-B IP datagram

Arsquos adapter sends frame Rrsquos adapter receives frame R removes IP datagram from Ethernet frame sees its destined to B R uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B

copy Kemal Akkaya

A

R

B

Routing to another LAN

copy Kemal Akkaya

EthernetldquoDominantrdquo wired LAN technology Cheap $20 for 100Mbs First widely used LAN technology Simpler cheaper than token LANs and ATM Kept up with speed race 10 Mbps ndash 10

Gbps

Metcalfersquos Ethernet sketch

Star Topology Bus topology popular

through mid 90s Now star topology prevails Connection choices hub or

switch (more later)

hub orswitch

copy Kemal Akkaya

Ethernet Frame Structure

Preamble 7 bytes with pattern 10101010 followed by one byte with pattern 10101011 Used to synchronize receiver sender clock rates

Addresses 6 bytes If adapter receives frame with matching destination address or with broadcast

address (eg ARP packet) it passes data in frame to net-layer protocol Otherwise adapter discards frame

Type Indicates the higher layer protocol (mostly IP but others may be supported

such as Novell IPX and AppleTalk) CRC

Checked at receiver if error is detected the frame is simply dropped

Sending adapter encapsulates IP datagram in Ethernet frame

copy Kemal Akkaya

Ethernet uses CSMACD Connectionless No handshaking between sending and receiving

adapter Unreliable receiving adapter doesnrsquot send acks or nacks to

sending adapter Stream of datagrams passed to network layer can have gaps Gaps will be filled if app is using TCP Otherwise app will see the gaps

No slots Adapter doesnrsquot transmit if it senses that some other adapter is

transmitting that is carrier sense Transmitting adapter aborts when it senses that another adapter is

transmitting that is collision detection Before attempting a retransmission adapter waits a random time

that is random access

copy Kemal Akkaya

Ethernet CSMACD algorithm1 Adaptor receives datagram from

net layer amp creates frame2 If adapter senses channel idle it

starts to transmit frame If it senses channel busy waits until channel idle and then transmits

3 If adapter transmits entire frame without detecting another transmission the adapter is done with frame

4 If adapter detects another transmission while transmitting aborts and sends jam signal

5 After aborting adapter enters exponential backoff after the mth collision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2

Jam Signal make sure all other transmitters are aware of collision 48 bits

Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec

Exponential Backoff Goal adapt retransmission

attempts to estimated current load Heavy load random wait will be

longer

First collision choose K from 01 delay is K 512 bit transmission times

After second collision choose K from 0123hellip

After ten collisions choose K from 01234hellip1023

copy Kemal Akkaya

CSMACD efficiency Tprop = max prop between 2 nodes in LAN ttrans = time to transmit max-size frame

Efficiency goes to 1 as tprop goes to 0 Goes to 1 as ttrans goes to infinity Much better than ALOHA but still decentralized

simple and cheap

Other Ethernets Switched Ethernet Fast Ethernet Gigabit Ethernet

transprop tt 51

1efficiency

copy Kemal Akkaya

Point to Point Data Link Control

One sender one receiver one link easier than broadcast link No Media Access Control No need for explicit MAC addressing eg dialup link ISDN line

Popular point-to-point DLC protocols PPP (point-to-point protocol) HDLC High level data link control (Data link used to be considered

ldquohigh layerrdquo in protocol stack

copy Kemal Akkaya

HDLC

Frame format for bit-oriented protocol HDLC

Control field of

(a) An information frame

(b) A supervisory frame

(c) An unnumbered frame

copy Kemal Akkaya

HDLC Exchange of information supervisory and unnumbered

frames Information - data to be transmitted to user (next layer up)

Flow and error control piggybacked on information frames Supervisory - ARQ when piggyback not used Unnumbered - supplementary link control

Three phases Initialization Data transfer Disconnect

Uses bit stuffing Flag delimit frame at both ends 01111110 is the Flag May close one frame and open another Bit stuffing to find flag data

copy Kemal Akkaya

Internet Datalink Protocol PPP Design Requirements [RFC 1557]

Packet framing encapsulation of network-layer datagram in data link frame Carry network layer data of any network layer protocol (not just IP) at

same time Ability to demultiplex upwards

Bit transparency must carry any bit pattern in the data field Error detection (no correction) Connection liveness detect signal link failure to network layer Network layer address negotiation endpoint can learnconfigure

each otherrsquos network address

PPP non-design requirements No error correctionrecovery No flow control Out of order delivery OK No need to support multipoint links (eg polling)

Error recovery flow control data re-ordering all relegated to higher layers

copy Kemal Akkaya

PPP Data Frame

Flag delimiter (framing) Address does nothing (only one option) Control does nothing in the future possible multiple

control fields Protocol upper layer protocol to which frame delivered

(eg PPP-LCP IP IPCP etc) Info upper layer data being carried Check cyclic redundancy check for error detection

copy Kemal Akkaya

Byte Stuffing ldquoData transparencyrdquo requirement data field must be allowed to include

flag pattern lt01111110gt Q is received lt01111110gt data or flag

Sender adds (ldquostuffsrdquo) extra lt 01111110gt byte after each lt 01111110gt data byte How about bit stuffing

Receiver Two 01111110 bytes in a row Discard first byte continue data reception Single 01111110 flag byte

flag bytepatternin datato send

flag byte pattern plusstuffed byte in transmitted data

copy Kemal Akkaya

PPP Data Control Protocol Before exchanging

network-layer data data link peers must configure PPP link

(max frame length authentication)

learnconfigure network layer information

For IP carry IP Control Protocol (IPCP) messages (protocol field 8021) to configurelearn IP address

Diagram for bring a line up and down

copy Kemal Akkaya

Channel Partitioning MAC protocols TDMA

TDMA Time division multiple access Access to channel in rounds Each station gets fixed length slot

Length = packet transmission time in each round

Unused slots go idle Example 6-station LAN

134 have packet Slots 256 idle

copy Kemal Akkaya

Channel Partitioning MAC protocols FDMA

FDMA Frequency division multiple access Channel spectrum divided into frequency bands Each station assigned fixed frequency band Unused transmission time in frequency bands go idle Example 6-station LAN 134 have packet frequency

bands 256 idle

frequ

ency

bands time

1

2

3

4

5

6

copy Kemal Akkaya

Random Access Protocols

When node has packet to send Transmit at full channel data rate R No a priori coordination among nodes

Two or more transmitting nodes ldquocollisionrdquo Random access MAC protocol specifies

How to detect collisions How to recover from collisions (eg via delayed retransmissions)

Examples of random access MAC protocols Slotted ALOHA ALOHA Carrier Sense Multiple Access (CSMA) CSMACollision Detection (CD) CSMACollision Avoidance (CA)

copy Kemal Akkaya

Slotted ALOHA

Pros Single active node can continuously

transmit at full rate of channel Highly decentralized only slots in

nodes need to be in sync Simple

Cons Collisions wasting slots Idle slots Nodes may be able to detect collision in

less than time to transmit packet Clock synchronization

Assumptions All frames same size Time is divided into equal size

slots time to transmit 1 frame Nodes start to transmit frames

only at beginning of slots Nodes are synchronized If 2 or more nodes transmit in

slot all nodes detect collision

Operation When node obtains fresh frame it

transmits in next slot No collision node can send new frame in

next slot If collision node retransmits frame in

each subsequent slot with prob p until success

copy Kemal Akkaya

Pure (unslotted) ALOHA Unslotted Aloha simpler no synchronization When frame first arrives

Transmit immediately

Collision probability increases Frame sent at t0 collides with other frames sent in [t0-1t0+1]

Even worse than slotted ALOHA

copy Kemal Akkaya

Collisions can still occur Propagation delay means two nodes may not hear each otherrsquos transmission Entire packet transmission time wasted Role of distance amp propagation delay in determining collision probability spatial layout of nodes

Idea listen before transmit If channel sensed idle

transmit entire frame If channel sensed busy defer

transmission Human analogy donrsquot

interrupt others

CSMA (Carrier Sense Multiple Access)

copy Kemal Akkaya

CSMACD (Collision Detection)

CSMACD carrier sensing deferral as in CSMA Collisions detected within short

time Send jam sequence Colliding transmissions aborted

reducing channel wastage

Collision detection

Easy in wired LANs measure signal strengths compare transmitted received signals

Difficult in wireless LANs receiver shut off while transmitting

Human analogy the polite conversationalist

copy Kemal Akkaya

ldquoTaking Turnsrdquo MAC protocols Channel partitioning MAC protocols

Share channel efficiently and fairly at high load Inefficient at low load

Delay in channel access 1N bandwidth allocated even if only 1 active node

Random access MAC protocols Efficient at low load

Single node can fully utilize channel

high load Collision overhead

ldquoTaking turnsrdquo protocols Look for best of both worlds

Two implementations Polling Token

copy Kemal Akkaya

ldquoTaking Turnsrdquo MAC protocolsPolling Master node ldquoinvitesrdquo slave

nodes to transmit in turn Concerns

Polling overhead Latency Single point of failure (master)

Token passing control token passed from

one node to next sequentially

token message concerns

token overhead latency single point of failure (token)

copy Kemal Akkaya

Summary of MAC protocols

What do you do with a shared media Channel Partitioning by time frequency or code

Time Division Frequency Division

Random partitioning (dynamic) ALOHA S-ALOHA CSMA CSMACD Carrier sensing easy in some technologies (wire) hard in others

(wireless) CSMACD used in Ethernet CSMACA used in 80211

Taking Turns Polling from a central site token passing

copy Kemal Akkaya

LAN technologies

Data link layer so farServicesError detectioncorrectionMultiple access

Next LAN technologiesAddressingEthernetHubs switchesPPP

copy Kemal Akkaya

MAC Addresses and ARP 32-bit IP address

Network-layer address Used to get datagram to

destination IP subnet

MAC (or LAN or physical or Ethernet) address Used to get datagram

from one interface to another physically-connected interface (same network)

48 bit MAC address (for most LANs) burned in the adapter ROM

Find your PCrsquos MAC address

1A-2F-BB-76-09-AD

58-23-D7-FA-20-B0

0C-C4-11-6F-E3-98

71-65-F7-2B-08-53

LAN(wired orwireless)

Each adapter on LAN has unique LAN address

Broadcast address =FF-FF-FF-FF-FF-FF = adapter

copy Kemal Akkaya

MAC Address (more)

MAC address allocation administered by IEEE Manufacturer buys portion of MAC address space (to

assure uniqueness) Analogy

(a) MAC address like Social Security Number

(b) IP address like postal address MAC flat address portability

Can move LAN card from one LAN to another

IP hierarchical address NOT portable Depends on IP subnet to which node is attached

copy Kemal Akkaya

ARP Address Resolution Protocol

Each IP node (Host Router) on LAN has ARP table

ARP Table IPMAC address mappings for some LAN nodes

lt IP address MAC address TTLgt TTL (Time To Live)

time after which address mapping will be forgotten (typically 20 min)

Question how to determineMAC address of Bknowing Brsquos IP address

1A-2F-BB-76-09-AD

58-23-D7-FA-20-B0

0C-C4-11-6F-E3-98

71-65-F7-2B-08-53

LAN

237196723

237196778

237196714

237196788

copy Kemal Akkaya

ARP protocol Same LAN (network) A wants to send datagram

to B and Brsquos MAC address not in Arsquos ARP table

A broadcasts ARP query packet containing Bs IP address Dest MAC address = FF-FF-

FF-FF-FF-FF All machines on LAN receive

ARP query

B receives ARP packet replies to A with its (Bs) MAC address Frame sent to Arsquos MAC

address (unicast)

A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) Soft state information that

times out (goes away) unless refreshed

ARP is ldquoplug-and-playrdquo Nodes create their ARP

tables without intervention from net administrator

copy Kemal Akkaya

Routing to another LAN Walkthrough send datagram from A to B via R

Assume A knows B IP address

Two ARP tables in router R one for each IP network (LAN) In routing table at source Host find router 111111111110 In ARP table at source find MAC address E6-E9-00-17-BB-4B etc A creates datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110

A creates link-layer frame with Rs MAC address as dest frame contains A-to-B IP datagram

Arsquos adapter sends frame Rrsquos adapter receives frame R removes IP datagram from Ethernet frame sees its destined to B R uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B

copy Kemal Akkaya

A

R

B

Routing to another LAN

copy Kemal Akkaya

EthernetldquoDominantrdquo wired LAN technology Cheap $20 for 100Mbs First widely used LAN technology Simpler cheaper than token LANs and ATM Kept up with speed race 10 Mbps ndash 10

Gbps

Metcalfersquos Ethernet sketch

Star Topology Bus topology popular

through mid 90s Now star topology prevails Connection choices hub or

switch (more later)

hub orswitch

copy Kemal Akkaya

Ethernet Frame Structure

Preamble 7 bytes with pattern 10101010 followed by one byte with pattern 10101011 Used to synchronize receiver sender clock rates

Addresses 6 bytes If adapter receives frame with matching destination address or with broadcast

address (eg ARP packet) it passes data in frame to net-layer protocol Otherwise adapter discards frame

Type Indicates the higher layer protocol (mostly IP but others may be supported

such as Novell IPX and AppleTalk) CRC

Checked at receiver if error is detected the frame is simply dropped

Sending adapter encapsulates IP datagram in Ethernet frame

copy Kemal Akkaya

Ethernet uses CSMACD Connectionless No handshaking between sending and receiving

adapter Unreliable receiving adapter doesnrsquot send acks or nacks to

sending adapter Stream of datagrams passed to network layer can have gaps Gaps will be filled if app is using TCP Otherwise app will see the gaps

No slots Adapter doesnrsquot transmit if it senses that some other adapter is

transmitting that is carrier sense Transmitting adapter aborts when it senses that another adapter is

transmitting that is collision detection Before attempting a retransmission adapter waits a random time

that is random access

copy Kemal Akkaya

Ethernet CSMACD algorithm1 Adaptor receives datagram from

net layer amp creates frame2 If adapter senses channel idle it

starts to transmit frame If it senses channel busy waits until channel idle and then transmits

3 If adapter transmits entire frame without detecting another transmission the adapter is done with frame

4 If adapter detects another transmission while transmitting aborts and sends jam signal

5 After aborting adapter enters exponential backoff after the mth collision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2

Jam Signal make sure all other transmitters are aware of collision 48 bits

Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec

Exponential Backoff Goal adapt retransmission

attempts to estimated current load Heavy load random wait will be

longer

First collision choose K from 01 delay is K 512 bit transmission times

After second collision choose K from 0123hellip

After ten collisions choose K from 01234hellip1023

copy Kemal Akkaya

CSMACD efficiency Tprop = max prop between 2 nodes in LAN ttrans = time to transmit max-size frame

Efficiency goes to 1 as tprop goes to 0 Goes to 1 as ttrans goes to infinity Much better than ALOHA but still decentralized

simple and cheap

Other Ethernets Switched Ethernet Fast Ethernet Gigabit Ethernet

transprop tt 51

1efficiency

copy Kemal Akkaya

Point to Point Data Link Control

One sender one receiver one link easier than broadcast link No Media Access Control No need for explicit MAC addressing eg dialup link ISDN line

Popular point-to-point DLC protocols PPP (point-to-point protocol) HDLC High level data link control (Data link used to be considered

ldquohigh layerrdquo in protocol stack

copy Kemal Akkaya

HDLC

Frame format for bit-oriented protocol HDLC

Control field of

(a) An information frame

(b) A supervisory frame

(c) An unnumbered frame

copy Kemal Akkaya

HDLC Exchange of information supervisory and unnumbered

frames Information - data to be transmitted to user (next layer up)

Flow and error control piggybacked on information frames Supervisory - ARQ when piggyback not used Unnumbered - supplementary link control

Three phases Initialization Data transfer Disconnect

Uses bit stuffing Flag delimit frame at both ends 01111110 is the Flag May close one frame and open another Bit stuffing to find flag data

copy Kemal Akkaya

Internet Datalink Protocol PPP Design Requirements [RFC 1557]

Packet framing encapsulation of network-layer datagram in data link frame Carry network layer data of any network layer protocol (not just IP) at

same time Ability to demultiplex upwards

Bit transparency must carry any bit pattern in the data field Error detection (no correction) Connection liveness detect signal link failure to network layer Network layer address negotiation endpoint can learnconfigure

each otherrsquos network address

PPP non-design requirements No error correctionrecovery No flow control Out of order delivery OK No need to support multipoint links (eg polling)

Error recovery flow control data re-ordering all relegated to higher layers

copy Kemal Akkaya

PPP Data Frame

Flag delimiter (framing) Address does nothing (only one option) Control does nothing in the future possible multiple

control fields Protocol upper layer protocol to which frame delivered

(eg PPP-LCP IP IPCP etc) Info upper layer data being carried Check cyclic redundancy check for error detection

copy Kemal Akkaya

Byte Stuffing ldquoData transparencyrdquo requirement data field must be allowed to include

flag pattern lt01111110gt Q is received lt01111110gt data or flag

Sender adds (ldquostuffsrdquo) extra lt 01111110gt byte after each lt 01111110gt data byte How about bit stuffing

Receiver Two 01111110 bytes in a row Discard first byte continue data reception Single 01111110 flag byte

flag bytepatternin datato send

flag byte pattern plusstuffed byte in transmitted data

copy Kemal Akkaya

PPP Data Control Protocol Before exchanging

network-layer data data link peers must configure PPP link

(max frame length authentication)

learnconfigure network layer information

For IP carry IP Control Protocol (IPCP) messages (protocol field 8021) to configurelearn IP address

Diagram for bring a line up and down

copy Kemal Akkaya

Channel Partitioning MAC protocols FDMA

FDMA Frequency division multiple access Channel spectrum divided into frequency bands Each station assigned fixed frequency band Unused transmission time in frequency bands go idle Example 6-station LAN 134 have packet frequency

bands 256 idle

frequ

ency

bands time

1

2

3

4

5

6

copy Kemal Akkaya

Random Access Protocols

When node has packet to send Transmit at full channel data rate R No a priori coordination among nodes

Two or more transmitting nodes ldquocollisionrdquo Random access MAC protocol specifies

How to detect collisions How to recover from collisions (eg via delayed retransmissions)

Examples of random access MAC protocols Slotted ALOHA ALOHA Carrier Sense Multiple Access (CSMA) CSMACollision Detection (CD) CSMACollision Avoidance (CA)

copy Kemal Akkaya

Slotted ALOHA

Pros Single active node can continuously

transmit at full rate of channel Highly decentralized only slots in

nodes need to be in sync Simple

Cons Collisions wasting slots Idle slots Nodes may be able to detect collision in

less than time to transmit packet Clock synchronization

Assumptions All frames same size Time is divided into equal size

slots time to transmit 1 frame Nodes start to transmit frames

only at beginning of slots Nodes are synchronized If 2 or more nodes transmit in

slot all nodes detect collision

Operation When node obtains fresh frame it

transmits in next slot No collision node can send new frame in

next slot If collision node retransmits frame in

each subsequent slot with prob p until success

copy Kemal Akkaya

Pure (unslotted) ALOHA Unslotted Aloha simpler no synchronization When frame first arrives

Transmit immediately

Collision probability increases Frame sent at t0 collides with other frames sent in [t0-1t0+1]

Even worse than slotted ALOHA

copy Kemal Akkaya

Collisions can still occur Propagation delay means two nodes may not hear each otherrsquos transmission Entire packet transmission time wasted Role of distance amp propagation delay in determining collision probability spatial layout of nodes

Idea listen before transmit If channel sensed idle

transmit entire frame If channel sensed busy defer

transmission Human analogy donrsquot

interrupt others

CSMA (Carrier Sense Multiple Access)

copy Kemal Akkaya

CSMACD (Collision Detection)

CSMACD carrier sensing deferral as in CSMA Collisions detected within short

time Send jam sequence Colliding transmissions aborted

reducing channel wastage

Collision detection

Easy in wired LANs measure signal strengths compare transmitted received signals

Difficult in wireless LANs receiver shut off while transmitting

Human analogy the polite conversationalist

copy Kemal Akkaya

ldquoTaking Turnsrdquo MAC protocols Channel partitioning MAC protocols

Share channel efficiently and fairly at high load Inefficient at low load

Delay in channel access 1N bandwidth allocated even if only 1 active node

Random access MAC protocols Efficient at low load

Single node can fully utilize channel

high load Collision overhead

ldquoTaking turnsrdquo protocols Look for best of both worlds

Two implementations Polling Token

copy Kemal Akkaya

ldquoTaking Turnsrdquo MAC protocolsPolling Master node ldquoinvitesrdquo slave

nodes to transmit in turn Concerns

Polling overhead Latency Single point of failure (master)

Token passing control token passed from

one node to next sequentially

token message concerns

token overhead latency single point of failure (token)

copy Kemal Akkaya

Summary of MAC protocols

What do you do with a shared media Channel Partitioning by time frequency or code

Time Division Frequency Division

Random partitioning (dynamic) ALOHA S-ALOHA CSMA CSMACD Carrier sensing easy in some technologies (wire) hard in others

(wireless) CSMACD used in Ethernet CSMACA used in 80211

Taking Turns Polling from a central site token passing

copy Kemal Akkaya

LAN technologies

Data link layer so farServicesError detectioncorrectionMultiple access

Next LAN technologiesAddressingEthernetHubs switchesPPP

copy Kemal Akkaya

MAC Addresses and ARP 32-bit IP address

Network-layer address Used to get datagram to

destination IP subnet

MAC (or LAN or physical or Ethernet) address Used to get datagram

from one interface to another physically-connected interface (same network)

48 bit MAC address (for most LANs) burned in the adapter ROM

Find your PCrsquos MAC address

1A-2F-BB-76-09-AD

58-23-D7-FA-20-B0

0C-C4-11-6F-E3-98

71-65-F7-2B-08-53

LAN(wired orwireless)

Each adapter on LAN has unique LAN address

Broadcast address =FF-FF-FF-FF-FF-FF = adapter

copy Kemal Akkaya

MAC Address (more)

MAC address allocation administered by IEEE Manufacturer buys portion of MAC address space (to

assure uniqueness) Analogy

(a) MAC address like Social Security Number

(b) IP address like postal address MAC flat address portability

Can move LAN card from one LAN to another

IP hierarchical address NOT portable Depends on IP subnet to which node is attached

copy Kemal Akkaya

ARP Address Resolution Protocol

Each IP node (Host Router) on LAN has ARP table

ARP Table IPMAC address mappings for some LAN nodes

lt IP address MAC address TTLgt TTL (Time To Live)

time after which address mapping will be forgotten (typically 20 min)

Question how to determineMAC address of Bknowing Brsquos IP address

1A-2F-BB-76-09-AD

58-23-D7-FA-20-B0

0C-C4-11-6F-E3-98

71-65-F7-2B-08-53

LAN

237196723

237196778

237196714

237196788

copy Kemal Akkaya

ARP protocol Same LAN (network) A wants to send datagram

to B and Brsquos MAC address not in Arsquos ARP table

A broadcasts ARP query packet containing Bs IP address Dest MAC address = FF-FF-

FF-FF-FF-FF All machines on LAN receive

ARP query

B receives ARP packet replies to A with its (Bs) MAC address Frame sent to Arsquos MAC

address (unicast)

A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) Soft state information that

times out (goes away) unless refreshed

ARP is ldquoplug-and-playrdquo Nodes create their ARP

tables without intervention from net administrator

copy Kemal Akkaya

Routing to another LAN Walkthrough send datagram from A to B via R

Assume A knows B IP address

Two ARP tables in router R one for each IP network (LAN) In routing table at source Host find router 111111111110 In ARP table at source find MAC address E6-E9-00-17-BB-4B etc A creates datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110

