Consider the implementation of sliding window protocol over a lossless link, with a window size of $W$ frames, where each frame is of size 1000 bits (including header). The bandwidth of the link is 100 kbps ($1\text{k} = 10^3$) and the one-way propagation delay is 100 milliseconds. Assume that processing times at the sender and receiver are zero and the transmission time of acknowledgements is also zero. Which one of the following options gives the minimum size of $W$ (in number of frames) required to achieve 100% link utilization?

To achieve $100\%$ link utilization (efficiency $\eta = 1$), the window size $W$ must satisfy:

Given:

• Frame size, $L = 1000\text{ bits}$

• Bandwidth, $B = 100\text{ kbps} = 10^5\text{ bps}$

• Propagation delay, $T_p = 100\text{ ms} = 0.1\text{ s}$

• Transmission time of acknowledgement, $T_{ack} = 0$

First, calculate the transmission time of a data frame ($T_t$):

Now calculate parameter $a$:

For $100\%$ utilization:

Therefore, the minimum window size required is 21 frames, which corresponds to option (B).