Ham Radio is a wonderful outlet for an old "Phone Phreak" like me. Not only can you crawl around inside a data network (the packet radio network) in ways the inventors hadn't intended, but when you tell people not only what you did, but how you did it, people thank you for getting the Word out!

The Cheshire Catalyst's
Explanation of Packet Radio
For Newbies

by The Cheshire Catalyst
(Richard Cheshire)
Cheshire@2600.com

Packet Radio is the data communications service of Amateur Radio, or Ham Radio. Anyone can get a Ham license by passing a written exam allowing you to operate on frequencies above 50 MHz (VHF and above, which limits you to "line of sight" transmissions). By learning Morse Code at a level of 5 words per minute, you can get a more advanced license allowing you to transmit in the HF (High Frequency) bands which can send messages across continents and oceans.

Packet on VHF (Very High Frequency) operates mostly at 1200 Baud. (1.2k Baud). There are now packet radio "modems" that operate at 9600 Baud (9.6k Baud). With landline modems being considered slow at 14.4k and 28.8k, Packet Radio is not a route into the Information Superhighway, but is more like the Information Super Bicycle Path. The difference is, with Packet Radio, you can go considerably Off Road!!

Steve Roberts N4RVE actually built a bicycle with packet radio onboard, and has traveled the country with it. It has a Radio Shack Model-100, a "keyboard" built into the handlebars (he types directly in Binary), and holds conversations via radio while riding down the road. there's no need to go to such extremes, though. The gear is small enough to fit into a backpack, and can be taken anywhere.

Information can be found on the Internet at <http://www.tapr.org>. TAPR, the Tucson Amateur Packet Radio group is a now nationwide club of Ham Radio operators that work to make packet radio even more useful. APRS, the Automatic Packet Reporting System created by Bob Bruninga WB4APR (Ham Operators are always identified by their FCC assigned call-signs), is a standard for transmitting a location's latitude & longitude via ham radio. While this is usually done by mobile stations using read-outs of their GPS (Global Positioning Satellite) receivers, using the APRS protocol means that any location can be sent, and displayed on a PC using freely downloadbale software available at <http://www.aprs.org>.

The orignal program is a DOS (Disk Operating system) based program, and can be a good dedicated use of an old PC (even an 8080 system!) that's otherwise lying around gathering dust. Macintosh & Windows users can download software at <http://aprs.rutgers.edu>. If you pay the shareware registration, you will not have to re-enter your own station information every time you start it up. Another TAPR project is the use of PIC (Programmable Interface Controllers) chip computers (such as found in the back of "Nuts & Volts magazine) to handle transmission of telemetry signals via packet radio under an APRS protocol.

Keep in mind too, you don't need a Ham license to receive any ham transmission. You can pick up a "scanner" type radio at any Radio Shack store, and pick up local Ham Radio repeaters, and packet radio transmissions. Tune it to the APRS frequency of 144.390 MHz, feed the audio stream into a TNC (Terminal Node Controller, the official name for the "radio modem"), and plug the TNC into a serial port on your computer. fire up the APRS software, and you'll start seeing "map icons" and call-signs appearing on your screen's map.

A TNC is not just a radio modem. The word "modem" is an engineering contraction for modulator/demodulator. What a modem does, is accept a serial data stream at the RS-232 voltage levels, and converts (modulates) it into sounds that can be carried over a communications channel that is capable of carrying voice frequencies (telephones for modems, radios for TNC's). At the other end, it demodulates (converts) the sounds into RS-232 voltage levels that the computer can receive through the serial port.

A TNC does more than the job of just a modem, since it also converts an asynchronous serial data stream into packets with a distinct Header (including call-signs of originating station, receiving station, and any stations used as digital repeaters(digipeaters) to get the packet from one end to the other), data content, and CRC (Cyclical Redundancy Check - a fancy Checksum). The protocol used is called AX-25, and Adapted radio version of the X-25 packet radio protocol. The major difference between the two are the Headers which have been adapted to the Amateur Radio world of call-signs for network identification.

If you're looking for help getting started, contact me at <mailto:cheshire@2600.com>, and include your city and zip code. I'll check a database of Ham Radio clubs and send you a list of what's nearby you.

This file can be found at: http://CheshireCatalyst.Com/packet.html

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