# MadeEasy Test Series: Computer Networks - Sliding Window

1 vote
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Assume 2 computers are connected via a network, The networks use GoBack7 SW protocol for maintaining flow control. It is observed that every $5^{th}$ packet that has been sent through the network is lost. The number of extra packets needs to resend for transmitting 10 such packet through the network is______________________.

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I am getting the answer as 7, the given answer is 11.
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7 seems correct to me.

The retransmitted packets will be 5,6,7,8,9,10,9 and 10.
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As size of window is 7 .You will send packets 1,2,3,4,5,6,7. Now, 5th packet will be lost.  Now,as 5th ack is lost it will resend 5,6,7,8,9,10 again. Now ,the 5th packet would be no. 7. (6,7,5,6,7) .Now it will resend 7,8,9,10 again. Now , 5th packet would be no 8 (8,9,10,7,8) .So it will resend 8,9,10 again. 5th packet would be (9,10,8,9,10) 10. It will resend 10 again. Now there will be no more 5th packet.So, total transmission will be 21.Extra =21-10=11.
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How is the 5th packet no.7?

We send 1 2 3 4 5 6 7 8 9 10.

In this, 5,6,7,8,9 and 10 have to be retransmitted. So the sequence becomes:
1 2 3 4 5 6 7 8 9 10 5 6 7 8 9 10. The 10th packet (or the second time a 5th packet is lost) would be packet number 10 (one between 9 and 5). But since that packet has already been transmitted again, we can ignore it. The 15th packet in this sequence will be 9 and for that, we'll transmit 9 and 10 again.
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You missed counting 6,7, when we have transmitted 1,2,3,4,5,6,7. After 5 6 will be counted as 1 and 7th will be counted as 2.Now again when 5,6,7,8,9,10 will be transmitted 5th will be number 3,6th will be no 4th and 7th will be number 5th which will be lost.
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I can't see what I missed.

When the first loss happens, we have acknowledged 1,2,3 and 4. Our current window now contains (5,6,7,8,9,10). In this window, both 5 and 10 will be lost.

Now since 5th packet is lost, we retransmit the entire window. So after 10, we start from 5 again i.e (5,6,7,8,9,10) and send these.
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im also getting 7 ...verify please
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@PiratedVirus

Yes 8 is the right answer.

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Great explanation

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