A creates link-layer frame with Rs MAC address as dest frame contains A-to-B IP datagram

Arsquos adapter sends frame Rrsquos adapter receives frame R removes IP datagram from Ethernet frame sees its destined to B R uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B

copy Kemal Akkaya

A

R

B

Routing to another LAN

copy Kemal Akkaya

EthernetldquoDominantrdquo wired LAN technology Cheap $20 for 100Mbs First widely used LAN technology Simpler cheaper than token LANs and ATM Kept up with speed race 10 Mbps ndash 10

Gbps

Metcalfersquos Ethernet sketch

Star Topology Bus topology popular

through mid 90s Now star topology prevails Connection choices hub or

switch (more later)

hub orswitch

copy Kemal Akkaya

Ethernet Frame Structure

Preamble 7 bytes with pattern 10101010 followed by one byte with pattern 10101011 Used to synchronize receiver sender clock rates

Addresses 6 bytes If adapter receives frame with matching destination address or with broadcast

address (eg ARP packet) it passes data in frame to net-layer protocol Otherwise adapter discards frame

Type Indicates the higher layer protocol (mostly IP but others may be supported

such as Novell IPX and AppleTalk) CRC

Checked at receiver if error is detected the frame is simply dropped

Sending adapter encapsulates IP datagram in Ethernet frame

copy Kemal Akkaya

Ethernet uses CSMACD Connectionless No handshaking between sending and receiving

adapter Unreliable receiving adapter doesnrsquot send acks or nacks to

sending adapter Stream of datagrams passed to network layer can have gaps Gaps will be filled if app is using TCP Otherwise app will see the gaps

No slots Adapter doesnrsquot transmit if it senses that some other adapter is

transmitting that is carrier sense Transmitting adapter aborts when it senses that another adapter is

transmitting that is collision detection Before attempting a retransmission adapter waits a random time

that is random access

copy Kemal Akkaya

Ethernet CSMACD algorithm1 Adaptor receives datagram from

net layer amp creates frame2 If adapter senses channel idle it

starts to transmit frame If it senses channel busy waits until channel idle and then transmits

3 If adapter transmits entire frame without detecting another transmission the adapter is done with frame

4 If adapter detects another transmission while transmitting aborts and sends jam signal

5 After aborting adapter enters exponential backoff after the mth collision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2

Jam Signal make sure all other transmitters are aware of collision 48 bits

Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec

Exponential Backoff Goal adapt retransmission

attempts to estimated current load Heavy load random wait will be

longer

First collision choose K from 01 delay is K 512 bit transmission times

After second collision choose K from 0123hellip

After ten collisions choose K from 01234hellip1023

copy Kemal Akkaya

CSMACD efficiency Tprop = max prop between 2 nodes in LAN ttrans = time to transmit max-size frame

Efficiency goes to 1 as tprop goes to 0 Goes to 1 as ttrans goes to infinity Much better than ALOHA but still decentralized

simple and cheap

Other Ethernets Switched Ethernet Fast Ethernet Gigabit Ethernet

transprop tt 51

1efficiency

copy Kemal Akkaya

Point to Point Data Link Control

One sender one receiver one link easier than broadcast link No Media Access Control No need for explicit MAC addressing eg dialup link ISDN line

Popular point-to-point DLC protocols PPP (point-to-point protocol) HDLC High level data link control (Data link used to be considered

ldquohigh layerrdquo in protocol stack

copy Kemal Akkaya

HDLC

Frame format for bit-oriented protocol HDLC

Control field of

(a) An information frame

(b) A supervisory frame

(c) An unnumbered frame

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HDLC Exchange of information supervisory and unnumbered

frames Information - data to be transmitted to user (next layer up)

Flow and error control piggybacked on information frames Supervisory - ARQ when piggyback not used Unnumbered - supplementary link control

Three phases Initialization Data transfer Disconnect

Uses bit stuffing Flag delimit frame at both ends 01111110 is the Flag May close one frame and open another Bit stuffing to find flag data

copy Kemal Akkaya

Internet Datalink Protocol PPP Design Requirements [RFC 1557]

Packet framing encapsulation of network-layer datagram in data link frame Carry network layer data of any network layer protocol (not just IP) at

same time Ability to demultiplex upwards

Bit transparency must carry any bit pattern in the data field Error detection (no correction) Connection liveness detect signal link failure to network layer Network layer address negotiation endpoint can learnconfigure

each otherrsquos network address

PPP non-design requirements No error correctionrecovery No flow control Out of order delivery OK No need to support multipoint links (eg polling)

Error recovery flow control data re-ordering all relegated to higher layers

copy Kemal Akkaya

PPP Data Frame

Flag delimiter (framing) Address does nothing (only one option) Control does nothing in the future possible multiple

control fields Protocol upper layer protocol to which frame delivered

(eg PPP-LCP IP IPCP etc) Info upper layer data being carried Check cyclic redundancy check for error detection

copy Kemal Akkaya

Byte Stuffing ldquoData transparencyrdquo requirement data field must be allowed to include

flag pattern lt01111110gt Q is received lt01111110gt data or flag

Sender adds (ldquostuffsrdquo) extra lt 01111110gt byte after each lt 01111110gt data byte How about bit stuffing

Receiver Two 01111110 bytes in a row Discard first byte continue data reception Single 01111110 flag byte

flag bytepatternin datato send

flag byte pattern plusstuffed byte in transmitted data

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PPP Data Control Protocol Before exchanging

network-layer data data link peers must configure PPP link

(max frame length authentication)

learnconfigure network layer information

For IP carry IP Control Protocol (IPCP) messages (protocol field 8021) to configurelearn IP address

Diagram for bring a line up and down

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Random Access Protocols

When node has packet to send Transmit at full channel data rate R No a priori coordination among nodes

Two or more transmitting nodes ldquocollisionrdquo Random access MAC protocol specifies

How to detect collisions How to recover from collisions (eg via delayed retransmissions)

Examples of random access MAC protocols Slotted ALOHA ALOHA Carrier Sense Multiple Access (CSMA) CSMACollision Detection (CD) CSMACollision Avoidance (CA)

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Slotted ALOHA

Pros Single active node can continuously

transmit at full rate of channel Highly decentralized only slots in

nodes need to be in sync Simple

Cons Collisions wasting slots Idle slots Nodes may be able to detect collision in

less than time to transmit packet Clock synchronization

Assumptions All frames same size Time is divided into equal size

slots time to transmit 1 frame Nodes start to transmit frames

only at beginning of slots Nodes are synchronized If 2 or more nodes transmit in

slot all nodes detect collision

Operation When node obtains fresh frame it

transmits in next slot No collision node can send new frame in

next slot If collision node retransmits frame in

each subsequent slot with prob p until success

copy Kemal Akkaya

Pure (unslotted) ALOHA Unslotted Aloha simpler no synchronization When frame first arrives

Transmit immediately

Collision probability increases Frame sent at t0 collides with other frames sent in [t0-1t0+1]

Even worse than slotted ALOHA

copy Kemal Akkaya

Collisions can still occur Propagation delay means two nodes may not hear each otherrsquos transmission Entire packet transmission time wasted Role of distance amp propagation delay in determining collision probability spatial layout of nodes

Idea listen before transmit If channel sensed idle

transmit entire frame If channel sensed busy defer

transmission Human analogy donrsquot

interrupt others

CSMA (Carrier Sense Multiple Access)

copy Kemal Akkaya

CSMACD (Collision Detection)

CSMACD carrier sensing deferral as in CSMA Collisions detected within short

time Send jam sequence Colliding transmissions aborted

reducing channel wastage

Collision detection

Easy in wired LANs measure signal strengths compare transmitted received signals

Difficult in wireless LANs receiver shut off while transmitting

Human analogy the polite conversationalist

copy Kemal Akkaya

ldquoTaking Turnsrdquo MAC protocols Channel partitioning MAC protocols

Share channel efficiently and fairly at high load Inefficient at low load

Delay in channel access 1N bandwidth allocated even if only 1 active node

Random access MAC protocols Efficient at low load

Single node can fully utilize channel

high load Collision overhead

ldquoTaking turnsrdquo protocols Look for best of both worlds

Two implementations Polling Token

copy Kemal Akkaya

ldquoTaking Turnsrdquo MAC protocolsPolling Master node ldquoinvitesrdquo slave

nodes to transmit in turn Concerns

Polling overhead Latency Single point of failure (master)

Token passing control token passed from

one node to next sequentially

token message concerns

token overhead latency single point of failure (token)

copy Kemal Akkaya

Summary of MAC protocols

What do you do with a shared media Channel Partitioning by time frequency or code

Time Division Frequency Division

Random partitioning (dynamic) ALOHA S-ALOHA CSMA CSMACD Carrier sensing easy in some technologies (wire) hard in others

(wireless) CSMACD used in Ethernet CSMACA used in 80211

Taking Turns Polling from a central site token passing

copy Kemal Akkaya

LAN technologies

Data link layer so farServicesError detectioncorrectionMultiple access

Next LAN technologiesAddressingEthernetHubs switchesPPP

copy Kemal Akkaya

MAC Addresses and ARP 32-bit IP address

Network-layer address Used to get datagram to

destination IP subnet

MAC (or LAN or physical or Ethernet) address Used to get datagram

from one interface to another physically-connected interface (same network)

48 bit MAC address (for most LANs) burned in the adapter ROM

Find your PCrsquos MAC address

1A-2F-BB-76-09-AD

58-23-D7-FA-20-B0

0C-C4-11-6F-E3-98

71-65-F7-2B-08-53

LAN(wired orwireless)

Each adapter on LAN has unique LAN address

Broadcast address =FF-FF-FF-FF-FF-FF = adapter

copy Kemal Akkaya

MAC Address (more)

MAC address allocation administered by IEEE Manufacturer buys portion of MAC address space (to

assure uniqueness) Analogy

(a) MAC address like Social Security Number

(b) IP address like postal address MAC flat address portability

Can move LAN card from one LAN to another

IP hierarchical address NOT portable Depends on IP subnet to which node is attached

copy Kemal Akkaya

ARP Address Resolution Protocol

Each IP node (Host Router) on LAN has ARP table

ARP Table IPMAC address mappings for some LAN nodes

lt IP address MAC address TTLgt TTL (Time To Live)

time after which address mapping will be forgotten (typically 20 min)

Question how to determineMAC address of Bknowing Brsquos IP address

1A-2F-BB-76-09-AD

58-23-D7-FA-20-B0

0C-C4-11-6F-E3-98

71-65-F7-2B-08-53

LAN

237196723

237196778

237196714

237196788

copy Kemal Akkaya

ARP protocol Same LAN (network) A wants to send datagram

to B and Brsquos MAC address not in Arsquos ARP table

A broadcasts ARP query packet containing Bs IP address Dest MAC address = FF-FF-

FF-FF-FF-FF All machines on LAN receive

ARP query

B receives ARP packet replies to A with its (Bs) MAC address Frame sent to Arsquos MAC

address (unicast)

A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) Soft state information that

times out (goes away) unless refreshed

ARP is ldquoplug-and-playrdquo Nodes create their ARP

tables without intervention from net administrator

copy Kemal Akkaya

Routing to another LAN Walkthrough send datagram from A to B via R

Assume A knows B IP address

Two ARP tables in router R one for each IP network (LAN) In routing table at source Host find router 111111111110 In ARP table at source find MAC address E6-E9-00-17-BB-4B etc A creates datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110

A creates link-layer frame with Rs MAC address as dest frame contains A-to-B IP datagram

Arsquos adapter sends frame Rrsquos adapter receives frame R removes IP datagram from Ethernet frame sees its destined to B R uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B

copy Kemal Akkaya

A

R

B

Routing to another LAN

copy Kemal Akkaya

EthernetldquoDominantrdquo wired LAN technology Cheap $20 for 100Mbs First widely used LAN technology Simpler cheaper than token LANs and ATM Kept up with speed race 10 Mbps ndash 10

Gbps

Metcalfersquos Ethernet sketch

Star Topology Bus topology popular

through mid 90s Now star topology prevails Connection choices hub or

switch (more later)

hub orswitch

copy Kemal Akkaya

Ethernet Frame Structure

Preamble 7 bytes with pattern 10101010 followed by one byte with pattern 10101011 Used to synchronize receiver sender clock rates

Addresses 6 bytes If adapter receives frame with matching destination address or with broadcast

address (eg ARP packet) it passes data in frame to net-layer protocol Otherwise adapter discards frame

Type Indicates the higher layer protocol (mostly IP but others may be supported

such as Novell IPX and AppleTalk) CRC

Checked at receiver if error is detected the frame is simply dropped

Sending adapter encapsulates IP datagram in Ethernet frame

copy Kemal Akkaya

Ethernet uses CSMACD Connectionless No handshaking between sending and receiving

adapter Unreliable receiving adapter doesnrsquot send acks or nacks to

sending adapter Stream of datagrams passed to network layer can have gaps Gaps will be filled if app is using TCP Otherwise app will see the gaps

No slots Adapter doesnrsquot transmit if it senses that some other adapter is

transmitting that is carrier sense Transmitting adapter aborts when it senses that another adapter is

transmitting that is collision detection Before attempting a retransmission adapter waits a random time

that is random access

copy Kemal Akkaya

Ethernet CSMACD algorithm1 Adaptor receives datagram from

net layer amp creates frame2 If adapter senses channel idle it

starts to transmit frame If it senses channel busy waits until channel idle and then transmits

3 If adapter transmits entire frame without detecting another transmission the adapter is done with frame

4 If adapter detects another transmission while transmitting aborts and sends jam signal

5 After aborting adapter enters exponential backoff after the mth collision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2

Jam Signal make sure all other transmitters are aware of collision 48 bits

Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec

Exponential Backoff Goal adapt retransmission

attempts to estimated current load Heavy load random wait will be

longer

First collision choose K from 01 delay is K 512 bit transmission times

After second collision choose K from 0123hellip

After ten collisions choose K from 01234hellip1023

copy Kemal Akkaya

CSMACD efficiency Tprop = max prop between 2 nodes in LAN ttrans = time to transmit max-size frame

Efficiency goes to 1 as tprop goes to 0 Goes to 1 as ttrans goes to infinity Much better than ALOHA but still decentralized

simple and cheap

Other Ethernets Switched Ethernet Fast Ethernet Gigabit Ethernet

transprop tt 51

1efficiency

copy Kemal Akkaya

Point to Point Data Link Control

One sender one receiver one link easier than broadcast link No Media Access Control No need for explicit MAC addressing eg dialup link ISDN line

Popular point-to-point DLC protocols PPP (point-to-point protocol) HDLC High level data link control (Data link used to be considered

ldquohigh layerrdquo in protocol stack

copy Kemal Akkaya

HDLC

Frame format for bit-oriented protocol HDLC

Control field of

(a) An information frame

(b) A supervisory frame

(c) An unnumbered frame

copy Kemal Akkaya

HDLC Exchange of information supervisory and unnumbered

frames Information - data to be transmitted to user (next layer up)

Flow and error control piggybacked on information frames Supervisory - ARQ when piggyback not used Unnumbered - supplementary link control

Three phases Initialization Data transfer Disconnect

Uses bit stuffing Flag delimit frame at both ends 01111110 is the Flag May close one frame and open another Bit stuffing to find flag data

copy Kemal Akkaya

Internet Datalink Protocol PPP Design Requirements [RFC 1557]

Packet framing encapsulation of network-layer datagram in data link frame Carry network layer data of any network layer protocol (not just IP) at

same time Ability to demultiplex upwards

Bit transparency must carry any bit pattern in the data field Error detection (no correction) Connection liveness detect signal link failure to network layer Network layer address negotiation endpoint can learnconfigure

each otherrsquos network address

PPP non-design requirements No error correctionrecovery No flow control Out of order delivery OK No need to support multipoint links (eg polling)

Error recovery flow control data re-ordering all relegated to higher layers

copy Kemal Akkaya

PPP Data Frame

Flag delimiter (framing) Address does nothing (only one option) Control does nothing in the future possible multiple

control fields Protocol upper layer protocol to which frame delivered

(eg PPP-LCP IP IPCP etc) Info upper layer data being carried Check cyclic redundancy check for error detection

copy Kemal Akkaya

Byte Stuffing ldquoData transparencyrdquo requirement data field must be allowed to include

flag pattern lt01111110gt Q is received lt01111110gt data or flag

Sender adds (ldquostuffsrdquo) extra lt 01111110gt byte after each lt 01111110gt data byte How about bit stuffing

Receiver Two 01111110 bytes in a row Discard first byte continue data reception Single 01111110 flag byte

flag bytepatternin datato send

flag byte pattern plusstuffed byte in transmitted data

copy Kemal Akkaya

PPP Data Control Protocol Before exchanging

network-layer data data link peers must configure PPP link

(max frame length authentication)

learnconfigure network layer information

For IP carry IP Control Protocol (IPCP) messages (protocol field 8021) to configurelearn IP address

Diagram for bring a line up and down

copy Kemal Akkaya

Slotted ALOHA

Pros Single active node can continuously

transmit at full rate of channel Highly decentralized only slots in

nodes need to be in sync Simple

Cons Collisions wasting slots Idle slots Nodes may be able to detect collision in

less than time to transmit packet Clock synchronization

Assumptions All frames same size Time is divided into equal size

slots time to transmit 1 frame Nodes start to transmit frames

only at beginning of slots Nodes are synchronized If 2 or more nodes transmit in

slot all nodes detect collision

Operation When node obtains fresh frame it

transmits in next slot No collision node can send new frame in

next slot If collision node retransmits frame in

each subsequent slot with prob p until success

copy Kemal Akkaya

Pure (unslotted) ALOHA Unslotted Aloha simpler no synchronization When frame first arrives

Transmit immediately

Collision probability increases Frame sent at t0 collides with other frames sent in [t0-1t0+1]

Even worse than slotted ALOHA

copy Kemal Akkaya

Collisions can still occur Propagation delay means two nodes may not hear each otherrsquos transmission Entire packet transmission time wasted Role of distance amp propagation delay in determining collision probability spatial layout of nodes

Idea listen before transmit If channel sensed idle

transmit entire frame If channel sensed busy defer

transmission Human analogy donrsquot

interrupt others

CSMA (Carrier Sense Multiple Access)

copy Kemal Akkaya

CSMACD (Collision Detection)

CSMACD carrier sensing deferral as in CSMA Collisions detected within short

time Send jam sequence Colliding transmissions aborted

reducing channel wastage

Collision detection

Easy in wired LANs measure signal strengths compare transmitted received signals

Difficult in wireless LANs receiver shut off while transmitting

Human analogy the polite conversationalist

copy Kemal Akkaya

ldquoTaking Turnsrdquo MAC protocols Channel partitioning MAC protocols

Share channel efficiently and fairly at high load Inefficient at low load

Delay in channel access 1N bandwidth allocated even if only 1 active node

Random access MAC protocols Efficient at low load

Single node can fully utilize channel

high load Collision overhead

ldquoTaking turnsrdquo protocols Look for best of both worlds

Two implementations Polling Token

copy Kemal Akkaya

ldquoTaking Turnsrdquo MAC protocolsPolling Master node ldquoinvitesrdquo slave

nodes to transmit in turn Concerns

Polling overhead Latency Single point of failure (master)

Token passing control token passed from

one node to next sequentially

token message concerns

token overhead latency single point of failure (token)

copy Kemal Akkaya

Summary of MAC protocols

What do you do with a shared media Channel Partitioning by time frequency or code

Time Division Frequency Division

Random partitioning (dynamic) ALOHA S-ALOHA CSMA CSMACD Carrier sensing easy in some technologies (wire) hard in others

(wireless) CSMACD used in Ethernet CSMACA used in 80211

Taking Turns Polling from a central site token passing

copy Kemal Akkaya

LAN technologies

Data link layer so farServicesError detectioncorrectionMultiple access

Next LAN technologiesAddressingEthernetHubs switchesPPP

copy Kemal Akkaya

MAC Addresses and ARP 32-bit IP address

Network-layer address Used to get datagram to

destination IP subnet

MAC (or LAN or physical or Ethernet) address Used to get datagram

from one interface to another physically-connected interface (same network)

48 bit MAC address (for most LANs) burned in the adapter ROM

Find your PCrsquos MAC address

1A-2F-BB-76-09-AD

58-23-D7-FA-20-B0

0C-C4-11-6F-E3-98

71-65-F7-2B-08-53

LAN(wired orwireless)

Each adapter on LAN has unique LAN address

Broadcast address =FF-FF-FF-FF-FF-FF = adapter

copy Kemal Akkaya

MAC Address (more)

MAC address allocation administered by IEEE Manufacturer buys portion of MAC address space (to

assure uniqueness) Analogy

(a) MAC address like Social Security Number

(b) IP address like postal address MAC flat address portability

Can move LAN card from one LAN to another

IP hierarchical address NOT portable Depends on IP subnet to which node is attached

copy Kemal Akkaya

ARP Address Resolution Protocol

Each IP node (Host Router) on LAN has ARP table

ARP Table IPMAC address mappings for some LAN nodes

lt IP address MAC address TTLgt TTL (Time To Live)

time after which address mapping will be forgotten (typically 20 min)

Question how to determineMAC address of Bknowing Brsquos IP address

1A-2F-BB-76-09-AD

58-23-D7-FA-20-B0

0C-C4-11-6F-E3-98

71-65-F7-2B-08-53

LAN

237196723

237196778

237196714

237196788

copy Kemal Akkaya

ARP protocol Same LAN (network) A wants to send datagram

to B and Brsquos MAC address not in Arsquos ARP table

A broadcasts ARP query packet containing Bs IP address Dest MAC address = FF-FF-

FF-FF-FF-FF All machines on LAN receive

ARP query

B receives ARP packet replies to A with its (Bs) MAC address Frame sent to Arsquos MAC

address (unicast)

A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) Soft state information that

times out (goes away) unless refreshed

ARP is ldquoplug-and-playrdquo Nodes create their ARP

tables without intervention from net administrator

copy Kemal Akkaya

Routing to another LAN Walkthrough send datagram from A to B via R

Assume A knows B IP address

Two ARP tables in router R one for each IP network (LAN) In routing table at source Host find router 111111111110 In ARP table at source find MAC address E6-E9-00-17-BB-4B etc A creates datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110

A creates link-layer frame with Rs MAC address as dest frame contains A-to-B IP datagram

Arsquos adapter sends frame Rrsquos adapter receives frame R removes IP datagram from Ethernet frame sees its destined to B R uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B

copy Kemal Akkaya

A

R

B

Routing to another LAN

copy Kemal Akkaya

EthernetldquoDominantrdquo wired LAN technology Cheap $20 for 100Mbs First widely used LAN technology Simpler cheaper than token LANs and ATM Kept up with speed race 10 Mbps ndash 10

Gbps

Metcalfersquos Ethernet sketch

Star Topology Bus topology popular

through mid 90s Now star topology prevails Connection choices hub or

switch (more later)

hub orswitch

copy Kemal Akkaya

Ethernet Frame Structure

Preamble 7 bytes with pattern 10101010 followed by one byte with pattern 10101011 Used to synchronize receiver sender clock rates

Addresses 6 bytes If adapter receives frame with matching destination address or with broadcast

address (eg ARP packet) it passes data in frame to net-layer protocol Otherwise adapter discards frame

