BY GUMSUM
BY AFTAB
The two alternatives are a trade off between bandwidth utilisation and buffer
space depending upon which resource is the most valuable (today RAM
memory is cheap but this was not the case during the 1 970's and early 1 980s).
If the number of error are small Go-Back-N ARQ is suitable whereas unreliable
long distance cables with long propagation delays would use selective-reject
ARQ (together with correspondingly large send and receive windows).
-- In your point of view which one is better "Go-Back-N" or "Selective Reject"?
Helping Material:
In the Go-back-n scheme, upon an error, the sender retransmits all the frames that came after the error. For example, sender may send frames 1,2,3,4 and get an NAK with a value of 2. The NAK acknowledges everything that came before it, and asks for frame 2 (and subsequent frames) to be resent.
Selective Reject allows the sender to reject any particular frame, without having the sender resend all the following frames. For example, the sender may send frames 1,2,3,4 and the receiver may send a NAK with value of 2. At which point, the sender retransmits frame 2. If frames 3,4 were received without any problems, then the receiver acknowledges them with a ACK 4 (note that ACK now points to the last successful frame, as opposed to the next
expected frame).
In order to support Selective reject, the sender needs to maintain all data that hasn't been acknowledged yet. The receiver also needs to have sorting (rearranging) logic to be able to re-order frames that may have arrived out of sequence (or some frames that were damaged and retransmitted out of sequence). The sender needs to be able to find any particular frame for retransmission.
Helping Material:
In the Go-back-n scheme, upon an error, the sender retransmits all the frames that came after the error. For example, sender may send frames 1,2,3,4 and get an NAK with a value of 2. The NAK acknowledges everything that came before it, and asks for frame 2 (and subsequent frames) to be resent.
Selective Reject allows the sender to reject any particular frame, without having the sender resend all the following frames. For example, the sender may send frames 1,2,3,4 and the receiver may send a NAK with value of 2. At which point, the sender retransmits frame 2. If frames 3,4 were received without any problems, then the receiver acknowledges them with a ACK 4 (note that ACK now points to the last successful frame, as opposed to the next
expected frame).
In order to support Selective reject, the sender needs to maintain all data that hasn't been acknowledged yet. The receiver also needs to have sorting (rearranging) logic to be able to re-order frames that may have arrived out of sequence (or some frames that were damaged and retransmitted out of sequence). The sender needs to be able to find any particular frame for retransmission.
BY AFTAB
The two alternatives are a trade off between bandwidth utilisation and buffer
space depending upon which resource is the most valuable (today RAM
memory is cheap but this was not the case during the 1 970's and early 1 980s).
If the number of error are small Go-Back-N ARQ is suitable whereas unreliable
long distance cables with long propagation delays would use selective-reject
ARQ (together with correspondingly large send and receive windows).
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