arq-error correction-lec12

8/7/2019 ARQ-Error correction-Lec12

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Error Control Through ARQError Control Through ARQ

Error Control


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Error control in the data link layer isError control in the data link layer is

based on automatic repeat request,based on automatic repeat request,(ARQ) which is the retransmission of(ARQ) which is the retransmission of

data.data.

NoteNote::


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Error Control mechanisms

Stop and wait ARQ

Go Back N ARQ

Selective repeat ARQ


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Stop and Wait ARQ Features

Sender keeps copy of the last frametransmitted until it receivesacknowledgment.

For identification, both data and ACKframes are numbered 1 and 0 alternately.Data 0 frame is acknowledged by ACK 1frame, Data 1 by ACK 0

Damaged or lost frame receiver sends noacknowledgment. Simply discards it. Italso discards out of order frames.


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Continued Sender has control variable S that holds

the number of recently sent frame either 0or 1.

Receiver has control variable R that holds

the number of next frame expected either0 or 1.

The sender starts a timer when it sends aframe, this is the time it waits for

acknowledgement frame. Receiver sends only positive

acknowledgements for frames received

safe and sound. else wise it stays silent.


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Stop-and-Wait ARQ


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StopStop--andand--Wait ARQWait ARQ

Operation

In the transmission of a frame, we can havefour situations.

Normal operation

The frame is lost The acknowledgment is lost

The acknowledgement is delayed


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Normal operation

Senderwaitsbeforesendingnextframe

Timer

is setagain


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Stop-and-Wait ARQ, lost or damaged frame

Receiverretainsvalueof R

Retransmission


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Stop-and-Wait ARQ

One frame received and handled at a time

If frame is damaged, receiver discards it andsends no acknowledgment

Sender uses timer to determine whether or not toretransmit

Sender must keep a copy of transmitted frame untilacknowledgment is received

If acknowledgment is damaged, sender willknow it because of numbering


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11.3 Stop-and-Wait ARQ, lost ACK frame


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Stop-and-Wait ARQ: Example


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In StopIn Stop--andand--Wait ARQ, numberingWait ARQ, numbering

frames prevents the retaining offrames prevents the retaining ofduplicate frames.duplicate frames.

NoteNote::


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11.4 Stop-and-Wait ARQ, delayed ACK

SupposeFrame lost

If there wereNo numberingSender wouldHave thoughtThis as ACK of

Frame 1.


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Numbered acknowledgments areNumbered acknowledgments are

needed if an acknowledgment isneeded if an acknowledgment isdelayed and the next frame is lost.delayed and the next frame is lost.

NoteNote::


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Go-Back-N ARQ

In stop and wait ARQ, at any point in timefor sender, there is only one frame, theoutstanding frame, that is sent and

waiting to be acknowledged. Not a good use of transmission medium.

Multiple frames should be in transitionwhile waiting for acknowledgement.

More than one outstanding frames. Thiswill improve the efficiency of system.


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GoGo--BackBack--N ARQ ConceptsN ARQ Concepts

Sequence Number

Sender and ReceiverSliding Window

Control Variables and Timers

Acknowledgment

Resending Frames

Operation


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Sequence Numbers

Frames from sender are numberedsequentially.

Sequence number of each frame is

included in header. If header allows m bits for sequence

number then sequence number range

would be from 0 to 2m

-1. If m=3 sequence numbers are

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


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Sender Sliding Window

At sender side, to hold outstanding framesuntil they are acknowledged, we useconcept of window.

Outstanding frames are enclosed inwindow.

Size of window is fixed in this protocol.

Size of window is at most 2m

-1. The frames on left are already sent and

acknowledged ones. These can be purged.


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Sender sliding window

Already sent and

acknowledged frames

Frames sent, waiting foracknowledgment

Waiting,Cannot sent until

Window slides onThem.


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Receiver sliding window

Receiver sliding window is always 1 in this protocol.

Receiver is looking for specific frames to arrive at specific order.

