arq-error correction-lec12
TRANSCRIPT
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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.
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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.
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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.
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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..
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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..
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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.