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Station A needs to send a message consisting of 10 packets to station B using a sliding window of size 4. All packets are ready and can be transferred immediately.Selective repeat and GBN are used at 2 different times and every 5th packet get lost for both protocols.(ACK's from B never get lost).Let x and y be the number of transmissions that A has to make in selective repeat and GBN respectively to ensure safe delivery to B. Then x+y= ?
asked in Computer Networks by Junior (943 points) | 50 views
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Answer given is 31 but i think that the ans has to be 21. Please confirm?
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12 for selective repeat and 19 in case of GBN
31 is correct
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for GBN, 

1234

          5678 (5 lost)

                   5678 (6 lost)

                               6789 (8 lost)

                                            8910

Total = 19

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thanks

2 Answers

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Best answer

In GBN,
First time 4,3,2,1 will be transmitted and received successfully.
Second time 8,7,6,5* will be transmitted and 5 will lost. so this along with the rest packets will be transmitted again.
Third Time 8,7,6*,5 will be transmitted and 6 will lost this time as it is the next fifth. So this along with the rest packets will be transmitted again.
Fourth time 9,8*,7,6 will be transmitted and 8 will lost as it is next fifth. so this along with the rest packets will be transmitted again.
Lastly 10,9,8 will be transmitted and reached successfully.
Therefore total 19 Transmissions in GBN.

In SR,

First time 4,3,2,1 will be transmitted and received successfully.
Second time 8,7,6,5* will be transmitted and 5 will lost and rest will reach. So 5 will be retransmitted
Third time 5 transmitted and recieved successfully.
Fourth time 10,9* will be transmitted and 9 will lost.
Lastly 9 will be transmitted and reaches successfully.
Therefore total 12 Transmissions in SR.

Then x+y = 31.

answered by Junior (943 points)
0 votes
31 is the correct answer
answered by (11 points)
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either explain the whole answer or post as a comment
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For SR i am getting 12 but in GBN why i am getting 26 transmissions ..(total both of Sr and gb4 i am getting 38 ) ..can you please explain @Mk Utkarsh..how GBN are 19 ??
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I added the new answer, please check that


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