GATE | GATE IT 2006 | Question 64

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)

Answer: (A)

Explanation: successfully delivered packets = (i-N) {transmission time of a packet =1 unit, so in i time i packets can be transmitted. And RTT = N, so only N packets are ACKed}
So, for time i, i-N packets are transmitted
Goodput = Successfully delivered data/ Time = (i-N)/i = 1- N/i

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