Question

In Go-Back-N protocol, if the maximum window size is 127, what is the range of the sequence number?

A. 0 to 127 B. 0 to 128 C. 1 to 127 D. 1 to 128

Answer given is A

Please explain :)

Thanks.

Answer 1

Here in Go Back - N :

N  refers to the senders window size

So window size here  =  127 as it is Go back - 127..

But we know  number of distinct sequence numbers for Go back N  =   max senders window size + 1 

                                                                                                  =    128

And starting sequence number    =    0..

Hence range of sequence numbers     =      0   to   127

Comments

is this approach correct

there window size is 127

so numbering is from 0 to 126 = 0 to (2^n)-1

so, (2^n)-1=126

      2^n=127

     so, n=7

now, the actual range will be

0 to (2^n)-1 == 0 to (2^7)-1

i.e=   0 to 127

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@Nandkishor Nangre 2^7 is 128 not 127

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why sequence number must start from 0, here D can also be correct

Answer 2

Sequence number >= sws+rws for any protocol.. If you go through gbn rws=1 Therefore seq n >= 127+1 Therefore the range would be from 0 to 127

Comments

Since 128 sequence numbers are there, then option d can also be the answer or is it that the sequence numbers start from 0 always?

Answer 3

 

https://stackoverflow.com/questions/28444055/go-back-n-window-size

Please refer this it should clear all doubts