Chapter 2 Datalink Layer & LAN Protocols

Ch.2: Link Layer &LAN

Chapter 2 Datalink Layer

& LAN Protocols

1


2

Physical Media physical link:

transmitted data bit propagates across link

guided media: signals propagate in

solid media: copper, fiber

unguided media: signals propagate

freely, e.g., radio

Twisted Pair (TP) two insulated copper

wires Category 3: traditional

phone wires, 10 Mbps ethernet

Category 5 TP: 100Mbps ethernet


3

Physical Media: coax, fiberCoaxial cable: wire (signal carrier)

within a wire (shield) baseband: single

channel on cable broadband: multiple

channel on cable bidirectional common use in

10Mbs Ethernet

Fiber optic cable: glass fiber carrying

light pulses high-speed operation:

100Mbps Ethernet high-speed point-to-

point transmission (eg, 40 Gps)

very low error rate


4

Physical media: Wireless signal carried in

electromagnetic spectrum

no physical “wire” bidirectional propagation

environment effects: reflection obstruction by objects interference

Wireless link types: microwave

e.g. up to 45 Mbps channels

LAN (e.g., 802.11b/g) 11/54 Mbps

wide-area (e.g., cellular) e.g. CDPD, 10’s Kbps 3G ~ 2.4 Mbps

satellite up to 50Mbps channel

• multiple smaller channels

270 Msec end-end delay geosynchronous versus

LEOS (low earth orbit)


5

Physical link types: Point to point link Shared medium link

- also called: Broadcast link Multi-access link LAN

Shared medium link: Many stations on same

medium segment Intermittent

transmission: only when needed

Qn: WHY? Collisions occur

unless protocol makes special arrangements for co-ordination of transmission

Bit synchronization done per frame

Point to point link Two stations only Continuous

transmission Needed to keep bit

clock synchronization Sends filler when no

data Full duplex


6

The Data Link LayerOur goals: understand principles

behind data link layer services: error detection,

correction sharing a broadcast

channel: multiple access

link layer addressing instantiation and

implementation of various link layer technologies

Overview: link layer services error detection, correction multiple access protocols

and LANs link layer addressing specific link layer

technologies: Ethernet


7

Link Layer: setting the context


8 8

Recap: The Hourglass Architecture of the Internet

IP

Ethernet FDDIWireless

TCP UDP

Telnet Email FTP WWW


9

Link Layer: setting the context two physically connected devices:

host-router, router-router, host-host unit of data: frame

applicationtransportnetwork

linkphysical

networklink

physical

MMMM

HtHtHnHtHnHl MHtHnHl

framephys. link

data linkprotocol

adapter card


10

Link layer: Context Data-link layer has

responsibility of transferring datagram from one node to another node over a link

Datagram transferred by different link protocols over different links, e.g., Ethernet on first link, frame relay on

intermediate links 802.11 on last link

transportation analogy

trip from New Haven to San Francisco taxi: home to union

station train: union station

to JFK plane: JFK to San

Francisco airport shuttle: airport to

hotel


11

Link Layer Services Framing, link access:

encapsulate datagram into frame, adding header, trailer

implement channel access if shared medium, ‘physical addresses’ used in frame headers to

identify source, destination • different from IP address!

(OPTIONAL) Reliable data delivery: seldom used on low bit-error link

• E.g., fiber, twisted pair wireless links: high error rates

•Qn: why both link-level and end-end reliability?


12

Link Layer Services (more) (OPTIONAL) Flow Control:

pacing between sender and receivers Error Detection:

errors caused by signal attenuation, noise. receiver detects presence of errors:

• signals sender for retransmission or drops frame(depending on protocol)

(OPTIONAL) Error Correction: receiver identifies and corrects bit error(s)

without resorting to retransmission


13

Adaptors Communicating

link layer implemented in “adaptor” (aka NIC) Ethernet card,

modem, 802.11 card adapter is semi-

autonomous, implementing link & physical layers

sending side: encapsulates datagram in a

frame, delimits frame adds error checking bits, rdt

param’s, flow ctrl, etc. receiving side

recognizes frame start /end checks errors, rdt, flow

ctrl, .. extracts datagram, passes

to L3

sendingnode

frame

receivingnode

datagram

frameadapter adapter

link layer protocol


14

Link Layer: Implementation implemented in “adapter”

e.g., PCMCIA card, Ethernet card typically includes: RAM, DSP chips, host bus

interface, and link interface

applicationtransportnetwork

linkphysical

networklink

physical

MMMM

HtHtHnHtHnHl MHtHnHl

framephys. link

data linkprotocol

adapter card


15

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

• Error detection not 100% reliable! Qn: why?• protocol may miss some errors, but rarely• larger EDC field yields better detection and correction

ChecksumGenerator

Checksum Generator

EDC”=?