Type Indicates the higher layer protocol (mostly IP but others may be supported

such as Novell IPX and AppleTalk) CRC

Checked at receiver if error is detected the frame is simply dropped

Sending adapter encapsulates IP datagram in Ethernet frame

copy Kemal Akkaya

Ethernet uses CSMACD Connectionless No handshaking between sending and receiving

adapter Unreliable receiving adapter doesnrsquot send acks or nacks to

sending adapter Stream of datagrams passed to network layer can have gaps Gaps will be filled if app is using TCP Otherwise app will see the gaps

No slots Adapter doesnrsquot transmit if it senses that some other adapter is

transmitting that is carrier sense Transmitting adapter aborts when it senses that another adapter is

transmitting that is collision detection Before attempting a retransmission adapter waits a random time

that is random access

copy Kemal Akkaya

Ethernet CSMACD algorithm1 Adaptor receives datagram from

net layer amp creates frame2 If adapter senses channel idle it

starts to transmit frame If it senses channel busy waits until channel idle and then transmits

3 If adapter transmits entire frame without detecting another transmission the adapter is done with frame

4 If adapter detects another transmission while transmitting aborts and sends jam signal

5 After aborting adapter enters exponential backoff after the mth collision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2

Jam Signal make sure all other transmitters are aware of collision 48 bits

Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec

Exponential Backoff Goal adapt retransmission

attempts to estimated current load Heavy load random wait will be

longer

First collision choose K from 01 delay is K 512 bit transmission times

After second collision choose K from 0123hellip

After ten collisions choose K from 01234hellip1023

copy Kemal Akkaya

CSMACD efficiency Tprop = max prop between 2 nodes in LAN ttrans = time to transmit max-size frame

Efficiency goes to 1 as tprop goes to 0 Goes to 1 as ttrans goes to infinity Much better than ALOHA but still decentralized

simple and cheap

Other Ethernets Switched Ethernet Fast Ethernet Gigabit Ethernet

transprop tt 51

1efficiency

copy Kemal Akkaya

Point to Point Data Link Control

One sender one receiver one link easier than broadcast link No Media Access Control No need for explicit MAC addressing eg dialup link ISDN line

Popular point-to-point DLC protocols PPP (point-to-point protocol) HDLC High level data link control (Data link used to be considered

ldquohigh layerrdquo in protocol stack

copy Kemal Akkaya

HDLC

Frame format for bit-oriented protocol HDLC

Control field of

(a) An information frame

(b) A supervisory frame

(c) An unnumbered frame

copy Kemal Akkaya

HDLC Exchange of information supervisory and unnumbered

frames Information - data to be transmitted to user (next layer up)

Flow and error control piggybacked on information frames Supervisory - ARQ when piggyback not used Unnumbered - supplementary link control

Three phases Initialization Data transfer Disconnect

Uses bit stuffing Flag delimit frame at both ends 01111110 is the Flag May close one frame and open another Bit stuffing to find flag data

copy Kemal Akkaya

Internet Datalink Protocol PPP Design Requirements [RFC 1557]

Packet framing encapsulation of network-layer datagram in data link frame Carry network layer data of any network layer protocol (not just IP) at

same time Ability to demultiplex upwards

Bit transparency must carry any bit pattern in the data field Error detection (no correction) Connection liveness detect signal link failure to network layer Network layer address negotiation endpoint can learnconfigure

each otherrsquos network address

PPP non-design requirements No error correctionrecovery No flow control Out of order delivery OK No need to support multipoint links (eg polling)

Error recovery flow control data re-ordering all relegated to higher layers

copy Kemal Akkaya

PPP Data Frame

Flag delimiter (framing) Address does nothing (only one option) Control does nothing in the future possible multiple

control fields Protocol upper layer protocol to which frame delivered

(eg PPP-LCP IP IPCP etc) Info upper layer data being carried Check cyclic redundancy check for error detection

copy Kemal Akkaya

Byte Stuffing ldquoData transparencyrdquo requirement data field must be allowed to include

flag pattern lt01111110gt Q is received lt01111110gt data or flag

Sender adds (ldquostuffsrdquo) extra lt 01111110gt byte after each lt 01111110gt data byte How about bit stuffing

Receiver Two 01111110 bytes in a row Discard first byte continue data reception Single 01111110 flag byte

flag bytepatternin datato send

flag byte pattern plusstuffed byte in transmitted data

copy Kemal Akkaya

PPP Data Control Protocol Before exchanging

network-layer data data link peers must configure PPP link

(max frame length authentication)

learnconfigure network layer information

For IP carry IP Control Protocol (IPCP) messages (protocol field 8021) to configurelearn IP address

Diagram for bring a line up and down

copy Kemal Akkaya

Pure (unslotted) ALOHA Unslotted Aloha simpler no synchronization When frame first arrives

Transmit immediately

Collision probability increases Frame sent at t0 collides with other frames sent in [t0-1t0+1]

Even worse than slotted ALOHA

copy Kemal Akkaya

Collisions can still occur Propagation delay means two nodes may not hear each otherrsquos transmission Entire packet transmission time wasted Role of distance amp propagation delay in determining collision probability spatial layout of nodes

Idea listen before transmit If channel sensed idle

transmit entire frame If channel sensed busy defer

transmission Human analogy donrsquot

interrupt others

CSMA (Carrier Sense Multiple Access)

copy Kemal Akkaya

CSMACD (Collision Detection)

CSMACD carrier sensing deferral as in CSMA Collisions detected within short

time Send jam sequence Colliding transmissions aborted

reducing channel wastage

Collision detection

Easy in wired LANs measure signal strengths compare transmitted received signals

Difficult in wireless LANs receiver shut off while transmitting

Human analogy the polite conversationalist

copy Kemal Akkaya

ldquoTaking Turnsrdquo MAC protocols Channel partitioning MAC protocols

Share channel efficiently and fairly at high load Inefficient at low load

Delay in channel access 1N bandwidth allocated even if only 1 active node

Random access MAC protocols Efficient at low load

Single node can fully utilize channel

high load Collision overhead

ldquoTaking turnsrdquo protocols Look for best of both worlds

Two implementations Polling Token

copy Kemal Akkaya

ldquoTaking Turnsrdquo MAC protocolsPolling Master node ldquoinvitesrdquo slave

nodes to transmit in turn Concerns

Polling overhead Latency Single point of failure (master)

Token passing control token passed from

one node to next sequentially

token message concerns

token overhead latency single point of failure (token)

copy Kemal Akkaya

Summary of MAC protocols

What do you do with a shared media Channel Partitioning by time frequency or code

Time Division Frequency Division

Random partitioning (dynamic) ALOHA S-ALOHA CSMA CSMACD Carrier sensing easy in some technologies (wire) hard in others

(wireless) CSMACD used in Ethernet CSMACA used in 80211

Taking Turns Polling from a central site token passing

copy Kemal Akkaya

LAN technologies

Data link layer so farServicesError detectioncorrectionMultiple access

Next LAN technologiesAddressingEthernetHubs switchesPPP

copy Kemal Akkaya

MAC Addresses and ARP 32-bit IP address

Network-layer address Used to get datagram to

destination IP subnet

MAC (or LAN or physical or Ethernet) address Used to get datagram

from one interface to another physically-connected interface (same network)

48 bit MAC address (for most LANs) burned in the adapter ROM

Find your PCrsquos MAC address

1A-2F-BB-76-09-AD

58-23-D7-FA-20-B0

0C-C4-11-6F-E3-98

71-65-F7-2B-08-53

LAN(wired orwireless)

Each adapter on LAN has unique LAN address

Broadcast address =FF-FF-FF-FF-FF-FF = adapter

copy Kemal Akkaya

MAC Address (more)

MAC address allocation administered by IEEE Manufacturer buys portion of MAC address space (to

assure uniqueness) Analogy

(a) MAC address like Social Security Number

(b) IP address like postal address MAC flat address portability

Can move LAN card from one LAN to another

IP hierarchical address NOT portable Depends on IP subnet to which node is attached

copy Kemal Akkaya

ARP Address Resolution Protocol

Each IP node (Host Router) on LAN has ARP table

ARP Table IPMAC address mappings for some LAN nodes

lt IP address MAC address TTLgt TTL (Time To Live)

time after which address mapping will be forgotten (typically 20 min)

Question how to determineMAC address of Bknowing Brsquos IP address

1A-2F-BB-76-09-AD

58-23-D7-FA-20-B0

0C-C4-11-6F-E3-98

71-65-F7-2B-08-53

LAN

237196723

237196778

237196714

237196788

copy Kemal Akkaya

ARP protocol Same LAN (network) A wants to send datagram

to B and Brsquos MAC address not in Arsquos ARP table

A broadcasts ARP query packet containing Bs IP address Dest MAC address = FF-FF-

FF-FF-FF-FF All machines on LAN receive

ARP query

B receives ARP packet replies to A with its (Bs) MAC address Frame sent to Arsquos MAC

address (unicast)

A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) Soft state information that

times out (goes away) unless refreshed

ARP is ldquoplug-and-playrdquo Nodes create their ARP

tables without intervention from net administrator

copy Kemal Akkaya

Routing to another LAN Walkthrough send datagram from A to B via R

Assume A knows B IP address

Two ARP tables in router R one for each IP network (LAN) In routing table at source Host find router 111111111110 In ARP table at source find MAC address E6-E9-00-17-BB-4B etc A creates datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110

A creates link-layer frame with Rs MAC address as dest frame contains A-to-B IP datagram

Arsquos adapter sends frame Rrsquos adapter receives frame R removes IP datagram from Ethernet frame sees its destined to B R uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B

copy Kemal Akkaya

A

R

B

Routing to another LAN

copy Kemal Akkaya

EthernetldquoDominantrdquo wired LAN technology Cheap $20 for 100Mbs First widely used LAN technology Simpler cheaper than token LANs and ATM Kept up with speed race 10 Mbps ndash 10

Gbps

Metcalfersquos Ethernet sketch

Star Topology Bus topology popular

through mid 90s Now star topology prevails Connection choices hub or

switch (more later)

hub orswitch

copy Kemal Akkaya

Ethernet Frame Structure

Preamble 7 bytes with pattern 10101010 followed by one byte with pattern 10101011 Used to synchronize receiver sender clock rates

Addresses 6 bytes If adapter receives frame with matching destination address or with broadcast

address (eg ARP packet) it passes data in frame to net-layer protocol Otherwise adapter discards frame

Type Indicates the higher layer protocol (mostly IP but others may be supported

such as Novell IPX and AppleTalk) CRC

Checked at receiver if error is detected the frame is simply dropped

Sending adapter encapsulates IP datagram in Ethernet frame

copy Kemal Akkaya

Ethernet uses CSMACD Connectionless No handshaking between sending and receiving

adapter Unreliable receiving adapter doesnrsquot send acks or nacks to

sending adapter Stream of datagrams passed to network layer can have gaps Gaps will be filled if app is using TCP Otherwise app will see the gaps

No slots Adapter doesnrsquot transmit if it senses that some other adapter is

transmitting that is carrier sense Transmitting adapter aborts when it senses that another adapter is

transmitting that is collision detection Before attempting a retransmission adapter waits a random time

that is random access

copy Kemal Akkaya

Ethernet CSMACD algorithm1 Adaptor receives datagram from

net layer amp creates frame2 If adapter senses channel idle it

starts to transmit frame If it senses channel busy waits until channel idle and then transmits

3 If adapter transmits entire frame without detecting another transmission the adapter is done with frame

4 If adapter detects another transmission while transmitting aborts and sends jam signal

5 After aborting adapter enters exponential backoff after the mth collision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2

Jam Signal make sure all other transmitters are aware of collision 48 bits

Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec

Exponential Backoff Goal adapt retransmission

attempts to estimated current load Heavy load random wait will be

longer

First collision choose K from 01 delay is K 512 bit transmission times

After second collision choose K from 0123hellip

After ten collisions choose K from 01234hellip1023

copy Kemal Akkaya

CSMACD efficiency Tprop = max prop between 2 nodes in LAN ttrans = time to transmit max-size frame

Efficiency goes to 1 as tprop goes to 0 Goes to 1 as ttrans goes to infinity Much better than ALOHA but still decentralized

simple and cheap

Other Ethernets Switched Ethernet Fast Ethernet Gigabit Ethernet

transprop tt 51

1efficiency

copy Kemal Akkaya

Point to Point Data Link Control

One sender one receiver one link easier than broadcast link No Media Access Control No need for explicit MAC addressing eg dialup link ISDN line

Popular point-to-point DLC protocols PPP (point-to-point protocol) HDLC High level data link control (Data link used to be considered

ldquohigh layerrdquo in protocol stack

copy Kemal Akkaya

HDLC

Frame format for bit-oriented protocol HDLC

Control field of

(a) An information frame

(b) A supervisory frame

(c) An unnumbered frame

copy Kemal Akkaya

HDLC Exchange of information supervisory and unnumbered

frames Information - data to be transmitted to user (next layer up)

Flow and error control piggybacked on information frames Supervisory - ARQ when piggyback not used Unnumbered - supplementary link control

Three phases Initialization Data transfer Disconnect

Uses bit stuffing Flag delimit frame at both ends 01111110 is the Flag May close one frame and open another Bit stuffing to find flag data

copy Kemal Akkaya

Internet Datalink Protocol PPP Design Requirements [RFC 1557]

Packet framing encapsulation of network-layer datagram in data link frame Carry network layer data of any network layer protocol (not just IP) at

same time Ability to demultiplex upwards

Bit transparency must carry any bit pattern in the data field Error detection (no correction) Connection liveness detect signal link failure to network layer Network layer address negotiation endpoint can learnconfigure

each otherrsquos network address

PPP non-design requirements No error correctionrecovery No flow control Out of order delivery OK No need to support multipoint links (eg polling)

Error recovery flow control data re-ordering all relegated to higher layers

copy Kemal Akkaya

PPP Data Frame

Flag delimiter (framing) Address does nothing (only one option) Control does nothing in the future possible multiple

control fields Protocol upper layer protocol to which frame delivered

(eg PPP-LCP IP IPCP etc) Info upper layer data being carried Check cyclic redundancy check for error detection

copy Kemal Akkaya

Byte Stuffing ldquoData transparencyrdquo requirement data field must be allowed to include

flag pattern lt01111110gt Q is received lt01111110gt data or flag

Sender adds (ldquostuffsrdquo) extra lt 01111110gt byte after each lt 01111110gt data byte How about bit stuffing

Receiver Two 01111110 bytes in a row Discard first byte continue data reception Single 01111110 flag byte

flag bytepatternin datato send

flag byte pattern plusstuffed byte in transmitted data

copy Kemal Akkaya

PPP Data Control Protocol Before exchanging

network-layer data data link peers must configure PPP link

(max frame length authentication)

learnconfigure network layer information

For IP carry IP Control Protocol (IPCP) messages (protocol field 8021) to configurelearn IP address

Diagram for bring a line up and down

copy Kemal Akkaya

Collisions can still occur Propagation delay means two nodes may not hear each otherrsquos transmission Entire packet transmission time wasted Role of distance amp propagation delay in determining collision probability spatial layout of nodes

Idea listen before transmit If channel sensed idle

transmit entire frame If channel sensed busy defer

transmission Human analogy donrsquot

interrupt others

CSMA (Carrier Sense Multiple Access)

copy Kemal Akkaya

CSMACD (Collision Detection)

CSMACD carrier sensing deferral as in CSMA Collisions detected within short

time Send jam sequence Colliding transmissions aborted

reducing channel wastage

Collision detection

Easy in wired LANs measure signal strengths compare transmitted received signals

Difficult in wireless LANs receiver shut off while transmitting

Human analogy the polite conversationalist

copy Kemal Akkaya

ldquoTaking Turnsrdquo MAC protocols Channel partitioning MAC protocols

Share channel efficiently and fairly at high load Inefficient at low load

Delay in channel access 1N bandwidth allocated even if only 1 active node

Random access MAC protocols Efficient at low load

Single node can fully utilize channel

high load Collision overhead

ldquoTaking turnsrdquo protocols Look for best of both worlds

Two implementations Polling Token

copy Kemal Akkaya

ldquoTaking Turnsrdquo MAC protocolsPolling Master node ldquoinvitesrdquo slave

nodes to transmit in turn Concerns

Polling overhead Latency Single point of failure (master)

Token passing control token passed from

one node to next sequentially

token message concerns

token overhead latency single point of failure (token)

copy Kemal Akkaya

Summary of MAC protocols

What do you do with a shared media Channel Partitioning by time frequency or code

Time Division Frequency Division

Random partitioning (dynamic) ALOHA S-ALOHA CSMA CSMACD Carrier sensing easy in some technologies (wire) hard in others

(wireless) CSMACD used in Ethernet CSMACA used in 80211

Taking Turns Polling from a central site token passing

copy Kemal Akkaya

LAN technologies

Data link layer so farServicesError detectioncorrectionMultiple access

Next LAN technologiesAddressingEthernetHubs switchesPPP

copy Kemal Akkaya

MAC Addresses and ARP 32-bit IP address

Network-layer address Used to get datagram to

destination IP subnet

MAC (or LAN or physical or Ethernet) address Used to get datagram

from one interface to another physically-connected interface (same network)

48 bit MAC address (for most LANs) burned in the adapter ROM

Find your PCrsquos MAC address

1A-2F-BB-76-09-AD

58-23-D7-FA-20-B0

0C-C4-11-6F-E3-98

71-65-F7-2B-08-53

LAN(wired orwireless)

Each adapter on LAN has unique LAN address

Broadcast address =FF-FF-FF-FF-FF-FF = adapter

copy Kemal Akkaya

MAC Address (more)

MAC address allocation administered by IEEE Manufacturer buys portion of MAC address space (to

assure uniqueness) Analogy

(a) MAC address like Social Security Number

(b) IP address like postal address MAC flat address portability

Can move LAN card from one LAN to another

IP hierarchical address NOT portable Depends on IP subnet to which node is attached

copy Kemal Akkaya

ARP Address Resolution Protocol

Each IP node (Host Router) on LAN has ARP table

ARP Table IPMAC address mappings for some LAN nodes

lt IP address MAC address TTLgt TTL (Time To Live)

time after which address mapping will be forgotten (typically 20 min)

Question how to determineMAC address of Bknowing Brsquos IP address

1A-2F-BB-76-09-AD

58-23-D7-FA-20-B0

0C-C4-11-6F-E3-98

71-65-F7-2B-08-53

LAN

237196723

237196778

237196714

237196788

copy Kemal Akkaya

ARP protocol Same LAN (network) A wants to send datagram

to B and Brsquos MAC address not in Arsquos ARP table

A broadcasts ARP query packet containing Bs IP address Dest MAC address = FF-FF-

FF-FF-FF-FF All machines on LAN receive

ARP query

B receives ARP packet replies to A with its (Bs) MAC address Frame sent to Arsquos MAC

address (unicast)

A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) Soft state information that

times out (goes away) unless refreshed

ARP is ldquoplug-and-playrdquo Nodes create their ARP

tables without intervention from net administrator

copy Kemal Akkaya

Routing to another LAN Walkthrough send datagram from A to B via R

Assume A knows B IP address

Two ARP tables in router R one for each IP network (LAN) In routing table at source Host find router 111111111110 In ARP table at source find MAC address E6-E9-00-17-BB-4B etc A creates datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110

A creates link-layer frame with Rs MAC address as dest frame contains A-to-B IP datagram

Arsquos adapter sends frame Rrsquos adapter receives frame R removes IP datagram from Ethernet frame sees its destined to B R uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B

copy Kemal Akkaya

A

R

B

Routing to another LAN

copy Kemal Akkaya

EthernetldquoDominantrdquo wired LAN technology Cheap $20 for 100Mbs First widely used LAN technology Simpler cheaper than token LANs and ATM Kept up with speed race 10 Mbps ndash 10

Gbps

Metcalfersquos Ethernet sketch

Star Topology Bus topology popular

through mid 90s Now star topology prevails Connection choices hub or

switch (more later)

hub orswitch

copy Kemal Akkaya

Ethernet Frame Structure

Preamble 7 bytes with pattern 10101010 followed by one byte with pattern 10101011 Used to synchronize receiver sender clock rates

Addresses 6 bytes If adapter receives frame with matching destination address or with broadcast

address (eg ARP packet) it passes data in frame to net-layer protocol Otherwise adapter discards frame

Type Indicates the higher layer protocol (mostly IP but others may be supported

such as Novell IPX and AppleTalk) CRC

Checked at receiver if error is detected the frame is simply dropped

Sending adapter encapsulates IP datagram in Ethernet frame

copy Kemal Akkaya

Ethernet uses CSMACD Connectionless No handshaking between sending and receiving

adapter Unreliable receiving adapter doesnrsquot send acks or nacks to

sending adapter Stream of datagrams passed to network layer can have gaps Gaps will be filled if app is using TCP Otherwise app will see the gaps

No slots Adapter doesnrsquot transmit if it senses that some other adapter is

transmitting that is carrier sense Transmitting adapter aborts when it senses that another adapter is

transmitting that is collision detection Before attempting a retransmission adapter waits a random time

that is random access

copy Kemal Akkaya

Ethernet CSMACD algorithm1 Adaptor receives datagram from

net layer amp creates frame2 If adapter senses channel idle it

starts to transmit frame If it senses channel busy waits until channel idle and then transmits

3 If adapter transmits entire frame without detecting another transmission the adapter is done with frame

4 If adapter detects another transmission while transmitting aborts and sends jam signal

5 After aborting adapter enters exponential backoff after the mth collision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2

Jam Signal make sure all other transmitters are aware of collision 48 bits

Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec

Exponential Backoff Goal adapt retransmission

attempts to estimated current load Heavy load random wait will be

longer

First collision choose K from 01 delay is K 512 bit transmission times

After second collision choose K from 0123hellip

After ten collisions choose K from 01234hellip1023

copy Kemal Akkaya

CSMACD efficiency Tprop = max prop between 2 nodes in LAN ttrans = time to transmit max-size frame

Efficiency goes to 1 as tprop goes to 0 Goes to 1 as ttrans goes to infinity Much better than ALOHA but still decentralized

simple and cheap

Other Ethernets Switched Ethernet Fast Ethernet Gigabit Ethernet

transprop tt 51

1efficiency

copy Kemal Akkaya

Point to Point Data Link Control

One sender one receiver one link easier than broadcast link No Media Access Control No need for explicit MAC addressing eg dialup link ISDN line

Popular point-to-point DLC protocols PPP (point-to-point protocol) HDLC High level data link control (Data link used to be considered

ldquohigh layerrdquo in protocol stack

copy Kemal Akkaya

HDLC

Frame format for bit-oriented protocol HDLC

Control field of

(a) An information frame

(b) A supervisory frame

(c) An unnumbered frame

copy Kemal Akkaya

HDLC Exchange of information supervisory and unnumbered

frames Information - data to be transmitted to user (next layer up)

Flow and error control piggybacked on information frames Supervisory - ARQ when piggyback not used Unnumbered - supplementary link control

Three phases Initialization Data transfer Disconnect

Uses bit stuffing Flag delimit frame at both ends 01111110 is the Flag May close one frame and open another Bit stuffing to find flag data

copy Kemal Akkaya

Internet Datalink Protocol PPP Design Requirements [RFC 1557]

Packet framing encapsulation of network-layer datagram in data link frame Carry network layer data of any network layer protocol (not just IP) at

same time Ability to demultiplex upwards

Bit transparency must carry any bit pattern in the data field Error detection (no correction) Connection liveness detect signal link failure to network layer Network layer address negotiation endpoint can learnconfigure

each otherrsquos network address

PPP non-design requirements No error correctionrecovery No flow control Out of order delivery OK No need to support multipoint links (eg polling)

