Selective-Repeat ARQ

Selective Repeat ARQ

For a system with ample memory space, Selective Reject Request ARQ may provide improved performance when compared to Go-Back-N ARQ. This error control scheme allows the receiver to selectively reject frames.

Go-Back-N ARQ simplifies the process at the receiver site. Receiver only keeps track of only one variable, and there is no need to buffer out-of-order frames, they are simply discarded.
However, Go-Back-N ARQ protocol is inefficient for noisy link. It bandwidth inefficient and slows down the transmission.
In Selective Repeat ARQ, only the damaged frame is resent. More bandwidth efficient but more complex processing at receiver.
It defines a negative ACK (NAK) to report the sequence number of a damaged frame before the timer expires.

What happens if the frame is lost?

Frames 0 and 1 are accepted when received because they are in the range specified by the receiver window. It is the same for frame 3.
Receiver sends a NAK2 to show that frame 2 has not been received and then sender resends only frame 2 and it is accepted as it is in the range of the window.

What are the sizes of sender and receiver windows?

Size of the sender and receiver windows must be at most one-half of 2 m. If m = 2, window size should be 2^m/2 = 2.
Figure compares a window size of 2 with a window size of 3.
Window size is 3 and all ACKs are lost, sender sends duplicate of frame 0, window of the receiver expect to receive frame 0 (part of the window), so accepts frame 0, as the 1^st frame of the next cycle – an error.