Define flow control. Explain Go-Back-N ARQ with suitable example. How is it different from Stop-and-Wait ARQ?

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Flow control is a technique that allows two stations working at different speeds to communicate with each other. It is a set of measures taken to regulate the amount of data that a sender sends so that a fast sender does not overwhelm a slow receiver. In data link layer, flow control restricts the number of frames the sender can send before it waits for an acknowledgment from the receiver.

Two method have been developed to control the follow of data:

  1. Stop and Wait
  2. Sliding Window

Go-back-N ARQ (Sliding Window):

  • It is an specific technique of ARQ protocol, in which the sending process continues to send a number of frames specified by a window size even without receiving an ACK packet from the reciver.
  • The receiver process keeps track of the sequence number of the neat expected frame and sent that number of the next with every ACK it sends.
  • The receiver will discord any frame that doesn’t have the exact sequence number it expects and will recent an ACK for last correct in-order frame.
  • It is more efficient use of a connection than wait and stop ARQ. Since unlike waiting for an ACK for each packets the connection is still being utilized as packets are being sent no matter what.


- Hamro CSIT

Here, m = 3 and sender’s window size is 2m – 1 = 23 – 1 = 7 i.e. 0 to 6

  1. Sender sends Frame 0, Frame 1, Frame 2, Frame 3 .  . . . . . . .
  2. Receier receives 0 and sends ACK for 1
  3. Receiver receives 1 and send ACK for 2
  4. ACK 2 is lost
  5. Sender has already send 2
  6. Receiver send ACK for 3
  7. Sender sends 3

Here there is no timeout event because all outstanding frames are acknowledged before the timer copies. Note that although ACK 2 is lost, ACK 3 servers as both ACK 2 and ACK 1.

Difference Between Stop-and-wait and Go-back ARQ:

Key Stop and Wait ARQ Go-Back-N ARQ
Sender window size In Stop and Wait, Sender window size is 1. In Go-Back-N, Sender window size is N.
Receiver Window size In Stop and Wait, Receiver window size is 1. In Go-Back-N, Receiver window size is 1.
Minimum Sequence Number In Stop and Wait, Minimum Sequence Number is 2. In Go-Back-N, Minimum Sequence Number is N+1 where N is number of packets sent.
Efficiency In Stop and Wait, Efficiency formular is 1/(1+2*a) where a is ratio of propagation delay vs
transmission delay.
In Go-Back-N, Efficiency formular is N/(1+2*a) where a is ratio of propagation delay vs
transmission delay and N is number of packets sent.
Acknowledgement Type In Stop and Wait, Acknowledgement type is individual. In Go-Back-N, Acknowledgement type is cumulative.
Supported Order In Stop and Wait, no specific order is needed at receiver end. In Go-Back-N, in-order delivery only are accepted at receiver end.
Retransmissions In Stop and Wait, in case of packet drop,number of retransmition is 1. In Go-Back-N, in case of packet drop,numbers of retransmitions are N.
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