420-10-09 - Go Back N (1) Based on sliding window If no...

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1 CS420/520 Axel Krings Sequence 9 Page 1 Go Back N (1) Based on sliding window If no error, ACK as usual with next frame expected Use window to control number of outstanding frames If error, reply with rejection Discard that frame and all future frames until error frame received correctly Transmitter must go back and retransmit that frame and all subsequent frames CS420/520 Axel Krings Sequence 9 Page 2 Go Back N - Damaged Frame Receiver detects error in frame i Receiver sends rejection- i Transmitter gets rejection- Transmitter retransmits frame and all subsequent
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2 CS420/520 Axel Krings Sequence 9 Page 3 Go Back N - Lost Frame (1) Frame i lost Transmitter sends i+1 Receiver gets frame i+1 out of sequence Receiver send reject Transmitter goes back to frame and retransmits CS420/520 Axel Krings Sequence 9 Page 4 Go Back N - Lost Frame (2) Frame lost and no additional frame sent Receiver gets nothing and returns neither acknowledgement nor rejection Transmitter times out and sends acknowledgement frame with P bit set to 1 Receiver interprets this as command which it acknowledges with the number of the next frame it expects (frame i ) Transmitter then retransmits frame
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3 CS420/520 Axel Krings Sequence 8 Page 5 Go Back N - Damaged Acknowledgement Receiver gets frame i and send acknowledgement ( i+1 ) which is lost Acknowledgements are cumulative, so next acknowledgement ( i+n ) may arrive before transmitter times out on frame i If transmitter times out, it sends acknowledgement with P bit set as before This can be repeated a number of times before a reset procedure is initiated CS420/520 Axel Krings Sequence 8 Page 6 Go Back N - Damaged Rejection As for lost frame (2)
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4 CS420/520 Axel Krings Sequence 8 Page 7 Go Back N - Diagram CS420/520 Axel Krings Sequence 8 Page 8 Selective Reject Also called selective retransmission Only rejected frames are retransmitted
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This note was uploaded on 12/20/2011 for the course COMPUTER 101 taught by Professor Dr.kahan during the Spring '11 term at Akademia Ekonomiczna w Krakowie.

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420-10-09 - Go Back N (1) Based on sliding window If no...

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