GATE | GATE-IT-2004 | Question 81

In a sliding window ARQ scheme, the transmitter’s window size is N and the receiver’s window size is M. The minimum number of distinct sequence numbers required to ensure correct operation of the ARQ scheme is

 
(A) min (M, N)
(B) max (M, N)
(C) M + N
(D) MN


Answer: (C)

Explanation:  

In general sliding window ARQ scheme , the sending process sends a number of frames without worrying about receiving an ACK(acknowledgement) packet from the receiver. The sending window size in general is N and receiver window is 1. This means it can transmit N frames to its peer before requiring an ACK. The receiver keeps track of the sequence number of the next frame it expects to receive and sends that number with ever ACK it sends. But in case of the question the sender window size is N and receiver is M so the receiver will accept M frames instead of 1 frame in general. Thus sending M sequence numbers attached with the acknowledgement.
Hence, for such a scheme to work properly we will need a total of M+ N distinct sequence numbers.

 

This solution is contributed by Namita Singh.

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