16

Parity CheckingSingle Bit Parity:Detect all single bit errors

0 0

Parity bit=1 iffNumber of 1’s even

Two dimensional Bit Parity:Correct all single bit errors, Detect all X bit errors X=?


17

Internet checksum

Sender: treat segment contents

as sequence of 16-bit integers

checksum: addition (1’s 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 detected.

But maybe errors nonetheless?

Goal: detect “errors” (e.g., flipped bits) in transmitted segment (note: used at transport layer only)


18

Checksumming: Cyclic Redundancy Check choose a (r+1) bit pattern (generator), G

G is fixed, known to Sender & Receiver Sender: Wants to send data bits D Finds r CRC bits, R, such that

(D || R) is exactly divisible by G (viewed as modulo 2 polynomials (*)) Sends D and R Receiver: divides (D || R) by G.

If remainder ≠ 0 : error detected! can detect all burst errors less than r+1 bits

widely used in practice (Ethernet, ATM, HDLC)

(*) This means that addition and subtraction use bitwise XOR


19

CRC ExampleWant:

D.2r XOR R = nGequivalently:

D.2r = nG XOR R equivalently: if we divide D.2r by

G, want remainder R

R = remainder[ ]D.2r

G


20

Example G(x) 16 bits CRC:

CRC-16: x16+x15+x2+1, CRC-CCITT: x16+x12+x5+1

both can catch • all single or double bit errors• all odd number of bit errors• all burst errors of length 16

or less• >99.99% of the 17 or 18 bits

burst errorsCRC-CCITT hardware implementation

Using shift and XOR registershttp://en.wikipedia.org/wiki/CRC-32#Implementation


21

Multiple Access Links and ProtocolsThree types of “links”: point-to-point (single wire, e.g. PPP, SLIP, HDLC) broadcast (shared wire or medium; e.g,

Ethernet, Token Ring, WiFi, WaveLAN, etc.)

switched (e.g., switched Ethernet, ATM etc)


22

Multiple Access protocols single shared communication channel two or more simultaneous transmissions by nodes:

interference only one node can send successfully at a time

multiple access protocol: distributed algorithm that determines how stations share

channel, i.e., determine when station can transmit communication about channel sharing must use channel itself! what to look for in multiple access protocols:

• synchronous or asynchronous • information needed about other stations • robustness (e.g., to channel errors) • performance


23

Multiple Access protocols claim: humans use multiple access

protocols all the time class can "guess" multiple access

protocols multiaccess protocol 1: multiaccess protocol 2: multiaccess protocol 3: multiaccess protocol 4:


24

MAC Protocols: a taxonomyThree broad classes: Channel Partitioning

divide channel into smaller “pieces” (time slots, frequency)

allocate piece to node for exclusive use Random Access

allow collisions “recover” from collisions

“Taking turns” tightly coordinate shared access to avoid collisions

Goal: efficient, fair, simple, decentralized


25

MAC Protocols: Measures Channel Rate = R bps Efficient:

Single user: Throughput R Fairness

N usersMin. user throughput R/N

Decentralized Fault tolerance

Simple


26

Channel Partitioning MAC protocols: TDMA

TDMA: time division multiple access access to channel in "rounds" each station gets fixed length slot (length = pkt trans time) in each round unused slots go idle example: 6-station LAN, 1,3,4 have pkt, slots 2,5,6 idle

TDM (Time Division Multiplexing): channel divided into N time slots, one per user; inefficient with low duty cycle users and at light load.

FDM (Frequency Division Multiplexing): frequency subdivided.


27

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, 1,3,4 have pkt, frequency bands 2,5,6 idle

TDM (Time Division Multiplexing): channel divided into N time slots, one per user; inefficient with low duty cycle users and at light load.

