GATE CS 2016 Set 2 — Question 65
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Computer Networks → Data Link Layer → Selective Repeat
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Question
Consider a bits/second satellite communication link with one way propagation delay of 150 milliseconds. Selective retransmission (repeat) protocol is used on this link to send data with a frame size of 1 kilobyte. Neglect the transmission time of acknowledgement. The minimum number of bits required for the sequence number field to achieve 100% utilization is ________.
Correct answer
4 to 4
Solution
Given:
Bandwidth bits/sec = 128 kbps.
Propagation delay ms.
Frame size kilobyte. Assuming .Transmission time .Let .For 100% utilization (efficiency ), the sender window size must be at least .Since the window size must be an integer, we take .In the Selective Repeat (SR) protocol, the receiver window size is typically equal to the sender window size . Thus, .The available sequence number space must satisfy the condition:The number of bits required for the sequence number field is:(Note: Even if we assume , , , , , leading to the same result.)
Bandwidth bits/sec = 128 kbps.
Propagation delay ms.
Frame size kilobyte. Assuming .Transmission time .Let .For 100% utilization (efficiency ), the sender window size must be at least .Since the window size must be an integer, we take .In the Selective Repeat (SR) protocol, the receiver window size is typically equal to the sender window size . Thus, .The available sequence number space must satisfy the condition:The number of bits required for the sequence number field is:(Note: Even if we assume , , , , , leading to the same result.)
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