Question

A sender sends a series of packets to the same destination using 5 bit sequence numbers. If the sequence number starts with 0, what is the sequence number after sending 100 packets ?

Comments

Arjun sir

can you tell me why each packet is consuming only 1 sequence number? There is no packet size or segment size given here

Answer 1

in GO backN arq protocol

 SWS < 2^m   and for 5 bits sequence no 2⁵ = 32(0-31) 

Sequence no.         Total No. of  frame

           0-30       = 31 frame          (1-31)

           31-29     = 31 frame          (32-62)

           30-28     = 31 frame          (63-93)

until by now 93 frames has been sent

now  sequence no. 29  30  31   0    1    2    3   4

        frame no.      94  95  96  97  98  99 100  101

so after 100 frame sequence number will be 4

Answer 2

With 5bit no of sequence no. Possible is 32 ie (0-31)

For frames 1-32 seq no 0-31

For frame 33-64 seq no 0-31

For frame 65-96 seq no 0-31

Now  97 98 99 100

           0   1    2     3

So after sending 100 packets sequence no is 4

Comments

You are getting right answer its your luck but it is completely wrong explanation 

you are using SWS=2^m   but in go backN arq we have to use SWS <2^m to avoid a drawkback as mentioned below-:

If you use  SWS=2^m  (0-31)

   1. then let sender sends the frame 1-32 using sequence no (0-31)

   2.now receiver receives the data and send the ACK 0

     (expecting the next frame 33-64 sequence no 0-31)

   3.let the ACK is lost sent by the receiver 

   4.now after time expires sender again send the same frame(1-32) using sequence 0-31 

   5.now receiver accept it thinking the next right data but its wrong protocol is not working.

Please refer FOROUZAN for this topic you would be more clear.

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where is Go-back-N mentioned in question?

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we have to use some protocol to transfer the data or not ?

Answer 3

Though I believe that answer should be 4, but following screenshot is taken from the solution manual:

Comments

according to manual's solution packet numbering is started from '1' which is leading seq#=5, but by default we take packet numbering from '0' which will give seq#=4