CN: Go Back N

Question

Station A needs to send a message consisting of 15 packets to station ‘B’ using a sling window (window size 4) and go-back-N error control strategy. All packets are ready and immediately available for transmission. If every 6th packet that ‘A’ transmits gets lost (but no Acks from ‘B’ ever gets lost), then what is the number of packets that ‘A’ will transmit for sending the message to ‘B’ ? (a) 29 (b) 33 (c) 27 (d) 25

Comments

@Rajat Sharma 1 Do you have any easy way to appraoch this problem. becoz i feel its time consuming

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I know only this method

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Can you enlighten the method ?

Answer 1

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 14 15

Total 33

Comments

Add some explanation.

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what about window size =4 ....

im confused plz explain litle more

Answer 2

answer will be 33...

solution is already given by many i m just writing for proper explaination...

as you can see the name suggest itself GO-BACK N where N is window size.In our question N=4 which means whenever a packet is lost you need to GO BACK 4 and retransmit from there again.

according to question every 6th packet is lost

1 2 3 4 5 6(lost) 7 8 9 when 6th packet is lost ,automatically receiver will discard other incoming packets i.e 7, 8  and 9 ....now why only 7, 8 and 9 because when 6th packet was transmitted window contain 4 packets(ws=4) including 6,7,8,9...therefore we need to GO BACK4 i.e upto 6 and retransmit again.....1 2 3 4 5 6 7 8 9 6 7 8 9... now you have to count after  first 6 for the 6th packet ...the 6th packet will be lost is (8) i.e  1 2 3 4 5 6 7 8 9 6 7 8(lost ) 9 10 11.. again follow the same procedure... and you get

1 2 3 4 5 6(lost) 7 8 9 6 7 8(lost) 9 10 11 8 9 10(lost) 11 12 13 10 11 12(lost) 13 14 15 12 13 14(lost) 15 14 15

Comments

Thanks for the explaination. 

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When the window size is 4 how are we sending 9 packets at a time? Please clear this doubt.

Answer 3

Answer is 33 ( The lost frames are 6,8,10,12,14). Many student get answer 31 but it is incorrect as they confuse with last transmission.