Packet Loss in GBN

Question

Q1 >> Station A wants to send 10 packets to station B using Go Back N ARQ, where N = 4. Moreover consider every 4 th packet lost while transmission (ACK's from B never get lost). Calculate the number of transmissions that A has to make in GBN respectively to ensure safe delivery to B.

Q2 >> Station A wants to send 10 packets to station B using Go Back N ARQ, where N = 4. Moreover consider every 3 rd packet lost while transmission (ACK's from B never get lost). Calculate the number of transmissions that A has to make in GBN respectively to ensure safe delivery to B.

Comments

Hello warlord , you know the concept , make then question yourself...

total 14 packets every 7 packet lost N=3

total 20 packets every 8packet lost N=7

total 13 packets , every 2 packer lost ,N=4

......whatever...

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@Shubhanshu 1,2,3,4,5,6,3,4,5,6,7,8,5,6,7,8,9,10, 7,8,9,10,9,10,10

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@Rupendra Okay got it thanks.

Answer 1

 This should be the required number of transmission  :

Comments

@Habibkhan sir, what is wrong with rupendra's solution??

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In the last u will be left with 2 frames only as the pointer is on frame having sequence no as '0' as shown above , hence we retransmit {0,1} but now in {0,1} as it is clear we have 2 frames only and it is given every 3rd frame is lost , hence in this case '1' need not be transmitted again..Had it been the case that at last we were left with {0,1,2} , we would have retransmitted the frame "2" as "2" is the 3rd frame w.r.t this final window of 3 frames. But here no such need arises in the last

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Hello Habib

i'm doubtful about your answer. Can you please tell me , if sender sent 1234 as form of window ans it's given that every second frame would be lost then which frame of this window will get lost? if i'm sending frame 1234 as window and if every 3rd frame would be lost then first two frame that would be lost are 3,6 but according to your explanation , that's 3,5. and in that way in last there won't be any 3rd frame so answer would be definitely 24.

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sorry @habibkhan sir for this comment

answer of b is wrong

it will be 25 not 24

Answer 2

1) We have GBN where N=4 , total packets to transmit =10 , every 4th packet lost so #total transmission to send those 10 packets...

after sending first 3 packets successfully the sliding window would be 4567 , now as packet 4 lost , sliding window won't slide until source get ACk for packet 4 ,after timeout for ACKWAIT , as it's GBN , sender will send 4567 again,

Now as it's given every 4th packet is lost , so here last 5,6,7 and now this 4 make 4th 4th packet so again packet number 4 lost , sender agian send 4567 , again 4th packet after last unsuccessful transmission is packet number 4 , which will get lost , so in that way every time 4th packet is packet number 4 ,so it will go on...never stop...

123(4*)567 (4*)567 (4*)567 (4*)567 (4*)567 (4*)567 (4*)567 (4*)567..............

2) here very 3rd packet will lost

so after losing 3 rd packer the sliding window will be 3456 , so sender will send again , in last 3456 window 3 and 6 got lost , but as whole sliding window will be resend , so no problem to take care about lost of 6 ....now 3456 is sent but here after last 6 lost the 3rd packet is packet number 5 , and at that time sliding window is 5678 so send 5678 , here ,after last lost packet (5) next 3rd packet is packet number 8, that will lost , but as 5678 are resent , 8 is ultimately retransmitted , so no problem , now after 8 the next 3rd packet is 7 , which will lost , now SW is 78910, next 3rd packet is 10 which will too lost but it's okay as 78910 resent ,so eventually 10 resent with net SW , but here next 3rd packet is packet number 9 , that will lost but SW now is 910 so send it...but here 3 packet after last loss is 10 number packet , so sender will send it again ...

12(3*)45(6*) 34(5*)67(8*) 56(7*)89(10*) 78(9*)10 9(10*) 10

Comments

Thanks a lot for this explanation..

Answer 3

Q1: W=4 , 4th packet lost, Total packet =10

 

1, 2, 3, 4, 5, 6, 7  4, 5, 6, 7  4, 5, 6, 7 ………………….. infinite loop

hence retransmission never stop

 

 

Q2: W=4 , 3rd packet lost, Total packet =10

 

1, 2, 3, 4, 5, 6, 3, 4, 5, 6, 7, 8, 5, 6, 7, 8, 9, 10, 7, 8, 9, 10, 9, 10, 10

 

Total transmission=25