CSE401N:Computer Networks

Lecture-15Data Link Layer

EDCMAC

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Preview: The Data Link LayerOur goals:

Understand principles behind data link layer services: error detection,

correction done! sharing a broadcast

channel: multiple access

reliable data transfer, flow control: done!

link layer forwarding

Instantiation and implementation of various link layer technologies

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

and LANs link layer addressing, ARP specific link layer

technologies: Ethernet hubs, bridges, switches IEEE 802.11 LANs and

wireless PPP ATM

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Link Layer: setting the context

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Link Layer: setting the context 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

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Link Layer: Introduction

Some terminology: hosts and routers are nodes (bridges and switches too) communication channels

that connect adjacent nodes along communication path are links wired links wireless links LANs

2-PDU is a frame, encapsulates datagram

“link”

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Link layer: Context

Data-link layer has responsibility of transferring datagram from one node to adjacent 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 limo: airport to hotel

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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, dest • different from IP address!

Reliable delivery between two physically connected devices: we learned how to do this already (chapter 3)! seldom used on low bit error link (fiber, some twisted

pair) wireless links: high error rates

• Q: why both link-level and end-end reliabilityQ: why both link-level and end-end reliability?

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Link Layer Services (more)

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

{RAM, DSP Chips, a host bus interface, link interface}

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

802.11 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 rcving node

adapter is semi-autonomous

link & physical layers

sendingnode

frame

rcvingnode

datagram

frame

adapter adapter

link layer protocol

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OSIApplication

Presentation

Session

Transport

Network

Data Link

Physical

TCP/IP

Application

Transport

Internet

Network Interface

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Outline

Link layer overview Error detection and correction

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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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Error Detection: Parity CheckingSingle Bit Parity:Detect single bit errors

Two Dimensional Bit Parity:Detect and correct single bit errors

0 0

Errors are generally bursty, i.e. several consecutive bits are flipped: column parity.

1

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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 checksum field

Receiver: compute checksum of

received segment check if computed

checksum equals checksum field value: NO - error detected YES - no error detected

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

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Cyclic Redundancy Check: Background Widely used in practice (ATM, Ethernet,

HDCL,) For a given data D, consider it as a

polynomial D(x) consider the string of 0 and 1 as the

coefficients of a polynomial• e.g. consider string 10011 as x4+x+1

addition and subtraction are modular 2, thus the same as xor

Choose generator polynomial G(x) with r+1 bits, where r is called the degree of G(x)

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Cyclic Redundancy Check: Objective Given data G(x) and D(x), choose R(x) with r

bits, such that D(x)xr+R(x) is exactly divisible by G(x)

The bits correspond to D(x)xr+R(x) are sent to the receiver

Since G(x) is global, when the receiver receives the transmission T’(x), it divides T’(x) by G(x) If non-zero remainder: error detected! If zero remainder, assumes no error

+x

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CRC: Steps and an Example1. Suppose the degree

of G(x) is r2. Append r zero to D(x),

i.e. consider D(x)xr

3. Divide D(x)xr by G(x). 4. Let R(x) denote the

reminder5. Send <D, R> to the

receiver

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CRC ExampleWant:

D.2r XOR R = nGequivalently:

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

remainder is R

R = remainder[ ]D.2r

G

Example: D=101110, G=1001, r=3

In CRC calculations, addition and subtraction are equivalent to bitwise exclusive-or (XOR) Sender transmit: 101110 011

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The Power of CRC Let T(x) denote D(x)xr+R(x), and E(x) the polynomial of the

error bits The received signal is T(x)+E(x)

Since T(x) is divisible by G(x), we only need to consider E(x) divided by G(x)

A single bit of error: E(x) = xi

If G(x) contains two or more terms, E(x) is not divisible by G(x)

An odd number of errors: E(x) has an odd number of terms: Lemma: if E(x) has an odd number of terms, E(x) cannot be

divisible by (x+1)• suppose E(x) = (x+1)F(x), let x=1, the left hand will be 1, while the

right hand will be 0 If G(x) contains x+1 as a factor, E(x) will not be divided by G(x)

Many more errors can be detected by designing the right G(x)

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Example G(x)

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

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Part-2

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Outline

Media access control (MAC) protocols: overview

Random MAC protocols

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Multiple Access Links and Protocols

