Chapter 3 - Data Link Layer

8/12/2019 Chapter 3 - Data Link Layer

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3a. Data Link Layer Protocols1. Introduction

2. DLL Design a. Network Layer Services

b. Error Controlc. Flow Control

3. Elementary Data LinkProtocols

a. Stop-and-Wait Protocol

b. Simplex Protocol for NoisyChannel; Time-out

c. Sliding Window Protocols

d. Sliding-window Flow Control

e. A One bit Sliding-Window

f. A Protocol Using Go-Back-N

g. Selective RejectHigh-Level Data Linc Control a. HDLC Operation

b. HDLC Protocol

The Internet Protocol a. PPP-The point-to-point protocol

(T. 183-229; 234-246)


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1. Data/control exchanged viaprotocols

a human protocol and a computer networkprotocol:

HiHi

Got thetime?

2:00

TCP connectionreq

TCP connectionresponse

Get http://www.awl.com/kurose-ross

time


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Data Link Layerapplication

transport

network

link

physical

Requirements and Objectives:Maintain and release data LinkFrame synchronizationError controlFlow controlAddressingLink management

DLL functions:

Providing service interface to the network layer.

Data Link Protocols must take circuit errors,

Flow regulating.

Data transfer betweenneighboring network elements


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Link Layer: Introduction

Some terminology:

Hosts, bridges, switches androutersare nodes

Communication channels that

connect adjacent nodes along

communication path are links wired links

wireless links

LANs

frame,encapsulates datagram

Data link

Data link layerhas responsibility oftransferring datagram from one node

to adjacent node over a data link


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2. Packet and Frame relationship

Packet

Header Payload fild Trai ler

Sending machine Receiving machine

PacketFrame

Header Payload field Trai ler

Network LayerNetwork Layer

In some cases, functions of error control and flow control areallocated in transport or other upper layer protocols and not in the

DLL, but principles are pretty much the same.


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Protocol layering and data

Each layer takes data from above

adds header information to create new data

unit

passes new data unit to layer below

applicationtransport

networklinkphysical

applicationtransport

networklinkphysical

source destination

M

M

MM

Ht

HtHnHtHnHl

M

M

M

M

Ht

HtH

n

HtHnHl

message

segment

datagram

frame


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Data flow-physical communication

applicationtransportnetwork

linkphysical


linkphysical


linkphysical


linkphysical

networklinkphysical

data

data


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list of the DLL requirements Frame synchronization. Data are sent in blocks

called frames. The beginning and endof each framemust be recognized.

Flow control. The sending station must notsendframes at a ratefaster then the receiving station canabsorb them.

Error control. Any bit errors introduced by thetransmission system must be checked & corrected.

Addressing. On a multipoint line, such as a LAN,the identity of the two stations involved in a

transmission must be specified. Link management. The initiation, maintenance, and

terminationof a data exchange requires a fairamount of coordination and cooperation amongstations.


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Services to the Network Layer (NL)

DLL processes data transfer using a data link

protocol. The actual services can vary from system to system.

Three reasonable services to the NL are:

1. Unacknowledged connectionless service.2. Acknowledged connectionless service.

3. Acknowledged connection-oriented service.


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1. Unacknowledged connectionless service The source machine send frames to the destination

machine withouthaving the destination machineacknowledged them.

No logical connectionis established beforehand orreleased afterward.

If a frame is lost due to noise on the line, no attempt

is made to detect the loss or recoverfrom it in theDLL.

This class of service is appropriate when the errorrate is very lowso that recovery task is left for

solution to higher layers. It is also appropriate for real-time traffic, such asvoice, in which late data are worse than bad data.

Most LANs use unacknowledged connectionlessservice in the DLL


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2. Acknowledged connectionless service Is more reliable.

Still no logical connectionsused, but each frame

sent is individually acknowledged. The sender knowswhether a frame has arrived

correctly.

If it has not arrived within a specific time interval, it

can be sent again. This service is useful over unreliable channels,

such as wireless system. If the large packet is broken up into frames, If

individual frames are acknowledgedorretransmitted, entire packets get through muchfaster than unbrokenframe that is lost, it may takea very long time for the packet to get through..


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3. ACKed connection-oriented service The service requires established connectionbetween

source/destination machines before data are transferred.

