If the sender does not receive the acknowledgment, it leads to the retransmission of all the current window frames.It does not accept the frames which are out of order and discards them. It does not consider the corrupted frames and simply discards them.In Go-Back-N, N determines the sender's window size, and the size of the receiver's window is always 1.Important points related to Go-Back-N ARQ: 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. Now, let's assume that the receiver is not acknowledging the frame no 2, either the frame is lost, or the acknowledgment is lost. 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). The receiver will then send the acknowledgment for the frame no 1. The sender will then send the next frame, i.e., 4, and the window slides containing four frames (1,2,3,4). Let's assume that the receiver has sent the acknowledgment for the 0 frame, and the receiver has successfully received it. Step 1: Firstly, 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. Let's consider the window size as 4, which means that the four frames can be sent at a time before expecting the acknowledgment of the first frame. But, for the better understanding, we took the running sequence numbers, i.e., 0,1,2,3,4,5,6,7,8,9,10. Mainly, the sequence number is decided by the sender's window size. 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. Suppose there are a sender and a receiver, and let's assume that there are 11 frames to be sent. The number of bits in the sequence number is 2 to generate the binary sequence 00,01,10,11. If the size of the sender's window is 4 then the sequence number will be 0,1,2,3,0,1,2,3,0,1,2, and so on.Suppose the sender's window size is 2, and we have ten frames to send, then the sequence numbers will not be 1,2,3,4,5,6,7,8,9,10. The sequence number of the outbound frames depends upon the size of the sender's window. Suppose we have sent the frame no 5, but we didn't receive the acknowledgment of frame no 5, and the current window is holding three frames, then these three frames will be retransmitted. 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. So, we can say that 'N' is the number of frames that can be sent at a time before receiving the acknowledgment from the receiver. The number of frames that can be sent at a time totally depends on the size of the sender's window. 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 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. It uses the principle of protocol pipelining in which the multiple frames can be sent before receiving the acknowledgment of the first frame. Suppose we say that Go-Back-3, which means that the three frames can be sent at a time before expecting the acknowledgment from the receiver. In Go-Back-N ARQ, N is the sender's window size. Let's understand 'what is Go-Back-N ARQ'. The variations of sliding window protocol are Go-Back-N ARQ and Selective Repeat ARQ. In the stop-and-wait protocol, the sender can send only one frame at a time and cannot send the next frame without receiving the acknowledgment of the previously sent frame, whereas, in the case of sliding window protocol, the multiple frames can be sent at a time. As we know that the sliding window protocol is different from the stop-and-wait protocol. Before understanding the working of Go-Back-N ARQ, we first look at the sliding window protocol.
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