Amateur Packet Radio

- an overview, summary or tutorial about the basics of packet radio a form of digimode used on the amateur radio bands above 30 MHz.


Amateur Radio Digital Modes Tutorial Includes:
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Packet radio is widely used on the amateur bands, particularly on frequencies above 30 MHz where it is now well established and forming one of the mainstays of communications within the amateur community. The basic system allows modern day computer technology to be used to enable error free communication combined with many useful facilities, making it a mode that is compatible with many computer style applications, and allows people to combine the hobby of amateur radio with computer technology.

Fortunately it is not difficult to convert an existing amateur radio station to be able to transmit packet radio. Typically most VHF / UHF FM equipment is capable of transmitting amateur radio packet signals. Even many small handheld radios provide an excellent means of communicating via this means. As many people already have computers, the investment required is often minimal.

 

... even many small handheld radios provide an excellent means of communicating via amateur packet radio ....


   VHF UHF Amateur Radio Handheld Transceiver  

How amateur packet radio started

The foundations for amateur packet radio were laid in the early 1980s when a group of radio amateurs in Tucson USA started to develop an amateur packet radio protocol based around the commercially available X.25 protocol. The group in Tucson was not the only group investigating this as others in Vancouver were developing a similar protocol. Fortunately the two groups decided to combine their work and the result was the AX.25 protocol used as the basis of amateur packet radio today.

The idea of amateur packet radio soon spread from North America around the world. People started to use the system, and TNCs (Terminal Node Controllers) soon became available to support the system. It must be remembered that at this time, PCs were not nearly as widely available as they are today. However with the spread of the PC, separate TNCs were no longer a requirement as the TNC functions could be incorporated within the PC making amateur packet radio much easier to use. Accordingly its use spread considerably with its use being mainly on the amateur radio bands above 30 MHz, although some operation took place at HF. Now with the availability of the Internet many prefer wire based emails to the radio based system offered by packet radio as they are faster and more reliable.

Packet radio basics

As the name implies this mode of transmission splits the data to be sent up into a series of packets which can be sent one at a time. As messages are usually much longer than the amount of data which can be sent in one packet, it takes several packets to complete the message.

One of the advantages of amateur packet radio is that one channel can be used by several amateur radio stations at the same time. This means that when sending data any station has to wait until the channel is clear. Once the frequency is free the first packet can be sent, and the receiving station will return an acknowledgement to say that all the data has been received correctly. If this acknowledgement is not received the transmitting station waits for the frequency to clear and re-sends the data. This process is repeated until the data has been correctly received. Once the first packet has been transferred, the second, and subsequent ones are all transmitted in the same way.

As the receiving radio station checks for errors and the transmitter repeats the data until it has been correctly received the system is very resilient and gives very high levels of accuracy. The other advantage is that the approach of waiting until the frequency is clear before transmitting allows many stations to use the same frequency, providing an efficient utilisation of the available spectrum. Nevertheless traffic is often high and as a result several channels may be allocated for amateur packet radio on a given band.

Details for amateur packet radio transmissions

Like other data modes or digimodes, packet radio uses frequency shift keying. A transmission speed of 1200 baud with tone frequencies of 2200 Hz for the space and 1200 Hz for the mark condition have been adopted for VHF. On HF where conditions are a little more difficult a speed of 300 baud with a 200 Hz shift is generally employed.

The format for each data packet is accurately defined so that the receiver can decipher the incoming data. Data is sent using ASCII (American Standard Code for Information Interchange) and each packet has five different elements or sections as shown below. There are flags at the beginning and end of each packet, an address, control information, a frame check sequence, and the data itself.

Packet radio transmission format

Packet radio transmission format

The flag at the beginning of the packet is used to allow the receiving decoder to synchronise to the incoming data. This is followed by a station address. This is used to define the callsign of the station to whom the data is being sent. Also included is the source or sending station callsign, and the callsigns of any repeaters or digipeaters which are to be used to relay the message. This means that any other station using the frequency will be able to ignore the data and only receive the signals intended for it.

The element within the data packet is the control byte. This is used to signal acknowledgements and requests to repeat transmissions. This is followed by the data itself. The length of this can be up to 256 bytes. Once it is complete the frame check sequence or FCS is sent. This is a check-sum whose value is calculated from the data. It is used by the receiving station to check that all the data has been correctly received. Only when the receiver is able to generate an identical code to match the received one is an acknowledgement sent.

The final part of the packet radio transmission is the terminating flag. This recognised by the receiver as the end of the message and enables it to check the data and send its acknowledgement.

Amateur packet radio features

Packet radio is able to utilise a number of features which were not present in previous types of data communication. One of the most widely used is the ability for other stations to relay messages, so that much greater distances can be covered. Stations which relay messages in this way are called digital repeaters or digipeaters for short.

Packet radio transmissions take place on a single frequency. This means that digipeaters have to receive and transmit on a single frequency. For them to be able to relay the messages, the message must first be received in full, stored and then transmitted. Once the final station in the chain has received the message the acknowledgement is sent back along the chain to the first station. This is known as an end to end acknowledgement. Only then is the next packet sent. This means that when a message is sent over a long path using several stations as repeaters, the message can take a very long time to get through, especially if any packets have to be repeated.

One powerful facility which amateur packet radio offers is the ability to read data from a mailbox. Sometimes called a bulletin board system (BBS), it enables messages to be sent to a particular mailbox and left for collection by a particular station. In many respects it is like a radio e-mail system.

A message is sent to the local mailbox. Once received it is stored and then it is passed on via a network of mailbox stations until the required destination mailbox is reached. The message is stored at this mailbox until it is read by the recipient station. The advantage of using the mailbox system is that it is not necessary to know the route required to be taken by the message. This is worked out by the system, as it has a knowledge of the stations and works out a suitable route. Data is generally sent at periods of low activity, often at night, and this means that messages can take a few days to arrive. However as many links exist between countries it is possible to send messages around the world.

In addition to the basic mail facility, many items of general interest are stored and can be accessed by any station.

Although much of the initial experimental and practical work regarding packet radio was carried out by radio amateurs many commercial packet systems are now in use around the world. They are particularly useful where a large number of users have to send small amounts of data at intervals which would not demand a separate frequency for each separate user. For example packet radio is ideal for monitoring systems where each outpost has to be polled or accessed at intervals, or where it periodically reports a status or other information to the main station.



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