Any framesent over the connection is numbered, and theDLL guarantees that each frame sent, is received, and arereceived in the same order.

With connectionless service, in contrast, it is possible that alost acknowledgement causes a packet to be sent severaltimes and thus received several times.

When connection-oriented service is used, transfers gothrough 3distinct phases:

1. The connection is establishedand counters needed to keep

trackof which frames have been received and which oneshave not.

2. One or more frames are transmitted andacknowledged.

3. Connection is released,freeing up the variables - buffersand

other resources used to maintain the connection.


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Link Layer JobFraming:

encapsulate datagram into frame, adding header, trailer

Error Detection:

errors caused by signal attenuation, noise.

receiver detects presence of errors:

signals sender for retransmission or drops frame

two types of errors: Lost frame

Damaged frame

Error Correction:

receiver identifies and correctsbit errors without

retransmission


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Example is a WAN subnet

Consisting of routers connected by point-to-point

leased telephone lines.1. When a frame arrives at a router, the hardwarechecksit for errors, (Passes the frame to the DLLsof twarewhich might be embedded in a chip on the

network interface board).2. The DLL software checks to see if it is the f rame

expected,

3. If so, givesthe packet (contained the payload field)

to the rout ing sof tware.4. The routing software then chooses the appropriate

ou tgoing l ine and passes the packetback down tothe DLL software, which then transmits it.

T h i f l


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Techniques for error control are: Error detection.

Positive Acknowledgment.

Retransmission after time-out.

Negative acknowledgment and retransmission

These 4mechanisms are all referred to as Automatic

Report reQuest (ARQ);the effect of ARQ is to turnan unreliable data link into a reliable one.

Three standardized versions Of ARQ:

Stop-and-wait ARQ Go-back-N ARQ

Selective-reject ARQ

L k L J b (C )


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Link Layer Job (Cont)Flow Control:

Two approaches are commonly used:1. Feedback-based flow control,the receiver

sends back information to the sender giving itpermission to send more data or at leasttelling the sender how the receiver is doing.

You may send me nframes now, but afterthey have been sent, do not send any more

until I have told you to continue.2. Rate-based flow control,the protocol has a

built-in mechanism that limits the rateat

which senders may transmit data. Since rate-based schemes are never used in the DLL


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Elementary Data Link Protocols Assumptions:

1).DLLand Networklayer are independent processes

that communicate by passing messages back and

forth trough the physical layer.

2).a.Machine Awants to send a long stream of data tomachine B, using a reliable, connection-oriented

service.

b. We will consider the case where Balso wants to

send data to Asimultaneously. Ais assumed tohave a data ready to send.

3).Machines do not crash.

P l 1 S d W i P l


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Prtcl.1. Stop-and Wait Protocol

Protocolin which the sender sends one frameand then waitsfor anACK: stop-and-wait.

t (timeout); Damaged ACK; ACK0, ACK1. bidirectionalinformation transfer.

Half duplexphysical channel.

It is often the case that a source will breakup a large block ofdata into smaller blocksand transmit the data in manyframes, Reason:

1. The buffersize of the receiver may be limited.

2. The larger the transmission, the more error,

With smaller frames, error aredetected sooner, Smaller

amount of data needs retransmission.3. On a shared medium, (LAN), it is usually desirable not topermitone station to occupythe medium for an extendedperiod, as this causes long delay at the other sendingstations.


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Stop-and-Wait ARQ

Timeout

Timeout

Frame lost A

retransmits

ACK1 lost A

retransmits

A B

B discards

duplicateframe

H t t th s d f


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How to prevent the sender fromflooding the receiver?

t= from_physical_layer +to_network_layer, Errors

Damaged:

Error detectionAcknowledgment

(Copies are maintained).,

Damaged ACK=Time-out+

Duplicates frame

Frame labeling (0 / 1).Positive ACK0= ready for 1;

ACK1= ready for 0.

Lost:

Timer

Time-out

Frame resend

(Copies are maintained)


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T T

T T

T T

T T

T T

R

R

R R

R

R

R

R

R

R

t0

t0+

t0+1+2

t0+1+

t0+1+2

t0+1+

t0+1

t0+

t0

t0+1

(a) >1 (b)


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Prtcl.2. Simplex prtcl for Noisy Channel; Time-out

Data are transmitted in one direction only (simplex

channel),that makes error. Frames may be eitherdamaged or lostcompletely.

