Amateur Radio in Australia (VKFAQ)

Packet Radio

Kantronics KAM XL

This multimode modem from Kantronics not only handles packet radio protocols but also many other modes.

Packet Radio

Anybody at all familiar with digital communications, will know of packets. Data are sent in discrete bundles, called packets, which can be individually checked for integrity and acknowledged (or NAK'ed), and processed, or routed on to somewhere else. Packet radio is exactly the same thing, except that the packets fly through the air with the greatest of ease, instead of being sent over wires.

Many people who have an interest both in computers and radio, find packet radio experimentation to be a natural marriage of the two fields. The basic protocol used in packet radio is called AX-25, an adaptation (some would say, bastardisation) of X.25. Error correction is achieved through a 16 bit cyclic redundancy check (CRC). Although AX-25 is by far the most common protocol in use, amateurs are free to experiment with other protocols which meet the ACA identification requirements. This is unlike some countries, which are forced to use AX-25.

Most VHF packet activity happens at the lightning fast speed [:-)] of 1200bps (bits per second) (though there is some use of a 4800 bps standard in areas such as Canberra and there is an increasing number of radios rated for 9600). To make matters worse, it's half duplex (because off-the-shelf radios themselves are half duplex devices). So if you're used to 56 kb/s from your internet modem, you're in for a bit of a shock with packet! And if that's not slow enough already, many people can (and usually do) use a single packet frequency at once, so the available bandwidth is divided between all the users.

The news isn't all bad, however. Some TNCs have built-in 2400bps modems, and there are readily available modem designs that operate at 4800, 9600, and 56K bps, and if you can find somebody using one of these faster modems, you don't have to talk at 1200. There's even amateurs in the US experimenting with 2Mbps full duplex over microwaves -- that's as fast as ISDN primary rate!

Packet bulletin board systems (BBSs)

Packet radio, being a digital communications mode, lends itself to unattended operations, such as BBSs. Packet BBSes are similar in many ways to the old dial-up bulletin boards, with file areas, messages, bulletins, automatic forwarding to other boards, and so on. Messages and files tend to be related to amateur radio. As with all forms of amateur communication, the amateur regulations restrict the content of material transmitted through packet BBSs. Packet radio is largely free of the vulgarity that characterises some internet newsgroups, though 'flame wars' can still break out.

The packet BBS network spans a large part of the globe, and is linked by VHF over short distances. Amateur satellites, HF radio and internet links carries traffic between countries. If you wish to sample the packet BBS network, look at the contents of the VK5BBS system at

Bandwidth limitations mean that you won't find large files on packet radio. Because of the mode's slowness (for most users), large messages (especially to international sites) are frowned upon. Packet Radio can be considered the 'poor man's internet' as use of the system is free to anyone with an amateur radio licence and suitable equipment.

How do I get into Packet?

Because packet radio uses isochronous communications (essentially start/stop synchronous), you can't just connect an internet modem up to the regular serial port in most PCs, and use it for packet.

Most packet operators use a device called a TNC (terminal node controller) which is about the size and shape of a telephone modem, and connects between a standard asynchronous serial port, and a radio transceiver. However, it's much more than a modem, it operates as a PAD (packet assembler-disassembler), and also provides a user interface to facilitate the setting up and tearing down of connections to other packet systems.

The advantage of the TNC approach is that it works with just about any computer (personal or otherwise) with a standard asynchronous serial port. Also, because it is dedicated to packet, and does all its framing and such in hardware, it can operate at relatively high speeds.

A cheaper alternative is to do all that in software, and bit-bang the packets by toggling a bit of an I/O port on and off. The only extra hardware you need for this is a simple FSK modem, which can be implemented with a single 16 pin chip, a crystal, and little else. Software to work with such modems is available for a variety of computer platforms, including the IBM PC range.

For further information on packet radio in Australia, please see the Australian Amateur Packet Radio Association's website. They have a software library online at too.

Can I use Packet to access the Internet?

No. There are some (indeed, many) connections between the amateur packet network and the Internet, but these are very carefully set up in order to prevent any breaches of licensing conditions. Specifically, most of these connections take the form of wormholes, which allow the Internet to serve as a backbone for amateur traffic over paths which can't be adequately served by radio.

The only traffic which passes out of the Internet and into the packet network through these wormholes is traffic which originated in other amateur packet networks, so no laws are broken. Nobody to our knowledge gateways news to packet, because each message has to be checked for suitability, and the packet networks currently in existence don't have the necessary bandwidth in any case.


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