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I realize that it is technically possible to multiplex APRS and other AX.25 traffic on a single frequency, but I'm wondering if this is frowned upon or not. My hunch is that it is, but I wanted to see what other folks had to say as well.

My use case is that I have a single TNC and I'd like to use it for both APRS digipeating and a packet-based BBS system if possible. What I'm concerned about most is the connectionless nature of APRS and that BBS traffic would interfere with APRS packets enough to become a nuisance.

Let me know what you think.

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APRS typically uses a fixed frequency (e.g. 144.39MHz in the US), and shared by all APRS users. So yes, it would be frowned upon to also run a BBS on that frequency as it would interfere with everyone else's APRS communication.

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  • $\begingroup$ Yeah, this is exactly what I thought too, but I wanted to make sure I wasn't misunderstanding anything. $\endgroup$ Aug 18, 2023 at 18:42
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The question is can that system work on the same frequency, and I'd say "yes". There's no legal framework in any country I'm aware of that would allow APRS but not other paket services.

Is that a good idea? Is it socially fine ?
It really depends. Is your secondary system going to deal nicely with the APRS that other people want to use? In that case, I'd say, go for it. APRS is technically very old and while I think it enables many a valuable application, having so much respect for it that you don't use a band because someone unspecific might potentially see interference with their antique system that has no real medium access control isn't worth it.

But that's a bit of a philosophical approach I have: I come from a communications engineering background, and I know how valuable the spectrum is that amateur radio is allowed to use. I'd much rather see the technology used in that spectrum actually being closer to state of the art, meaning that the technology uses the spectrum as efficiently and briefly as possible to maximize utility. The reason is simple: if governments decided that tomorrow, the, say 2m band was instantly assigned to machine-to-machine low power wide area networks, that would definitely enable a lot of societally beneficial applications from automated agriculture to automatic bridge maintenance to better control of small photovoltaic plants through early demand information from regional electricity consumers... We don't do all these nice things with the ham bands because we assume there is cultural and technological value in what hams do with that spectrum.

AX.25 is a bit of a spit in the face of anyone who would like to use these bands for the general good (or even just to make money), in my humble opinion, it being an utterly unsuitable protocol for wide-range wireless communications. So, if you asked me, if ask you to do something modern, efficient in that band, to offer your BBS through that less wasteful physical layer. Nobody forces you to transport data using a >50 year old standard that was already heavily criticized back in the day for lack of appropriate forward error correction and checksumming, and on a physical layer that every student of electrical engineering could tell you after their digital communications 101 lecture is very spectrally inefficient.

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  • $\begingroup$ True, but then you need software and/or hardware that supports a more modern stack and enough other folks with that software/hardware to make it usable. $\endgroup$ Aug 18, 2023 at 18:31
  • $\begingroup$ I got into amateur radio because I was working on satellite ground systems and we were communicating largely with cubesats. As someone without an electrical engineering or signal processing background I learned a lot by talking to cubesats on the ham bands. A lot of them are sending telemetry with AX.25 over APSK or GMSK, but others implement things the CCSDS channel coding standards. Implementing AX.25 helped me understand why things like forward error correction are important. And the irony isn't lost of me that Reed-Solomon is 50 years old too, but AX.25 was "good enough" at the time. $\endgroup$ Aug 18, 2023 at 18:38
  • $\begingroup$ @AndrewYoung and honestly, there's enough applications where it still is good enough. The point which I (probably using 12 dB too many words) tried to make is: if you're messing in a band where others already operate a service, be at least state-of-the-art efficient, because that way you can morally argue you're making the most with your usage of the spectrum. $\endgroup$ Aug 18, 2023 at 20:37

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