L11 - Error control with multiple frames outstanding...

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Error control with multiple frames outstanding Communication is faster if multiple frames are sent out before an acknowledgment of the first is returned. This requires more sequence number bits. Let b be the number of bits in the sequence number. Sending "next expected" sequence number acts as a cumulative acknowledgment . It implies all prior frames have been correctly received. There is a certain maximum window W of number of unacknowledged frames that can be outstanding, as a function of b, so as to ensure no numbering ambiguity.
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There are two different protocol types, that have different maximum value of W. One type can only accept the next frame in order. It is often called the “ go-back ” protocol - if the sender needs to repeat a frame, it must go back and repeat all frames it had sent after that frame. The ambiguity window limit for the “go- back” protocol is: W # 2 - 1. b
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Why is W # 2 - 1 for the go-back protocol? b Consider b = 3. You could number 8 different frames, so at first thought a window of 8 would be OK. Recall that the round trip acknowledgment time can vary, so you don’t know when a cumulative acknowledgment will return. Frames are numbered 0,1,2,3,4,5,6,7,0,1,2,3,4,. ..... Suppose you send out 0,1,2,3,4,5,6,7 (none yet acknowledged) and then get back “expecting 0". Possibilities: a) None of the 8 have been received correctly. b) all 8 have been received correctly.
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But if you were only allowed to send 0,1,2,3,4,5,6, any return acknowledgment number would be unambiguous.
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L11 - Error control with multiple frames outstanding...

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