FDM (Frequency Division Multiplexing): frequency subdivided.

frequ

ency

ban

ds

time


28

TDMA & FDMA: Performance Channel Rate = R bps Single user

Throughput R/N Fairness

Each user gets the same allocationDepends on maximum number of users

Decentralized Requires resource division

Simple


29

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 -> “collision”, random access MAC protocol specifies:

how to detect collisions how to recover from collisions (e.g., via delayed

retransmissions) Examples of random access MAC protocols:

slotted ALOHA ALOHA CSMA and CSMA/CD


30

Slotted Aloha [Norm Abramson]

time is divided into equal size slots (= pkt trans. time)

node with new arriving pkt: transmit at beginning of next slot

if collision: retransmit pkt in future slots with probability p, until successful.

Success (S), Collision (C), Empty (E) slots


31

Slotted Aloha efficiencyQ: what is max fraction slots successful?A: Suppose N stations have packets to send

each transmits in slot with probability p prob. successful transmission S is:

by single node: S= p (1-p)(N-1)

by any of N nodes S = Prob (only one of the nodes transmits)

= N p (1-p)(N-1)

… choosing optimum p =1/Nas N -> infinity ...

S≈ 1/e = .37 as N -> infinity

At best: channeluse for useful transmissions 37%of time!


32

Goodput vs. Offered LoadS

= th

roug

hpu t

=

“goo

dput

”

(su

c ces

s ra t

e)

G = offered load = Np0.5 1.0 1.5 2.0

Slotted Aloha

when pN < 1, as p (or N) increases probability of empty slots reduces probability of collision is still low, thus goodput increases

when pN > 1, as p (or N) increases, probability of empty slots does not reduce much, but probability of collision increases, thus goodput decreases

goodput is optimal when pN = 1


33

Maximum Efficiency vs. n

0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0.4

2 7 12 17 n

max

imum

effi

cien

cy1/e = 0.37

At best: channeluse for useful transmissions 37%of time!


34

Pure (unslotted) ALOHA unslotted Aloha: simpler, no synchronization pkt needs transmission:

send without awaiting for beginning of slot collision probability increases:

pkt sent at t0 collide with other pkts sent in [t0-1, t0+1]


35

Pure Aloha (cont.)P(success by given node) = P(node transmits) . P(no other node transmits in [t0-

1,t0] . P(no other node transmits in [t0,t0+1] = p . (1-p)N-1 . (1-p)N-1

P(success by any of N nodes) = N p . (1-p)N-1 . (1-p)N-1 … choosing optimum p=1/(2N-1)

as N -> infty ... S≈ 1/(2e) = .18

S =

thro

ughp

u t =

“g

oodp

ut”

(

suc c

ess r

a te)

G = offered load = Np0.5 1.0 1.5 2.0

0.1

0.2

0.3

0.4

Pure Aloha

Slotted Aloha protocol constrainseffective channelthroughput!


36

Aloha: Performance Channel Rate = R bps Single user

Throughput R Fairness

Multiple usersCombined throughput only 0.37*R

Decentralized Slotted Aloha needs slot synchronization

Simple


37

CSMA: Carrier Sense Multiple AccessCSMA: listen before transmit: If channel sensed idle: transmit entire pkt If channel sensed busy, defer transmission

Persistent CSMA: retry immediately with probability p when channel becomes idle

Non-persistent CSMA: retry after random interval human analogy: don’t interrupt others!


38

CSMA collisionscollisions can occur:propagation delay means two nodes may not yethear each other’s transmissioncollision:entire packet transmission time wasted

spatial layout of nodes along Ethernet

note:role of distance and propagation delay in determining collision prob.


39

spatial layout of nodes along EthernetA B C D

time

t0

spatial layout of nodes along EthernetA B C D

time

t0

B detectscollision, aborts

D detectscollision,aborts

CSMA/CD: Collision Detection

instead of wasting the whole packettransmission time, abort after detection.

