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

Stop-and-wait ARQ:

Characteristics:

  • Used in Connection-oriented communication.
  • It offers error and flows control
  • It is used in Data Link and Transport Layers
  • Stop and Wait for ARQ mainly implements the Sliding Window Protocol concept with Window Size 1

Working:

1) Sender A sends a data frame or packet with sequence number 0.
2) Receiver B, after receiving the data frame, sends an acknowledgement with sequence number 1 (the sequence number of the next expected data frame or packet)
There is only a one-bit sequence number that implies that both sender and receiver have a buffer for one frame or packet only.

Stop and Wait ARQ

The Stop and Wait ARQ solves the problems but may cause big performance issues as the sender always waits for acknowledgement even if it has the next packet ready to send. Consider a situation where you have a high bandwidth connection and propagation delay is also high (you are connected to some server in some other country through a high-speed connection). To solve this problem, we can send more than one packet at a time with a larger sequence number.

So Stop and Wait ARQ may work fine where propagation delay is very less for example LAN connections but performs badly for distant connections like satellite connections.

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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