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Q.   If maximum sequence number in Go back N ( arq) protocol is 'n' then  window size (sender) is ?

If your answer is 'n' or 'n'+1' then please design GBN for sender window 5 and 11 .

4 Answers

Best answer
7 votes
7 votes

Maximum sequence number in GBN is same as window size.

For sequence bits = n, number of sequence numbers = 2n and window size = 2n - 1

Consider n = 3, sequence numbers will be 23 = 8 ( i.e. from 0 - 7 )

and maximum window size = 23 - 1 = 7 i.e  window will carry frames  from 0 to 6 which are 7 in number.

Now, for sender window = 5

number of sequence bits needed = ceil (log2(1+window size)) = 3

So, sequence numbers generated would be 0 to 7 but window will carry frames from 0 to 4 which is 5 in number (same as window size)

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1 votes
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in any of the flow control mechanism, it is always true: sequence number >= (sws + rws) however for GBN we know rws=1 always.

as per question sequence number =n out of which 1 is given to rws, therefore, sws must be less or equal to n-1.

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Maximum window size at sender will be 'n-1'(As in question,no of sequence no is given not the no of bits used in sequence no)
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if maximum sequence number in Go back N ( arq) protocol is 'n' then  window size  is 2^n-1 is sender window size

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