What is Packet Radio?
PACKET RADIO is a means by which error-free information can be
transmitted and received. It requires a computer for its operation, and
normally the operators communicate by means of keyboards and monitor
screens. Facilities to store and forward messages are usually available,
as is access to bulletin boards and databases. The mode of Packet is an
ideal marriage for the radio amateur who dabbles with computers and is
an interesting way to obtain ERROR FREE communication in either rag
chewing, programme or information transfer and satellite communication.
If however, you are not computer minded but wish to be involved in
Packet, there is really no problem as the role of the computer can be
Packet in detail
Packet is a development of the original modes of radio
communication such as Morse and Teletype. These modes are essentially
digital patterns of ON-OFF signals and are used to represent items of
information such as letters of the alphabet, numbers, punctuation etc.
The signals are transmitted between stations by methods such as radio or
cables, and are written down or printed out.
Morse is a relatively slow but simple mode. The brain and ears
form a sensing device that is superior to other means of receiving Morse
code patterns, as they are much better at pattern recognition in the
presence of extraneous signals and noise.
In order better to automate communication, teletype was developed. It
involved more complex patterns generated by mechanical
This gave improved speed and good recognition of patterns. But it is
susceptible to production of false patterns when extraneous signals and
noise are received. Because of the speed involved, brain and ears no
longer help, although there is still use for the brain in guessing what
corrupted messages were meant to say. The same problems occur even after
electronic devices have supplanted the mechanical ones.
RTTY (Radio TeleTYpe) characters are made up of groups of five
ON-OFF signals - the standard code now used was devised by a New Zealand
sheep farmer named Murray, though generally it is misnamed Baudot code
which is a less efficient earlier code.
With the same sort of electronic equipment, ASCII code which
provides up to 255 characters using a seven or eight unit code, can be
employed. It is subject to the same kind of interference problems as
Baudot when used in radio communications, so it is rarely used
unadorned. Within a computer there is no interference, so as you
probably know, all computers talk ASCII of some sort.
To improve message transfer, error correction was incorporated, and AMTOR
(Amateur Microprocessor Teleprinting Over Radio) is one such
development. It uses code in groups of three characters, so that the
value of each group must total a fixed sum. If it does not, when the
receiver adds it up, it is rejected and retransmission is requested.
When it is correct it accepts it and asks for the next group. Using this
mode, almost error free messages can be exchanged. When propagation
conditions are bad, the rate of information transfer will slow down but
it will still be error free. However, the number of character types is
limited by the five unit code, and there has to be a reactive link
between both ends of the communication path.
Here is where PACKET comes in.
Packet uses ASCII code, so can have 255 symbols. Messages are sent in
groups of up to 255 units as compared with the three of AMTOR. Complex
error checking is installed, as is information to guide the Packets
through the system. The system is comprised of Terminal Node Controllers
(TNC) connected to transceivers, each linked with its own
computer that gives commands to the TNC and writes and displays the
messages. The packets are dispatched entirely automatically. They
provide an error-free means of communication that permits multiple use
of a single frequency. Anything that can be put into digital form can be
sent over Packet radio; for example, digital sound or television.
At the present time it is very easily used to transmit written
messages, computer programmes and files that are written in ASCII or
even binary code, between radio amateurs. And the result is a screen
full of characters in front of your eyes.
Bulletin Board Systems (BBS) form a useful part of the system,
providing the source of lots of information and can act as
"mailboxes". They also help in the spread of information by
communicating with each other, exchanging files and messages. Most
Packet is best used on VHF, and using the property of digipeating, VHF
links over long distances form a useful communication network. HF radio
is less efficient with Packet due to propagation difficulties often
giving rise to many errors, hence retries, so that information transfer
is much slower. Packet lengths need to be reduced. In these conditions
AMTOR may be better, but the restricted number of characters used by
AMTOR creates difficulties in linking Packet and AMTOR systems. A system
to join the two, called APLINK, is now being used to connect
Packet systems more effectively across long distances.
This very brief description leaves a lot unsaid, but it should lift
some of the veil of mystery surrounding Packet. It is just the thing for
people who are interested in both radio and computers, but it also is a
fine means of communication for people who can remember about three
simple keyboard commands.
How to get on Packet Radio
Although Packet radio involves high technology and the latest
state of the art components, the basic equipment required is
relatively simple to assemble (as many amateurs have found). You
can of course obtain pre-assembled units at a most reasonable
|A Personal Computer and a Terminal program OR
|A Personal Computer with TNC-emulating software such as
TPK or BayCom|
|A TNC (black box) such as the PacComm Tiny-2 OR
|A simple modem such as the BayCom modem|
|An FM transceiver|
There are of course many breeds and varieties of computer.
Essentially the only requirement is that the computer be able to
run a communications program. There are many such programs.
paKet, YAPP, PKTWin and WINPACK can be found in public
domain/shareware libraries and are also available from the AAPRA
software library. The vital requirements are that its parameters
can be set to:-
baud rate 1200 or higher
parity even or nil
data bits 7 or 8
stop bits 1 or 2
It is via this program that data is sent between you and the
TNC. The commands issued to the TNC are also via the same
TNC and MODEM
These two units are sometimes combined into a neat black box
which contains its own microprocessor, usually a Z80. It will
also include memory chips for holding its program and the data
you will be transmitting and receiving. Another chip, the HDLC
integrated circuit, does all the checking and handling of the
data as it enters and leaves the TNC. The advantage of using a
TNC is that your PMS (Personal Message System) is accessable
even when your computer is turned off or being used for some
These modems are available in both kit and built up form and the
only disadvantage is that if you want to run a PMS (Personal
Message System) your machine has to be on all the time. The
answer to this is use the many BBSs for leaving messages on as
they do operate continuously.
The bulk of operation at this time is on VHF-FM. The original
channel set aside for the mode was 147.575 MHz and usage has now
spread to other frequencies such as 147.600, 144.800, 144.850,
144.900 & 145.050. This list is not exhaustive and specific
operating frequencies depend on the area in which you are
located. Typical packet action is in the lower half of the two
metre band (144.700 to 145.175), so have a listen, or ask some
HF operation is increasing; this can be helpful to those
living away from repeater-serviced areas. The BBS service also
makes good use of frequencies within these bands. 10.147 and
14.107MHz and surrounding frequencies are popular. Gateways to
40m are also in use.
UHF usage is less popular than VHF. These frequencies are
used to provide links between main centres for through traffic
flow, rather than have this traffic clutter the local VHF
frequencies. This is most desirable in the higher density areas.
a transceiver for Packet should have such a frequency response
as to keep the levels of the Packet tones equal. PacComm have a
radio tailored to the needs of digital radio only. On many BBS
systems you can find radio modification files, however for 1200
baud operation the connection of your radio to the TNC is simply
made by connecting to the MIC and Speaker socket and adjusting
the levels to provide satisfactory performance. Full
instructions will be supplied with the device you choose.