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In your point of view which one is better “Go back n” or “Selective Reject”? Give reasons to support your opinion

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In your point of view which one is better “Go-Back-N” or “Selective Reject”?


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.

the following stuff may b helppful !!

we present the performance of go-back-N and selective-reject automatic repeat request modes of high level data link control (HDLC) and the effect of the window size, packet error rate and processor speed in the maximum utilization in infrared wireless transmission systems. The maximum utilization (efficiency) performance of these protocols in half-duplex and full-duplex environments are being driven under different channel conditions. We demonstrate which link parameter is dominant in the performance under different link conditions, and also optimal packet sizes for ARQ schemes. It is also concluded that processing time affects the performance more than channel capacity after accomplishing a certain bit error rate
Read out Lecture#39 of CS601 to get idea !
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).

BY GUMSUM

 


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.

 

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).

Helping Material

Slide # 17 to 25

See the attached file please

Attachments:
In Go Back n ARQ, if one frame is lost or damaged, all frames sent since last frame acknowledged are retransmitted. In selective-reject ARQ, only the specific damaged or loss frame is retransmitted Due to the complexity of sorting and storage required by the receiver and
Extra logic needed by sender to select specific frames for retransmission,

Selective Reject ARQ is EXPENSIVE and not often used Selective reject

gives better performance but in practice it is

Usually it is discarded in fever of go-back–n for simplicity of implementation
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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