![]() Let understand selective repeat protocol with the help of an example. In selective repeat, at the sender side, a timer is attached to each sent packet and if the acknowledgement is not received before the timer expire the corresponding packet is resent. It waits until a set of consecutive packets are received so that they can be delivered to the application layer. The receiver transport layer does not deliver packets out of order to the application layer. The receiver window covers the sequence number of the packets that are received out of order and are waiting for the packets that were sent earlier but are not yet received. The maximum size of the receiver window is 2 m-1 which is the same as the sender window. ![]() The sender window covers the packets that are sent but not yet acknowledged, one that is acknowledged out of order and the one that can be sent once the data for the corresponding are received by the sender’s application layer. Here m is the number of bits used by the packet header to express the sequence number of the corresponding packet. The size of the sender window here is 2 m-1. The sender window in selective repeat is much smaller as compared to the go-back-n protocol. Let us study the sender window and receiver window in brief. In selective repeat, the size of the sender and receiver window is the same. At the receiver, the sliding window covers the sequence number of the packets that are either received or are expected to be received. On the sender side, the window covers the sequence of packets that are either sent or can be sent. In selective repeat, both sender and receiver have a sliding window. Let us first discuss the windows in selective repeat. Unlike the go-back-n protocol, the selective repeat protocol resends only a selective packet that is either lost or corrupted. Like go-back-n, selective repeat protocol is also a sliding window protocol. In the section ahead we will discuss the working of this selective repeat protocol. The alternative approach for this is selective repeat protocol. This retransmission of packets increases the traffic on the network creating a cumulative increase in congestion. And the sender has to retransmit all the outstanding packets though some of these may have arrived at the receiver safely but out of order. In go-back-n, the size of the receiver window is 1 so it buffers only one packet in order that it has to acknowledge next and if this expected packet is lost or corrupted all the received out of order packets are discarded. This wastes the bandwidth of the channel. If the connection is poor, there will be frequent loss of packets and the sender would have to retransmit all the outstanding packets. The go-back-n protocol that we have studied in our previous content is efficient if the error rate and loss of packets is less. We will also discuss the working of selective repeat with the help of an example. In this section, we will discuss the need for selective repeat ARQ (Automatic Repeat Request) and how it overcomes the shortcomings of the go-back-n protocol.
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