ACK 28 has been received by the sender in a Go-Back-N system where packets are n

Question

ACK 28 has been received by the sender in a Go-Back-N system where packets are numbered modulo-32. While 3 packets are still acknowledgement pending, 5 more packets are sent. You may assume that the window size is the largest allowed.(a).Draw a diagram below to show the sender window. On the diagram, clearly indicate the values of sendBase, nextSeqNum, and the last sequence number in the window.(b).If the sender has more data to send, how many more packets is the sender allowed to send before it has to stop and wait for the acks? Why?(c).If the sender has more data to send, how many more packets is the sender allowed to send before it has to stop and wait for the acks? Why?(d).There is no more ACK received by the sender. When timer expires, which packets will have to be retransmitted? Why? PLEASE HELP ME WITH (c) and (d)!!

Explanation / Answer

c)

Last ACK recieved is 28 and 3 more acknowledgements are pending that means packet number 29, 30, 31 are also sent and other than this 5 more packets are sent that is 0,1, 2, 3, 4 (as packets are numbered modulo 32) Now sender can send packets numbered 5, 6, 7, 8......28 that is 24 more packets before it has to wait for acknowledgement This is because if he will sent packet number 29 than collision will occur as he has already sent this packet Consider the case that initial packet numbered 29 was lost then the reciever will ignore all the corresponding packets as it is waiting for the retransmission of packet numbered 29 and if sender has sent new packet numbered 29 then reciever will think it is the reuiqred packet which was lost but actually it was a new packet different than the previous one Hence sender is allowed to send 24 more packets before it has to stop and wait for the acks

d)

In Go-Back-N, If timer runs out for a particular packet, then all the packets from the lost packet are re-sent Now say no more ACK is received by the sender In this case, all he packets that is packet numbered 29, 30, 31, 0,1, 2, 3, 4 are retransmitted That is a total of 8 packets are retransmitted

I have tried to explain it to you in very simple language Hope i have answered your question satisfactorily.Leave doubts in comment section if any.

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