GATE | GATE MOCK 2017 | Question 43

In Go–back 3 flow control protocol every 6th packet is lost. If we have to send 11 packets. How many transmissions will be needed ?

(A) 10
(B) 17
(C) 12
(D) 9

Answer: (B)

Explanation: In Go back N, if we don’t receive acknowledgement for a packet, whole window of that packet is sent again. As a packet is received window is slided.
Here, window size is 3. Initially window will contain 1,2,3 then as acknowledgement of 1 is received window slides so 4 is transmitted. Now,when 4th packet’s acknowledgement is received 7th packet is sent and when 5th packet’s acknowledgement is received 8th packet is sent. Now, as acknowledgement of 6 is not received so the window of 6 i.e. 6,7,8 packets are retransmitted.Now the 6th packet from there is 9, so 9,10 will be retransmitted.

These are the serial transmissions of packets: 1 2 3 4 5 6 7 8 6 7 8 9 10 11 9 10 11 . Hence total 17 transmissions are needed.

Packets in bold in the above were failed transmissions. Hence their window (underlined) was resent.

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