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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.

There have been: What Is Packet count accesses since 23/12/98
Last update to this page: 22/2/00