Pax25

Abstract

Pax25 is a Python library for building packet radio applications using Python. Documentation/website here and source code here.

Role

I designed the application layer, and implemented the AX.25 networking protocol from scratch based on reverse engineering existing implementations and working with a partial specification.

Background

In the 90s, amateur radio operators began experimenting with data modes and protocols for transmission over the air. A data network was constructed via enthusiast stations over most major metropolitan areas and acted as a parallel communications system to the Internet. In many respects, the radio networks mimicked early Internet applications, with text-based bulletin board systems and basic chat capabilities.

Over time this network has faded, with the Internet outclassing it in every respect. However, much like BBS and MUD systems retain a die-hard retrocomputing fanbase, there remain a handful of packet radio stations over most US cities. These have acted as backup communications during emergencies.

However, the development landscape has become quite stagnant. In 2023, I’d gotten my amateur radio license on the encouragement of a friend when I was curious about how I could perform backup long-range communications when traditional networks like the Internet are unavailable. This introduced me to packet radio and the problem of its stagnation.

Solving a Decades-Long Problem

My friend had extensive experience with the AX.25 protocol which powered these systems, and he enlisted my help in designing a library that could be used by developers to make their own packet radio applications– potentially reviving interest in packet radio among amateur operators.

The initial plan was for him to work on the protocol part of the development, and I work on the application layer. However, after delivering the application layer, he was not able to find the free time to build the protocol implementation. It became apparent that if I wanted to see this project see the light of day, I would have to build it myself.

The protocol is not consistently implemented or specified. There are published documents that detail different versions, none of which are correct. To build an implementation, one must use both the specifications available and existing implementations to test against, and eventually come up with their own implementation that interoperates well enough with various available hardware.

Eventually, I had the protocol working well enough to connect to existing hardware and jump between nodes across my city. I integrated this with my application layer, and now the two of them can work together, allowing anyone to build their own AX.25 applications. It includes extensive documentation, allowing anyone to get started using tutorials and references.

Work continues on pax25 to continually improve it, and it is now building an audience in the packet radio community.

Tools/Technologies used

• Python
• dataclasses
• Bitmasks/bit twiddling
• asyncio (Python’s async/await event loop)
• Linux
• Windows
• VHF/UHF radio station operation
• AX.25
• MkDocs