Selective repeat ARQ

Question

In selective -repeat when how receiver will know that a frame is lost and send NAK ?Is it when he recieves an out of order packet .?

Answer 1

Yes,When Receiver received out of order frame. 

In selective repeat protocol, a frame can be received out of order and further those frames are sorted to maintain a proper sequence of the frames. While sorting, if a frame number is skipped, then receiver recognize that a frame is lost and it sends NAK for that frame to the sender.(for example if frame No.-2 lost then Receiver send NAK2 to Sender) After receiving NAK for the lost frame the sender search that frame in its window and retransmits that frame(Frame No.-2).

Comments

When receiver will send a NAK just after it receives out of order frame or after receiving all frames ? And second question is what will be the acknowledgement number after it receives lost frame in the above example is it be 5 ? If it so then how SR follow independent acknowledgement only .? Please tell me

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Receiver will send NAK after it receive out of order frame.

Yes ,ACK 5 will send by Receiver because it already recieved frame 4.(In SR out of order packet received.)

For more -http://www.ccs-labs.org/teaching/rn/animations/gbn_sr/