Error recovery flow control data re-ordering all relegated to higher layers

copy Kemal Akkaya

PPP Data Frame

Flag delimiter (framing) Address does nothing (only one option) Control does nothing in the future possible multiple

control fields Protocol upper layer protocol to which frame delivered

(eg PPP-LCP IP IPCP etc) Info upper layer data being carried Check cyclic redundancy check for error detection

copy Kemal Akkaya

Byte Stuffing ldquoData transparencyrdquo requirement data field must be allowed to include

flag pattern lt01111110gt Q is received lt01111110gt data or flag

Sender adds (ldquostuffsrdquo) extra lt 01111110gt byte after each lt 01111110gt data byte How about bit stuffing

Receiver Two 01111110 bytes in a row Discard first byte continue data reception Single 01111110 flag byte

flag bytepatternin datato send

flag byte pattern plusstuffed byte in transmitted data

copy Kemal Akkaya

PPP Data Control Protocol Before exchanging

network-layer data data link peers must configure PPP link

(max frame length authentication)

learnconfigure network layer information

For IP carry IP Control Protocol (IPCP) messages (protocol field 8021) to configurelearn IP address

Diagram for bring a line up and down

copy Kemal Akkaya

CSMACD (Collision Detection)

CSMACD carrier sensing deferral as in CSMA Collisions detected within short

time Send jam sequence Colliding transmissions aborted

reducing channel wastage

Collision detection

Easy in wired LANs measure signal strengths compare transmitted received signals

Difficult in wireless LANs receiver shut off while transmitting

Human analogy the polite conversationalist

copy Kemal Akkaya

ldquoTaking Turnsrdquo MAC protocols Channel partitioning MAC protocols

Share channel efficiently and fairly at high load Inefficient at low load

Delay in channel access 1N bandwidth allocated even if only 1 active node

Random access MAC protocols Efficient at low load

Single node can fully utilize channel

high load Collision overhead

ldquoTaking turnsrdquo protocols Look for best of both worlds

Two implementations Polling Token

copy Kemal Akkaya

ldquoTaking Turnsrdquo MAC protocolsPolling Master node ldquoinvitesrdquo slave

nodes to transmit in turn Concerns

Polling overhead Latency Single point of failure (master)

Token passing control token passed from

one node to next sequentially

token message concerns

token overhead latency single point of failure (token)

copy Kemal Akkaya

Summary of MAC protocols

What do you do with a shared media Channel Partitioning by time frequency or code

Time Division Frequency Division

Random partitioning (dynamic) ALOHA S-ALOHA CSMA CSMACD Carrier sensing easy in some technologies (wire) hard in others

(wireless) CSMACD used in Ethernet CSMACA used in 80211

Taking Turns Polling from a central site token passing

copy Kemal Akkaya

LAN technologies

Data link layer so farServicesError detectioncorrectionMultiple access

Next LAN technologiesAddressingEthernetHubs switchesPPP

copy Kemal Akkaya

MAC Addresses and ARP 32-bit IP address

Network-layer address Used to get datagram to

destination IP subnet

MAC (or LAN or physical or Ethernet) address Used to get datagram

from one interface to another physically-connected interface (same network)

48 bit MAC address (for most LANs) burned in the adapter ROM

Find your PCrsquos MAC address

1A-2F-BB-76-09-AD

58-23-D7-FA-20-B0

0C-C4-11-6F-E3-98

71-65-F7-2B-08-53

LAN(wired orwireless)

Each adapter on LAN has unique LAN address

Broadcast address =FF-FF-FF-FF-FF-FF = adapter

copy Kemal Akkaya

MAC Address (more)

MAC address allocation administered by IEEE Manufacturer buys portion of MAC address space (to

assure uniqueness) Analogy

(a) MAC address like Social Security Number

(b) IP address like postal address MAC flat address portability

Can move LAN card from one LAN to another

IP hierarchical address NOT portable Depends on IP subnet to which node is attached

copy Kemal Akkaya

ARP Address Resolution Protocol

Each IP node (Host Router) on LAN has ARP table

ARP Table IPMAC address mappings for some LAN nodes

lt IP address MAC address TTLgt TTL (Time To Live)

time after which address mapping will be forgotten (typically 20 min)

Question how to determineMAC address of Bknowing Brsquos IP address

1A-2F-BB-76-09-AD

58-23-D7-FA-20-B0

0C-C4-11-6F-E3-98

71-65-F7-2B-08-53

LAN

237196723

237196778

237196714

237196788

copy Kemal Akkaya

ARP protocol Same LAN (network) A wants to send datagram

to B and Brsquos MAC address not in Arsquos ARP table

A broadcasts ARP query packet containing Bs IP address Dest MAC address = FF-FF-

FF-FF-FF-FF All machines on LAN receive

ARP query

B receives ARP packet replies to A with its (Bs) MAC address Frame sent to Arsquos MAC

address (unicast)

A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) Soft state information that

times out (goes away) unless refreshed

ARP is ldquoplug-and-playrdquo Nodes create their ARP

tables without intervention from net administrator

copy Kemal Akkaya

Routing to another LAN Walkthrough send datagram from A to B via R

Assume A knows B IP address

Two ARP tables in router R one for each IP network (LAN) In routing table at source Host find router 111111111110 In ARP table at source find MAC address E6-E9-00-17-BB-4B etc A creates datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110

A creates link-layer frame with Rs MAC address as dest frame contains A-to-B IP datagram

Arsquos adapter sends frame Rrsquos adapter receives frame R removes IP datagram from Ethernet frame sees its destined to B R uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B

copy Kemal Akkaya

A

R

B

Routing to another LAN

copy Kemal Akkaya

EthernetldquoDominantrdquo wired LAN technology Cheap $20 for 100Mbs First widely used LAN technology Simpler cheaper than token LANs and ATM Kept up with speed race 10 Mbps ndash 10

Gbps

Metcalfersquos Ethernet sketch

Star Topology Bus topology popular

through mid 90s Now star topology prevails Connection choices hub or

switch (more later)

hub orswitch

copy Kemal Akkaya

Ethernet Frame Structure

Preamble 7 bytes with pattern 10101010 followed by one byte with pattern 10101011 Used to synchronize receiver sender clock rates

Addresses 6 bytes If adapter receives frame with matching destination address or with broadcast

address (eg ARP packet) it passes data in frame to net-layer protocol Otherwise adapter discards frame

Type Indicates the higher layer protocol (mostly IP but others may be supported

such as Novell IPX and AppleTalk) CRC

Checked at receiver if error is detected the frame is simply dropped

Sending adapter encapsulates IP datagram in Ethernet frame

copy Kemal Akkaya

Ethernet uses CSMACD Connectionless No handshaking between sending and receiving

adapter Unreliable receiving adapter doesnrsquot send acks or nacks to

sending adapter Stream of datagrams passed to network layer can have gaps Gaps will be filled if app is using TCP Otherwise app will see the gaps

No slots Adapter doesnrsquot transmit if it senses that some other adapter is

transmitting that is carrier sense Transmitting adapter aborts when it senses that another adapter is

transmitting that is collision detection Before attempting a retransmission adapter waits a random time

that is random access

copy Kemal Akkaya

Ethernet CSMACD algorithm1 Adaptor receives datagram from

net layer amp creates frame2 If adapter senses channel idle it

starts to transmit frame If it senses channel busy waits until channel idle and then transmits

3 If adapter transmits entire frame without detecting another transmission the adapter is done with frame

4 If adapter detects another transmission while transmitting aborts and sends jam signal

5 After aborting adapter enters exponential backoff after the mth collision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2

Jam Signal make sure all other transmitters are aware of collision 48 bits

Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec

Exponential Backoff Goal adapt retransmission

attempts to estimated current load Heavy load random wait will be

longer

First collision choose K from 01 delay is K 512 bit transmission times

After second collision choose K from 0123hellip

After ten collisions choose K from 01234hellip1023

copy Kemal Akkaya

CSMACD efficiency Tprop = max prop between 2 nodes in LAN ttrans = time to transmit max-size frame

Efficiency goes to 1 as tprop goes to 0 Goes to 1 as ttrans goes to infinity Much better than ALOHA but still decentralized

simple and cheap

Other Ethernets Switched Ethernet Fast Ethernet Gigabit Ethernet

transprop tt 51

1efficiency

copy Kemal Akkaya

Point to Point Data Link Control

One sender one receiver one link easier than broadcast link No Media Access Control No need for explicit MAC addressing eg dialup link ISDN line

Popular point-to-point DLC protocols PPP (point-to-point protocol) HDLC High level data link control (Data link used to be considered

ldquohigh layerrdquo in protocol stack

copy Kemal Akkaya

HDLC

Frame format for bit-oriented protocol HDLC

Control field of

(a) An information frame

(b) A supervisory frame

(c) An unnumbered frame

copy Kemal Akkaya

HDLC Exchange of information supervisory and unnumbered

frames Information - data to be transmitted to user (next layer up)

Flow and error control piggybacked on information frames Supervisory - ARQ when piggyback not used Unnumbered - supplementary link control

Three phases Initialization Data transfer Disconnect

Uses bit stuffing Flag delimit frame at both ends 01111110 is the Flag May close one frame and open another Bit stuffing to find flag data

copy Kemal Akkaya

Internet Datalink Protocol PPP Design Requirements [RFC 1557]

Packet framing encapsulation of network-layer datagram in data link frame Carry network layer data of any network layer protocol (not just IP) at

same time Ability to demultiplex upwards

Bit transparency must carry any bit pattern in the data field Error detection (no correction) Connection liveness detect signal link failure to network layer Network layer address negotiation endpoint can learnconfigure

each otherrsquos network address

PPP non-design requirements No error correctionrecovery No flow control Out of order delivery OK No need to support multipoint links (eg polling)

Error recovery flow control data re-ordering all relegated to higher layers

copy Kemal Akkaya

PPP Data Frame

Flag delimiter (framing) Address does nothing (only one option) Control does nothing in the future possible multiple

control fields Protocol upper layer protocol to which frame delivered

(eg PPP-LCP IP IPCP etc) Info upper layer data being carried Check cyclic redundancy check for error detection

copy Kemal Akkaya

Byte Stuffing ldquoData transparencyrdquo requirement data field must be allowed to include

flag pattern lt01111110gt Q is received lt01111110gt data or flag

Sender adds (ldquostuffsrdquo) extra lt 01111110gt byte after each lt 01111110gt data byte How about bit stuffing

Receiver Two 01111110 bytes in a row Discard first byte continue data reception Single 01111110 flag byte

flag bytepatternin datato send

flag byte pattern plusstuffed byte in transmitted data

copy Kemal Akkaya

PPP Data Control Protocol Before exchanging

network-layer data data link peers must configure PPP link

(max frame length authentication)

learnconfigure network layer information

For IP carry IP Control Protocol (IPCP) messages (protocol field 8021) to configurelearn IP address

Diagram for bring a line up and down

copy Kemal Akkaya

ldquoTaking Turnsrdquo MAC protocols Channel partitioning MAC protocols

Share channel efficiently and fairly at high load Inefficient at low load

Delay in channel access 1N bandwidth allocated even if only 1 active node

Random access MAC protocols Efficient at low load

Single node can fully utilize channel

high load Collision overhead

ldquoTaking turnsrdquo protocols Look for best of both worlds

Two implementations Polling Token

copy Kemal Akkaya

ldquoTaking Turnsrdquo MAC protocolsPolling Master node ldquoinvitesrdquo slave

nodes to transmit in turn Concerns

Polling overhead Latency Single point of failure (master)

Token passing control token passed from

one node to next sequentially

token message concerns

token overhead latency single point of failure (token)

copy Kemal Akkaya

Summary of MAC protocols

What do you do with a shared media Channel Partitioning by time frequency or code

Time Division Frequency Division

Random partitioning (dynamic) ALOHA S-ALOHA CSMA CSMACD Carrier sensing easy in some technologies (wire) hard in others

(wireless) CSMACD used in Ethernet CSMACA used in 80211

Taking Turns Polling from a central site token passing

copy Kemal Akkaya

LAN technologies

Data link layer so farServicesError detectioncorrectionMultiple access

Next LAN technologiesAddressingEthernetHubs switchesPPP

copy Kemal Akkaya

MAC Addresses and ARP 32-bit IP address

Network-layer address Used to get datagram to

destination IP subnet

MAC (or LAN or physical or Ethernet) address Used to get datagram

from one interface to another physically-connected interface (same network)

48 bit MAC address (for most LANs) burned in the adapter ROM

Find your PCrsquos MAC address

1A-2F-BB-76-09-AD

58-23-D7-FA-20-B0

0C-C4-11-6F-E3-98

71-65-F7-2B-08-53

LAN(wired orwireless)

Each adapter on LAN has unique LAN address

Broadcast address =FF-FF-FF-FF-FF-FF = adapter

copy Kemal Akkaya

MAC Address (more)

MAC address allocation administered by IEEE Manufacturer buys portion of MAC address space (to

assure uniqueness) Analogy

(a) MAC address like Social Security Number

(b) IP address like postal address MAC flat address portability

Can move LAN card from one LAN to another

IP hierarchical address NOT portable Depends on IP subnet to which node is attached

copy Kemal Akkaya

ARP Address Resolution Protocol

Each IP node (Host Router) on LAN has ARP table

ARP Table IPMAC address mappings for some LAN nodes

lt IP address MAC address TTLgt TTL (Time To Live)

time after which address mapping will be forgotten (typically 20 min)

Question how to determineMAC address of Bknowing Brsquos IP address

1A-2F-BB-76-09-AD

58-23-D7-FA-20-B0

0C-C4-11-6F-E3-98

71-65-F7-2B-08-53

LAN

237196723

237196778

237196714

237196788

copy Kemal Akkaya

ARP protocol Same LAN (network) A wants to send datagram

to B and Brsquos MAC address not in Arsquos ARP table

A broadcasts ARP query packet containing Bs IP address Dest MAC address = FF-FF-

FF-FF-FF-FF All machines on LAN receive

ARP query

B receives ARP packet replies to A with its (Bs) MAC address Frame sent to Arsquos MAC

address (unicast)

A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) Soft state information that

times out (goes away) unless refreshed

ARP is ldquoplug-and-playrdquo Nodes create their ARP

tables without intervention from net administrator

copy Kemal Akkaya

Routing to another LAN Walkthrough send datagram from A to B via R

Assume A knows B IP address

Two ARP tables in router R one for each IP network (LAN) In routing table at source Host find router 111111111110 In ARP table at source find MAC address E6-E9-00-17-BB-4B etc A creates datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110

A creates link-layer frame with Rs MAC address as dest frame contains A-to-B IP datagram

Arsquos adapter sends frame Rrsquos adapter receives frame R removes IP datagram from Ethernet frame sees its destined to B R uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B

copy Kemal Akkaya

A

R

B

Routing to another LAN

copy Kemal Akkaya

EthernetldquoDominantrdquo wired LAN technology Cheap $20 for 100Mbs First widely used LAN technology Simpler cheaper than token LANs and ATM Kept up with speed race 10 Mbps ndash 10

Gbps

Metcalfersquos Ethernet sketch

Star Topology Bus topology popular

through mid 90s Now star topology prevails Connection choices hub or

switch (more later)

hub orswitch

copy Kemal Akkaya

Ethernet Frame Structure

Preamble 7 bytes with pattern 10101010 followed by one byte with pattern 10101011 Used to synchronize receiver sender clock rates

Addresses 6 bytes If adapter receives frame with matching destination address or with broadcast

address (eg ARP packet) it passes data in frame to net-layer protocol Otherwise adapter discards frame

Type Indicates the higher layer protocol (mostly IP but others may be supported

such as Novell IPX and AppleTalk) CRC

Checked at receiver if error is detected the frame is simply dropped

Sending adapter encapsulates IP datagram in Ethernet frame

copy Kemal Akkaya

Ethernet uses CSMACD Connectionless No handshaking between sending and receiving

adapter Unreliable receiving adapter doesnrsquot send acks or nacks to

sending adapter Stream of datagrams passed to network layer can have gaps Gaps will be filled if app is using TCP Otherwise app will see the gaps

No slots Adapter doesnrsquot transmit if it senses that some other adapter is

transmitting that is carrier sense Transmitting adapter aborts when it senses that another adapter is

transmitting that is collision detection Before attempting a retransmission adapter waits a random time

that is random access

copy Kemal Akkaya

Ethernet CSMACD algorithm1 Adaptor receives datagram from

net layer amp creates frame2 If adapter senses channel idle it

starts to transmit frame If it senses channel busy waits until channel idle and then transmits

3 If adapter transmits entire frame without detecting another transmission the adapter is done with frame

4 If adapter detects another transmission while transmitting aborts and sends jam signal

5 After aborting adapter enters exponential backoff after the mth collision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2

Jam Signal make sure all other transmitters are aware of collision 48 bits

Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec

Exponential Backoff Goal adapt retransmission

attempts to estimated current load Heavy load random wait will be

longer

First collision choose K from 01 delay is K 512 bit transmission times

After second collision choose K from 0123hellip

After ten collisions choose K from 01234hellip1023

copy Kemal Akkaya

CSMACD efficiency Tprop = max prop between 2 nodes in LAN ttrans = time to transmit max-size frame

Efficiency goes to 1 as tprop goes to 0 Goes to 1 as ttrans goes to infinity Much better than ALOHA but still decentralized

simple and cheap

Other Ethernets Switched Ethernet Fast Ethernet Gigabit Ethernet

transprop tt 51

1efficiency

copy Kemal Akkaya

Point to Point Data Link Control

One sender one receiver one link easier than broadcast link No Media Access Control No need for explicit MAC addressing eg dialup link ISDN line

Popular point-to-point DLC protocols PPP (point-to-point protocol) HDLC High level data link control (Data link used to be considered

ldquohigh layerrdquo in protocol stack

copy Kemal Akkaya

HDLC

Frame format for bit-oriented protocol HDLC

Control field of

(a) An information frame

(b) A supervisory frame

(c) An unnumbered frame

copy Kemal Akkaya

HDLC Exchange of information supervisory and unnumbered

frames Information - data to be transmitted to user (next layer up)

Flow and error control piggybacked on information frames Supervisory - ARQ when piggyback not used Unnumbered - supplementary link control

Three phases Initialization Data transfer Disconnect

Uses bit stuffing Flag delimit frame at both ends 01111110 is the Flag May close one frame and open another Bit stuffing to find flag data

copy Kemal Akkaya

Internet Datalink Protocol PPP Design Requirements [RFC 1557]

Packet framing encapsulation of network-layer datagram in data link frame Carry network layer data of any network layer protocol (not just IP) at

same time Ability to demultiplex upwards

Bit transparency must carry any bit pattern in the data field Error detection (no correction) Connection liveness detect signal link failure to network layer Network layer address negotiation endpoint can learnconfigure

each otherrsquos network address

PPP non-design requirements No error correctionrecovery No flow control Out of order delivery OK No need to support multipoint links (eg polling)

Error recovery flow control data re-ordering all relegated to higher layers

copy Kemal Akkaya

PPP Data Frame

Flag delimiter (framing) Address does nothing (only one option) Control does nothing in the future possible multiple

control fields Protocol upper layer protocol to which frame delivered

(eg PPP-LCP IP IPCP etc) Info upper layer data being carried Check cyclic redundancy check for error detection

copy Kemal Akkaya

Byte Stuffing ldquoData transparencyrdquo requirement data field must be allowed to include

flag pattern lt01111110gt Q is received lt01111110gt data or flag

Sender adds (ldquostuffsrdquo) extra lt 01111110gt byte after each lt 01111110gt data byte How about bit stuffing

Receiver Two 01111110 bytes in a row Discard first byte continue data reception Single 01111110 flag byte

flag bytepatternin datato send

flag byte pattern plusstuffed byte in transmitted data

copy Kemal Akkaya

PPP Data Control Protocol Before exchanging

network-layer data data link peers must configure PPP link

(max frame length authentication)

learnconfigure network layer information

For IP carry IP Control Protocol (IPCP) messages (protocol field 8021) to configurelearn IP address

Diagram for bring a line up and down

copy Kemal Akkaya

ldquoTaking Turnsrdquo MAC protocolsPolling Master node ldquoinvitesrdquo slave

nodes to transmit in turn Concerns

Polling overhead Latency Single point of failure (master)

Token passing control token passed from

one node to next sequentially

token message concerns

token overhead latency single point of failure (token)

copy Kemal Akkaya

Summary of MAC protocols

What do you do with a shared media Channel Partitioning by time frequency or code

Time Division Frequency Division

Random partitioning (dynamic) ALOHA S-ALOHA CSMA CSMACD Carrier sensing easy in some technologies (wire) hard in others

(wireless) CSMACD used in Ethernet CSMACA used in 80211

Taking Turns Polling from a central site token passing

copy Kemal Akkaya

LAN technologies

Data link layer so farServicesError detectioncorrectionMultiple access

Next LAN technologiesAddressingEthernetHubs switchesPPP

copy Kemal Akkaya

MAC Addresses and ARP 32-bit IP address

Network-layer address Used to get datagram to

destination IP subnet

MAC (or LAN or physical or Ethernet) address Used to get datagram

from one interface to another physically-connected interface (same network)

48 bit MAC address (for most LANs) burned in the adapter ROM

Find your PCrsquos MAC address

1A-2F-BB-76-09-AD

58-23-D7-FA-20-B0

0C-C4-11-6F-E3-98

71-65-F7-2B-08-53

LAN(wired orwireless)

Each adapter on LAN has unique LAN address

Broadcast address =FF-FF-FF-FF-FF-FF = adapter

copy Kemal Akkaya

MAC Address (more)

MAC address allocation administered by IEEE Manufacturer buys portion of MAC address space (to

assure uniqueness) Analogy

(a) MAC address like Social Security Number

(b) IP address like postal address MAC flat address portability

Can move LAN card from one LAN to another

IP hierarchical address NOT portable Depends on IP subnet to which node is attached

copy Kemal Akkaya

ARP Address Resolution Protocol

Each IP node (Host Router) on LAN has ARP table

ARP Table IPMAC address mappings for some LAN nodes

lt IP address MAC address TTLgt TTL (Time To Live)

time after which address mapping will be forgotten (typically 20 min)

Question how to determineMAC address of Bknowing Brsquos IP address

1A-2F-BB-76-09-AD

58-23-D7-FA-20-B0

0C-C4-11-6F-E3-98

71-65-F7-2B-08-53

LAN

237196723

237196778

237196714

237196788

copy Kemal Akkaya

ARP protocol Same LAN (network) A wants to send datagram

to B and Brsquos MAC address not in Arsquos ARP table

A broadcasts ARP query packet containing Bs IP address Dest MAC address = FF-FF-

FF-FF-FF-FF All machines on LAN receive

ARP query

B receives ARP packet replies to A with its (Bs) MAC address Frame sent to Arsquos MAC

address (unicast)

A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) Soft state information that

times out (goes away) unless refreshed

ARP is ldquoplug-and-playrdquo Nodes create their ARP

tables without intervention from net administrator

copy Kemal Akkaya

Routing to another LAN Walkthrough send datagram from A to B via R

Assume A knows B IP address

Two ARP tables in router R one for each IP network (LAN) In routing table at source Host find router 111111111110 In ARP table at source find MAC address E6-E9-00-17-BB-4B etc A creates datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110

A creates link-layer frame with Rs MAC address as dest frame contains A-to-B IP datagram

