Radio Watch and the Missing Acknowledgement
During a night exercise, a signalman sends a coded message of 240 characters over a radio link. The transmitter sends at 8 characters per second, but the link drops 1 character in every 40 due to interference, and each dropped character must be resent once. The receiving station must acknowledge within 30 seconds of the message's final character arriving, or the exercise controller logs a failure. The signalman begins transmitting at 22:14:00. Separately, the receiving operator reports hearing only part of the message and asks for a repeat of the entire message rather than the missing characters. The signalman must judge whether a full repeat will breach the 30-second acknowledgement window.
1. Calculate when the final character of the first transmission arrives, accounting for resends.
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Dropped characters = 240 / 40 = 6. Total characters sent = 240 + 6 = 246. Time = 246 / 8 = 30.75 seconds. Start 22:14:00 + 30.75 s = 22:14:30.75 for the final character to arrive.
2. If the operator requests a full repeat, will the acknowledgement still fall within 30 seconds of the repeat's final character?
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A full repeat takes the same 30.75 s to transmit. The 30-second acknowledgement window is measured from the final character of that transmission, not the original. So the repeat finishes and the receiver has 30 s from that point - the window itself is not breached by repeating; only the total exercise time grows. The repeat does not violate the acknowledgement rule as stated, though it doubles elapsed time to about 61.5 s.
3. The controller instead says the whole exchange must finish within 60 seconds of 22:14:00. Does the full-repeat plan fit?
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First transmission ends at 30.75 s. Full repeat adds another 30.75 s → total 61.5 s. 61.5 s > 60 s, so the full-repeat plan fails by 1.5 s. Requesting only the 6 missing characters would take 6/8 = 0.75 s, finishing at about 31.5 s - comfortably inside 60 s. The correct judgement is to request selective resend, not a full repeat.