

Digital Communications via Radio

by Thomas Icom

Within recent years the state of the art in digital radio communications has
reached the point where consistently reliable high speed systems can be set up
at a very reasonable cost.  Furthermore, the elimination of morse code as a
requirement for an amateur radio license now allows for some interesting
possibilities.
The advantage of radio over telephone for digital communications is the simple
fact that you don't have to worry about the phone companies or phone bills.
Using radio costs nothing except for the initial outlay in equipment.  Also,
activities such as networks and large-scale digital conferences are
accomplished easier when done via radio. Radio communications eliminates the
need to commit telecommunications fraud, a common downfall of phone users.
Finally, freed from the restriction of phone lines, we can begin to see truly
portable operation.  You can literally be anywhere and be able to get into the
net.  Complete digital radio communications systems exist that can be fit into
a briefcase.
A radio signal is also better suited for transmission than a phone line.  Due
to the restricted bandwidth and noise encountered on a phone circut the speed
of digital commo is severely restricted.  With radio one can have an almost
unlimited bandwidth for incredible data communications speeds (some microwave
links have data transmission speeds upwards of 1.4 million bits per second),
and can increase power to compensate for noise conditions.
While it is often said that radio isn't as secure as telecom, in reality with
the proper encryption it is more secure than listening to a phone due to the
simple fact that you are no longer using a telecommunications carrier's
network.  However , seeing the &quot;security measures&quot; on many &quot;underground&quot; BBSes
leads me to believe that even an unencrypted radio data network running on
152.24 Mhz would offer an increase in security.  One an also randomly change
the physical characteristics of the digital signal (mark/space frequencies,
modulation type, communications frequency, RF power level) easier on radio to
confuse an intercepter.
There are various protocols being used via radio today. All of these protocols
are in common use by amateur radio operators, are easy to employ, and are
readily and inexpensively available.  In fact, a complete low-end
communications digital amateur radio setup can be had for under $100.
The oldest communications protocol used is Baudot. This is a five bit code
running at either 45 or 75 baud and is mostly used on the shortwave
frequencies (under 30 Mhz.) It is a five bit code and as such is unsuitable
for any type of computer file transfers.  It is also very inexpensive.
Traditional ASCII is also used in various forms. Besides being used with
different error checking protocols it is also used in the traditional form on
shortwave running at 110 and 300 baud.  While not as noise resistant and
reliable as Baudot on shortwave it does allow for faster communications and
for file transfers at nation/worldwide distances with a minimum of equipment.
ASCII also doesn't require 5 to 8 bit translation as does baudot for computer
use. (In the old days, people used mechanical teletypewriters for data commo
and a 5 bit code was simple to work with, given the equipment.  Computers were
still big clunky things that took up whole rooms.) Someone with moderate
knowledge in electronics could make an ASCII radio modem with an XR-2206 and
XR-2211 FSK chips for under $20!
There is also an error checking form of ASCII used on shortwave called AMTOR
(AMateur Teleprinting Over Radio) which is a variation of the STIOR system
used by ships.
Finally, we have packet.  Packet is a radio based data communications protocol
used by ham radio operators for their own computer network that stretches over
most of the country. It uses ASCII with a communications protocol called
AX.25. AX.25 is a ham radio adaptation of the X.25 protocol used by Telenet
and other telecom based data networks.  Packet is very active on VHF and UHF
frequencies (2 meters, 220 Mhz, 440 Mhz., and above) and is ideally suited to
these frequencies.  The major east coast ham packet network EASTNET runs from
Maine to Miami and contains hundreds of relay stations (digipeaters) and BBSes
as well as gateways; stations which provide crossband operation to smaller
more local nets running on a different frequency or to shortwave stations
which provide connections to the west coast network. There are now also ham
satellites in orbit which can extend the range to Europe and other parts of
the globe.  Access is either directly through the satellite or via a land
based gateway in the net.  Packet allows for more reliable connections and
more portable operation.  Instead of running a shortwave transceiver that
requires a large power supply and antenna one can fit a laptop computer, TNC
(Terminal Node Controller; a modem for packet operation), a VHF/UHF handheld,
and a small gel-cell battery in a briefcase.  This set-up will go 10-20 miles
which is enough of a distance to get to a digipeater from anywhere in the U.S.
and access the net.
Radio modems/TNCs are available from ham suppliers along with the respective
radios.  Check out ham magazines (73, QST, CQ) for supplier addresses and
equipment reviews. Also check out the sources listed in my previous articles
for more genera l info on radio communications and getting a ham license.
As you see, radio offers as good if not better digital communications
capability than the telecommunications network does, with better capability in
security.  Should you be seriously interested in digital commo it worth your
while to check out radio as a means of transmission.
 
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