Arsquos adapter sends frame Rrsquos adapter receives frame R removes IP datagram from Ethernet frame sees its destined to B R uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B

copy Kemal Akkaya

A

R

B

Routing to another LAN

copy Kemal Akkaya

EthernetldquoDominantrdquo wired LAN technology Cheap $20 for 100Mbs First widely used LAN technology Simpler cheaper than token LANs and ATM Kept up with speed race 10 Mbps ndash 10

Gbps

Metcalfersquos Ethernet sketch

Star Topology Bus topology popular

through mid 90s Now star topology prevails Connection choices hub or

switch (more later)

hub orswitch

copy Kemal Akkaya

Ethernet Frame Structure

Preamble 7 bytes with pattern 10101010 followed by one byte with pattern 10101011 Used to synchronize receiver sender clock rates

Addresses 6 bytes If adapter receives frame with matching destination address or with broadcast

address (eg ARP packet) it passes data in frame to net-layer protocol Otherwise adapter discards frame

Type Indicates the higher layer protocol (mostly IP but others may be supported

such as Novell IPX and AppleTalk) CRC

Checked at receiver if error is detected the frame is simply dropped

Sending adapter encapsulates IP datagram in Ethernet frame

copy Kemal Akkaya

Ethernet uses CSMACD Connectionless No handshaking between sending and receiving

adapter Unreliable receiving adapter doesnrsquot send acks or nacks to

sending adapter Stream of datagrams passed to network layer can have gaps Gaps will be filled if app is using TCP Otherwise app will see the gaps

No slots Adapter doesnrsquot transmit if it senses that some other adapter is

transmitting that is carrier sense Transmitting adapter aborts when it senses that another adapter is

transmitting that is collision detection Before attempting a retransmission adapter waits a random time

that is random access

copy Kemal Akkaya

Ethernet CSMACD algorithm1 Adaptor receives datagram from

net layer amp creates frame2 If adapter senses channel idle it

starts to transmit frame If it senses channel busy waits until channel idle and then transmits

3 If adapter transmits entire frame without detecting another transmission the adapter is done with frame

4 If adapter detects another transmission while transmitting aborts and sends jam signal

5 After aborting adapter enters exponential backoff after the mth collision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2

Jam Signal make sure all other transmitters are aware of collision 48 bits

Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec

Exponential Backoff Goal adapt retransmission

attempts to estimated current load Heavy load random wait will be

longer

First collision choose K from 01 delay is K 512 bit transmission times

After second collision choose K from 0123hellip

After ten collisions choose K from 01234hellip1023

copy Kemal Akkaya

CSMACD efficiency Tprop = max prop between 2 nodes in LAN ttrans = time to transmit max-size frame

Efficiency goes to 1 as tprop goes to 0 Goes to 1 as ttrans goes to infinity Much better than ALOHA but still decentralized

simple and cheap

Other Ethernets Switched Ethernet Fast Ethernet Gigabit Ethernet

transprop tt 51

1efficiency

copy Kemal Akkaya

Point to Point Data Link Control

One sender one receiver one link easier than broadcast link No Media Access Control No need for explicit MAC addressing eg dialup link ISDN line

Popular point-to-point DLC protocols PPP (point-to-point protocol) HDLC High level data link control (Data link used to be considered

ldquohigh layerrdquo in protocol stack

copy Kemal Akkaya

HDLC

Frame format for bit-oriented protocol HDLC

Control field of

(a) An information frame

(b) A supervisory frame

(c) An unnumbered frame

copy Kemal Akkaya

HDLC Exchange of information supervisory and unnumbered

frames Information - data to be transmitted to user (next layer up)

Flow and error control piggybacked on information frames Supervisory - ARQ when piggyback not used Unnumbered - supplementary link control

Three phases Initialization Data transfer Disconnect

Uses bit stuffing Flag delimit frame at both ends 01111110 is the Flag May close one frame and open another Bit stuffing to find flag data

copy Kemal Akkaya

Internet Datalink Protocol PPP Design Requirements [RFC 1557]

Packet framing encapsulation of network-layer datagram in data link frame Carry network layer data of any network layer protocol (not just IP) at

same time Ability to demultiplex upwards

Bit transparency must carry any bit pattern in the data field Error detection (no correction) Connection liveness detect signal link failure to network layer Network layer address negotiation endpoint can learnconfigure

each otherrsquos network address

PPP non-design requirements No error correctionrecovery No flow control Out of order delivery OK No need to support multipoint links (eg polling)

Error recovery flow control data re-ordering all relegated to higher layers

copy Kemal Akkaya

PPP Data Frame

Flag delimiter (framing) Address does nothing (only one option) Control does nothing in the future possible multiple

control fields Protocol upper layer protocol to which frame delivered

(eg PPP-LCP IP IPCP etc) Info upper layer data being carried Check cyclic redundancy check for error detection

copy Kemal Akkaya

Byte Stuffing ldquoData transparencyrdquo requirement data field must be allowed to include

flag pattern lt01111110gt Q is received lt01111110gt data or flag

Sender adds (ldquostuffsrdquo) extra lt 01111110gt byte after each lt 01111110gt data byte How about bit stuffing

Receiver Two 01111110 bytes in a row Discard first byte continue data reception Single 01111110 flag byte

flag bytepatternin datato send

flag byte pattern plusstuffed byte in transmitted data

copy Kemal Akkaya

PPP Data Control Protocol Before exchanging

network-layer data data link peers must configure PPP link

(max frame length authentication)

learnconfigure network layer information

For IP carry IP Control Protocol (IPCP) messages (protocol field 8021) to configurelearn IP address

Diagram for bring a line up and down

copy Kemal Akkaya

Summary of MAC protocols

What do you do with a shared media Channel Partitioning by time frequency or code

Time Division Frequency Division

Random partitioning (dynamic) ALOHA S-ALOHA CSMA CSMACD Carrier sensing easy in some technologies (wire) hard in others

(wireless) CSMACD used in Ethernet CSMACA used in 80211

Taking Turns Polling from a central site token passing

copy Kemal Akkaya

LAN technologies

Data link layer so farServicesError detectioncorrectionMultiple access

Next LAN technologiesAddressingEthernetHubs switchesPPP

copy Kemal Akkaya

MAC Addresses and ARP 32-bit IP address

Network-layer address Used to get datagram to

destination IP subnet

MAC (or LAN or physical or Ethernet) address Used to get datagram

from one interface to another physically-connected interface (same network)

48 bit MAC address (for most LANs) burned in the adapter ROM

Find your PCrsquos MAC address

1A-2F-BB-76-09-AD

58-23-D7-FA-20-B0

0C-C4-11-6F-E3-98

71-65-F7-2B-08-53

LAN(wired orwireless)

Each adapter on LAN has unique LAN address

Broadcast address =FF-FF-FF-FF-FF-FF = adapter

copy Kemal Akkaya

MAC Address (more)

MAC address allocation administered by IEEE Manufacturer buys portion of MAC address space (to

assure uniqueness) Analogy

(a) MAC address like Social Security Number

(b) IP address like postal address MAC flat address portability

Can move LAN card from one LAN to another

IP hierarchical address NOT portable Depends on IP subnet to which node is attached

copy Kemal Akkaya

ARP Address Resolution Protocol

Each IP node (Host Router) on LAN has ARP table

ARP Table IPMAC address mappings for some LAN nodes

lt IP address MAC address TTLgt TTL (Time To Live)

time after which address mapping will be forgotten (typically 20 min)

Question how to determineMAC address of Bknowing Brsquos IP address

1A-2F-BB-76-09-AD

58-23-D7-FA-20-B0

0C-C4-11-6F-E3-98

71-65-F7-2B-08-53

LAN

237196723

237196778

237196714

237196788

copy Kemal Akkaya

ARP protocol Same LAN (network) A wants to send datagram

to B and Brsquos MAC address not in Arsquos ARP table

A broadcasts ARP query packet containing Bs IP address Dest MAC address = FF-FF-

FF-FF-FF-FF All machines on LAN receive

ARP query

B receives ARP packet replies to A with its (Bs) MAC address Frame sent to Arsquos MAC

address (unicast)

A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) Soft state information that

times out (goes away) unless refreshed

ARP is ldquoplug-and-playrdquo Nodes create their ARP

tables without intervention from net administrator

copy Kemal Akkaya

Routing to another LAN Walkthrough send datagram from A to B via R

Assume A knows B IP address

Two ARP tables in router R one for each IP network (LAN) In routing table at source Host find router 111111111110 In ARP table at source find MAC address E6-E9-00-17-BB-4B etc A creates datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110

A creates link-layer frame with Rs MAC address as dest frame contains A-to-B IP datagram

Arsquos adapter sends frame Rrsquos adapter receives frame R removes IP datagram from Ethernet frame sees its destined to B R uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B

copy Kemal Akkaya

A

R

B

Routing to another LAN

copy Kemal Akkaya

EthernetldquoDominantrdquo wired LAN technology Cheap $20 for 100Mbs First widely used LAN technology Simpler cheaper than token LANs and ATM Kept up with speed race 10 Mbps ndash 10

Gbps

Metcalfersquos Ethernet sketch

Star Topology Bus topology popular

through mid 90s Now star topology prevails Connection choices hub or

switch (more later)

hub orswitch

copy Kemal Akkaya

Ethernet Frame Structure

Preamble 7 bytes with pattern 10101010 followed by one byte with pattern 10101011 Used to synchronize receiver sender clock rates

Addresses 6 bytes If adapter receives frame with matching destination address or with broadcast

address (eg ARP packet) it passes data in frame to net-layer protocol Otherwise adapter discards frame

Type Indicates the higher layer protocol (mostly IP but others may be supported

such as Novell IPX and AppleTalk) CRC

Checked at receiver if error is detected the frame is simply dropped

Sending adapter encapsulates IP datagram in Ethernet frame

copy Kemal Akkaya

Ethernet uses CSMACD Connectionless No handshaking between sending and receiving

adapter Unreliable receiving adapter doesnrsquot send acks or nacks to

sending adapter Stream of datagrams passed to network layer can have gaps Gaps will be filled if app is using TCP Otherwise app will see the gaps

No slots Adapter doesnrsquot transmit if it senses that some other adapter is

transmitting that is carrier sense Transmitting adapter aborts when it senses that another adapter is

transmitting that is collision detection Before attempting a retransmission adapter waits a random time

that is random access

copy Kemal Akkaya

Ethernet CSMACD algorithm1 Adaptor receives datagram from

net layer amp creates frame2 If adapter senses channel idle it

starts to transmit frame If it senses channel busy waits until channel idle and then transmits

3 If adapter transmits entire frame without detecting another transmission the adapter is done with frame

4 If adapter detects another transmission while transmitting aborts and sends jam signal

5 After aborting adapter enters exponential backoff after the mth collision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2

Jam Signal make sure all other transmitters are aware of collision 48 bits

Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec

Exponential Backoff Goal adapt retransmission

attempts to estimated current load Heavy load random wait will be

longer

First collision choose K from 01 delay is K 512 bit transmission times

After second collision choose K from 0123hellip

After ten collisions choose K from 01234hellip1023

copy Kemal Akkaya

CSMACD efficiency Tprop = max prop between 2 nodes in LAN ttrans = time to transmit max-size frame

Efficiency goes to 1 as tprop goes to 0 Goes to 1 as ttrans goes to infinity Much better than ALOHA but still decentralized

simple and cheap

Other Ethernets Switched Ethernet Fast Ethernet Gigabit Ethernet

transprop tt 51

1efficiency

copy Kemal Akkaya

Point to Point Data Link Control

One sender one receiver one link easier than broadcast link No Media Access Control No need for explicit MAC addressing eg dialup link ISDN line

Popular point-to-point DLC protocols PPP (point-to-point protocol) HDLC High level data link control (Data link used to be considered

ldquohigh layerrdquo in protocol stack

copy Kemal Akkaya

HDLC

Frame format for bit-oriented protocol HDLC

Control field of

(a) An information frame

(b) A supervisory frame

(c) An unnumbered frame

copy Kemal Akkaya

HDLC Exchange of information supervisory and unnumbered

frames Information - data to be transmitted to user (next layer up)

Flow and error control piggybacked on information frames Supervisory - ARQ when piggyback not used Unnumbered - supplementary link control

Three phases Initialization Data transfer Disconnect

Uses bit stuffing Flag delimit frame at both ends 01111110 is the Flag May close one frame and open another Bit stuffing to find flag data

copy Kemal Akkaya

Internet Datalink Protocol PPP Design Requirements [RFC 1557]

Packet framing encapsulation of network-layer datagram in data link frame Carry network layer data of any network layer protocol (not just IP) at

same time Ability to demultiplex upwards

Bit transparency must carry any bit pattern in the data field Error detection (no correction) Connection liveness detect signal link failure to network layer Network layer address negotiation endpoint can learnconfigure

each otherrsquos network address

PPP non-design requirements No error correctionrecovery No flow control Out of order delivery OK No need to support multipoint links (eg polling)

Error recovery flow control data re-ordering all relegated to higher layers

copy Kemal Akkaya

PPP Data Frame

Flag delimiter (framing) Address does nothing (only one option) Control does nothing in the future possible multiple

control fields Protocol upper layer protocol to which frame delivered

(eg PPP-LCP IP IPCP etc) Info upper layer data being carried Check cyclic redundancy check for error detection

copy Kemal Akkaya

Byte Stuffing ldquoData transparencyrdquo requirement data field must be allowed to include

flag pattern lt01111110gt Q is received lt01111110gt data or flag

Sender adds (ldquostuffsrdquo) extra lt 01111110gt byte after each lt 01111110gt data byte How about bit stuffing

Receiver Two 01111110 bytes in a row Discard first byte continue data reception Single 01111110 flag byte

flag bytepatternin datato send

flag byte pattern plusstuffed byte in transmitted data

copy Kemal Akkaya

PPP Data Control Protocol Before exchanging

network-layer data data link peers must configure PPP link

(max frame length authentication)

learnconfigure network layer information

For IP carry IP Control Protocol (IPCP) messages (protocol field 8021) to configurelearn IP address

Diagram for bring a line up and down

copy Kemal Akkaya

LAN technologies

Data link layer so farServicesError detectioncorrectionMultiple access

Next LAN technologiesAddressingEthernetHubs switchesPPP

copy Kemal Akkaya

MAC Addresses and ARP 32-bit IP address

Network-layer address Used to get datagram to

destination IP subnet

MAC (or LAN or physical or Ethernet) address Used to get datagram

from one interface to another physically-connected interface (same network)

48 bit MAC address (for most LANs) burned in the adapter ROM

Find your PCrsquos MAC address

1A-2F-BB-76-09-AD

58-23-D7-FA-20-B0

0C-C4-11-6F-E3-98

71-65-F7-2B-08-53

LAN(wired orwireless)

Each adapter on LAN has unique LAN address

Broadcast address =FF-FF-FF-FF-FF-FF = adapter

copy Kemal Akkaya

MAC Address (more)

MAC address allocation administered by IEEE Manufacturer buys portion of MAC address space (to

assure uniqueness) Analogy

(a) MAC address like Social Security Number

(b) IP address like postal address MAC flat address portability

Can move LAN card from one LAN to another

IP hierarchical address NOT portable Depends on IP subnet to which node is attached

copy Kemal Akkaya

ARP Address Resolution Protocol

Each IP node (Host Router) on LAN has ARP table

ARP Table IPMAC address mappings for some LAN nodes

lt IP address MAC address TTLgt TTL (Time To Live)

time after which address mapping will be forgotten (typically 20 min)

Question how to determineMAC address of Bknowing Brsquos IP address

1A-2F-BB-76-09-AD

58-23-D7-FA-20-B0

0C-C4-11-6F-E3-98

71-65-F7-2B-08-53

LAN

237196723

237196778

237196714

237196788

copy Kemal Akkaya

ARP protocol Same LAN (network) A wants to send datagram

to B and Brsquos MAC address not in Arsquos ARP table

A broadcasts ARP query packet containing Bs IP address Dest MAC address = FF-FF-

FF-FF-FF-FF All machines on LAN receive

ARP query

B receives ARP packet replies to A with its (Bs) MAC address Frame sent to Arsquos MAC

address (unicast)

A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) Soft state information that

times out (goes away) unless refreshed

ARP is ldquoplug-and-playrdquo Nodes create their ARP

tables without intervention from net administrator

copy Kemal Akkaya

Routing to another LAN Walkthrough send datagram from A to B via R

Assume A knows B IP address

Two ARP tables in router R one for each IP network (LAN) In routing table at source Host find router 111111111110 In ARP table at source find MAC address E6-E9-00-17-BB-4B etc A creates datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110

A creates link-layer frame with Rs MAC address as dest frame contains A-to-B IP datagram

Arsquos adapter sends frame Rrsquos adapter receives frame R removes IP datagram from Ethernet frame sees its destined to B R uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B

copy Kemal Akkaya

A

R

B

Routing to another LAN

copy Kemal Akkaya

EthernetldquoDominantrdquo wired LAN technology Cheap $20 for 100Mbs First widely used LAN technology Simpler cheaper than token LANs and ATM Kept up with speed race 10 Mbps ndash 10

Gbps

Metcalfersquos Ethernet sketch

Star Topology Bus topology popular

through mid 90s Now star topology prevails Connection choices hub or

switch (more later)

hub orswitch

copy Kemal Akkaya

Ethernet Frame Structure

Preamble 7 bytes with pattern 10101010 followed by one byte with pattern 10101011 Used to synchronize receiver sender clock rates

Addresses 6 bytes If adapter receives frame with matching destination address or with broadcast

address (eg ARP packet) it passes data in frame to net-layer protocol Otherwise adapter discards frame

Type Indicates the higher layer protocol (mostly IP but others may be supported

such as Novell IPX and AppleTalk) CRC

Checked at receiver if error is detected the frame is simply dropped

Sending adapter encapsulates IP datagram in Ethernet frame

copy Kemal Akkaya

Ethernet uses CSMACD Connectionless No handshaking between sending and receiving

adapter Unreliable receiving adapter doesnrsquot send acks or nacks to

sending adapter Stream of datagrams passed to network layer can have gaps Gaps will be filled if app is using TCP Otherwise app will see the gaps

No slots Adapter doesnrsquot transmit if it senses that some other adapter is

transmitting that is carrier sense Transmitting adapter aborts when it senses that another adapter is

transmitting that is collision detection Before attempting a retransmission adapter waits a random time

that is random access

copy Kemal Akkaya

Ethernet CSMACD algorithm1 Adaptor receives datagram from

net layer amp creates frame2 If adapter senses channel idle it

starts to transmit frame If it senses channel busy waits until channel idle and then transmits

3 If adapter transmits entire frame without detecting another transmission the adapter is done with frame

4 If adapter detects another transmission while transmitting aborts and sends jam signal

5 After aborting adapter enters exponential backoff after the mth collision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2

Jam Signal make sure all other transmitters are aware of collision 48 bits

Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec

Exponential Backoff Goal adapt retransmission

attempts to estimated current load Heavy load random wait will be

longer

First collision choose K from 01 delay is K 512 bit transmission times

After second collision choose K from 0123hellip

After ten collisions choose K from 01234hellip1023

copy Kemal Akkaya

CSMACD efficiency Tprop = max prop between 2 nodes in LAN ttrans = time to transmit max-size frame

Efficiency goes to 1 as tprop goes to 0 Goes to 1 as ttrans goes to infinity Much better than ALOHA but still decentralized

simple and cheap

Other Ethernets Switched Ethernet Fast Ethernet Gigabit Ethernet

transprop tt 51

1efficiency

copy Kemal Akkaya

Point to Point Data Link Control

One sender one receiver one link easier than broadcast link No Media Access Control No need for explicit MAC addressing eg dialup link ISDN line

Popular point-to-point DLC protocols PPP (point-to-point protocol) HDLC High level data link control (Data link used to be considered

ldquohigh layerrdquo in protocol stack

copy Kemal Akkaya

HDLC

Frame format for bit-oriented protocol HDLC

Control field of

(a) An information frame

(b) A supervisory frame

(c) An unnumbered frame

copy Kemal Akkaya

HDLC Exchange of information supervisory and unnumbered

frames Information - data to be transmitted to user (next layer up)

Flow and error control piggybacked on information frames Supervisory - ARQ when piggyback not used Unnumbered - supplementary link control

Three phases Initialization Data transfer Disconnect

Uses bit stuffing Flag delimit frame at both ends 01111110 is the Flag May close one frame and open another Bit stuffing to find flag data

copy Kemal Akkaya

Internet Datalink Protocol PPP Design Requirements [RFC 1557]

Packet framing encapsulation of network-layer datagram in data link frame Carry network layer data of any network layer protocol (not just IP) at

same time Ability to demultiplex upwards

Bit transparency must carry any bit pattern in the data field Error detection (no correction) Connection liveness detect signal link failure to network layer Network layer address negotiation endpoint can learnconfigure

each otherrsquos network address

PPP non-design requirements No error correctionrecovery No flow control Out of order delivery OK No need to support multipoint links (eg polling)

Error recovery flow control data re-ordering all relegated to higher layers

copy Kemal Akkaya

PPP Data Frame

Flag delimiter (framing) Address does nothing (only one option) Control does nothing in the future possible multiple

control fields Protocol upper layer protocol to which frame delivered

(eg PPP-LCP IP IPCP etc) Info upper layer data being carried Check cyclic redundancy check for error detection

copy Kemal Akkaya

Byte Stuffing ldquoData transparencyrdquo requirement data field must be allowed to include

flag pattern lt01111110gt Q is received lt01111110gt data or flag

Sender adds (ldquostuffsrdquo) extra lt 01111110gt byte after each lt 01111110gt data byte How about bit stuffing

Receiver Two 01111110 bytes in a row Discard first byte continue data reception Single 01111110 flag byte

flag bytepatternin datato send

flag byte pattern plusstuffed byte in transmitted data

copy Kemal Akkaya

PPP Data Control Protocol Before exchanging

network-layer data data link peers must configure PPP link

(max frame length authentication)

learnconfigure network layer information

For IP carry IP Control Protocol (IPCP) messages (protocol field 8021) to configurelearn IP address

Diagram for bring a line up and down

copy Kemal Akkaya

MAC Addresses and ARP 32-bit IP address

Network-layer address Used to get datagram to

destination IP subnet

MAC (or LAN or physical or Ethernet) address Used to get datagram

from one interface to another physically-connected interface (same network)

48 bit MAC address (for most LANs) burned in the adapter ROM

Find your PCrsquos MAC address

1A-2F-BB-76-09-AD

58-23-D7-FA-20-B0

0C-C4-11-6F-E3-98

71-65-F7-2B-08-53

LAN(wired orwireless)

Each adapter on LAN has unique LAN address

Broadcast address =FF-FF-FF-FF-FF-FF = adapter

copy Kemal Akkaya

MAC Address (more)

MAC address allocation administered by IEEE Manufacturer buys portion of MAC address space (to

assure uniqueness) Analogy

(a) MAC address like Social Security Number

(b) IP address like postal address MAC flat address portability

Can move LAN card from one LAN to another

IP hierarchical address NOT portable Depends on IP subnet to which node is attached

copy Kemal Akkaya

ARP Address Resolution Protocol

Each IP node (Host Router) on LAN has ARP table

ARP Table IPMAC address mappings for some LAN nodes

lt IP address MAC address TTLgt TTL (Time To Live)

time after which address mapping will be forgotten (typically 20 min)