Stop-and-wait protocol would work: adding a timer.

a. The sender could send a frame, but the receiver

would only send an ACK frame if the data werecorrectly received.

b. If a damaged frame arrived at the receiver, itwould be discarded.

c.After a while the sender would time out andsends the frame again. This process would berepeated until the frame finally arrives intact.

1-bit sequence number (0 or 1)


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TCP Round Trip Time and Timeout

Q:how to set TCP

timeout value?

too short: premature

timeout

=unnecessary

retransmissions

too long: slow

reaction =time

wasting

Q:how to estimate RTT?

SampleRTT:measured time

from segment transmission

until ACK receipt

ignore retransmissions

SampleRTTwill vary, want

estimated RTT smoother

average several recent

measurements, not just

current SampleRTT


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Fast Retransmit Time-out period often

relatively long: long delay beforeresending lost packet

Detect lost frame via

duplicate ACKs. Sender often sends

many frames back-to-

back

If frame is lost, therewill likely be many

duplicate ACKs.

If sender receives 3

ACKs for the samedata, it presumes that

frame after ACKed

data was lost:

fast retransmit: resend

frame immediately,

before timer expires

P t l i


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Protocol scenario:1. The network layer on Agives packet 1 to its DLL. The

packet is correctly received at Band passed to the

network layer on B.Bsends anACKframe back to A.

2. The ACK frame gets lost completely. It just neverarrives at all.

3. The DLLon Atimes out. Not having received an ACK,it (incorrectly) assumes that its data frame was lost ordamaged and sends the frame containing packet 1again.

4. The duplicate frame also arrives at the DLLonBperfectly and is randomly passed to the network layerthere. If Ais sending a file to B, part of the file will beduplicated (i.e., the copy of the file made by Bwill beincorrect and the error will not have been detected). In

other words, the protocol will fail.

Sliding Window


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Sliding-Window Better idea is to use the Duplex Channel.

Data frame from Ato Bare intermixed with theacknowledgment frames from Bto A.

By looking at the kind field in the header of an

incoming frame, the receiver can tell whetherthe frame is dataorACK.

Station B,-buffer space for n frames. Thus, B

can accept nframes, and A is allowed to sendnframes withoutwaiting for anyACK.

3-bitfield, the sequence number can range

from 0to 7 0 through , from 0 to12

k


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Sliding-Window

0 1 2 3 5 6 7 0 14 0765432

0

4321

554321 6 2107 43

Frames already received

Frames already received

Window of frames that

may be transmitted

Window of frames that

may be accepted

321076 4

Frame

Sequence

number

Last frame

transmittedWindow shrinks

from trailing edge

as frames are sent

Window expands from

leading edge as received

acknowledgment

Last frame

acknowledgedWindow shrinks

from trailing edge

as frames are received

Window expands from

leading edge as sent

acknowledgment

(a) Transmitters perspective

(b) Receivers perspective

P

ipeline

P 3 E l Slidi i d t l


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Pr.3. Example: Sliding-window protocol

0 1 2 3 4 5 6 7 0 1 2 3 0 1 2 3 4 5 6 7 0 1 2 3

0 1 2 3 4 5 6 7 0 1 2 3

0 1 2 3 4 5 6 7 0 1 2 3

0 1 2 3 4 5 6 7 0 1 2 3

0 1 2 3 4 5 6 7 0 1 2 3

0 1 2 3 4 5 6 7 0 1 2 3

0 1 2 3 4 5 6 7 0 1 2 3

0 1 2 3 4 5 6 7 0 1 2 3

0 1 2 3 4 5 6 7 0 1 2 3

F0F1

F2

RR3

F3

F4

F5

F6

RR7

Source system A Destination system B

Maximum window size=7

(RR

6);(RNR)

P 4 O Bit Slidi Wi d


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Pr.4 x mple One-Bit Sliding Window(piggybacking)

A sends(0,1,A0)

A gets (0,0,B0)*

A sends(1,0,A1)

A gets (1,1,B1)*

A sends(0,1,A2)

A gets (0,0,B2)*

A sends(1,0,A3)

B gets (0,1,A0)*

B sends(0,0,B0)

B gets (1,0,A1)*

B sends(1,1,B1)

B gets (0,1,A2)*

B sends(0,0,B2)

