ccnet lec 08 error detection and flow control

8/6/2019 Ccnet Lec 08 Error Detection and Flow Control

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Lecture

08Data Communication & Networks

Error Detection & Flow Control

Muhammad Yousaf


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Errors on the network:

We know real life is very uncertainData sent might be lost in networkOr receiver receive data with errorsNoise may be higher enough to invert bit-values

How receiver will decide that data receivedcorrectly or erroneouslyNeed to develop some mechanism that enables the

receiver to discriminate between correct data anderroneous data


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Error Detection:

Idea is to add some extra information with userdataReceiver uses these information to infer about thecorrectness of dataThere are many schemes that implement errordetection

Parity Checking

Cyclic Redundancy Check (CRC)Checksum


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Parity Check:

Used with character-wise transmissionAppend an extra bit with the character bits0 1 0 0 0 0 1 0 P

Count the number of ones & set the P-bitaccordinglyEven Parity:

Number of ones including P-bit should be even

0 1 0 0 0 0 1 0 00 1 0 0 0 0 1 1 1

Similarly in Odd Parity number of ones are odd


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Problem in Parity Check:

Odd number of error-bits can be detected easilySend data with even parity: 0 1 0 0 0 0 1 0 0Error occurred during transmission: 0 1 0 0 0 1 1 0 0Receiver checked the parity: 0 1 0 0 0 1 1 0 0

Number of ones are not even: ERROR DETECTEDEven number of error- bits cant be detected

Send data with even parity: 0 1 0 0 0 0 1 0 0

Error occurred during transmission: 0 1 0 0 1 1 1 0 0Receiver checked the parity: 0 1 0 0 1 1 1 0 0Number of ones are even: ERROR NOT DETECTED


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Cyclic Redundancy Check: (CRC)

Used with block-wise transmissionTransmitter & receiver agree on a common divisorTransmitter divides the data with this divisorCalculates the remainder

Appends this remainder with the data as CRC valueTransmit (data + CRC value)Receiver divides this (data + CRC) value with the commondivisor

If remainder is zero then data received error freeIf remainder is non-zero then data received erroneously


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CRC Calculation: (sender side)

1011

10111

11100

10111

10000

10111

10011

10111

11110

10111

10110

10111

11100

10111

010000110010011110111

Let user data is:110010011101Common divisor is:

10111

Start with 0000110010011101 0000

Dividing using XORRemainder is 1011Append it with data &transmit

110010011101 1011


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CRC Verification: (receiver side)

0000

10111

10111

10111

10010

10111

10011

10111

11110

10111

10110

10111

11100

10111

011011110010011110111

Receiver received thedata + CRC valuePerformed samecalculationIf remainder is zero

Data received error free

Else if Non-Zero

There is some error inreceived data


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Frame transmission scenarios:


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Flow Control


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Why Flow Control ?

In this world, every one is not of same capabilitySender might be having more resources thanreceiver in terms of:

Processing powerMemory spaceBandwidth

Sender can send so much data packets that

receiver might not be able to handlePackets more than the receivers buffer space willbe dropped


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Means of Flow Control:

Need a mechanism in which:Receiver can limit the sender from overwhelming himThere are two popular schemes:

Stop & Wait Flow ControlSliding Window Flow Control


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Stop and Wait:

Source transmits frameDestination receives frame and replies withAcknowledgementSource waits for ACK before sending next frameDestination can stop flow by not send ACKVery simple scheme


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Stop and Wait: cont

Frames are alternatively numbered as frame-0 & frame-1Similarly receiver uses two ACK messages:

ACK-0 = receiver has received frame-1 and is now asking for frame-0ACK-1 = receiver is asking for frame-1

If sender doesnt receive any ACK then:Either receiver didnt received the frameOr ACK has been lost in the waySender waits for the period of TimeoutAfter the expiry of Timeout sender retransmits the previous frame

Often Timeout = 2 * RTTRTT = Round Trip Time


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Stop and Wait:


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Stop and Wait: (Link Utilization)


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Stop & Wait: (pros & cons)

Advantages:Very simple schemeImplementation is easy

Disadvantages:Poor link utilizationHigher delays

Poor throughput


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Sliding Windows:

We want to improve link utilizationAllow multiple frames to be in transit at the sametimeEach frame is labeled with a sequence numberReceiver can send Accumulative ACKThis ACK includes number of next frame expected

ACK-6 acknowledges the reception of previous 5 frames

& is now asking for frame-6


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Sliding Windows: cont

Both sender & receiver maintain two windowsCommon window sizes are:3-bits = HDLC (High-level Data Link Control Protocol)7-bits = HDLC enhancement for satellite communication

16-bits = TCP (Transmission Control Protocol)Sender maintains a list of sequence numbers thathe can sendReceiver maintains a list of sequence numbers thathe can receiveSender sends the number of frames usually lessthan the maximum window size


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Example Sliding Window:


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Automatic Repeat Request: (ARQ)

If some frames are lost in transit or received witherror then what to do?Definitely perform retransmission

In sliding window for retransmissions there are twopossibilities:

Go back NSelective repeat


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Go Back N:

If frame N is lost or received with error;Receiver side:Receiver discards that frame and all subsequent framesuntil error frame received correctly

Receiver doesnt buffer the out of order framesSends ACK-N to indicate that receiver is asking forframe-N

Sender side:Goes back to sequence number N and sends allsubsequent frames


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Go Back N: cont


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Selective Repeat:

Minimizes retransmissionOnly lost/damaged frames are retransmittedIf frame N is lost or received with error:Receiver side:

Receiver discards that frame but buffers the subsequentframesReceiver must maintain a buffer to store out of orderframes

Sender side:Only the frame that has been asked by the receiver isretransmitted


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Selective Repeat:


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HDLC:

High-level Data Link Control ProtocolLayer 2 protocolDefines the services provided by data link layerAppends layer-2 header and trailer with the dataOnce heavily used protocolTodays data link protocols are based on HDLC


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Frame Structure of HDLC:


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Control Field: (information frame)


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Questions ???

ccnet lec 08 error detection and flow control

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