Question how to determineMAC address of Bknowing Brsquos IP address

1A-2F-BB-76-09-AD

58-23-D7-FA-20-B0

0C-C4-11-6F-E3-98

71-65-F7-2B-08-53

LAN

237196723

237196778

237196714

237196788

copy Kemal Akkaya

ARP protocol Same LAN (network) A wants to send datagram

to B and Brsquos MAC address not in Arsquos ARP table

A broadcasts ARP query packet containing Bs IP address Dest MAC address = FF-FF-

FF-FF-FF-FF All machines on LAN receive

ARP query

B receives ARP packet replies to A with its (Bs) MAC address Frame sent to Arsquos MAC

address (unicast)

A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) Soft state information that

times out (goes away) unless refreshed

ARP is ldquoplug-and-playrdquo Nodes create their ARP

tables without intervention from net administrator

copy Kemal Akkaya

Routing to another LAN Walkthrough send datagram from A to B via R

Assume A knows B IP address

Two ARP tables in router R one for each IP network (LAN) In routing table at source Host find router 111111111110 In ARP table at source find MAC address E6-E9-00-17-BB-4B etc A creates datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110

A creates link-layer frame with Rs MAC address as dest frame contains A-to-B IP datagram

Arsquos adapter sends frame Rrsquos adapter receives frame R removes IP datagram from Ethernet frame sees its destined to B R uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B

copy Kemal Akkaya

A

R

B

Routing to another LAN

copy Kemal Akkaya

EthernetldquoDominantrdquo wired LAN technology Cheap $20 for 100Mbs First widely used LAN technology Simpler cheaper than token LANs and ATM Kept up with speed race 10 Mbps ndash 10

Gbps

Metcalfersquos Ethernet sketch

Star Topology Bus topology popular

through mid 90s Now star topology prevails Connection choices hub or

switch (more later)

hub orswitch

copy Kemal Akkaya

Ethernet Frame Structure

Preamble 7 bytes with pattern 10101010 followed by one byte with pattern 10101011 Used to synchronize receiver sender clock rates

Addresses 6 bytes If adapter receives frame with matching destination address or with broadcast

address (eg ARP packet) it passes data in frame to net-layer protocol Otherwise adapter discards frame

Type Indicates the higher layer protocol (mostly IP but others may be supported

such as Novell IPX and AppleTalk) CRC

Checked at receiver if error is detected the frame is simply dropped

Sending adapter encapsulates IP datagram in Ethernet frame

copy Kemal Akkaya

Ethernet uses CSMACD Connectionless No handshaking between sending and receiving

adapter Unreliable receiving adapter doesnrsquot send acks or nacks to

sending adapter Stream of datagrams passed to network layer can have gaps Gaps will be filled if app is using TCP Otherwise app will see the gaps

No slots Adapter doesnrsquot transmit if it senses that some other adapter is

transmitting that is carrier sense Transmitting adapter aborts when it senses that another adapter is

transmitting that is collision detection Before attempting a retransmission adapter waits a random time

that is random access

copy Kemal Akkaya

Ethernet CSMACD algorithm1 Adaptor receives datagram from

net layer amp creates frame2 If adapter senses channel idle it

starts to transmit frame If it senses channel busy waits until channel idle and then transmits

3 If adapter transmits entire frame without detecting another transmission the adapter is done with frame

4 If adapter detects another transmission while transmitting aborts and sends jam signal

5 After aborting adapter enters exponential backoff after the mth collision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2

Jam Signal make sure all other transmitters are aware of collision 48 bits

Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec

Exponential Backoff Goal adapt retransmission

attempts to estimated current load Heavy load random wait will be

longer

First collision choose K from 01 delay is K 512 bit transmission times

After second collision choose K from 0123hellip

After ten collisions choose K from 01234hellip1023

copy Kemal Akkaya

CSMACD efficiency Tprop = max prop between 2 nodes in LAN ttrans = time to transmit max-size frame

Efficiency goes to 1 as tprop goes to 0 Goes to 1 as ttrans goes to infinity Much better than ALOHA but still decentralized

simple and cheap

Other Ethernets Switched Ethernet Fast Ethernet Gigabit Ethernet

transprop tt 51

1efficiency

copy Kemal Akkaya

Point to Point Data Link Control

One sender one receiver one link easier than broadcast link No Media Access Control No need for explicit MAC addressing eg dialup link ISDN line

Popular point-to-point DLC protocols PPP (point-to-point protocol) HDLC High level data link control (Data link used to be considered

ldquohigh layerrdquo in protocol stack

copy Kemal Akkaya

HDLC

Frame format for bit-oriented protocol HDLC

Control field of

(a) An information frame

(b) A supervisory frame

(c) An unnumbered frame

copy Kemal Akkaya

HDLC Exchange of information supervisory and unnumbered

frames Information - data to be transmitted to user (next layer up)

Flow and error control piggybacked on information frames Supervisory - ARQ when piggyback not used Unnumbered - supplementary link control

Three phases Initialization Data transfer Disconnect

Uses bit stuffing Flag delimit frame at both ends 01111110 is the Flag May close one frame and open another Bit stuffing to find flag data

copy Kemal Akkaya

Internet Datalink Protocol PPP Design Requirements [RFC 1557]

Packet framing encapsulation of network-layer datagram in data link frame Carry network layer data of any network layer protocol (not just IP) at

same time Ability to demultiplex upwards

Bit transparency must carry any bit pattern in the data field Error detection (no correction) Connection liveness detect signal link failure to network layer Network layer address negotiation endpoint can learnconfigure

each otherrsquos network address

PPP non-design requirements No error correctionrecovery No flow control Out of order delivery OK No need to support multipoint links (eg polling)

Error recovery flow control data re-ordering all relegated to higher layers

copy Kemal Akkaya

PPP Data Frame

Flag delimiter (framing) Address does nothing (only one option) Control does nothing in the future possible multiple

control fields Protocol upper layer protocol to which frame delivered

(eg PPP-LCP IP IPCP etc) Info upper layer data being carried Check cyclic redundancy check for error detection

copy Kemal Akkaya

Byte Stuffing ldquoData transparencyrdquo requirement data field must be allowed to include

flag pattern lt01111110gt Q is received lt01111110gt data or flag

Sender adds (ldquostuffsrdquo) extra lt 01111110gt byte after each lt 01111110gt data byte How about bit stuffing

Receiver Two 01111110 bytes in a row Discard first byte continue data reception Single 01111110 flag byte

flag bytepatternin datato send

flag byte pattern plusstuffed byte in transmitted data

copy Kemal Akkaya

PPP Data Control Protocol Before exchanging

network-layer data data link peers must configure PPP link

(max frame length authentication)

learnconfigure network layer information

For IP carry IP Control Protocol (IPCP) messages (protocol field 8021) to configurelearn IP address

Diagram for bring a line up and down

copy Kemal Akkaya

MAC Address (more)

MAC address allocation administered by IEEE Manufacturer buys portion of MAC address space (to

assure uniqueness) Analogy

(a) MAC address like Social Security Number

(b) IP address like postal address MAC flat address portability

Can move LAN card from one LAN to another

IP hierarchical address NOT portable Depends on IP subnet to which node is attached

copy Kemal Akkaya

ARP Address Resolution Protocol

Each IP node (Host Router) on LAN has ARP table

ARP Table IPMAC address mappings for some LAN nodes

lt IP address MAC address TTLgt TTL (Time To Live)

time after which address mapping will be forgotten (typically 20 min)

Question how to determineMAC address of Bknowing Brsquos IP address

1A-2F-BB-76-09-AD

58-23-D7-FA-20-B0

0C-C4-11-6F-E3-98

71-65-F7-2B-08-53

LAN

237196723

237196778

237196714

237196788

copy Kemal Akkaya

ARP protocol Same LAN (network) A wants to send datagram

to B and Brsquos MAC address not in Arsquos ARP table

A broadcasts ARP query packet containing Bs IP address Dest MAC address = FF-FF-

FF-FF-FF-FF All machines on LAN receive

ARP query

B receives ARP packet replies to A with its (Bs) MAC address Frame sent to Arsquos MAC

address (unicast)

A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) Soft state information that

times out (goes away) unless refreshed

ARP is ldquoplug-and-playrdquo Nodes create their ARP

tables without intervention from net administrator

copy Kemal Akkaya

Routing to another LAN Walkthrough send datagram from A to B via R

Assume A knows B IP address

Two ARP tables in router R one for each IP network (LAN) In routing table at source Host find router 111111111110 In ARP table at source find MAC address E6-E9-00-17-BB-4B etc A creates datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110

A creates link-layer frame with Rs MAC address as dest frame contains A-to-B IP datagram

Arsquos adapter sends frame Rrsquos adapter receives frame R removes IP datagram from Ethernet frame sees its destined to B R uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B

copy Kemal Akkaya

A

R

B

Routing to another LAN

copy Kemal Akkaya

EthernetldquoDominantrdquo wired LAN technology Cheap $20 for 100Mbs First widely used LAN technology Simpler cheaper than token LANs and ATM Kept up with speed race 10 Mbps ndash 10

Gbps

Metcalfersquos Ethernet sketch

Star Topology Bus topology popular

through mid 90s Now star topology prevails Connection choices hub or

switch (more later)

hub orswitch

copy Kemal Akkaya

Ethernet Frame Structure

Preamble 7 bytes with pattern 10101010 followed by one byte with pattern 10101011 Used to synchronize receiver sender clock rates

Addresses 6 bytes If adapter receives frame with matching destination address or with broadcast

address (eg ARP packet) it passes data in frame to net-layer protocol Otherwise adapter discards frame

Type Indicates the higher layer protocol (mostly IP but others may be supported

such as Novell IPX and AppleTalk) CRC

Checked at receiver if error is detected the frame is simply dropped

Sending adapter encapsulates IP datagram in Ethernet frame

copy Kemal Akkaya

Ethernet uses CSMACD Connectionless No handshaking between sending and receiving

adapter Unreliable receiving adapter doesnrsquot send acks or nacks to

sending adapter Stream of datagrams passed to network layer can have gaps Gaps will be filled if app is using TCP Otherwise app will see the gaps

No slots Adapter doesnrsquot transmit if it senses that some other adapter is

transmitting that is carrier sense Transmitting adapter aborts when it senses that another adapter is

transmitting that is collision detection Before attempting a retransmission adapter waits a random time

that is random access

copy Kemal Akkaya

Ethernet CSMACD algorithm1 Adaptor receives datagram from

net layer amp creates frame2 If adapter senses channel idle it

starts to transmit frame If it senses channel busy waits until channel idle and then transmits

3 If adapter transmits entire frame without detecting another transmission the adapter is done with frame

4 If adapter detects another transmission while transmitting aborts and sends jam signal

5 After aborting adapter enters exponential backoff after the mth collision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2

Jam Signal make sure all other transmitters are aware of collision 48 bits

Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec

Exponential Backoff Goal adapt retransmission

attempts to estimated current load Heavy load random wait will be

longer

First collision choose K from 01 delay is K 512 bit transmission times

After second collision choose K from 0123hellip

After ten collisions choose K from 01234hellip1023

copy Kemal Akkaya

CSMACD efficiency Tprop = max prop between 2 nodes in LAN ttrans = time to transmit max-size frame

Efficiency goes to 1 as tprop goes to 0 Goes to 1 as ttrans goes to infinity Much better than ALOHA but still decentralized

simple and cheap

Other Ethernets Switched Ethernet Fast Ethernet Gigabit Ethernet

transprop tt 51

1efficiency

copy Kemal Akkaya

Point to Point Data Link Control

One sender one receiver one link easier than broadcast link No Media Access Control No need for explicit MAC addressing eg dialup link ISDN line

Popular point-to-point DLC protocols PPP (point-to-point protocol) HDLC High level data link control (Data link used to be considered

ldquohigh layerrdquo in protocol stack

copy Kemal Akkaya

HDLC

Frame format for bit-oriented protocol HDLC

Control field of

(a) An information frame

(b) A supervisory frame

(c) An unnumbered frame

copy Kemal Akkaya

HDLC Exchange of information supervisory and unnumbered

frames Information - data to be transmitted to user (next layer up)

Flow and error control piggybacked on information frames Supervisory - ARQ when piggyback not used Unnumbered - supplementary link control

Three phases Initialization Data transfer Disconnect

Uses bit stuffing Flag delimit frame at both ends 01111110 is the Flag May close one frame and open another Bit stuffing to find flag data

copy Kemal Akkaya

Internet Datalink Protocol PPP Design Requirements [RFC 1557]

Packet framing encapsulation of network-layer datagram in data link frame Carry network layer data of any network layer protocol (not just IP) at

same time Ability to demultiplex upwards

Bit transparency must carry any bit pattern in the data field Error detection (no correction) Connection liveness detect signal link failure to network layer Network layer address negotiation endpoint can learnconfigure

each otherrsquos network address

PPP non-design requirements No error correctionrecovery No flow control Out of order delivery OK No need to support multipoint links (eg polling)

Error recovery flow control data re-ordering all relegated to higher layers

copy Kemal Akkaya

PPP Data Frame

Flag delimiter (framing) Address does nothing (only one option) Control does nothing in the future possible multiple

control fields Protocol upper layer protocol to which frame delivered

(eg PPP-LCP IP IPCP etc) Info upper layer data being carried Check cyclic redundancy check for error detection

copy Kemal Akkaya

Byte Stuffing ldquoData transparencyrdquo requirement data field must be allowed to include

flag pattern lt01111110gt Q is received lt01111110gt data or flag

Sender adds (ldquostuffsrdquo) extra lt 01111110gt byte after each lt 01111110gt data byte How about bit stuffing

Receiver Two 01111110 bytes in a row Discard first byte continue data reception Single 01111110 flag byte

flag bytepatternin datato send

flag byte pattern plusstuffed byte in transmitted data

copy Kemal Akkaya

PPP Data Control Protocol Before exchanging

network-layer data data link peers must configure PPP link

(max frame length authentication)

learnconfigure network layer information

For IP carry IP Control Protocol (IPCP) messages (protocol field 8021) to configurelearn IP address

Diagram for bring a line up and down

copy Kemal Akkaya

ARP Address Resolution Protocol

Each IP node (Host Router) on LAN has ARP table

ARP Table IPMAC address mappings for some LAN nodes

lt IP address MAC address TTLgt TTL (Time To Live)

time after which address mapping will be forgotten (typically 20 min)

Question how to determineMAC address of Bknowing Brsquos IP address

1A-2F-BB-76-09-AD

58-23-D7-FA-20-B0

0C-C4-11-6F-E3-98

71-65-F7-2B-08-53

LAN

237196723

237196778

237196714

237196788

copy Kemal Akkaya

ARP protocol Same LAN (network) A wants to send datagram

to B and Brsquos MAC address not in Arsquos ARP table

A broadcasts ARP query packet containing Bs IP address Dest MAC address = FF-FF-

FF-FF-FF-FF All machines on LAN receive

ARP query

B receives ARP packet replies to A with its (Bs) MAC address Frame sent to Arsquos MAC

address (unicast)

A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) Soft state information that

times out (goes away) unless refreshed

ARP is ldquoplug-and-playrdquo Nodes create their ARP

tables without intervention from net administrator

copy Kemal Akkaya

Routing to another LAN Walkthrough send datagram from A to B via R

Assume A knows B IP address

Two ARP tables in router R one for each IP network (LAN) In routing table at source Host find router 111111111110 In ARP table at source find MAC address E6-E9-00-17-BB-4B etc A creates datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110

A creates link-layer frame with Rs MAC address as dest frame contains A-to-B IP datagram

Arsquos adapter sends frame Rrsquos adapter receives frame R removes IP datagram from Ethernet frame sees its destined to B R uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B

copy Kemal Akkaya

A

R

B

Routing to another LAN

copy Kemal Akkaya

EthernetldquoDominantrdquo wired LAN technology Cheap $20 for 100Mbs First widely used LAN technology Simpler cheaper than token LANs and ATM Kept up with speed race 10 Mbps ndash 10

Gbps

Metcalfersquos Ethernet sketch

Star Topology Bus topology popular

through mid 90s Now star topology prevails Connection choices hub or

switch (more later)

hub orswitch

copy Kemal Akkaya

Ethernet Frame Structure

Preamble 7 bytes with pattern 10101010 followed by one byte with pattern 10101011 Used to synchronize receiver sender clock rates

Addresses 6 bytes If adapter receives frame with matching destination address or with broadcast

address (eg ARP packet) it passes data in frame to net-layer protocol Otherwise adapter discards frame

Type Indicates the higher layer protocol (mostly IP but others may be supported

such as Novell IPX and AppleTalk) CRC

Checked at receiver if error is detected the frame is simply dropped

Sending adapter encapsulates IP datagram in Ethernet frame

copy Kemal Akkaya

Ethernet uses CSMACD Connectionless No handshaking between sending and receiving

adapter Unreliable receiving adapter doesnrsquot send acks or nacks to

sending adapter Stream of datagrams passed to network layer can have gaps Gaps will be filled if app is using TCP Otherwise app will see the gaps

No slots Adapter doesnrsquot transmit if it senses that some other adapter is

transmitting that is carrier sense Transmitting adapter aborts when it senses that another adapter is

transmitting that is collision detection Before attempting a retransmission adapter waits a random time

that is random access

copy Kemal Akkaya

Ethernet CSMACD algorithm1 Adaptor receives datagram from

net layer amp creates frame2 If adapter senses channel idle it

starts to transmit frame If it senses channel busy waits until channel idle and then transmits

3 If adapter transmits entire frame without detecting another transmission the adapter is done with frame

4 If adapter detects another transmission while transmitting aborts and sends jam signal

5 After aborting adapter enters exponential backoff after the mth collision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2

Jam Signal make sure all other transmitters are aware of collision 48 bits

Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec

Exponential Backoff Goal adapt retransmission

attempts to estimated current load Heavy load random wait will be

longer

First collision choose K from 01 delay is K 512 bit transmission times

After second collision choose K from 0123hellip

After ten collisions choose K from 01234hellip1023

copy Kemal Akkaya

CSMACD efficiency Tprop = max prop between 2 nodes in LAN ttrans = time to transmit max-size frame

Efficiency goes to 1 as tprop goes to 0 Goes to 1 as ttrans goes to infinity Much better than ALOHA but still decentralized

simple and cheap

Other Ethernets Switched Ethernet Fast Ethernet Gigabit Ethernet

transprop tt 51

1efficiency

copy Kemal Akkaya

Point to Point Data Link Control

One sender one receiver one link easier than broadcast link No Media Access Control No need for explicit MAC addressing eg dialup link ISDN line

Popular point-to-point DLC protocols PPP (point-to-point protocol) HDLC High level data link control (Data link used to be considered

ldquohigh layerrdquo in protocol stack

copy Kemal Akkaya

HDLC

Frame format for bit-oriented protocol HDLC

Control field of

(a) An information frame

(b) A supervisory frame

(c) An unnumbered frame

copy Kemal Akkaya

HDLC Exchange of information supervisory and unnumbered

frames Information - data to be transmitted to user (next layer up)

Flow and error control piggybacked on information frames Supervisory - ARQ when piggyback not used Unnumbered - supplementary link control

Three phases Initialization Data transfer Disconnect

Uses bit stuffing Flag delimit frame at both ends 01111110 is the Flag May close one frame and open another Bit stuffing to find flag data

copy Kemal Akkaya

Internet Datalink Protocol PPP Design Requirements [RFC 1557]

Packet framing encapsulation of network-layer datagram in data link frame Carry network layer data of any network layer protocol (not just IP) at

same time Ability to demultiplex upwards

Bit transparency must carry any bit pattern in the data field Error detection (no correction) Connection liveness detect signal link failure to network layer Network layer address negotiation endpoint can learnconfigure

each otherrsquos network address

PPP non-design requirements No error correctionrecovery No flow control Out of order delivery OK No need to support multipoint links (eg polling)

Error recovery flow control data re-ordering all relegated to higher layers

copy Kemal Akkaya

PPP Data Frame

Flag delimiter (framing) Address does nothing (only one option) Control does nothing in the future possible multiple

control fields Protocol upper layer protocol to which frame delivered

(eg PPP-LCP IP IPCP etc) Info upper layer data being carried Check cyclic redundancy check for error detection

copy Kemal Akkaya

Byte Stuffing ldquoData transparencyrdquo requirement data field must be allowed to include

flag pattern lt01111110gt Q is received lt01111110gt data or flag

Sender adds (ldquostuffsrdquo) extra lt 01111110gt byte after each lt 01111110gt data byte How about bit stuffing

Receiver Two 01111110 bytes in a row Discard first byte continue data reception Single 01111110 flag byte

flag bytepatternin datato send

flag byte pattern plusstuffed byte in transmitted data

copy Kemal Akkaya

PPP Data Control Protocol Before exchanging

network-layer data data link peers must configure PPP link

(max frame length authentication)

learnconfigure network layer information

For IP carry IP Control Protocol (IPCP) messages (protocol field 8021) to configurelearn IP address

Diagram for bring a line up and down

copy Kemal Akkaya

ARP protocol Same LAN (network) A wants to send datagram

to B and Brsquos MAC address not in Arsquos ARP table

A broadcasts ARP query packet containing Bs IP address Dest MAC address = FF-FF-

FF-FF-FF-FF All machines on LAN receive

ARP query

B receives ARP packet replies to A with its (Bs) MAC address Frame sent to Arsquos MAC

address (unicast)

A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) Soft state information that

times out (goes away) unless refreshed

ARP is ldquoplug-and-playrdquo Nodes create their ARP

tables without intervention from net administrator

copy Kemal Akkaya

Routing to another LAN Walkthrough send datagram from A to B via R

Assume A knows B IP address

Two ARP tables in router R one for each IP network (LAN) In routing table at source Host find router 111111111110 In ARP table at source find MAC address E6-E9-00-17-BB-4B etc A creates datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110

A creates link-layer frame with Rs MAC address as dest frame contains A-to-B IP datagram

Arsquos adapter sends frame Rrsquos adapter receives frame R removes IP datagram from Ethernet frame sees its destined to B R uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B

copy Kemal Akkaya

A

R

B

Routing to another LAN

copy Kemal Akkaya

EthernetldquoDominantrdquo wired LAN technology Cheap $20 for 100Mbs First widely used LAN technology Simpler cheaper than token LANs and ATM Kept up with speed race 10 Mbps ndash 10

Gbps

Metcalfersquos Ethernet sketch

Star Topology Bus topology popular

through mid 90s Now star topology prevails Connection choices hub or

switch (more later)

hub orswitch

copy Kemal Akkaya

Ethernet Frame Structure

Preamble 7 bytes with pattern 10101010 followed by one byte with pattern 10101011 Used to synchronize receiver sender clock rates

Addresses 6 bytes If adapter receives frame with matching destination address or with broadcast

address (eg ARP packet) it passes data in frame to net-layer protocol Otherwise adapter discards frame

Type Indicates the higher layer protocol (mostly IP but others may be supported

such as Novell IPX and AppleTalk) CRC

Checked at receiver if error is detected the frame is simply dropped

Sending adapter encapsulates IP datagram in Ethernet frame

copy Kemal Akkaya

Ethernet uses CSMACD Connectionless No handshaking between sending and receiving

adapter Unreliable receiving adapter doesnrsquot send acks or nacks to

sending adapter Stream of datagrams passed to network layer can have gaps Gaps will be filled if app is using TCP Otherwise app will see the gaps

No slots Adapter doesnrsquot transmit if it senses that some other adapter is

transmitting that is carrier sense Transmitting adapter aborts when it senses that another adapter is

transmitting that is collision detection Before attempting a retransmission adapter waits a random time

that is random access

copy Kemal Akkaya

Ethernet CSMACD algorithm1 Adaptor receives datagram from

net layer amp creates frame2 If adapter senses channel idle it

starts to transmit frame If it senses channel busy waits until channel idle and then transmits

