DATA COMMUNICATION(ELA…)

DATA LINK CONTROL PROTOCOLS1

INTRODUCTION

Synchronization and Interfacing are insufficient due toProbability of transmission errorsNeed of receiver to regulate arrival rate of

data Layer of logic is added

Data Link Control

Key mechanisms

2

FLOW CONTROL

FLOW CONTROL

4

Rcv Data from Tx

Is there buffercapacity left?

Stop Rcv’ng Data from Tx

Transfer Data to next upper layer

Process Data

No

Yes

Begin

Clear Buffer More data

to Tx?Yes

No

Stop

MODEL OF FRAME TRANSMISSION

5

FLOW CONTROL

DefinitionsTransmission time

Time taken to emit all bits onto the medium Proportional to length of frame

Propagation time Time for a bit to traverse the link

OptionsStop-and-WaitSliding Window

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STOP-AND-WAIT FLOW CONTROL

How does it work? 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 Not good when fragmentation is used

7

PERFORMANCE ISSUES

Where d Link distance (m) V Velocity of propagation (m/s) L Length of frame in bits R Data Rate (bps)

8

a =Propagation TimeTransmission Time

d/VL/R

RdVL

= =

STOP-AND-WAIT – LINK UTILIZATION

9

Tx Time = 1, Propagation time = a

a > 1 a < 1

SLIDING WINDOW FLOW CONTROL

How does it work?Allows multiple frames to be in transitReceiver has buffer of length = WTransmitter can send up to W frames

without ACKEach frame is numberedACK includes number of next frame

expectedSequence number bounded by size of field

(k) Frames are numbered modulo 2k

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SLIDING WINDOW DIAGRAM

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SLIDING WINDOW ENHANCEMENTS

Receiver can acknowledge frames without permitting further transmission RNR Receive Not Ready Must send a normal ACK to resume

If duplex, use piggybacking If no data to send, use ACK frame If data but no ACK to send

Send last ACK number again, or have ACK valid flag (TCP)

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ERROR CONTROL

ERROR CONTROL

Detection and correction of errorsLost framesDamaged frames

Automatic Repeat RequestError detectionPositive acknowledgmentRetransmission after timeoutNegative acknowledgement and

retransmission

14

AUTOMATIC REPEAT REQUEST (ARQ) These techniques are all based on either

Stop-and-wait or Sliding Window flow control techniques Stop-and-wait ARQ Go-back-N ARQ Selective reject ARQ

Selective retransmission

15

STOP-AND-WAIT ARQ

Basic Operation

16

T

RF(0)

F(0)

AC

K(1

)

t<To ….t=To

F(1) F(1)

AC

K(0

)

t<To

Duplicate detected/discarded

t

tF(1) F(1) F(1)

t=To….

AC

K(0

)

STOP-AND-WAIT ARQ

Pros and Cons

SimpleInefficient

Referred to as Idle RQ

17

GO-BACK-N ARQ

Most common implementation of Sliding Window flow control

Uses window to control number of outstanding frames

If no error, ACK as usual with next frame expected

If error, reply with rejection (REJ)Discard that frame and all future frames until

error frame received correctlyTransmitter must go back and retransmit

that frame and all subsequent frames18

GO-BACK-N ARQ

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SELECTIVE REJECT

Also called selective retransmission Advantages

Only rejected frames are retransmittedSubsequent frames are accepted by the

receiver and bufferedMinimizes retransmission

DisadvantagesReceiver must maintain large enough

bufferMore complex logic in transmitter

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SELECTIVE REJECT ARQ

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PERFORMANCE ISSUES

PERFORMANCE ISSUES

Stop-and-Wait Flow Control (No Errors)A sends a frame to BAfter receiving frame, B sends and ACK to AProcess is repeated n timesThus, total time to transmit n frames can be

expressed as

WhereTF Time to send a frame and receive

and ACK

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PERFORMANCE ISSUES

Where tprop Propagation time from A to B

tframe Time to transmit a frame

tproc Processing time at each station

tack Time to transmit and ACK

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procackpropprocframepropF ttttttT

PERFORMANCE ISSUES

Since a = tprop / tframe

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procackpropprocframepropF ttttttT

)2( framepropF ttnnTT

frameprop

frame

frameprop

frame

tt

t

ttn

tnU

2)2(

aU

21

1

PERFORMANCE ISSUES

Sliding Window Flow Control (No Errors)

26

U

12 aW

12 aW12 a

W

1

PERFORMANCE ISSUES

Stop-and-Wait ARQ (With Errors)

Where Nr Expected # of transmissions of a frame

27

)21(

1

aNU

r

a

P

aNU

r 21

1

)21(

1

PNr

1

1

PERFORMANCE ISSUES

Selective Reject ARQ (With Errors)

28

U

12 aW

12 aW12

)1(

a

PW

P1

PERFORMANCE ISSUES

Go-back-N ARQ (With Errors)

29

U

12 aW

12 aW)1)(12(

)1(

WPPa

PW

aP

P

21

1