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 very elementary.

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 "computers".

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 price.

 

Requirements

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

The Computer

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 program.

TNC and MODEM

Hardware Type
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 other task.
Software Type
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.

Transceiver

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 local operators.

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.

Ideally 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.

 

                                                            CQ FREQUENCIES

 

SSTV PSK31 MT63 MFSK
3730 1838.15 14110 1838
14230 3580.15 21120 3580
21340 7835.15 28130 KHz USB 7037
28680 KHz 14070.15   14080
 

21080.15.28120.15 KHz

USB

  21080
      28080 KHz USB