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Consider a 128 × 103 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 ________ .
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- 10
- 8
- 4
- 12
Correct Option: C
To achieve 100% efficiency, the number of frames that we should send = 1 + 2 × Tp / Tt where, Tp is propagation delay, and Tt is transmission delay. So, Number of frames sent = 1 + 4.687 = 5.687 (approx 6). As the protocol used is Selective repeat, Receiver window size should be equal to Sender window size. Then, Number of distinct sequence numbers required = 6 + 6 = 12 Number of bits required to represent 12 distinct numbers = 4
