# Go Back N, Computer Networks

#1

Packets are being transmitted using GB5 and here every 4th packet is lost.How many packets need to be transmitted to transmit 10 packets?

#2

Ans is 20.

1 2 3 [4 5 6 7 8] 4 5 6 [7 8 9 10] 7 8 9 [10] 10

#3

#4

Packets are being transmitted using GB5 and here every 6th packet is lost.How many packets need to be transmitted to transmit 10 packets?

In this case 6th packet is lost and N=5.
Here as N<(lost packet number) or 5<6
So you start counting from the next number itself.
Like this.

1 2 3 4 5 [6 7 8 9 10] 6 [7 8 9 10] 7 8 [9 10] 9 10

The packet loss was at 6. Then you start counting from the next number, i.e.7. Again when your count becomes 6, re-transmit it.

So re-transmissions required is 21 in this case.

#5

Packets are being transmitted using GB5 and here every 4th packet is lost.How many packets need to be transmitted to transmit 10 packets?

Now coming to given question.

Here N>(lost packet number) , 5>4

So in this case you have to start counting after the current go back call is successfully executed.
Like this.

1 2 3 [4 5 6 7 8] 4 5 6 [7 8 9 10] 7 8 9 [10] 10

You started counting from 1. You encountered 1st packet loss at number 4. Now you re-transmitted till 8. Then again start counting from the next number, i.e. 4. and continue this process till you successfully send packet number 10.

If you count the total number of numbers in the above sequence it would be 20. So it is the answer.