Consider the sliding window flow-control protocol operating between a sender and a receiver over a full-duplex error-fre...

Question

Consider the sliding window flow-control protocol operating between a sender and a receiver over a full-duplex error-free link. Assume the following:
- The time taken for processing the data frame by the receiver is negligible.
- The time taken for processing the acknowledgement frame by the sender is negligible.
- The sender has infinite number of frames available for transmission.
- The size of the data frame is 2,000 bits and the size of the acknowledgement frame is 10 bits.
- The link data rate in each direction is 1 Mbps (= 10^6 bits per second).
- The propagation delay of the link is 100 milliseconds.

The minimum number of frames, the window size (rounded to the nearest integer) needed to achieve a link utilization of 50% is ___ .

Accepted Answer

The minimum window size is 51 frames. In a sliding window protocol, the sender can have multiple unacknowledged frames 'in flight'. Link utilization depends on how many frames can be in transit during one round-trip time. If the window is too small, the sender must stop and wait for ACKs, wasting bandwidth. We need to find the smallest window size that keeps the link at least 50% utilized, accounting for both the data frame and ACK transmission times.