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.
Speed:
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:
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has been replaced by software in the PC.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 to use 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:
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.
19200:
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)[d4art.html - MISSING]. 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 €251 (US $217) and the T-M19 19.2k modem costs €100
($86.50).
The Symek F-T4S 440 Mhz
data transciever costs €936 ($809), and will run up to 150k baud!
check current exchange rates here
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?).
56k:
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. $414 US as of 3 Mar 2002). 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.
CB Packet:
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)