B gets (1,0,A3)*B sends(1,1,B3)

A sends(0,1,A0)

A gets (0,1,B0)*

A sends(0,0,A0)

A gets (0,0,B0)

A sends(1,0,A1)

A gets (1,0,B1)*

A sends(1,1,A1)

B sends(0,1,B0)B gets (0,1,A0)*

B sends(0,0,B0)

B gets (0,0,A0)

B sends(1,1,B1)

B gets (1,0,A1)*

B sends(1,1,B1)

B gets (1,1,A1)B sends(0,1,B2)

a b

Two scenario: (a) Normal case. (b) Abnormal case. The notation is (seq, ack,

packetnumber).An asterisk indicates where a network layer accepts a packet.

A; T-O short

P t l 5 A P t l U i G B k N


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Prtcl.5. A Protocol Using Go Back N

For efficiency of the bandwidth utilization:

59 kbpssatellite channel-500-msecround-trip delay.

Sent 1000-bitframe. At t=0msec-the frame has been

Started and t=20 msecsent. Received t=270 msec

frame fully arrived at the receiver; t=520 msec- ACKto

the sender; So, sender was blocked during 500/520or96%of the time. 4 %of the bandwidth was used.

The solution: the sender transmits up to wframes

before blocking, instead of just 1 frame.

The example, wshould be at least 26. The senderbegins sending Fr. 0as before. Finishes sending 26frames, at t=520 msec, the ACK for frame 0will have

just arrived.ACKarrive every 20 msec, (PIPLINING)so the sender always gets permission to continuewhen it needs it.

P 5 A P t l U i G B k N (C t)


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Pr.5.A Protocol Using Go Back N (Cont)

If the channel capacity is bbits/sec, the frame

size lbits, and the round-trip propagation timeRsec, the time required to transmit a single

frame is l /bsec. After the last bit of data

frame has been sent, there is a delay of R/2before that bit arrives at the receiver and

another delay of at least R/2for ACK to come

back, for a total delay of R.

In stop-and-wait the line is busy for l/band

idle for R, giving:

L ine uti l izat ion = l / (l+bR).=4%

Pr 5 A P t l U i G B k N


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Pr.5. A Protocol Using Go Back N

0 31 2 4 5 6 7 8

0 1 E

765432

532

D

0 1 2

5432

4

DD D DD

Time interval, Time out

9876 13121110

53E

0 1 24 9876 13121110

Error Frames discarded by DLL Time

Error Frames buffered by DLL

a

b

0 1 NAK2 1 1 5 6 7 8 9 10 11

0 1 2 3 4 5

Data flow ACK flowError recovery, when:

(a) receivers window size is 1 and(b)receivers window size is large; Selective Repeat

Selective

repeat

(NAK)

Go Back N

With size

window 1

Pr 5 Go Back N


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Pr.5.Go-Back-NSender:

k-bit seq # in packet header

window of up to N, consecutive unACKed pkts allowed

ACK(n): ACKs all pkts up to, including seq # n -

cumulative ACK

may receive duplicate ACKs (see receiver)

timer for each in-flight pkt

timeout(n):retransmit pkt n and all higher seq # pkts in

window

Prtcl 5 S l ti t d


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Prtcl.5. Selective repeat: sender-receiver windows


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Prtcl.7.High-Level Data Link Control

High-Level Data Link Control (HDLC)subsets:

(Synchronous Data Link Control (SDLC)

Link Access Procedurefor DChannel (LAPD)

Advanced Data Communication Control

Procedure (ADCCP) Link Access Procedure (LAP).

These protocols are based on the same principles.

P 7 HDLC F m F m t


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Pr.7.HDLC Frame Format

Flag

8 Bits

Address

8/16 Bits

Control

8/16 Bits

Data

Variable Length

CRC

8/16 Bits

Flag

8 Bits

bit oriented; bit stuffing

Master Slave

Commends

Response

Flag- synchronization.

Address- address of the secondary station.

Control- keep track of transmitted and received frames foracknowledgment and flow control.

CRC- contains a checksum to ensure data integrity.

Flag- used to signal the end of a frame, and possibly the start

of the next frame.

P 7 High Level Data Link Control


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Pr.7.High-Level Data Link Control Three kinds of control fields:

a. Informationb. Supervisory

c. Unnumbered.

