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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:
Go-back-N ARQ (Sliding Window):
Here, m = 3 and sender’s window size is 2m – 1 = 23 – 1 = 7 i.e. 0 to 6
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
|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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