Frame arriving out of order are discarded and needs to be resent.


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Control variables

Sender has 3 control variables,1. S holds sequence number of recently sent frame

2. SFholds sequence number first frame in the window3. SL holds sequence number last frame in the window

Size of window = SL- S

F+1

Receiver has only one variable R holds sequence numberof frame it expects to receive.


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Acknowledgment

Receiver sends positive acknowledgment ifa frame has arrived safe and in order.

If a frame is damaged or out of order,

receiver stay silent and will discard allsubsequent frames, until it gets the one itwants.

This causes sender to go back and

resends all frames. Now receiver sends one cumulative ACK

for several frames.


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Resending Frame

When frame is damaged sender goesback and sends a set if frames startingfrom damaged one up to the last one

sent. Suppose sender has already send frame 5

but time for frame 2 expires.Frame 2 not acknowledged. Sendersends 2,3,4,5 again.

Thats why we call it Go-Back-N ARQ


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Go-Back-N ARQ, normal operation


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Go-Back-N ARQ, lost frame


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Damaged or lost Acknowledgement

1. If the next acknowledgement occurs beforeexpiration of any timer no need ofretransmission Acknowledgments arecumulative.

ACK 3 means ACK1, ACK2, ACK3(So if ACK1 & 2 are lost, ACK4 can cover them).

2. IF next ACK comes after the timeout, the frameand all the frame after that are resent.

Note: Receiver never resends an ACK.


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In GoIn Go--BackBack--NARQ, the size of theNARQ, the size of the

sender window must be less thansender window must be less than 22m;m;the size of the receiver window isthe size of the receiver window is

alwaysalways 11..

NoteNote::


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SelectiveSelective--Repeat ARQRepeat ARQ

Sender and Receiver Windows

Operation

Sender WindowSize


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Selective Repeat ARQ, sender and receiver windows


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Selective Repeat ARQ S

ender window size

Receive window size


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In Selective Repeat ARQ, the size ofIn Selective Repeat ARQ, the size of

the sender and receiver window mustthe sender and receiver window mustbe at most onebe at most one--half ofhalf of 22mm..

NoteNote::


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Selective-reject ARQ

Most efficient error controlprotocol

If a frame is received in

error, the receiver askstransmitter to resendONLY the frame that wasin error

Subsequent framesfollowing the Nth frameare not retransmitted


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Selective Repeat ARQ, lost frame


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Selective Repeat ARQ


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Selective Repeat ARQ, sender window size


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Selective Repeat ARQ: Example


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Bandwidth Delay Product

Measure of efficiency of an ARQ system

Bandwidth delay product is measure ofthe number of bits we can send out of our

system while waiting for news fromreceiver.

Bandwidth delay product = bandwidth (in

bits per second) X round trip delay (inseconds)


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ExampleExample 11

In a Stop-and-Wait ARQ system, the bandwidth of the line is 1 Mbps, and 1 bit

takes 20 ms to make a round trip. What is the bandwidth-delay product? If the

system data frames are 1000 bits in length, what is the utilization percentage of

the link?

SolutionSolutionThe bandwidth-delay product is

1v 106 v 20v 10-3 =20,000 bits

The system can send 20,000 bits during the time it takes for the data to go

from the sender to the receiver and then back again. However, the system

sends only 1000 bits. We can say that the link utilization is only 1000/20,000,

or 5%. For this reason, for a link withhigh bandwidth or long delay, use of

Stop-and-Wait ARQ wastes the capacity of the link.


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ExampleExample 22

What is the utilization percentage of the link in Example 1 if the link uses Go-

Back-N ARQ with a 15-frame sequence?

SolutionSolution

The bandwidth-delay product is still 20,000. The system can send up to 15

frames or 15,000 bits during a round trip. This means the utilization is

15,

000/20,000

, or 75 percent. O

f course, if th

ere are damaged frames, th

eutilization percentage is much less because frames have to be resent.

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