3 If adapter transmits entire frame without detecting another transmission the adapter is done with frame

4 If adapter detects another transmission while transmitting aborts and sends jam signal

5 After aborting adapter enters exponential backoff after the mth collision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2

Jam Signal make sure all other transmitters are aware of collision 48 bits

Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec

Exponential Backoff Goal adapt retransmission

attempts to estimated current load Heavy load random wait will be

longer

First collision choose K from 01 delay is K 512 bit transmission times

After second collision choose K from 0123hellip

After ten collisions choose K from 01234hellip1023

copy Kemal Akkaya

CSMACD efficiency Tprop = max prop between 2 nodes in LAN ttrans = time to transmit max-size frame

Efficiency goes to 1 as tprop goes to 0 Goes to 1 as ttrans goes to infinity Much better than ALOHA but still decentralized

simple and cheap

Other Ethernets Switched Ethernet Fast Ethernet Gigabit Ethernet

transprop tt 51

1efficiency

copy Kemal Akkaya

Point to Point Data Link Control

One sender one receiver one link easier than broadcast link No Media Access Control No need for explicit MAC addressing eg dialup link ISDN line

Popular point-to-point DLC protocols PPP (point-to-point protocol) HDLC High level data link control (Data link used to be considered

ldquohigh layerrdquo in protocol stack

copy Kemal Akkaya

HDLC

Frame format for bit-oriented protocol HDLC

Control field of

(a) An information frame

(b) A supervisory frame

(c) An unnumbered frame

copy Kemal Akkaya

HDLC Exchange of information supervisory and unnumbered

frames Information - data to be transmitted to user (next layer up)

Flow and error control piggybacked on information frames Supervisory - ARQ when piggyback not used Unnumbered - supplementary link control

Three phases Initialization Data transfer Disconnect

Uses bit stuffing Flag delimit frame at both ends 01111110 is the Flag May close one frame and open another Bit stuffing to find flag data

copy Kemal Akkaya

Internet Datalink Protocol PPP Design Requirements [RFC 1557]

Packet framing encapsulation of network-layer datagram in data link frame Carry network layer data of any network layer protocol (not just IP) at

same time Ability to demultiplex upwards

Bit transparency must carry any bit pattern in the data field Error detection (no correction) Connection liveness detect signal link failure to network layer Network layer address negotiation endpoint can learnconfigure

each otherrsquos network address

PPP non-design requirements No error correctionrecovery No flow control Out of order delivery OK No need to support multipoint links (eg polling)

Error recovery flow control data re-ordering all relegated to higher layers

copy Kemal Akkaya

PPP Data Frame

Flag delimiter (framing) Address does nothing (only one option) Control does nothing in the future possible multiple

control fields Protocol upper layer protocol to which frame delivered

(eg PPP-LCP IP IPCP etc) Info upper layer data being carried Check cyclic redundancy check for error detection

copy Kemal Akkaya

Byte Stuffing ldquoData transparencyrdquo requirement data field must be allowed to include

flag pattern lt01111110gt Q is received lt01111110gt data or flag

Sender adds (ldquostuffsrdquo) extra lt 01111110gt byte after each lt 01111110gt data byte How about bit stuffing

Receiver Two 01111110 bytes in a row Discard first byte continue data reception Single 01111110 flag byte

flag bytepatternin datato send

flag byte pattern plusstuffed byte in transmitted data

copy Kemal Akkaya

PPP Data Control Protocol Before exchanging

network-layer data data link peers must configure PPP link

(max frame length authentication)

learnconfigure network layer information

For IP carry IP Control Protocol (IPCP) messages (protocol field 8021) to configurelearn IP address

Diagram for bring a line up and down

copy Kemal Akkaya

Routing to another LAN Walkthrough send datagram from A to B via R

Assume A knows B IP address

Two ARP tables in router R one for each IP network (LAN) In routing table at source Host find router 111111111110 In ARP table at source find MAC address E6-E9-00-17-BB-4B etc A creates datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110

A creates link-layer frame with Rs MAC address as dest frame contains A-to-B IP datagram

Arsquos adapter sends frame Rrsquos adapter receives frame R removes IP datagram from Ethernet frame sees its destined to B R uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B

copy Kemal Akkaya

A

R

B

Routing to another LAN

copy Kemal Akkaya

EthernetldquoDominantrdquo wired LAN technology Cheap $20 for 100Mbs First widely used LAN technology Simpler cheaper than token LANs and ATM Kept up with speed race 10 Mbps ndash 10

Gbps

Metcalfersquos Ethernet sketch

Star Topology Bus topology popular

through mid 90s Now star topology prevails Connection choices hub or

switch (more later)

hub orswitch

copy Kemal Akkaya

Ethernet Frame Structure

Preamble 7 bytes with pattern 10101010 followed by one byte with pattern 10101011 Used to synchronize receiver sender clock rates

Addresses 6 bytes If adapter receives frame with matching destination address or with broadcast

address (eg ARP packet) it passes data in frame to net-layer protocol Otherwise adapter discards frame

Type Indicates the higher layer protocol (mostly IP but others may be supported

such as Novell IPX and AppleTalk) CRC

Checked at receiver if error is detected the frame is simply dropped

Sending adapter encapsulates IP datagram in Ethernet frame

copy Kemal Akkaya

Ethernet uses CSMACD Connectionless No handshaking between sending and receiving

adapter Unreliable receiving adapter doesnrsquot send acks or nacks to

sending adapter Stream of datagrams passed to network layer can have gaps Gaps will be filled if app is using TCP Otherwise app will see the gaps

No slots Adapter doesnrsquot transmit if it senses that some other adapter is

transmitting that is carrier sense Transmitting adapter aborts when it senses that another adapter is

transmitting that is collision detection Before attempting a retransmission adapter waits a random time

that is random access

copy Kemal Akkaya

Ethernet CSMACD algorithm1 Adaptor receives datagram from

net layer amp creates frame2 If adapter senses channel idle it

starts to transmit frame If it senses channel busy waits until channel idle and then transmits

3 If adapter transmits entire frame without detecting another transmission the adapter is done with frame

4 If adapter detects another transmission while transmitting aborts and sends jam signal

5 After aborting adapter enters exponential backoff after the mth collision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2

Jam Signal make sure all other transmitters are aware of collision 48 bits

Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec

Exponential Backoff Goal adapt retransmission

attempts to estimated current load Heavy load random wait will be

longer

First collision choose K from 01 delay is K 512 bit transmission times

After second collision choose K from 0123hellip

After ten collisions choose K from 01234hellip1023

copy Kemal Akkaya

CSMACD efficiency Tprop = max prop between 2 nodes in LAN ttrans = time to transmit max-size frame

Efficiency goes to 1 as tprop goes to 0 Goes to 1 as ttrans goes to infinity Much better than ALOHA but still decentralized

simple and cheap

Other Ethernets Switched Ethernet Fast Ethernet Gigabit Ethernet

transprop tt 51

1efficiency

copy Kemal Akkaya

Point to Point Data Link Control

One sender one receiver one link easier than broadcast link No Media Access Control No need for explicit MAC addressing eg dialup link ISDN line

Popular point-to-point DLC protocols PPP (point-to-point protocol) HDLC High level data link control (Data link used to be considered

ldquohigh layerrdquo in protocol stack

copy Kemal Akkaya

HDLC

Frame format for bit-oriented protocol HDLC

Control field of

(a) An information frame

(b) A supervisory frame

(c) An unnumbered frame

copy Kemal Akkaya

HDLC Exchange of information supervisory and unnumbered

frames Information - data to be transmitted to user (next layer up)

Flow and error control piggybacked on information frames Supervisory - ARQ when piggyback not used Unnumbered - supplementary link control

Three phases Initialization Data transfer Disconnect

Uses bit stuffing Flag delimit frame at both ends 01111110 is the Flag May close one frame and open another Bit stuffing to find flag data

copy Kemal Akkaya

Internet Datalink Protocol PPP Design Requirements [RFC 1557]

Packet framing encapsulation of network-layer datagram in data link frame Carry network layer data of any network layer protocol (not just IP) at

same time Ability to demultiplex upwards

Bit transparency must carry any bit pattern in the data field Error detection (no correction) Connection liveness detect signal link failure to network layer Network layer address negotiation endpoint can learnconfigure

each otherrsquos network address

PPP non-design requirements No error correctionrecovery No flow control Out of order delivery OK No need to support multipoint links (eg polling)

Error recovery flow control data re-ordering all relegated to higher layers

copy Kemal Akkaya

PPP Data Frame

Flag delimiter (framing) Address does nothing (only one option) Control does nothing in the future possible multiple

control fields Protocol upper layer protocol to which frame delivered

(eg PPP-LCP IP IPCP etc) Info upper layer data being carried Check cyclic redundancy check for error detection

copy Kemal Akkaya

Byte Stuffing ldquoData transparencyrdquo requirement data field must be allowed to include

flag pattern lt01111110gt Q is received lt01111110gt data or flag

Sender adds (ldquostuffsrdquo) extra lt 01111110gt byte after each lt 01111110gt data byte How about bit stuffing

Receiver Two 01111110 bytes in a row Discard first byte continue data reception Single 01111110 flag byte

flag bytepatternin datato send

flag byte pattern plusstuffed byte in transmitted data

copy Kemal Akkaya

PPP Data Control Protocol Before exchanging

network-layer data data link peers must configure PPP link

(max frame length authentication)

learnconfigure network layer information

For IP carry IP Control Protocol (IPCP) messages (protocol field 8021) to configurelearn IP address

Diagram for bring a line up and down

copy Kemal Akkaya

A

R

B

Routing to another LAN

copy Kemal Akkaya

EthernetldquoDominantrdquo wired LAN technology Cheap $20 for 100Mbs First widely used LAN technology Simpler cheaper than token LANs and ATM Kept up with speed race 10 Mbps ndash 10

Gbps

Metcalfersquos Ethernet sketch

Star Topology Bus topology popular

through mid 90s Now star topology prevails Connection choices hub or

switch (more later)

hub orswitch

copy Kemal Akkaya

Ethernet Frame Structure

Preamble 7 bytes with pattern 10101010 followed by one byte with pattern 10101011 Used to synchronize receiver sender clock rates

Addresses 6 bytes If adapter receives frame with matching destination address or with broadcast

address (eg ARP packet) it passes data in frame to net-layer protocol Otherwise adapter discards frame

Type Indicates the higher layer protocol (mostly IP but others may be supported

such as Novell IPX and AppleTalk) CRC

Checked at receiver if error is detected the frame is simply dropped

Sending adapter encapsulates IP datagram in Ethernet frame

copy Kemal Akkaya

Ethernet uses CSMACD Connectionless No handshaking between sending and receiving

adapter Unreliable receiving adapter doesnrsquot send acks or nacks to

sending adapter Stream of datagrams passed to network layer can have gaps Gaps will be filled if app is using TCP Otherwise app will see the gaps

No slots Adapter doesnrsquot transmit if it senses that some other adapter is

transmitting that is carrier sense Transmitting adapter aborts when it senses that another adapter is

transmitting that is collision detection Before attempting a retransmission adapter waits a random time

that is random access

copy Kemal Akkaya

Ethernet CSMACD algorithm1 Adaptor receives datagram from

net layer amp creates frame2 If adapter senses channel idle it

starts to transmit frame If it senses channel busy waits until channel idle and then transmits

3 If adapter transmits entire frame without detecting another transmission the adapter is done with frame

4 If adapter detects another transmission while transmitting aborts and sends jam signal

5 After aborting adapter enters exponential backoff after the mth collision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2

Jam Signal make sure all other transmitters are aware of collision 48 bits

Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec

Exponential Backoff Goal adapt retransmission

attempts to estimated current load Heavy load random wait will be

longer

First collision choose K from 01 delay is K 512 bit transmission times

After second collision choose K from 0123hellip

After ten collisions choose K from 01234hellip1023

copy Kemal Akkaya

CSMACD efficiency Tprop = max prop between 2 nodes in LAN ttrans = time to transmit max-size frame

Efficiency goes to 1 as tprop goes to 0 Goes to 1 as ttrans goes to infinity Much better than ALOHA but still decentralized

simple and cheap

Other Ethernets Switched Ethernet Fast Ethernet Gigabit Ethernet

transprop tt 51

1efficiency

copy Kemal Akkaya

Point to Point Data Link Control

One sender one receiver one link easier than broadcast link No Media Access Control No need for explicit MAC addressing eg dialup link ISDN line

Popular point-to-point DLC protocols PPP (point-to-point protocol) HDLC High level data link control (Data link used to be considered

ldquohigh layerrdquo in protocol stack

copy Kemal Akkaya

HDLC

Frame format for bit-oriented protocol HDLC

Control field of

(a) An information frame

(b) A supervisory frame

(c) An unnumbered frame

copy Kemal Akkaya

HDLC Exchange of information supervisory and unnumbered

frames Information - data to be transmitted to user (next layer up)

Flow and error control piggybacked on information frames Supervisory - ARQ when piggyback not used Unnumbered - supplementary link control

Three phases Initialization Data transfer Disconnect

Uses bit stuffing Flag delimit frame at both ends 01111110 is the Flag May close one frame and open another Bit stuffing to find flag data

copy Kemal Akkaya

Internet Datalink Protocol PPP Design Requirements [RFC 1557]

Packet framing encapsulation of network-layer datagram in data link frame Carry network layer data of any network layer protocol (not just IP) at

same time Ability to demultiplex upwards

Bit transparency must carry any bit pattern in the data field Error detection (no correction) Connection liveness detect signal link failure to network layer Network layer address negotiation endpoint can learnconfigure

each otherrsquos network address

PPP non-design requirements No error correctionrecovery No flow control Out of order delivery OK No need to support multipoint links (eg polling)

Error recovery flow control data re-ordering all relegated to higher layers

copy Kemal Akkaya

PPP Data Frame

Flag delimiter (framing) Address does nothing (only one option) Control does nothing in the future possible multiple

control fields Protocol upper layer protocol to which frame delivered

(eg PPP-LCP IP IPCP etc) Info upper layer data being carried Check cyclic redundancy check for error detection

copy Kemal Akkaya

Byte Stuffing ldquoData transparencyrdquo requirement data field must be allowed to include

flag pattern lt01111110gt Q is received lt01111110gt data or flag

Sender adds (ldquostuffsrdquo) extra lt 01111110gt byte after each lt 01111110gt data byte How about bit stuffing

Receiver Two 01111110 bytes in a row Discard first byte continue data reception Single 01111110 flag byte

flag bytepatternin datato send

flag byte pattern plusstuffed byte in transmitted data

copy Kemal Akkaya

PPP Data Control Protocol Before exchanging

network-layer data data link peers must configure PPP link

(max frame length authentication)

learnconfigure network layer information

For IP carry IP Control Protocol (IPCP) messages (protocol field 8021) to configurelearn IP address

Diagram for bring a line up and down

copy Kemal Akkaya

EthernetldquoDominantrdquo wired LAN technology Cheap $20 for 100Mbs First widely used LAN technology Simpler cheaper than token LANs and ATM Kept up with speed race 10 Mbps ndash 10

Gbps

Metcalfersquos Ethernet sketch

Star Topology Bus topology popular

through mid 90s Now star topology prevails Connection choices hub or

switch (more later)

hub orswitch

copy Kemal Akkaya

Ethernet Frame Structure

Preamble 7 bytes with pattern 10101010 followed by one byte with pattern 10101011 Used to synchronize receiver sender clock rates

Addresses 6 bytes If adapter receives frame with matching destination address or with broadcast

address (eg ARP packet) it passes data in frame to net-layer protocol Otherwise adapter discards frame

Type Indicates the higher layer protocol (mostly IP but others may be supported

such as Novell IPX and AppleTalk) CRC

Checked at receiver if error is detected the frame is simply dropped

Sending adapter encapsulates IP datagram in Ethernet frame

copy Kemal Akkaya

Ethernet uses CSMACD Connectionless No handshaking between sending and receiving

adapter Unreliable receiving adapter doesnrsquot send acks or nacks to

sending adapter Stream of datagrams passed to network layer can have gaps Gaps will be filled if app is using TCP Otherwise app will see the gaps

No slots Adapter doesnrsquot transmit if it senses that some other adapter is

transmitting that is carrier sense Transmitting adapter aborts when it senses that another adapter is

transmitting that is collision detection Before attempting a retransmission adapter waits a random time

that is random access

copy Kemal Akkaya

Ethernet CSMACD algorithm1 Adaptor receives datagram from

net layer amp creates frame2 If adapter senses channel idle it

starts to transmit frame If it senses channel busy waits until channel idle and then transmits

3 If adapter transmits entire frame without detecting another transmission the adapter is done with frame

4 If adapter detects another transmission while transmitting aborts and sends jam signal

5 After aborting adapter enters exponential backoff after the mth collision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2

Jam Signal make sure all other transmitters are aware of collision 48 bits

Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec

Exponential Backoff Goal adapt retransmission

attempts to estimated current load Heavy load random wait will be

longer

First collision choose K from 01 delay is K 512 bit transmission times

After second collision choose K from 0123hellip

After ten collisions choose K from 01234hellip1023

copy Kemal Akkaya

CSMACD efficiency Tprop = max prop between 2 nodes in LAN ttrans = time to transmit max-size frame

Efficiency goes to 1 as tprop goes to 0 Goes to 1 as ttrans goes to infinity Much better than ALOHA but still decentralized

simple and cheap

Other Ethernets Switched Ethernet Fast Ethernet Gigabit Ethernet

transprop tt 51

1efficiency

copy Kemal Akkaya

Point to Point Data Link Control

One sender one receiver one link easier than broadcast link No Media Access Control No need for explicit MAC addressing eg dialup link ISDN line

Popular point-to-point DLC protocols PPP (point-to-point protocol) HDLC High level data link control (Data link used to be considered

ldquohigh layerrdquo in protocol stack

copy Kemal Akkaya

HDLC

Frame format for bit-oriented protocol HDLC

Control field of

(a) An information frame

(b) A supervisory frame

(c) An unnumbered frame

copy Kemal Akkaya

HDLC Exchange of information supervisory and unnumbered

frames Information - data to be transmitted to user (next layer up)

Flow and error control piggybacked on information frames Supervisory - ARQ when piggyback not used Unnumbered - supplementary link control

Three phases Initialization Data transfer Disconnect

Uses bit stuffing Flag delimit frame at both ends 01111110 is the Flag May close one frame and open another Bit stuffing to find flag data

copy Kemal Akkaya

Internet Datalink Protocol PPP Design Requirements [RFC 1557]

Packet framing encapsulation of network-layer datagram in data link frame Carry network layer data of any network layer protocol (not just IP) at

same time Ability to demultiplex upwards

Bit transparency must carry any bit pattern in the data field Error detection (no correction) Connection liveness detect signal link failure to network layer Network layer address negotiation endpoint can learnconfigure

each otherrsquos network address

PPP non-design requirements No error correctionrecovery No flow control Out of order delivery OK No need to support multipoint links (eg polling)

Error recovery flow control data re-ordering all relegated to higher layers

copy Kemal Akkaya

PPP Data Frame

Flag delimiter (framing) Address does nothing (only one option) Control does nothing in the future possible multiple

control fields Protocol upper layer protocol to which frame delivered

(eg PPP-LCP IP IPCP etc) Info upper layer data being carried Check cyclic redundancy check for error detection

copy Kemal Akkaya

Byte Stuffing ldquoData transparencyrdquo requirement data field must be allowed to include

flag pattern lt01111110gt Q is received lt01111110gt data or flag

Sender adds (ldquostuffsrdquo) extra lt 01111110gt byte after each lt 01111110gt data byte How about bit stuffing

Receiver Two 01111110 bytes in a row Discard first byte continue data reception Single 01111110 flag byte

flag bytepatternin datato send

flag byte pattern plusstuffed byte in transmitted data

copy Kemal Akkaya

PPP Data Control Protocol Before exchanging

network-layer data data link peers must configure PPP link

(max frame length authentication)

learnconfigure network layer information

For IP carry IP Control Protocol (IPCP) messages (protocol field 8021) to configurelearn IP address

Diagram for bring a line up and down

copy Kemal Akkaya

Ethernet Frame Structure

Preamble 7 bytes with pattern 10101010 followed by one byte with pattern 10101011 Used to synchronize receiver sender clock rates

Addresses 6 bytes If adapter receives frame with matching destination address or with broadcast

address (eg ARP packet) it passes data in frame to net-layer protocol Otherwise adapter discards frame

Type Indicates the higher layer protocol (mostly IP but others may be supported

such as Novell IPX and AppleTalk) CRC

Checked at receiver if error is detected the frame is simply dropped

Sending adapter encapsulates IP datagram in Ethernet frame

copy Kemal Akkaya

Ethernet uses CSMACD Connectionless No handshaking between sending and receiving

adapter Unreliable receiving adapter doesnrsquot send acks or nacks to

sending adapter Stream of datagrams passed to network layer can have gaps Gaps will be filled if app is using TCP Otherwise app will see the gaps

No slots Adapter doesnrsquot transmit if it senses that some other adapter is

transmitting that is carrier sense Transmitting adapter aborts when it senses that another adapter is

transmitting that is collision detection Before attempting a retransmission adapter waits a random time

that is random access

copy Kemal Akkaya

Ethernet CSMACD algorithm1 Adaptor receives datagram from

net layer amp creates frame2 If adapter senses channel idle it

starts to transmit frame If it senses channel busy waits until channel idle and then transmits

3 If adapter transmits entire frame without detecting another transmission the adapter is done with frame

4 If adapter detects another transmission while transmitting aborts and sends jam signal

5 After aborting adapter enters exponential backoff after the mth collision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2

Jam Signal make sure all other transmitters are aware of collision 48 bits

Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec

Exponential Backoff Goal adapt retransmission

attempts to estimated current load Heavy load random wait will be

longer

First collision choose K from 01 delay is K 512 bit transmission times

After second collision choose K from 0123hellip

After ten collisions choose K from 01234hellip1023

copy Kemal Akkaya

CSMACD efficiency Tprop = max prop between 2 nodes in LAN ttrans = time to transmit max-size frame

Efficiency goes to 1 as tprop goes to 0 Goes to 1 as ttrans goes to infinity Much better than ALOHA but still decentralized

simple and cheap

Other Ethernets Switched Ethernet Fast Ethernet Gigabit Ethernet

transprop tt 51

1efficiency

copy Kemal Akkaya

Point to Point Data Link Control

One sender one receiver one link easier than broadcast link No Media Access Control No need for explicit MAC addressing eg dialup link ISDN line

Popular point-to-point DLC protocols PPP (point-to-point protocol) HDLC High level data link control (Data link used to be considered

ldquohigh layerrdquo in protocol stack

copy Kemal Akkaya

HDLC

Frame format for bit-oriented protocol HDLC

Control field of

(a) An information frame

(b) A supervisory frame

(c) An unnumbered frame

copy Kemal Akkaya

HDLC Exchange of information supervisory and unnumbered

frames Information - data to be transmitted to user (next layer up)

Flow and error control piggybacked on information frames Supervisory - ARQ when piggyback not used Unnumbered - supplementary link control

Three phases Initialization Data transfer Disconnect

Uses bit stuffing Flag delimit frame at both ends 01111110 is the Flag May close one frame and open another Bit stuffing to find flag data

copy Kemal Akkaya

Internet Datalink Protocol PPP Design Requirements [RFC 1557]

