Selective Repeat Protocol – sliding window

Selective Repeat Protocol

When a data packet is transmitted, its corresponding timer will start and when an acknowledgement is received then the timer will be destroyed. On the other hand, if an acknowledgement packet is not received, then the timer expires after certain time period and then the packet will be resend.

The receiver stores out of order packets as well, unlike GBN protocol, but it will not deliver those packets to upper layer till the received packets are in order and thus a delay will occur.

Therefore, we can say that in GBN protocol, there is no concept of storing at receiver end, but in Selective Repeat protocol, this concept exists.

In SR protocol, the packets in the consecutive order are delivered to the upper layer and if the packets are not consecutive, then the receiver will wait, order the packets and then will send it to the upper layer. After this, the window will slide and move to other not received packets.

Advantage of Selective Repeat Protocol Over Go Back N Protocol

Following are some of the advantages of Selective Repeat protocol over Go Back N protocol:

• SR protocols avoids unnecessary retransmission of packets.
• When receiver gets an out of order packet it is acknowledged, but buffered by the receiver. When the receiver gets all the packets, it orders them and then sends to upper layer.
• The receiver re-acknowledges already received packets with certain sequence numbers below the current window case, if the receiver does not do so, then the sender’s window would not move forward.

 

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