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A satellite communication example using a stop-and-wait-style ARQ. In this task, the goal is to change the protocol of the example using some sliding window method so that the sender is active at least 50% of the time instead of 3% of the example.

• 64 kbps, 1000 bits/packet, distance 72 000 km

• Time to transmit one packet: 1 kb / 64 kbps = 15,6 ms

• Propagation delay 72 000 km / 300 000 km/s = 240 ms

• First packet arrives: t = transmission + propagation = 255,6 ms

• ACK received by sender: t = transmission + 2x propagation = 495,6 ms

• Transmitter idle: 480 / 495,6 = 96,8% of total time
1. What equation describes the capacity (k) with respect to the transmitter activity time (x)? Use a period as a decimal separator. (Hint: the correct answer is exactly the form a = b / c, without any other parentheses or characters.)
2. How many bits should the window size be at least if the sender's activity percentage (ratio of transmission time to total time) was at least 50%?
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