Preventing Failure to Communicate- Part 3, by JMD

We’re continuing to evaluate how to prevent a failure to communicate in the event that our normal, electronic communications equipment are not available to us. We are exploring our options. Yesterday, I wrote about common content items and encoding. Let’s continue.


The medium defines what carries the communication through the channel. For written communication, the medium will usually be paper. For most signal-based communications, the medium will be inseparable from the channel. In the case where communications is sent by a flashing light, light is the medium.


The channel is critical to all communications. It determines how the message is actually transferred to the receiver. Thus, how much bandwidth is available and how noise may impact the communication. It also tells us likely it is the communication will be noticed/intercepted by someone other than the intended recipient. Channels can also be mixed to improve the overall communications process. For example, say you’re out on a roving patrol and need to use semaphore flags to send a message to someone. How do you know they’ll be watching and see your message?

One option would be to send up from your location a certain color flare. This would alert the receiver that a communication will be coming. They should grab their binoculars and look for flags in the area the flare came from. The receiver can then send up a different color flare when they’re ready to receive communications. (This would be a “handshake” using an alternate channel.) The following sections highlight some potential communications channels along with their pros and cons.

Visual Communications Channels

Visual communications channels are one of the most common forms of communications channels. Human beings tend to be heavily driven by visual influences. Their biggest advantages are that they can typically be seen over long distances by anyone with reasonable vision or binoculars. They tend to be easy to transport and use. Their biggest disadvantages can be that their effectiveness is subject to environmental conditions. Particularly the amount of available light. They can be difficult to restrict to just the desired recipient. A good example of the latter is when you shine a flashlight in the fog. Another example is when there are a lot of suspended particles in the air (pollen, dust, et cetera). The beam of light becomes very visible to everyone around.

Common Visual Channels

Some of the most common forms of visual channels are:

  • Signal Light – A signal light send signals using the light it produces. This can be something as simple as a flashlight pointed at the receiver, or as complex as an Aldis lamp. Signal lights can be used to produce different lengths of transmission “pulses”. This is usually done by either moving the light towards and away from the receiver. You can also cover up the front of the light. You can also turn the light source on and off. They can also be configured to send different colors of light, which can be part of the encoding, and multiple light sources can be positioned to have different meanings, such as “one if by land, two if by sea”. The light source can be anything that produces light, such as a flashlight, a fire, or a hand-held signal flare. Signal lights tend to work best in low-light situations, such as dusk or night time. It can also be difficult to narrowly focus a light source so only the intended receiver can see it. This can be overcome to some degree by limiting the “spill” of the light source. Or you can focuse it more narrowly at the receiver. This may be done by placing a tube over the end of your flashlight. Perhaps by building a signal fire in an “alcove” of trees or rocks,. You will need one clear opening towards the receiver. You can also use a special lens to focus the light. A good example of the latter is a coastal lighthouse, which sends a communication toward the ocean of “there be rocks here – stay away!”.
  • Signal Mirror – A signal mirror is similar to a signal light, except that it doesn’t produce its own light; it uses reflections from another light source, typically the sun, to send signals. The biggest advantage to a signal mirror is that there are no moving or electronic parts, so it’s incredibly reliable, light, and easy to use. The down side is that it requires a strong light source, like the sun, to work. Therefore, it’s typically no good at night or when the skies are cloudy. A great example of using signal mirrors for structured communications is the heliograph, which, despite popular myths, didn’t really come into use until the 1800s. As with a signal light, limiting the viewing angle to minimize the risk of interception can be difficult. Also, signal mirrors can be challenging to use, so practicing is a must.
  • Flags – Flags can provide a visual communication channel either by how they look (size, shape, color, sequence, location, et cetera), how they move (semaphore), or a combination of the two. Since they require the receiver to see them, their use is typically limited to daytime. You also need good line-of-sight between sender and receiver. A semaphore signaler with reflective flags can be lit up at night and seen for quite a distance. A set of maritime signal flags can be easily carried and run up a line connected to a tree, or connected between two vertical poles in an elevated location, to send a message that can also be seen for quite a distance. Note that signal “flags” don’t have to be made of cloth; they could also be a couple of pieces of brightly-colored plastic, for example. As with light-based signals, limiting the viewing angle can be accomplished to some degree by positioning the flags somewhere with the field of view limited towards the intended receiver.
  • Airborne Signal – An airborne signal is a signal that is sent via a device launched in the air. This can be an aerial flare, some fireworks, a sky lantern, or a flaming arrow, for example. With a bright enough signal source, airborne signals can work just as effectively during the day as they do at night, and they can typically be seen for a long distance. The downsides are that they are very limited in the amount of information they can communicate, and it’s almost impossible to limit who can see them.
  • Smoke Signal – A smoke signal involves using smoke to send messages. The color, location, or structure of the smoke can be used to communicate different messages. Smoke signals have been used as far back as ancient China and are still in use today (for example, at the Vatican when a new Pope is elected). Smoke signals can be used anywhere you can start a smoky fire, and they can be seen omni-directionally for a long distance, assuming there’s minimal wind. Their downside is that they require visibility, which limits their use during daytime when it’s not raining. Smoke flares can be a viable alternative to having to start a fire to send a smoke signal.
  • Signs – Signs communicate through their location, appearance, or content, and are typically used in fixed locations. This can be a “trespassers will be shot” sign at the edge of your property or a simple blaze on a tree along a trail to indicate the patrol headed east at this point. The advantage to signs as a channel is that they tend to be persistent; for example, they continue to communicate information as long as they’re present. The biggest limitations are that the receiver has to be able to see the sign (proximity), which is usually in a fixed location, and the amount of information they can carry.
  • Body Language – Body language transmits information based on the location, position, or movement of body parts. Most of us are familiar with common body language, such as someone holding up their hand to signal “stop”, or shrugging your shoulders to indicate “I don’t know”. Most structured body language tends to utilize the arms, hands, and fingers, since those are the most flexible and visible parts of the body. One good example of encoding information to send via body movement is the hand signals utilized by military and police tactical teams to communicate during an engagement. These are great for their intended purpose, since they can silently convey a wide range of relevant information, but they’re less useful outside of tactical situations. A better general-purpose example is American Sign Language (ASL), which allows the user to convey a much wider range of information but at the expense of speed. Another example would be the use of some simple hand signals behind your back to covertly communicate with team members when interacting with a group of strangers with unknown intent.