CSMA CSMA/CD


40

CSMA/CD (Collision Detection)CSMA/CD: carrier sensing, deferral as in CSMA

collisions detected within short time colliding transmissions aborted, reducing

channel wastage persistent or non-persistent retransmission

collision detection: easy in wired LANs: measure signal strengths,

compare transmitted, received signals in wireless LAN:

• receiver closed when transmitting• the interfering station may not be heard by contender

human analogy: the polite conversationalist


41

CSMA/CD collision detection


42

CDMA/CD Channel Rate = R bps Single user

Throughput R Fairness

Multiple usersDepends on Detection Time

Decentralized Completely

Simple Needs collision detection hardware


43

“Taking Turns” MAC protocolschannel partitioning MAC protocols:

share channel efficiently at high load inefficient at low load: delay in channel

access, 1/N 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

“taking turns” protocolslook for best of both worlds!


44

“Taking Turns” MAC protocolsPolling: master node

“invites” slave nodes to transmit in turn

Request to Send, Clear to Send msgs

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)


45

Reservation-based protocolsDistributed Polling: time divided into slots begins with N short dedicated reservation slots

reservation slot time equals to channel end-end propagation delay Qn: WHY?

station with message to send posts reservation reservation seen by all stations

after reservation slots, message transmissions ordered by known priority


46

Summary of MAC protocols What do you do with a shared media?

Channel Partitioning: by time, frequency or code

• Time Division, Frequency Division, Code Division Random partitioning (dynamic),

• ALOHA, S-ALOHA, CSMA, CSMA/CD• carrier sensing: easy in some technologies (wire),

hard in others (wireless)• CSMA/CD used in Ethernet

Taking Turns• polling from a central cite, token passing• Popular in cellular 3G/4G networks where

base station is the master


47

LAN technologiesData link layer so far:

services, error detection/correction, multiple access

Next: LAN technologies addressing Ethernet hubs, bridges, switches 802.11 PPP ATM


48

LAN Addresses32-bit IP address: network-layer address used to get datagram to destination networkLAN (or MAC or physical) 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 at production time


49

LAN Address (more) MAC address allocation administered by IEEE manufacturer buys portion of MAC address space

(to assure uniqueness) Analogy: (a) MAC address: like ID number תעודת זהות (b) IP address: like postal address כתובת מגורים MAC flat address => portability

can move LAN card from one LAN to another IP hierarchical address NOT portable

depends on network to which one attaches ARP protocol translates IP address to MAC address


50

Comparison of IP address and MAC Address IP address is

hierarchical for routing scalability

IP address needs to be globally unique (if no NAT)

IP address depends on IP network to which an interface is attached NOT portable

MAC address is flat

MAC address: no need for global uniqueness, but in fact is globally unique

MAC address is assigned to a device portable

5-51 Ch.2: Link

Layer &LA

N

LAN Addresses and ARPEach adaptor on LAN has unique MAC address

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

= adaptor card (NIC)

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)

5-52 Ch.2: Link

Layer &LA

N

ARP: Address Resolution Protocol

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

ARP Table: IPMAC addr mapping for LAN nodes

ARP protocol: used to get new entries in ARP table when needed

ARP message has following parameters: Source IP addr + MAC addr. Dest. IP addr + MAC addr. TTL (Time To Live): time after

which address mapping will be discarded (typically 20 min)

ARP Messages: Query, Reply

Question: how to determineMAC address of Bknowing B’s 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

237.196.7.23

237.196.7.78

237.196.7.14

237.196.7.88

5-53 Ch.2: Link

Layer &LA

N

ARP protocol usage node A wants to send datagram

to B, but doesn’t find B’s MAC address in its ARP table.

A broadcasts an ARP query containing B's IP address and asking for B’s MAC address frame dest MAC address =

FF-FF-FF-FF-FF-FF all nodes on LAN receive query only B answers (ARP reply)

reply sent to A’s MAC address only

other nodes ignore query the reply shows B's MAC address see messages in next slide

A caches the (IP,MAC) address pair in its ARP table until TTL expires (timeout) soft state: info deleted

unless refreshed Qn1: Which other node can

update its ARP table? Qn2: What happens if the

ARP query has dest IP = src IP ? Qn3: What happens if A sends

query with My_IP = IP address of C andSrc_MAC=My_MAC= MAC of A ?

ARP is “plug-and-play”: i.e. nodes create their ARP

tables without action of network administrator

5-54


ARP Messages A (a, α ) knows B’s IP addr. (b) & wants to know B’s MAC

addr (β) 1. A sends ARP Query Message for B’s MAC address:

message sent as broadcast frame on Ethernet

2. B reads the message and sends ARP reply to A reply sent as a unicast frame to A’s MAC address

Src MAC Dest MAC Type Source IP Src MAC Dest IP Dest MACα FF-…-FF Query a α b ?

