Solution

The maximum efficiency of the sliding window protocol is achieved when the sender window size is equal to the product of the bandwidth-delay product and the packet size.

The bandwidth-delay product is calculated as the product of the bandwidth (in bits per second) and the propagation delay (in seconds). In this case, the bandwidth is 1.536 Mbps and the propagation delay is 6 μs/km * 3000 km = 18000 μs.

So the bandwidth-delay product is 1.536 * 10⁶ * 18000 * 10^-6 = 27648 bits.

The packet size is 64 bytes, which is 64 * 8 = 512 bits.

So the maximum efficiency is achieved when the sender window size is equal to the bandwidth-delay product divided by the packet size, or 27648 / 512 = 54.

Therefore, the sender window size to get the maximum efficiency is 54.