Applied Test Series

Question

In the GO back N ARQ sender is sending the 20 packets to the destination with a window size of 4. Every sixth packet is lost and after a packet loss sender is sending one dummy packet outside the window for ensuring everything is correct [ dummy packet is an outside packet not the part of the window]. How many number of transmissions will be there?

Answer 1

Total number of transmission = 54

1 2 3 4 5 6 7 8 9 6 7 8 9 10 11 8 9 10 11 12 13 10 11 12 13 14 15 12 13 14 15 16 17 14 15 16 17 18 19 16 17 18 19 20 18 19 20

(Bold numbers represent one window)

= 47

Also we need to send 7 dummy packet for 7 windows.

Therefore total = 47 + 7

= 54 transmissions.

Comments

As the question is saying after a packet loss sender is sending one dummy packet outside the window for ensuring everything is correct

As Every sixth packet is lost then when every six packet is lost why are you not counting the dummy packet as one of the transmission like

1 2 3 4 5 6 7 8 9 D1 6 7 8 9 10 D2 7 8 9 10 11 D3 8 9 10 11 12 D4 9 10 11 12 13 D5 10 11 12 13 14 D6 11 12 13 14 15 D7 12 13 14 15 16 D8 13 14 15 16 17 D9 14 15 16 17 18 D10 15 16 17 18 19 D11 16 17 18 19 20 D12 17 18 19 20  D13 18 19 20 D14 20

Here Di for i=1,2,3...14 are the Dummy packets.

Don’t you think that even if the dummy packet is outside the window it will still be counted every time when a packet is lost due to which when the 2^nd time the packet is lost it should be the packet number 7 like i have explained above.

Hence total transmission should be= 86

Answer 2

total 54 transmission requires