Two types of “links”: point-to-point

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

broadcast (shared wire or medium) traditional Ethernet 802.11 wireless LAN satellite

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

interference only one node can send successfully at a time (see CDMA for

an exception)

multiple access protocol Algorithm that determines how nodes share channel, i.e.,

determines when node 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

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Ideal Mulitple Access Protocol

Broadcast channel of rate R bpsrate R bps1. Efficiency: when one node wants to transmit,

it can send at rate R.2. Fairness: when M nodes want to transmit,

each can send at average rate R/M3. Decentralized:

no special node to coordinate transmissions no synchronization of clocks

4. Simple

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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:

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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, decentralized, simple

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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 =

pkt trans time) in each round Unused slots go idle Example: 6-station LAN, 1,3,4 have pkt, slots

2,5,6 idle

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Channel Partitioning MAC protocols: FDMAFDMA: 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

frequ

ency

bands time

5

1

4

3

2

6

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FDMA and TDMA

FDMA

frequency

time

TDMA

frequency

time

4 users

Example:

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TDMA and FDMA

Often combined in practice, for example, in cellular phone networks: TDMA cellular phones

use 30 KHz channels, with each channel divided into three time slots. A single handset uses one timeslot for sending and the other for receiving.

• e.g. Cingular (Nokia 8265, TDMA 800/ 1900 MHz, AMPS 800 mHz ), AT&T Wireless

GSM uses 200 KHz channels divided into eight time slots. A single handset uses one slot in two channels for sending and receiving.

• Cingular, T-Mobile, AT&T are converting to it

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Channel Partitioning (CDMA)

CDMA (Code Division Multiple Access) Used mostly in wireless broadcast

channels (cellular, satellite, etc) Unique “code” assigned to each user; i.e.,

code set partitioning All users share same frequency, but each

user has its own “chipping” sequence (i.e., code) to encode data e.g. code = 1 1 1 1 -1 -1 1 1

Examples: MetroPCS, Sprint and Verizon

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CDMA: Encoding and Decoding

Assume original data are represented by 1 and -1

Encoded signal = (original data) x (chipping sequence)

Decoding: inner-product (summation of bit-by-bit product) of encoded signal and chipping sequence if inner-product > threshold, the data is 1; else

-1

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CDMA Encode/Decode

Code: 1 1 1 -1 1 -1 -1 -1

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CDMA: Deal with Multiple-User Interference

Two codes Ci and Cj are orthogonal, if , where we use “.” to denote inner

product, e.g.

If codes are orthogonal, multiple users can “coexist” and transmit simultaneously with minimal interference:

iiij

jj cdccd )(

0 ij cc

C1: 1 1 1 -1 1 -1 -1 -1 C2: 1 -1 1 1 1 -1 1 1-----------------------------------------C1 . C2 = 1 +(-1) + 1 + (-1) +1 + 1+ (-1)+(-1)=0

Analogy: Speak in different languages!

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CDMA: Two-Sender Interference

Code 1: 1 1 1 -1 1 -1 -1 -1Code 2: 1 -1 1 1 1 -1 1 1 . . . .

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Outline

Media access control (MAC) protocols: overview

Random MAC protocols Random Access

• Compete for each packet• Aloha, CSMA, CSMA/CD

“Taking-turns”

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Random Access Protocols

When node has packets 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 and pure ALOHA CSMA and CSMA/CD, CSMA/CA

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Slotted Aloha

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

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Slotted Aloha EfficiencyQ: What is the fraction of successful slots?A: Suppose N stations have packets to send

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

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

by any one of the N nodes

S(p) = Prob (only one transmits) = N p (1-p)(N-1)

when p=1/N, S(p) achieves the maximum (1-1/N)(N-1)

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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

imu

m e

ffic

ien

cy1/e = 0.37

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

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

send without awaiting for the beginning of slot

Collision probability increases: pkt sent at t0 collide with other pkts sent in [t0-1,

t0+1]

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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

Bound: 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!

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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 (may cause instability)

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

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CSMA collisions

collisions can occur:propagation delay means two nodes may not yearhear 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.

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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 difficult in wireless LANs: receiver shut off while

transmitting human analogy: the polite conversationalist

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CSMA/CD Collision Detection

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

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MAC Rules & Collision Detection/Backoff

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MAC Rules and Collision Detection/Backoff

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Efficiency of CSMA/CD Given collision detection, instead of wasting the

whole packet transmission time (a slot), we only waste the time needed to detect collision.

