Discussion
Consider the stop and wait protocol, if transmission time is 'a' at the source and propagation delay is 'b' then after what time the sender can send the second packet ?
Assume data packet and ACK packet of the same size with no pissy backing :
Answer: A
The sender can send the second packet after 2a + 2b.
In stop-and-wait the sender must receive the acknowledgement before sending the next packet. With the ACK the same size as the data packet and no piggybacking:
\[ \begin{aligned} T &= a\ (\text{send data}) + b\ (\text{data travels}) \\ &+ a\ (\text{send ACK}) + b\ (\text{ACK travels}) \\ &= 2a + 2b \end{aligned} \]
a + 2b is the answer when the ACK's transmission time is negligible, which the question rules out by making the ACK as large as the data. 2b + a is the same mistake written differently, and (a + b)/2 has no meaning here.
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