Sound-based communications are arguably the most common communications channel utilized by human beings. We speak to each other on a daily basis, listen to radio and TV, and listen for sounds when we walk through the woods at night. A sound communications channel has the advantage of being easy to use (voice, whistle, drum, et cetera), and sound tends to carry quite a distance in most circumstances. Sounds can also provide a way for one person to communicate with a lot of other people simultaneously. The biggest disadvantage is that it’s very difficult to make sound directional, meaning that anyone within hearing distance will be aware of the communication, even if you don’t want them to.

Common Audio Channels

Some examples of audio channels are:

  • Speech – Human vocal sound is the most common form of communication, and it allows a lot of information to be effectively exchanged. This can be normal speech, whistling, or various other sounds that can be produced by the human mouth (including yodeling). If you’re concerned about sensitive information being overheard, you can come up with code words for common actions, locations, et cetera. Speech in an open area can be amplified and, to some degree, directionalized, by using something like a megaphone (electronic or a simple rolled-up piece of plastic). For transmitting speech more effectively and securely over short distances in a fixed location, you can create a series of speaking tubes (aka “voice pipes”) using PVC pipe. These can be run between buildings in a compound, to watch towers and close-by observation posts. By using a hood over the ends to speak into, you can limit the amount of sound that will be overheard by anyone nearby. You can also use a whistle or other sound-making device as a handshake to alert the recipient that you need to send a message. (Just make sure people don’t keep the pipe up to their ear waiting for a message!)
  • Impersonation – You’ve all probably seen at least one movie where someone mimics the sound of a bird or other animal in the woods to communicate with their other members of their team or tribe. This can be a very effective way to securely communicate over short distances, but it requires a lot of practice and is somewhat limited in the amount of information that can be exchanged.
  • Wind Device – A wind device produces sound by air moving over a surface and causing vibrations. A whistle is a good example of this; it’s small, inexpensive, and easy to carry and use. Wind musical instruments, such as horns, kazoos, or bugles can also be good for communicating by producing loud sounds; bugles have been used by the military for hundreds of years, since they’re simple (no moving parts), loud, and with practice enable a person to produce a wide range of sounds. If you’re interested in producing loud sounds that are more unique, you could adopt the use of bagpipes. A more obscure example of a wind device is the bullroarer, which has also been used for thousands of years in places like Australia, Greece, and Scandinavia. (Watch Crocodile Dundee II for an example.) One of the loudest examples of a wind device is an air horn, which can produce extremely loud sounds. Wind devices typically allow the user to produce different sound durations, which allow a wide range of encoding options, such as Morse code.
  • Percussion Device – A percussion device transmits information by the sound vibrations it makes when two components come in contact. One of the more common forms is a drum, which has been used for communications for thousands of years. Another common example is the classic American dinner bell, which has called millions of children (and spouses) to dinner over the years. One disadvantage to percussion device is that they can only produce a fixed-length sound (one strike); however, this can be somewhat overcome by using spacing of the sounds to represent different encodings; e.g. a strike with a pause represents a dot, two quick strikes with a pause represent a dash.
  • Explosions – Explosions happen when a gas expands at a rapid rate, and these generate sound when the resulting gas expansion pushes out a pressure wave. Explosive sounds can be created by actual explosives (I know, “Captain Obvious”), firecrackers, gunshots, compressed gasses, et cetera and can be typically heard over a long distance. As with percussion devices, explosions are typically limited to producing only fixed-length sounds. Another disadvantage is that you probably don’t want to waste explosives or ammo for anything but the most critical emergency communications.

Tomorrow we’ll continue looking at other types of communications.

See Also:

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