Question

In relation to the use of error control techniques: 4. From the perspective of a Receiving station only: Explain how the use of the Go- back-N technique differs from the Selective Reject technique when an out-of sequence frame arrives (i) (5 marks) (ii) From the perspective of the Transmitting station responding to the Receiver station as per i) above: Explain how the use of the Go-back-N technique differs from the Selective Reject technique (5 marks) From the perspective of a Transmitting station only: Explain how the use of the Go-back-N technique differs from the Stop-and-Wait technique when a timer expires for a frame previously transmitted (ii) (5 marks

Answer 1

i) In Go Back N if out of sequence frame arrives then it is discarded whereas in selective repeat it is not discarded but protocol waits for correct sequence number frame and then arranges all the frames.

ii) If out of sequence frame is received, most probably the required frame was lost. So at transmitting station timer times out for that frame and is sent again and all other frames are also sent. For instance receiver expects frame number 2 but instead it gets 3, 4, and 5. They all are discarded at receiver and 2, 3,4, 5 are sent again.

In selective repeat only the frame lost is sent again if it gets lost. No other frame is sent as it is stored by the receiver and later rearranged. For example, For instance receiver expects frame number 2 but instead it receives 3 , 4, 5, so it keeps all of these but keeps sending request for frame number 2 until it receives it.

iii) In stop and wait only one packet whose timer is timed out is retransmitted again whereas in Go Back N, all N packets present in the window are sent again to the receiver.

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