Suppose that it takes 1 unit of time to transmit a packet (of fixed size) on a…
2006
Suppose that it takes 1 unit of time to transmit a packet (of fixed size) on a communication link. The link layer uses a window flow control protocol with a window size of N packets. Each packet causes an ack or a nak to be generated by the receiver, and ack/nak transmission times are negligible. Further, the round trip time on the link is equal to N units. Consider time i > N. If only acks have been received till time i(no naks), then the goodput evaluated at the transmitter at time i(in packets per unit time) is
- A.
1 - N/i
- B.
i/(N + i)
- C.
1
- D.
1 - e(i/N)
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Correct answer: A
Answer: 1 - N/i (i.e., (i - N)/i)
Each packet transmitted at time t is acknowledged at time t + N because the round-trip time is N.
Therefore the first acknowledgment arrives at time N + 1, and by time i (> N) the number of acknowledgments received is i - N.
Goodput at the transmitter at time i is acknowledged packets divided by elapsed time: (i - N)/i = 1 - N/i.
Example: if N = 5 and i = 10, acknowledgments = 10 - 5 = 5, so goodput = 5/10 = 0.5.
Note: As i becomes very large, the term N/i → 0 and the measured goodput approaches 1 packet per unit time (the steady-state transmission rate).