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Consider an unreliable channel that guarantees a propagation delay of at most d for every packet that gets through, but a packet is only successful with probability p. Furthermore, consider a transport layer protocol that works according to the stop and wait principle.

Given that the transmission delay of a packet is t and apart from the propagation delay all other delays are negligible. Time out interval would be the smallest possible for the transport layer to ensure that only packets get re transmitted for which either the packet itself or its acknowledgement got lost.

The average time needed for the sender to be sure that it successfully transmitted a packet to the receiver is ?

(A)$\frac{2d+t}{1-p}$

(B)$\frac{(2d+p)(1-p)}{p}$

(C)$\frac{(2d+p)p}{1-p}$

(D)$\frac{2d+t}{p}$

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P is the probability that the packet is successfully transmitted.

Hence Expected number of retransmissions in case of unsuccessful transmission of a packet is 1/P.

So for each packet, we can 'expect' that it won't take (t+ 2d) * 1/P time to reach successfully considering the retransmissions. Hence D is correct.

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