Packet framing encapsulation of network-layer datagram in data link frame Carry network layer data of any network layer protocol (not just IP) at

same time Ability to demultiplex upwards

Bit transparency must carry any bit pattern in the data field Error detection (no correction) Connection liveness detect signal link failure to network layer Network layer address negotiation endpoint can learnconfigure

each otherrsquos network address

PPP non-design requirements No error correctionrecovery No flow control Out of order delivery OK No need to support multipoint links (eg polling)

Error recovery flow control data re-ordering all relegated to higher layers

copy Kemal Akkaya

PPP Data Frame

Flag delimiter (framing) Address does nothing (only one option) Control does nothing in the future possible multiple

control fields Protocol upper layer protocol to which frame delivered

(eg PPP-LCP IP IPCP etc) Info upper layer data being carried Check cyclic redundancy check for error detection

copy Kemal Akkaya

Byte Stuffing ldquoData transparencyrdquo requirement data field must be allowed to include

flag pattern lt01111110gt Q is received lt01111110gt data or flag

Sender adds (ldquostuffsrdquo) extra lt 01111110gt byte after each lt 01111110gt data byte How about bit stuffing

Receiver Two 01111110 bytes in a row Discard first byte continue data reception Single 01111110 flag byte

flag bytepatternin datato send

flag byte pattern plusstuffed byte in transmitted data

copy Kemal Akkaya

PPP Data Control Protocol Before exchanging

network-layer data data link peers must configure PPP link

(max frame length authentication)

learnconfigure network layer information

For IP carry IP Control Protocol (IPCP) messages (protocol field 8021) to configurelearn IP address

Diagram for bring a line up and down

copy Kemal Akkaya

Ethernet uses CSMACD Connectionless No handshaking between sending and receiving

adapter Unreliable receiving adapter doesnrsquot send acks or nacks to

sending adapter Stream of datagrams passed to network layer can have gaps Gaps will be filled if app is using TCP Otherwise app will see the gaps

No slots Adapter doesnrsquot transmit if it senses that some other adapter is

transmitting that is carrier sense Transmitting adapter aborts when it senses that another adapter is

transmitting that is collision detection Before attempting a retransmission adapter waits a random time

that is random access

copy Kemal Akkaya

Ethernet CSMACD algorithm1 Adaptor receives datagram from

net layer amp creates frame2 If adapter senses channel idle it

starts to transmit frame If it senses channel busy waits until channel idle and then transmits

3 If adapter transmits entire frame without detecting another transmission the adapter is done with frame

4 If adapter detects another transmission while transmitting aborts and sends jam signal

5 After aborting adapter enters exponential backoff after the mth collision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2

Jam Signal make sure all other transmitters are aware of collision 48 bits

Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec

Exponential Backoff Goal adapt retransmission

attempts to estimated current load Heavy load random wait will be

longer

First collision choose K from 01 delay is K 512 bit transmission times

After second collision choose K from 0123hellip

After ten collisions choose K from 01234hellip1023

copy Kemal Akkaya

CSMACD efficiency Tprop = max prop between 2 nodes in LAN ttrans = time to transmit max-size frame

Efficiency goes to 1 as tprop goes to 0 Goes to 1 as ttrans goes to infinity Much better than ALOHA but still decentralized

simple and cheap

Other Ethernets Switched Ethernet Fast Ethernet Gigabit Ethernet

transprop tt 51

1efficiency

copy Kemal Akkaya

Point to Point Data Link Control

One sender one receiver one link easier than broadcast link No Media Access Control No need for explicit MAC addressing eg dialup link ISDN line

Popular point-to-point DLC protocols PPP (point-to-point protocol) HDLC High level data link control (Data link used to be considered

ldquohigh layerrdquo in protocol stack

copy Kemal Akkaya

HDLC

Frame format for bit-oriented protocol HDLC

Control field of

(a) An information frame

(b) A supervisory frame

(c) An unnumbered frame

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HDLC Exchange of information supervisory and unnumbered

frames Information - data to be transmitted to user (next layer up)

Flow and error control piggybacked on information frames Supervisory - ARQ when piggyback not used Unnumbered - supplementary link control

Three phases Initialization Data transfer Disconnect

Uses bit stuffing Flag delimit frame at both ends 01111110 is the Flag May close one frame and open another Bit stuffing to find flag data

copy Kemal Akkaya

Internet Datalink Protocol PPP Design Requirements [RFC 1557]

Packet framing encapsulation of network-layer datagram in data link frame Carry network layer data of any network layer protocol (not just IP) at

same time Ability to demultiplex upwards

Bit transparency must carry any bit pattern in the data field Error detection (no correction) Connection liveness detect signal link failure to network layer Network layer address negotiation endpoint can learnconfigure

each otherrsquos network address

PPP non-design requirements No error correctionrecovery No flow control Out of order delivery OK No need to support multipoint links (eg polling)

Error recovery flow control data re-ordering all relegated to higher layers

copy Kemal Akkaya

PPP Data Frame

Flag delimiter (framing) Address does nothing (only one option) Control does nothing in the future possible multiple

control fields Protocol upper layer protocol to which frame delivered

(eg PPP-LCP IP IPCP etc) Info upper layer data being carried Check cyclic redundancy check for error detection

copy Kemal Akkaya

Byte Stuffing ldquoData transparencyrdquo requirement data field must be allowed to include

flag pattern lt01111110gt Q is received lt01111110gt data or flag

Sender adds (ldquostuffsrdquo) extra lt 01111110gt byte after each lt 01111110gt data byte How about bit stuffing

Receiver Two 01111110 bytes in a row Discard first byte continue data reception Single 01111110 flag byte

flag bytepatternin datato send

flag byte pattern plusstuffed byte in transmitted data

copy Kemal Akkaya

PPP Data Control Protocol Before exchanging

network-layer data data link peers must configure PPP link

(max frame length authentication)

learnconfigure network layer information

For IP carry IP Control Protocol (IPCP) messages (protocol field 8021) to configurelearn IP address

Diagram for bring a line up and down

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Ethernet CSMACD algorithm1 Adaptor receives datagram from

net layer amp creates frame2 If adapter senses channel idle it

starts to transmit frame If it senses channel busy waits until channel idle and then transmits

3 If adapter transmits entire frame without detecting another transmission the adapter is done with frame

4 If adapter detects another transmission while transmitting aborts and sends jam signal

5 After aborting adapter enters exponential backoff after the mth collision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2

Jam Signal make sure all other transmitters are aware of collision 48 bits

Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec

Exponential Backoff Goal adapt retransmission

attempts to estimated current load Heavy load random wait will be

longer

First collision choose K from 01 delay is K 512 bit transmission times

After second collision choose K from 0123hellip

After ten collisions choose K from 01234hellip1023

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CSMACD efficiency Tprop = max prop between 2 nodes in LAN ttrans = time to transmit max-size frame

Efficiency goes to 1 as tprop goes to 0 Goes to 1 as ttrans goes to infinity Much better than ALOHA but still decentralized

simple and cheap

Other Ethernets Switched Ethernet Fast Ethernet Gigabit Ethernet

transprop tt 51

1efficiency

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Point to Point Data Link Control

One sender one receiver one link easier than broadcast link No Media Access Control No need for explicit MAC addressing eg dialup link ISDN line

Popular point-to-point DLC protocols PPP (point-to-point protocol) HDLC High level data link control (Data link used to be considered

ldquohigh layerrdquo in protocol stack

copy Kemal Akkaya

HDLC

Frame format for bit-oriented protocol HDLC

Control field of

(a) An information frame

(b) A supervisory frame

(c) An unnumbered frame

copy Kemal Akkaya

HDLC Exchange of information supervisory and unnumbered

frames Information - data to be transmitted to user (next layer up)

Flow and error control piggybacked on information frames Supervisory - ARQ when piggyback not used Unnumbered - supplementary link control

Three phases Initialization Data transfer Disconnect

Uses bit stuffing Flag delimit frame at both ends 01111110 is the Flag May close one frame and open another Bit stuffing to find flag data

copy Kemal Akkaya

Internet Datalink Protocol PPP Design Requirements [RFC 1557]

Packet framing encapsulation of network-layer datagram in data link frame Carry network layer data of any network layer protocol (not just IP) at

same time Ability to demultiplex upwards

Bit transparency must carry any bit pattern in the data field Error detection (no correction) Connection liveness detect signal link failure to network layer Network layer address negotiation endpoint can learnconfigure

each otherrsquos network address

PPP non-design requirements No error correctionrecovery No flow control Out of order delivery OK No need to support multipoint links (eg polling)

Error recovery flow control data re-ordering all relegated to higher layers

copy Kemal Akkaya

PPP Data Frame

Flag delimiter (framing) Address does nothing (only one option) Control does nothing in the future possible multiple

control fields Protocol upper layer protocol to which frame delivered

(eg PPP-LCP IP IPCP etc) Info upper layer data being carried Check cyclic redundancy check for error detection

copy Kemal Akkaya

Byte Stuffing ldquoData transparencyrdquo requirement data field must be allowed to include

flag pattern lt01111110gt Q is received lt01111110gt data or flag

Sender adds (ldquostuffsrdquo) extra lt 01111110gt byte after each lt 01111110gt data byte How about bit stuffing

Receiver Two 01111110 bytes in a row Discard first byte continue data reception Single 01111110 flag byte

flag bytepatternin datato send

flag byte pattern plusstuffed byte in transmitted data

copy Kemal Akkaya

PPP Data Control Protocol Before exchanging

network-layer data data link peers must configure PPP link

(max frame length authentication)

learnconfigure network layer information

For IP carry IP Control Protocol (IPCP) messages (protocol field 8021) to configurelearn IP address

Diagram for bring a line up and down

copy Kemal Akkaya

CSMACD efficiency Tprop = max prop between 2 nodes in LAN ttrans = time to transmit max-size frame

Efficiency goes to 1 as tprop goes to 0 Goes to 1 as ttrans goes to infinity Much better than ALOHA but still decentralized

simple and cheap

Other Ethernets Switched Ethernet Fast Ethernet Gigabit Ethernet

transprop tt 51

1efficiency

copy Kemal Akkaya

Point to Point Data Link Control

One sender one receiver one link easier than broadcast link No Media Access Control No need for explicit MAC addressing eg dialup link ISDN line

Popular point-to-point DLC protocols PPP (point-to-point protocol) HDLC High level data link control (Data link used to be considered

ldquohigh layerrdquo in protocol stack

copy Kemal Akkaya

HDLC

Frame format for bit-oriented protocol HDLC

Control field of

(a) An information frame

(b) A supervisory frame

(c) An unnumbered frame

copy Kemal Akkaya

HDLC Exchange of information supervisory and unnumbered

frames Information - data to be transmitted to user (next layer up)

Flow and error control piggybacked on information frames Supervisory - ARQ when piggyback not used Unnumbered - supplementary link control

Three phases Initialization Data transfer Disconnect

Uses bit stuffing Flag delimit frame at both ends 01111110 is the Flag May close one frame and open another Bit stuffing to find flag data

copy Kemal Akkaya

Internet Datalink Protocol PPP Design Requirements [RFC 1557]

Packet framing encapsulation of network-layer datagram in data link frame Carry network layer data of any network layer protocol (not just IP) at

same time Ability to demultiplex upwards

Bit transparency must carry any bit pattern in the data field Error detection (no correction) Connection liveness detect signal link failure to network layer Network layer address negotiation endpoint can learnconfigure

each otherrsquos network address

PPP non-design requirements No error correctionrecovery No flow control Out of order delivery OK No need to support multipoint links (eg polling)

Error recovery flow control data re-ordering all relegated to higher layers

copy Kemal Akkaya

PPP Data Frame

Flag delimiter (framing) Address does nothing (only one option) Control does nothing in the future possible multiple

control fields Protocol upper layer protocol to which frame delivered

(eg PPP-LCP IP IPCP etc) Info upper layer data being carried Check cyclic redundancy check for error detection

copy Kemal Akkaya

Byte Stuffing ldquoData transparencyrdquo requirement data field must be allowed to include

flag pattern lt01111110gt Q is received lt01111110gt data or flag

Sender adds (ldquostuffsrdquo) extra lt 01111110gt byte after each lt 01111110gt data byte How about bit stuffing

Receiver Two 01111110 bytes in a row Discard first byte continue data reception Single 01111110 flag byte

flag bytepatternin datato send

flag byte pattern plusstuffed byte in transmitted data

copy Kemal Akkaya

PPP Data Control Protocol Before exchanging

network-layer data data link peers must configure PPP link

(max frame length authentication)

learnconfigure network layer information

For IP carry IP Control Protocol (IPCP) messages (protocol field 8021) to configurelearn IP address

Diagram for bring a line up and down

copy Kemal Akkaya

Point to Point Data Link Control

One sender one receiver one link easier than broadcast link No Media Access Control No need for explicit MAC addressing eg dialup link ISDN line

Popular point-to-point DLC protocols PPP (point-to-point protocol) HDLC High level data link control (Data link used to be considered

ldquohigh layerrdquo in protocol stack

copy Kemal Akkaya

HDLC

Frame format for bit-oriented protocol HDLC

Control field of

(a) An information frame

(b) A supervisory frame

(c) An unnumbered frame

copy Kemal Akkaya

HDLC Exchange of information supervisory and unnumbered

frames Information - data to be transmitted to user (next layer up)

Flow and error control piggybacked on information frames Supervisory - ARQ when piggyback not used Unnumbered - supplementary link control

Three phases Initialization Data transfer Disconnect

Uses bit stuffing Flag delimit frame at both ends 01111110 is the Flag May close one frame and open another Bit stuffing to find flag data

copy Kemal Akkaya

Internet Datalink Protocol PPP Design Requirements [RFC 1557]

Packet framing encapsulation of network-layer datagram in data link frame Carry network layer data of any network layer protocol (not just IP) at

same time Ability to demultiplex upwards

Bit transparency must carry any bit pattern in the data field Error detection (no correction) Connection liveness detect signal link failure to network layer Network layer address negotiation endpoint can learnconfigure

each otherrsquos network address

PPP non-design requirements No error correctionrecovery No flow control Out of order delivery OK No need to support multipoint links (eg polling)

Error recovery flow control data re-ordering all relegated to higher layers

copy Kemal Akkaya

PPP Data Frame

Flag delimiter (framing) Address does nothing (only one option) Control does nothing in the future possible multiple

control fields Protocol upper layer protocol to which frame delivered

(eg PPP-LCP IP IPCP etc) Info upper layer data being carried Check cyclic redundancy check for error detection

copy Kemal Akkaya

Byte Stuffing ldquoData transparencyrdquo requirement data field must be allowed to include

flag pattern lt01111110gt Q is received lt01111110gt data or flag

Sender adds (ldquostuffsrdquo) extra lt 01111110gt byte after each lt 01111110gt data byte How about bit stuffing

Receiver Two 01111110 bytes in a row Discard first byte continue data reception Single 01111110 flag byte

flag bytepatternin datato send

flag byte pattern plusstuffed byte in transmitted data

copy Kemal Akkaya

PPP Data Control Protocol Before exchanging

network-layer data data link peers must configure PPP link

(max frame length authentication)

learnconfigure network layer information

For IP carry IP Control Protocol (IPCP) messages (protocol field 8021) to configurelearn IP address

Diagram for bring a line up and down

copy Kemal Akkaya

HDLC

Frame format for bit-oriented protocol HDLC

Control field of

(a) An information frame

(b) A supervisory frame

(c) An unnumbered frame

copy Kemal Akkaya

HDLC Exchange of information supervisory and unnumbered

frames Information - data to be transmitted to user (next layer up)

Flow and error control piggybacked on information frames Supervisory - ARQ when piggyback not used Unnumbered - supplementary link control

Three phases Initialization Data transfer Disconnect

Uses bit stuffing Flag delimit frame at both ends 01111110 is the Flag May close one frame and open another Bit stuffing to find flag data

copy Kemal Akkaya

Internet Datalink Protocol PPP Design Requirements [RFC 1557]

Packet framing encapsulation of network-layer datagram in data link frame Carry network layer data of any network layer protocol (not just IP) at

same time Ability to demultiplex upwards

Bit transparency must carry any bit pattern in the data field Error detection (no correction) Connection liveness detect signal link failure to network layer Network layer address negotiation endpoint can learnconfigure

each otherrsquos network address

PPP non-design requirements No error correctionrecovery No flow control Out of order delivery OK No need to support multipoint links (eg polling)

Error recovery flow control data re-ordering all relegated to higher layers

copy Kemal Akkaya

PPP Data Frame

Flag delimiter (framing) Address does nothing (only one option) Control does nothing in the future possible multiple

control fields Protocol upper layer protocol to which frame delivered

(eg PPP-LCP IP IPCP etc) Info upper layer data being carried Check cyclic redundancy check for error detection

copy Kemal Akkaya

Byte Stuffing ldquoData transparencyrdquo requirement data field must be allowed to include

flag pattern lt01111110gt Q is received lt01111110gt data or flag

Sender adds (ldquostuffsrdquo) extra lt 01111110gt byte after each lt 01111110gt data byte How about bit stuffing

Receiver Two 01111110 bytes in a row Discard first byte continue data reception Single 01111110 flag byte

flag bytepatternin datato send

flag byte pattern plusstuffed byte in transmitted data

copy Kemal Akkaya

PPP Data Control Protocol Before exchanging

network-layer data data link peers must configure PPP link

(max frame length authentication)

learnconfigure network layer information

For IP carry IP Control Protocol (IPCP) messages (protocol field 8021) to configurelearn IP address

Diagram for bring a line up and down

copy Kemal Akkaya

HDLC Exchange of information supervisory and unnumbered

frames Information - data to be transmitted to user (next layer up)

Flow and error control piggybacked on information frames Supervisory - ARQ when piggyback not used Unnumbered - supplementary link control

Three phases Initialization Data transfer Disconnect

Uses bit stuffing Flag delimit frame at both ends 01111110 is the Flag May close one frame and open another Bit stuffing to find flag data

copy Kemal Akkaya

Internet Datalink Protocol PPP Design Requirements [RFC 1557]

Packet framing encapsulation of network-layer datagram in data link frame Carry network layer data of any network layer protocol (not just IP) at

same time Ability to demultiplex upwards

Bit transparency must carry any bit pattern in the data field Error detection (no correction) Connection liveness detect signal link failure to network layer Network layer address negotiation endpoint can learnconfigure

each otherrsquos network address

PPP non-design requirements No error correctionrecovery No flow control Out of order delivery OK No need to support multipoint links (eg polling)

Error recovery flow control data re-ordering all relegated to higher layers

copy Kemal Akkaya

PPP Data Frame

Flag delimiter (framing) Address does nothing (only one option) Control does nothing in the future possible multiple

control fields Protocol upper layer protocol to which frame delivered

(eg PPP-LCP IP IPCP etc) Info upper layer data being carried Check cyclic redundancy check for error detection

copy Kemal Akkaya

Byte Stuffing ldquoData transparencyrdquo requirement data field must be allowed to include

flag pattern lt01111110gt Q is received lt01111110gt data or flag

Sender adds (ldquostuffsrdquo) extra lt 01111110gt byte after each lt 01111110gt data byte How about bit stuffing

Receiver Two 01111110 bytes in a row Discard first byte continue data reception Single 01111110 flag byte

flag bytepatternin datato send

flag byte pattern plusstuffed byte in transmitted data

copy Kemal Akkaya

PPP Data Control Protocol Before exchanging

network-layer data data link peers must configure PPP link

(max frame length authentication)

learnconfigure network layer information

For IP carry IP Control Protocol (IPCP) messages (protocol field 8021) to configurelearn IP address

Diagram for bring a line up and down

copy Kemal Akkaya

Internet Datalink Protocol PPP Design Requirements [RFC 1557]

Packet framing encapsulation of network-layer datagram in data link frame Carry network layer data of any network layer protocol (not just IP) at

same time Ability to demultiplex upwards

Bit transparency must carry any bit pattern in the data field Error detection (no correction) Connection liveness detect signal link failure to network layer Network layer address negotiation endpoint can learnconfigure

each otherrsquos network address

PPP non-design requirements No error correctionrecovery No flow control Out of order delivery OK No need to support multipoint links (eg polling)

Error recovery flow control data re-ordering all relegated to higher layers

copy Kemal Akkaya

PPP Data Frame

Flag delimiter (framing) Address does nothing (only one option) Control does nothing in the future possible multiple

control fields Protocol upper layer protocol to which frame delivered

(eg PPP-LCP IP IPCP etc) Info upper layer data being carried Check cyclic redundancy check for error detection

copy Kemal Akkaya

Byte Stuffing ldquoData transparencyrdquo requirement data field must be allowed to include

flag pattern lt01111110gt Q is received lt01111110gt data or flag

Sender adds (ldquostuffsrdquo) extra lt 01111110gt byte after each lt 01111110gt data byte How about bit stuffing

Receiver Two 01111110 bytes in a row Discard first byte continue data reception Single 01111110 flag byte

flag bytepatternin datato send

flag byte pattern plusstuffed byte in transmitted data

copy Kemal Akkaya

PPP Data Control Protocol Before exchanging

network-layer data data link peers must configure PPP link

(max frame length authentication)

learnconfigure network layer information

For IP carry IP Control Protocol (IPCP) messages (protocol field 8021) to configurelearn IP address

Diagram for bring a line up and down

copy Kemal Akkaya

PPP Data Frame

Flag delimiter (framing) Address does nothing (only one option) Control does nothing in the future possible multiple

control fields Protocol upper layer protocol to which frame delivered

(eg PPP-LCP IP IPCP etc) Info upper layer data being carried Check cyclic redundancy check for error detection

copy Kemal Akkaya

Byte Stuffing ldquoData transparencyrdquo requirement data field must be allowed to include

flag pattern lt01111110gt Q is received lt01111110gt data or flag

Sender adds (ldquostuffsrdquo) extra lt 01111110gt byte after each lt 01111110gt data byte How about bit stuffing

Receiver Two 01111110 bytes in a row Discard first byte continue data reception Single 01111110 flag byte

flag bytepatternin datato send

flag byte pattern plusstuffed byte in transmitted data

copy Kemal Akkaya

PPP Data Control Protocol Before exchanging

network-layer data data link peers must configure PPP link

(max frame length authentication)

learnconfigure network layer information

For IP carry IP Control Protocol (IPCP) messages (protocol field 8021) to configurelearn IP address

Diagram for bring a line up and down

copy Kemal Akkaya

Byte Stuffing ldquoData transparencyrdquo requirement data field must be allowed to include

flag pattern lt01111110gt Q is received lt01111110gt data or flag

Sender adds (ldquostuffsrdquo) extra lt 01111110gt byte after each lt 01111110gt data byte How about bit stuffing

Receiver Two 01111110 bytes in a row Discard first byte continue data reception Single 01111110 flag byte

flag bytepatternin datato send

flag byte pattern plusstuffed byte in transmitted data

copy Kemal Akkaya

PPP Data Control Protocol Before exchanging

network-layer data data link peers must configure PPP link

(max frame length authentication)

learnconfigure network layer information

For IP carry IP Control Protocol (IPCP) messages (protocol field 8021) to configurelearn IP address

Diagram for bring a line up and down

copy Kemal Akkaya

PPP Data Control Protocol Before exchanging

network-layer data data link peers must configure PPP link

(max frame length authentication)

learnconfigure network layer information

For IP carry IP Control Protocol (IPCP) messages (protocol field 8021) to configurelearn IP address

Diagram for bring a line up and down