Go Back N Protocol

'Go-Back-N' Protocol:

The main theme of this protocol, sender sends multiple packets (Such As 1, 2, 3, 4, 5).

1

2

3

4

5

6

7

8

9

10

Receiver will give ACK for every packet sequentially. ACK1, ACK2, ACK3, ACK4, ACK5. Then sender will slide its window another 5 packets and send packets 6, 7, 8, 9, 10

5

6

7

8

9

10

11

12

13

14

In these ways GBN slide window, so GBN itself referred as a Sliding-window protocol.

Now we will see how sender and receiver send and receive data

Problem Studies 1:

If receiver gets 6, 7, 8, 10 packets and misses packet 9 then receiver will send ACK6, ACK7, ACK8

Then sender will slide its window another 5 packets and send packets 11, 12, 13 because sender knows he sent 9 and 10

8

9

10

11

12

13

14

15

16

17

But receiver end will discard packet 11, 12, 13 and will send no ACK. Sender will wait for a time and then send again packets 9, 10, 11, 12, 14. Receiver will send ACK sequentially for all packets.

Sender will slide its window from 14- 19 and will send 5 packets.

Problem Studies 2:

If receiver gets 6, 7, 8, 9, 10 packets then receiver will send ACK6, ACK7, ACK8, ACK9, ACK10

But ACK9 lost. Then sender will slide its window another 5 packets and send packets 9, 10, 11, 12, 13

8

9

10

11

12

13

14

15

16

17

There are some problem in GBN that if 1-100 packet send and packet 3 missing in this protocol sender have to retransmit again 3-100 packets. That is a wasting time. Cause receiver has no buffer to store. If there is buffer receiver can store 4-100 data and sender then has to retransmit only packet 3.

So there is another protocol which is doing this. This protocol is Selective Repeat Protocol.

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