Packet Radio Under Linux

My first introduction to packet was using the sound card modem driver with a hand-held 2-meter band transceiver for 1200bps packet. The audio connects from the PC sound card to the radio's microphone and speaker jacks. A signal from a serial or parallel port in conjunction with a simple (one transistor) circuit is used to control the radio's PTT (Press To Talk) circuit. This approach requires no TNC and no packet modem—if you already have a computer and sound card this costs almost nothing. Figure 1 shows a block diagram of my setup.

Configuring the system was straightforward—I just followed the detailed instructions in the AX25 HOWTO. Utility programs included with the AX25 package allow me to monitor the packets being broadcast and received, and to call and answer other packet stations.

I obtained an IP address from the local IP coordinator (the 44.x.x.x ampr.org Internet domain is assigned for packet radio) and configured my system for TCP/IP over packet. All the standard network tools then operated over packet. Assuming they are configured for TCP/IP, I can ping or finger other stations and connect to them using telnet. Similarly, I can log on to my home Linux machine over the Internet.

Next I plan to set up a simple BBS system users can log on to via packet radio. I'd also like to look at more sophisticated packet networking tools supported under Linux, such as NetRom, NOS and Rose. In the future I may even explore options for higher-speed packet such as the 56 Kbps Ottawa PI2 card.

As well as being fun, packet radio under Linux taught me a lot about networking, much of which is also applicable to Ethernet, X.25 and other network protocols.

Linux is a great platform for packet, particularly since it is fully integrated with the rest of the networking subsystem. Its reliability lets you leave a Linux system up for long periods of time without crashing (ideal for a BBS environment). As an example, one local Linux system has been on the air continuously for over 310 days without interruption. Some of the software I used was still in alpha release yet was stable enough to use. Finally, packet radio has opened up a whole new area of Linux for me to explore, ensuring that I won't run out of things to do in the foreseeable future.

Thanks go to Gord Dey, VE3PPE, for reviewing this article and adding many valuable suggestions. I also wish to thank Terry Dawson for writing the AX25 HOWTO and utilities, Alan Cox, Jonathan Naylor and others for writing the Linux packet code and Thomas Sailer for writing the sound card packet modem driver.

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