3295: Semaphore

Semaphore

|< < Prev #3295 (September 7, 2026)

Title text: The speed of light in air is 50% faster than in fiber, but it's a challenge to get the arms to move fast enough to realize the latency advantage.

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Explanation

This is one of 50 incomplete explanations: This page was created by an AIR-FREE SEMAPHORE GAP. Don't remove this notice too soon. You can help by editing the explanation!

Semaphore is a maritime signaling system involving the use of patterned flags held at specific angles.

This comic is similar to 2651: Air Gap.

Semaphore towers have historically been used for long-distance communication, with the Optical Telegraph chaining towers capable of displaying sempaphore-style signals, with a message being relayed between towers to allow cross-country communication faster than it was possible to travel, before electric communication technologies were available

The title text notes that the speed of light is faster in air than in an optical fibre. This is correct. In air, light travels at close to the same speed as in a vacuum, about 300000000 m/s. But optical fibres are made of glass that is optically dense, and refraction causes the effective speed of light in glass to be much slower. So in principle it is quicker to transmit a signal by light through air than by light through a fibre. This is the "latency" of the connection. However the transmission of the signal is limited by how fast the flags can move, and this is much much slower than the speed of light.

Wireless communications using radio waves (like wi-fi, bluetooth, 5G, ...) do travel at the speed of light in air, making them technically faster than fibre optics.

Transcript

This is one of 34 incomplete transcripts: Don't remove this notice too soon. You can help by, editing the transcript!

[Two identical machines sit facing each other. Each one consists of a box with a wire connecting it offscreen, a webcam, and a machine with 2 flags. Each webcam is pointing at the other machine's 2 flags. Motion lines indicate that the machines are able to move the 2 flags independently.]

[Caption below the panel:]

Networking Tip: Separate sensitive components with a semaphore gap.

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Discussion

It's not directly to the speed of light that the speed of moving flags should be compared (in terms of signal transmission, as Randall rightly points, semaphore is faster), it is rather the rate at which a new semaphore symbol can be dialled (typically one of the half-a-dozen to a dozen patterns a given semaphore can present). If calculating the information flow, that dialling rate (thus speed of moving the flags divided by a typical size of semaphore, ie about 10 per second if moving a 10 cm semaphore at 1 m/s) times the number of possible patterns (let's say 8) should be compared to the baud rate (typically $10^11$ per second) times 2 to the power of the number of lanes (let's say 3). For the data chosen, we're comparing approximately 80 bits per second for semaphore to $8\cdot10^11$ for the optical fibre. This is the rate at which a permanent flow of information can be transmitted (if we consider that deciphering the semaphore pattern is faster than dialling it, so in the situation where the operator or software getting the image from the webcam is at least as fast as the dialling). However, this doesn't tell us about the latency time: how much time does it take for an elementary 8-bit signal to reach destination. That time is obtained by summing the time of "dialling" (or modulating the numerical signal for the optic fibre), plus the time it travels in air or fibre (faster in the former, that's where semaphore can try to catch up!), plus the deciphering/demodulating at reception.

--2001:1C04:2F0B:5300:3C9D:4D7A:7E6C:66E7 20:37, 7 September 2026 (UTC)
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