GMRS/FRS data communication

Question

Is it possible to use an ordinary GMRS/FRS UHF handheld set for data communications? I hope to use a Motorola UHF GMRS handheld with a terminal node controller device to for data communication. Are there any terminal node controller devices for this task?

Answer 1

The short answer: this is probably technically feasible, but it's not legal.

Part 95 of the Code of Federal Regulations is where all the rules on personal radio services (which includes FRS, GRMS and amateur services) are found. §95.631 specifically regulates permitted emission types.

Here are the relevant passages for GMRS:

• §95.631 (a): "A non-voice emission is limited to selective calling or tone-operated squelch tones to establish or continue voice communications."
• §95.631 (e): "No GMRS or CB transmitter shall employ a digital modulation or emission."
• §95.631 (f): "No GMRS, CB or R/C transmitter shall transmit non-voice data."

And for FRS:

• §95.631 (d): "A non-voice emission is limited to selective calling or tone-operated squelch tones to establish or continue voice communications, digital data transmission of location information or text messaging."

In summary: non-voice transmissions are limited to tone squelch/tone burst for using repeaters, and a very narrow range of location/text data. It's worth reading the rules to see all the specifics, but I don't think using a TNC would be permitted. I hope this is helpful!

Answer 2

Is it possible to use an ordinary GMRS/FRS UHF handheld set for data communications?

In principle: yes.

You see, you're talking about voice communication devices. If now, on one end, someone spoke "zero one zero one…", obviously, you could communicate arbitrary data through that voice-optimized channel.

In reality, you'd use a device that modulates the data onto audio on one side, and demodulates it on the other – a modem.

There's modems that actually work over voice channels. See the acoustic couplers that predate the little grey modem boxes that connected everyone and their mom to the internet over telephone lines in the 90s.

Now, using something that is designed to transport speech as understandably as possible (that does not mean "as close as possible to the original") to transport data is a pretty bad approach – basically, you're using the wrong transport. That's why telephone line modems went away from the acoustic coupling principle (which put the telephone's speaker/mic and amplifiers in between the data and the telephone line channel) and switched over to modems that used the available channel as efficiently as possible, at the cost of it spectrally no longer looking like voice.

So, you could design a pretty simple e.g. FSK (frequency shift keying) modulator to act as "fake" microphone to your voice communication device, and do the opposite (FSK demodulator as fake speaker) on your receiving end. But frankly:

The FM, AM or AM/SSB modulation done by the UHF handset is pretty simple compared to implementing a working modem. So, you're most likely better off just designing an UHF modem directly, and not be limited by the audio-centric behaviour of your handset. In fact, such devices are extremely common – your remote car key, your remote door bell, your wireless temperature sensor going with your kitchen thermometer, all do such things at miniature cost, using freely (and cheaply) available RF modules. You really don't have to use the large, battery-eating walkie talkie.

Note that this might not be entirely true for all use cases – if you really only need to transport a few bit ever minute or so, then implementing a short sequence of beep codes would be totally sufficient – just as Morse code is really sufficient if you just need to get a few words across.