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Sliding Window Protocol | Set 3 (Selective Repeat)

Prerequisite : Sliding Window Protocol – Set 1 (Sender Side), Set 2 (Receiver Side) Why Selective Repeat Protocol? The go-back-n protocol works well if errors are less, but if the line is poor it wastes a lot of bandwidth on retransmitted frames. An alternative strategy, the selective repeat protocol, is to allow the receiver to accept and buffer the frames following a damaged or lost one. Selective Repeat attempts to retransmit only those packets that are actually lost (due to errors) :

Retransmission requests :

Selective Repeat Protocol (SRP) : This protocol(SRP) is mostly identical to GBN protocol, except that buffers are used and the receiver, and the sender, each maintains a window of size. SRP works better when the link is very unreliable. Because in this case, retransmission tends to happen more frequently, selectively retransmitting frames is more efficient than retransmitting all of them. SRP also requires full-duplex link. backward acknowledgements are also in progress.

Figure – the sender only retransmits frames, for which a NAK is receivedEfficiency of Selective Repeat Protocol (SRP) is same as GO-Back-N’s efficiency :

Efficiency = N/(1+2a)
Where a = Propagation delay / Transmission delay
Buffers = N + N
Sequence number = N(sender side) + N ( Receiver Side)

if  Tt(ack) :Transmission delay for acknowledgment , Tq: Queuing delay and  Tpro: Processing delay is mention

We know that the Efficiency (?),

=Useful time / Total cycle time

=Tt(data) /Tt(data) + 2*Tp + Tq + Tpro + Tt(ack)

Tt(data) : Transmission delay for Data packet
Tp : propagation delay for Data packet
Tq: Queuing delay
Tpro: Processing delay
Tt(ack): Transmission delay for acknowledgment

Above formula is applicable for any condition, if any of the things are not given we assume it to be 0.

References – Slideshare Youtube MIT article

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