In the normal case, we try approximately e times before each successful transmission, then for each successful transmission, which takes P/C, we waste a total of 2eT (5T) seconds on collision, where T is one-way propagation delay

P/C

P: packet size, e.g. 1000 bitsC: link capacity, e.g. 10Mbps

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Efficiency of CSMA/CD The efficiency (the percentage of useful

time) is

The value of a plays a fundamental role in the efficiency of CSMA/CD protocols.

Question: you want to increase the bandwidth of the network, but still want to maintain the same efficiency, what do you do?

PTC

aTTCPCP a

CP where,51

1/51

15/

/

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Review CSMA protocols “Take-turn” MAC protocols Example protocols

Outline

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“Taking Turns” MAC Protocols Channel partitioning MAC protocols:

share channel efficiently and fairly 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” protocols look for the best of both worlds!

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“Taking Turns” MAC protocolsPolling: master node

“invites” slave nodes to transmit in turn

reserve or request/reply

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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Polling Example: Distributed Polling Time divided into slots Begins with N short reservation slots

reservation slot time equal to channel end-end propagation delay

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

After reservation slots, message transmissions ordered by

known priority

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Token Passing Example: Token Ring A token rotates around a ring to each node in

turn All nodes (computers, routers, etc.) copy all

data and tokens, and repeat them along the link of the stations

When a node wishes to transmit packet(s), it grabs the token as it passes

It holds the token while it transmits When it is done, it releases the token and

sends it on its way

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Token Passing: Illustration

data token/data

l1

l3 l2

l4

Listen:

Talk:

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Token Ring: Two Variants

Release After Reception (RAR) Example: IEEE 802.5

Token Rings (4Mbps)

Release After Transmissions (RAT) Example: Fiber

Distributed Data Interface (FDDI) (100Mbps)

station 1 station 1station 2 station N

Packet 1

Packet 2

station 1 station 1station 2 station N

Packet 1

Packet 2token

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Efficiency of Token Ring: RAR Release After Reception (RAR)

maximum efficiency RAR achieved when a station continuously transmits:

if the sender always releases its token after a data transmission:

1 12 N

Packet 1

Packet 2

where

a: on timeTransmissiPacket

delayn propagatio E-2-E

1 12 n

Packet 1

Packet 2

where

a: on timeTransmissiPacket

delayn propagatio E-2-E

aRAR

1

1

a

aNNRAR

1

1

1

11

N: number of hosts

1

a

1

a

a/N

Figure assumes- packet time is 1- end-to-end propagation delay a

Assume token very small, stations equally spaced

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Efficiency of Token Ring: RAT Release After Transmission (RAT)

maximum efficiency achieved when a station continuously transmits:

assume a station always releases its token after a packet transmission:

1 12 n

Packet 1

Packet 2

1 12 n

Packet 1

Packet 2Therefore, the efficiency is approaching 1

NaRAT /1

1

where

a:

N: number of hosts

on timeTransmissiPacket

delayn propagatio E-2-E

1

a

aa/NTime 0

Time 1+a/N

Packet 3

3

Time 2+2a/N

Assume token very small, stations equally spaced

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Comparison of Efficiency

Na

a

a

RAT

RAR

CDCSMA

/1

11

151

1/

where

a:

N: number of hosts

on timeTransmissiPacket

delayn propagatio E-2-E

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Summary of MAC Protocols

How do you access a shared media? Channel Partitioning, by time, frequency or

code• Time Division, Code Division, Frequency Division

Random partitioning (dynamic), • ALOHA, S-ALOHA, CSMA, CSMA/CD

“Taking-turns”• polling• token passing

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Media Access Control (MAC) Protocols

Ethernet (IEEE 802.3) Logical bus topology Physical star or extended

star Nondeterministic

• First-come, first-served Token Ring (IEEE 802.5)

Logical ring Physical star topology Deterministic

• Token controls traffic Older declining technology

FDDI (IEEE 802.5) Fiber Distributed Data

Interface Logical ring topology Physical dual-ring topology Deterministic

• Token controls traffic Near-end-of-life technology

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THANX

HOME WORK: What is inside the Router?

CSE 402N:QUIZ 3rd May, 2006