![]() Step 6 − If the sender doesn’t receive the ACK2 from the receiver within the time slot, it declares timeout for frame 2 and resends the frame 2 again, because it thought the frame2 may be lost or damaged. Step 5 − whenever the sender receives the ACK1 from the receiver, within the frame 1 timer then it is closed and sent to the next frame, frame 4. Step 4 − Whenever sender receives the ACK0 from receiver, within the frame 0 timer then it is closed and sent to the next frame, frame 3. Step 3 − In the same way frame2 is also sent to the receiver by setting the timer without waiting for previous acknowledgement. Step 2 − Without waiting for acknowledgement from the receiver another frame, Frame1 is sent by sender by setting the timer for it. Step 1 − Frame 0 sends from sender to receiver and set timer. Given below is an example of the Selective Repeat ARQ − The sender will send/retransmit a packet for which NACK is received. The receiver while keeping track of sequence numbers buffers the frames in memory and sends NACK for only frames which are missing or damaged. In Selective Repeat ARQ only the lost or error frames are retransmitted, whereas correct frames are received and buffered. In selective repeat protocol, the retransmitted frame is received out of sequence. In the selective repeat, the sender sends several frames specified by a window size even without the need to wait for individual acknowledgement from the receiver as in Go-Back-N ARQ. It is also known as Sliding Window Protocol and used for error detection and control in the data link layer. Let us see the working condition of Selective Repeat ARQ. A Protocol Using Go Back N - Go-Back-N protocol, also called Go-Back-N Automatic Repeat reQuest, is a data link layer protocol that uses a sliding window method for reliable and sequential delivery of data frames. SR protocol is superior to other protocols but because of its. SR protocol may be considered as a combination of advantages of Stop and Wait ARQ and Go back N. Go back N and Selective Repeat are similar in terms of efficiency if sender window sizes are same. If an ACK of data-frame previously transmitted does not arrive before the timeout, the sender retransmits the frame, thinking that the frame or it’s ACK is lost in transit Selective Repeat and Stop and Wait ARQ are similar in terms of retransmissions. Retransmission − The sender always maintains a clock and sets a timeout period. ![]() ![]() Negative ACK − Whenever the receiver receives a damaged frame or a duplicate frame, it sends a NACK back to the sender and sender must retransmit the correct frame. Positive ACK − Whenever a receiver receives a correct frame, it should acknowledge it. There are some requirements for error control mechanisms and they are as follows −Įrror detection − The sender and receiver, or any must ascertain that there is some error in the transit. Generally, there are three types of techniques which control the errors by Automatic Repeat Request (ARQ) they are − Selective-repeat Automatic Repeat Request (ARQ) is one of the techniques where a data link layer may deploy to control errors.
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