Question on finding minium number of bits required ,Selective retransmission (repeat) protocol is used,how to approach?


#1

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 _________.


#2

Let Tp= propagation delay
and Tt= transmission delay.
For 100% efficiency,
no of frames to be send = 1 + 2 * Tp / Tt
Number of frames sent = 1 + 4.687
= 5.687 (approx 6)
Since protocol used is Selective repeat,
Receiver window size = Sender window size.
No of distinct sequence numbers required
= 6 + 6 = 12
No of bits required to represent 12 distinct numbers = 4
Answer (B)