0 Seq P/F Next

1

1

1

0

Bits 1 3 1 3

Type P/F Next

Type P/F Modifier

(a)

(b)

(c)

The protocol uses a slidingwindow, with 3-bit sequence

number. Up to seven

unacknowledged frames may be

outstanding at any instant.

For ACK is used the number of the first frame not

yet received (i.e.., the next frame expected).

P-polling data

F-finished polling.

(a)-nACK (reject)

(b)-RNR

(c)-Selective reject-

retransmit specified

P 7 Hi h L l D t Li k C t l


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Pr.7.High-Level Data Link Control Different types of frames use different ACKs:

Type

1

REJECT Transmission error

has been detected

Type

2

RECEIVENOT

READY

Acknowledges all

frames, but not

including Next.

Stop sending

Type

3

SELECTIE

REJECT

Retransmission

of only the frame

specified.

ACK Definition UsedFrame with

error

Problems withthe receiver

shortage of

buffer

senders

window size is

half or lessthesequence space


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A Network Layer in the Internet

Leased

Lines toAsia

A U.S. backbone

Regional

network

IP Ethernet

LAN

IP tokenRing LAN

Regional

network

A European backbone

A1 C

D

B

2

D Li k L i h I


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Data Link Layer in the Internet

Subnetrouter

Host

ATC

PC

Service

provider



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PC

modemClient process

Using TCP/IP

Users home

Dial-upTelephone

line

TCP/IP

ConnectionUsing PPP

modems

RouterRouting

process

Internet providers office

A home personal computer acting as an Internet host

PPP Situation

Pr 8 PPP-The Point-to-point


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Pr.8.PPP-The Point-to-pointProtocol

PPP provides three features:

A framing method that clearly determines the:

end of one frame and the start of the next one,

Error detection.

A link control protocol for bringing lines up, testing them,

negotiating options, and bringing them down again when theyare no longer needed, This protocol is calledLCP (LinkControl Protocol).It supports synchronous andasynchronous circuits and byte-oriented and bit-orientedencodings.

A way to negotiate network-layer options in a way that isindependent of the network layer protocol to be used. Themethod chosen is to have a differentNCP (Network ControlProtocol)for each network layer supported.

P PPP


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Pr.8.PPP- Steps

ATC

Router 1. PC calls the providers router via a modem.

2. The routers modem has answered the

phone and established a physical connection

3. PC sends to the router a series of LCP

packets in the payload field of one or more

PPP frames

These packets and their responses select the PPP

parameters to be used.

Once the parameters have been agreed upon, a series of

Network Control Protocol packets are sent to configure the

network layer.

Typically, the PC wants to run a TCP/IP protocol stack, so it

needs an IP address.

PPP d HDLC


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Difference betweenPPPandHDLC

Flag

8 Bits01111110

Address

8/16

Bits

Control

8/16

Bits

Data

Variable

Length

CRC

8/16

Bits

Flag

8 Bits01111110

High-Level Data Link Frame; Bit-Oriented

Flag

01111110

Address

11111111

Control

00000011

Protocol Payload Checksum Flag

01111110

1 1 1 1 or 2 variable 2 or 4 1

Bytes

PPP Frame; Byte Oriented

Pr 8 PPP Protocol field


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Pr.8.PPP-Protocol field The Protocolfields job is to tell what kind of

packet is in the Payloadfield.

Codes are defined for LCP, NCP, IP, and other

protocols.

Protocols starting with a 0 bitare network layer

protocols such as IP, IPX, OSI CLANP. Those starting with a 1 bitare used to negotiate

other protocols. These include LCPand a different

NCPfor each network layer protocol supported.

The default size of the protocol field is 2 bytes, but it

can be negotiated down to 1 byteusing LCP.

PPP


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PPP-summary PPP is a multiprotocol framing mechanism suitable for use

over:Modems, HDLC,SONET, Other physical layers. It supports:Error detection, Option negotiating, Header

compression.

DLL converts the raw bit stream (from physical layer) into a

stream of frames (for network layer). Various framing methods are used:character count, byte

stuffing, and bit stuffing.

Data link protocols can provide:1. Error control to retransmit

damaged or lost frames. 2.To prevent a fast sender fromoverrunning a slow receiver.

The data link protocol also provide flow control.

The sliding window mechanism is used to integrate error

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Chapter 3 - Data Link Layer