Src MAC Dest MAC Type Source IP Src MAC Dest IP Dest MACβ α Query b β a α

ARP MessageEthernet Header

ARP MessageEthernet Header


55

Ethernet“dominant” LAN technology: cheap $5-10 for 10/100/1000 Mbs! first widely used LAN technology Simpler, cheaper than token LANs and ATM Kept up with speed race: 1, 10, 100, 1000 Mbps

Metcalfe’s Etheretsketch


56

Ethernet Frame StructureSending adapter encapsulates IP datagram (or

other network layer protocol packet) in Ethernet frame

Preamble: 7 bytes with pattern 10101010 followed by one

byte with pattern 10101011 used to synchronize receiver, sender clock rates


57

Ethernet Frame Structure (more) Addresses: 6 bytes, frame is received by all

adapters on a LAN and dropped if address does not match

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


58

Ethernet: uses CSMA/CDA: sense channel, if idle

then { transmit and monitor the channel;

If detect another transmission then { abort and send jam signal;

update # collisions; delay as required by exponential backoff algorithm; goto A}

else {done with the frame; set collisions to zero}}

else {wait until ongoing transmission is over and goto A}


59

Ethernet’s CSMA/CD (more)Jam Signal: make sure all other transmitters are

aware of collision; 48 bits; Exponential Backoff: Goal: adapt retransmission attempts to

estimated current load heavy load: random wait will be longer

first collision: choose K from {0,1}; delay is K x 512 bit transmission times

after n-th collision: choose K from {0,1,…, 2n-1} after ten or more collisions, choose K from

{0,1,2,3,4,…,1023}


60

Exponential Backoff (simplified) N users Interval of size 2n

Prob Node/slot is 1/2n

Prob of success N(1/2n)(1 – 1/2n)N-1

Average slot success N(1 – 1/2n)N-1

Intervals size: 1, 2, 4, 8, 16 … Fraction (out of N) of success:

2n = N/8 -> 0.03 % 2n = N/4 -> 2% 2n = N/2 -> 15% 2n = N -> 37 % 2n = 2N -> 60%


61

Ethernet Technologies: 10Base2 10: 10Mbps; 2: under 200 meters max cable length thin coaxial cable in a bus topology

repeaters used to connect up to multiple segments repeater repeats bits it hears on one interface to its other interfaces: physical layer device only!


62

10BaseT and 100BaseT 10/100 Mbps rate; latter called “fast ethernet” T stands for Twisted Pair Hub to which nodes are connected by twisted

pair, thus “star topology” (multi-port repeater) Hub acts as a multi-legged (broadcast)

repeater Effectively same as a single segment


63

10BaseT and 100BaseT (more) Max distance from node to Hub is 100 meters Hub can disconnect “jabbering” adapter Hub can gather monitoring information,

statistics for display to LAN administrators


64

Gbit Ethernet use standard Ethernet frame format allows for point-to-point links and shared

broadcast channels in shared mode, CSMA/CD is used; short

distances between nodes to be efficient uses hubs, called here “Buffered Distributors” Full-Duplex at 1 Gbps for point-to-point links

Wide area networks


65

Token Rings (IEEE 802.5) A ring topology is a single

unidirectional loop connecting a series of stations in sequence

Each bit is stored and forwarded by each station’s network interface


66

Token Ring: IEEE802.5 standard 4 Mbps (also 16 Mbps) max token holding time: 10 ms, limiting frame

length

SD, ED mark start, end of packet AC: access control byte:

token bit: value 0 means token can be seized, value 1 means data follows FC

priority bits: priority of packet reservation bits: station can write these bits to prevent

stations with lower priority packet from seizing token after token becomes free


67

Token Ring: IEEE802.5 standard

FC: frame control used for monitoring and maintenance

source, destination address: 48 bit physical address, as in Ethernet

data: packet from network layer checksum: CRC FS: frame status: set by dest., read by sender

set to indicate destination up, frame copied OK from ring

DLC-level ACKing

Chapter 2 Datalink Layer & LAN Protocols

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