Ch.2: Link Layer &LAN Chapter 2 Datalink Layer & LAN Protocols 1.

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: e.g copper, fiber

unguided media: signals propagate

freely, in air or vacuume.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, fiber

Coaxial 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

L2

L3

L4

L5


9

Link Layer: setting the context acts between two physically connected devices:

host-router, router-router, host-host

unit of data: frame

applicationtransportnetwork

linkphysical

networklink

physical

M

M

M

M

Ht

HtHn

HtHnHl 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

(IF SHARED LINK) link access: implement channel access, ‘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 use both link-level and end-end reliability?


12

Link Layer Services (more)

(OPTIONAL) Flow Control: pacing between sender and receivers

(OPTIONAL) 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 Qn: Why congestion ctrl not listed here?


13

Adaptors Communicating

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

modem, 802.11 card

adaptor is semi-autonomous, implementing link & physical layers

sending side: encapsulates datagram in a

frame, delimits frame adds error checking bits,

Optionally: rdt param’s, etc.

receiving side recognizes frame start /end checks errors, Optionally:

check rdt, send Ack+flow ctrl info, ..

extracts datagram, passes to L3

sendingnode

frame

receivingnode

datagram

frame

adapter adapter

link layer protocol


14

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”=?


15

Parity Checking

Single 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=?


16

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)


17

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


18

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


19

Examples of 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 errors

CRC-CCITT hardware implementationUsing shift and XOR registers

http://en.wikipedia.org/wiki/CRC-32#Implementation


20

Multiple Access Links and Protocols

Three 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)


21

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


22

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:


23

MAC Protocols: a taxonomy

Three 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


24

MAC Protocols: Measures

Channel Rate = R bps Efficient:

Single user: Throughput R Fairness

N usersMin. user throughput R/N

Decentralized Fault tolerance

Simple


25

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.


26

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

bands time


27

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


28

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


29

Slotted Aloha [Norm Abramson]

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

node w. new pkt: transmit at beginning of next slot a satellite acts as Access Point, and sends Ack to

sender if successful. It also sends sync signal to all stations

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

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


30

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/N

as N -> infinity ...

S≈ 1/e = .37 as N -> infinity

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


31

Goodput vs. Offered LoadS =

thro

ughput

=

“goodput”

(

succ

ess

rate

)

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


32

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

ma

xim

um

eff

icie

nc

y1/e = 0.37

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


33

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]


34

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

ughput

=

“goodput”

(

succ

ess

rate

)

G = offered load = Np0.5 1.0 1.5 2.0

0.1

0.2

0.3

0.4

Pure Aloha

Slotted Alohaprotocol constrainseffective channelthroughput!


35

Aloha: Performance


Throughput R Fairness

Multiple usersCombined throughput only 0.37*R

Decentralized Slotted Aloha needs slot synchronization

Simple


36

CSMA: Carrier Sense Multiple Access

CSMA: 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!


37

CSMA collisions

collisions 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.


38

spatial layout of nodes along EthernetA B C D

tim

e

t0

spatial layout of nodes along EthernetA B C D

tim

e

t0

B detectscollision, aborts

D detectscollision,aborts

CSMA/CD: Collision Detection

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

CSMA CSMA/CD


39

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


40

CSMA/CD collision detection


41

CDMA/CD


Throughput R Fairness

Multiple usersDepends on Detection Time

Decentralized Completely

Simple Needs collision detection hardware


42

“Taking Turns” MAC protocols

channel 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!


43

“Taking Turns” MAC protocols

Polling: 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)


44

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


45

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


46

LAN technologies

Data link layer so far: services, error detection/correction, multiple

access

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


47

LAN Addresses

32-bit IP address: network-layer address used to get datagram to destination network

LAN (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


48

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


49

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

5-50 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)

5-51 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

5-52 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

5-53


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 MAC

α FF-…-FF Query a α b ?

Src MAC Dest MAC Type Source IP Src MAC Dest IP Dest MAC

β α Query b β a α

ARP MessageEthernet Header

ARP MessageEthernet Header


54

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


55

Ethernet Frame Structure

Sending 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


56

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


57

Ethernet: uses CSMA/CD

A: 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}


58

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 10 collisions, choose K from {0,1 … ,

1023} after 16 collisions, give up


59

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%

DataLink Layer

5-60

Ethernet length limitations Ethernet defines:

MAX. allowed distance between stations on LAN

MIN. allowed frame size

The rule is:Ttrans(frame) > 2 * Tprop(Max)

Ensures that all collisions will be detected by sender

1 st bit arrives at B(no transmission allowed from this

time)

Max.propagation time

propagation time

back

After this time the transmitter is sure that no collision occurred for

this frame


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

Ch.2: Link Layer &LAN Chapter 2 Datalink Layer & LAN Protocols 1.

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Transcript of Ch.2: Link Layer &LAN Chapter 2 Datalink Layer & LAN Protocols 1.