Go-Back-n Sliding Window Protocol

In case of satellite communication, it takes 270 msec to transmit a frame and 520 msec is needed before the acknowledgement arrives. The solution here is the sender is allowed to send up to 'w' frames and the acknowledgment will arrive after the roundtrip time gets equal. The technique of allowing the sender to send the data and receiver acknowledging them parallel is called Pipelining.

If the channel capacity is b bits/sec and frame size 1 bits and the roundtrip propagation time R sec, the time required to transmit a single frame is 1/b sec. There is a delay of R/2 before the last bit arrives and another R/2 before acknowledgement arrives. The line utilization for the stop-and-wait is 1(1+bR). Pipelining has a serious drawback when one of the frames gets damaged in the middle. This could be overcome by the Go Back n protocol.

This approach is mainly used for dealing with errors when the frames are pipelined. The receiver simply discards all subsequent frames, sending no acknowledgments. Here the receiver window is of size 1. In other words, the data link layer refuses to accept any frame except the next one it must give to the network layer. If the sender's window fills up before the timer runs out, the pipeline will begin to empty. Eventually, the sender will time out and retransmit all unacknowledged frames in order, starting with damaged or lost one. This approach can waste a lot of bandwidth if the error rate is high.