Go-Back-N ARQ

Go-Back-N ARQ (Go-Back-N automatic repeat request) is a flow control protocol where the sender continues to send several frames specified by a window size even without receiving feedback from the receiver node.

It is a special case of the general sliding window protocol where the transmitter or sender window size is N and the receiver's window size is 1, which means that it can transmit N frames to the receiving node before waiting for a feedback.

It uses the principle of protocol pipelining in which the multiple frames can be sent before receiving the acknowledgment of the first frame. If we have five frames and the concept is Go-Back-3, which means that the three frames can be sent, i.e., frame no 1, frame no 2, frame no 3 can be sent before expecting the acknowledgment of frame no 1.

In Go-Back-N ARQ, the frames are numbered sequentially as Go-Back-N ARQ sends the multiple frames at a time that requires the numbering approach to distinguish the frame from another frame, and these numbers are known as the sequential numbers.

If the acknowledgment of a frame is not received within an agreed-upon time period, then all the frames available in the current window will be retransmitted. 

Working of Go-Back-N ARQ

Suppose there are a sender and a receiver, and let's assume that there are 11 frames to be sent. These frames are represented as 0,1,2,3,4,5,6,7,8,9,10, and these are the sequence numbers of the frames.

Consider the window size as 4, which mean that the four frames can be sent at a time before expecting the acknowledgment of the first frame.

First, the sender will send the first four frames to the receiver, i.e., 0,1,2,3, and now the sender is expected to receive the acknowledgment of the 0^th frame.

Assume that the receiver has sent the acknowledgment for the 0 frame, and the receiver has successfully received it. The sender will then send the next frame, i.e., 4, and the window slides containing four frames (1,2,3,4).

The receiver will then send the acknowledgment for the frame no 1. After receiving the acknowledgment, the sender will send the next frame, i.e., frame no 5, and the window will slide having four frames (2,3,4,5).

Assume that the receiver is not acknowledging the frame no 2, either the frame is lost, or the acknowledgment is lost.

Instead of sending the frame no 6, the sender Go-Back to 2, which is the first frame of the current window, retransmits all the frames in the current window, i.e., 2,3,4,5.

Points to remember:

• In Go-Back-N, N determines the sender's window size, and the size of the receiver's window is always 1.
• It does not consider the corrupted frames and simply discards them.
• It does not accept the frames which are out of order and discards them.
• If the sender does not receive the acknowledgment, it leads to the retransmission of all the current window frames.