Question

Compute the fraction of the bandwidth that is wasted on overhead (headers and retransmissions) for protocol on a heavily loaded 50-kbps satellite channel with data frames consisting of 40 bits header and 3960 data bits. Assume that the signal propagation time from the earth to the satellite is 270 msec. ACK frames never occur. NAK frames are 40 bits. The error rate for data frames is 1%, and the error rate for NAK frames is negligible.

1.   1.21%
2.   2.12%
3.   1.99%
4.   1.71%

Comments

No offence, but shouldn't it be 8040/396000 ?

100*40 + 4000*1 + 40(NAK because 1 frame  is corrupt out of 100) = 8040 ?

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https://gateoverflow.in/84137/tanenbaum --this is crct

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Yes!!! Right!

Answer 1

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Error Rate = 1\100 ie, there are totally 100 frames from which 1 frame is retransmited.

[1] Header = 40 bits. For 100 Frames = 4000 bits(n*h)

[2] Total useful Data = 3960 *100 = 396000 bits(m)

[3] Now one Frame is Retransmitted = 3960(data) + 40 bits(header) => 4000 bits

[4] Also NAK is sent = 40 bits

Overhead = Headers[1] + Retranmissions[3] + NAK[4] / Total Data[[1] + [2] + [3] + [4]

=> 4000 + 4000 + 40 / 396000 + 4000 + 4000 + 40

=> 0.0198

% => 0.0198 * 100 => 1.98 %

Comments

NAK are sent for corrupted data only.