What is packet radio?
Packet radio sends and receives data over a radio link, first transmitting a burst of data (a packet), and then listening for a response. A majority of packet radio activity takes place on the VHF and UHF bands ("upper" bands, frequencies above 30 Mhz.) where more "space" (frequencies) is available. A characteristic of radio transmission is that the higher the frequency, the less radio waves will "bend" and therefore a direct path from transmitter to receiver becomes necessary.
DigiPeaters ("DIGItal rePEATER") are stations located in a good location (typically on the top of a skyscraper, a mountain, or someone's well-located house) that retransmit packets it can hear between two (or more) stations that can not hear each other. They are often operated by radio clubs, and available for public use.
Today's amateur packet radio is quite slow. Most packet radio stations use 1200 baud. For those of you who remember how slow a 1200 baud telephone modem is, I have bad news: it is even worse. This is because 1200 baud packet modems can't send and receive at the same time like telephone modems can (i.e. they are half-duplex, while telephone modems are full-duplex), and there is also a small delay switching between transmit and receive. It gets worse using a digipeater, where several people may be using the channel at the same time and your transmission has to wait its turn. 1200 is used because it is common and cheap. However, 9600 baud and faster speed are becoming more common. It is the opinion of this author that 1200 baud should be junked in favor of higher speeds. However, I will detail 1200 because it is a good starting point for beginners.
1200 baud packet radio is easy. Almost any old VHF or UHF ham radio can be used. However, a regular telephone modem can not be used on the radio (not without extensive modifications which are not covered here). In the past, as special controller called a TNC (Terminal Node Controller) would be used between the modem and the computer. Kantronics makes a popular series of TNCs and modems. However, with today's more powerful computers, the TNC has been replaced by software in the PC. A typical example of this is the TigerTronics BP-2 "BayPac" Modem, which connects to the computers serial port one one end and the radio on the other, has a list price of $49.95. Commercial, ShareWare and Freeware software is available.
A better solution is available if you have a higher-speed computer (486/66 or better) and a sound card. You won't need a hardware modem or a TNC! The best thing to do is to use Linux instead of Microsoft Windows, but failing that you can use PC/FlexNet or the original Linux Baycom and Soundcard modem driver by Thomas Sailer.
9600 is, in my opinion, the best speed to use right now. Not only is it 8 times faster than 1200, it uses the bandwidth more efficiently. It is inexpensive and easy to implement without having to build your own equipment. The biggest drawback to it is that only relatively new radios can be used without modification. here is a short list I have compiled of radios that work with 9600. If you don't have problems handling a soldering iron, you can modify an older radio. However, there are still problems and tradeoffs with different strategies on 9600, check out these posts. The good news is the sound-card modem stuff in Both Linux and MS-DOS/Windows works at 9600. It actually uses less CPU time, because it is a more efficient transmission method! There are also 9600 modems available for Kantronics TNCs, and Tigertronics makes a $96 BP-96A 9600 baud modem. TAPR (Tucson Amateur Packet Radio Corp.) has a kit. The assembly and operations for it are online (Adobe Acrobat format, 265K). You'll also need the PEEL, GAL, and EEPROM images (4K) if you don't buy the kit.
Radios that are sold are being 9600 baud capable can often be upgraded to use 19.2k. The Kantronics D4-10 comes 19200 ready (mostly). The Tekk 900 and 960 series can be modified for this rate. Unfortunately, the sound-card & software method which is so cheap and reliable at 9600 doesn't work at 19200, because of limitations of the sound card. Standard TNC/Modem combinations such as the DRSI DPK-2, and other TNC-2 clones, are used. Another option is the $249 Gracellis PackeTwin PC-based TNC and the $160 PT-19K modem. Two modems can be used at the PackeTwin at the same time, and modems are available for up to 56k. Symek also makes 19.2k capable radios and TNCs. The Symek TNC3S costs 490 Marks (US $252) and the T-M19 19.2k modem costs 175 Marks ($90). The Symek F-T4S 440 Mhz data transciever costs 1830 marks ($942), and will run up to 150k baud! The Tiny-2 is available from Paccomm for $149. You will need the 9600 baud modem ($119), which MAY do 19200 (some confusion here...up to 100 kbps?).
At this speed things start to get more complicated and expensive. The original GRAPES Modem costs $250 in kit form from GRAPES. An updated design costs $350 from PacComm, or can be assembled from instructions on the WA4DSY page. It requires a transverter, a kit for a 440Mhz transverter including transceiver is available at UBC ARS for $659 CDN (appx. $446 US as of 12 Dec 99). Down East Microwave offers a 220 Mhz transverter/transceiver for $395 US, Assembled. A TNC is also required. 56K Networks utilizing the GRAPES modem exist in Vienna, Austria
Once you have the radio and modem, you'll need to hook it all together. Radio to TNC/modem Diagrams
You may want to look at this list of radio dealers.
Packet over CB radio is not legal in the united states (yet), but it is in other countries, such as Sweden and the Netherlands. Such operation might require modifications (another site)
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