Question

Discuss various categories of flow – control

Answer 1

Stop and Wait Flow control:
The sender has to wait for an acknowledgment of every frame that it sends.Only when a acknowledgment has been received is the next frame sent. This process continues until the sender transmits an End of Transmission (EOT) frame.In Stop-and-Wait flow control, the receiver indicates its readiness to receive data for each frame.For every frame that is sent, there needs to be an acknowledgment, which takes a similar amount of propagation time to get back to the sender.Only one frame can be in transmission at a time. This leads to inefficiency if propagation delay is much longer than the transmission delay
Advantages of Stop and Wait:
It's simple and each frame is checked and acknowledged well.
Disadvantages of Stop and Wait:
Only one frame can be in transmission at a time.It is inefficient, if the distance between devices is long. Reason is propagation delay is much longer than the transmission delay.The time spent for waiting acknowledgements between each frame can add significant amount to the total transmission time.
Sliding Window Flow Control:
It works by having the sender and receiver have a “window” of frames.Each frame has to be numbered in relation to the sliding window. For a window of size n, frames get a number from 0 to n - 1.  Subsequent frames get a number mod n.The sender can send as many frames as would fit into a window.The receiver, upon receiving enough frames, will respond with an acknowledgment of all frames up to a certain point in the window. It is called slide.This window can hold frames at either end and provides the upper limit on the number of frames that can be transmitted before requiring an acknowledgement.For example, if n = 8, the frames are numbered 0, 1, 2, 3, 4, 5, 6, 7, 0, 1, 2, 3, 4, 5, 6, 7, 0, 1...so on. The size of the window is (n -1) = 7.When the receiver sends an ACK, it includes the number of the next frame it expects to receive. When the receiver sends an ACK containing the number 5, it means all frames upto number 4 have been received.If the window size is sufficiently large the sender can continuously transmit packets:If W >= (2a+1), sender can transmit continuously. (Efficiency =1)If W < (2a+1), sender can transmit W frames every (2a+1) time units. (Efficiency = W/(1+2a) )