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. 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 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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