Packet radio is a form of digital data transmission used in amateur radio to construct wireless computer networks. Its name is a reference to the use of packet switching between network nodes, which allows multiple virtual circuits to coexist on a single radio channel. Packet radio networks use the AX.25 data link layer protocol, derived from the X.25 protocol suite and designed for amateur radio use.
(The term "packet radio" also was used for early version of mobile ad hoc networks, but this technology has little in common with the description below.)
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A basic packet radio station consists of a computer, a modem, and a transceiver with an antenna. Traditionally, the computer and modem are combined in one unit, the terminal node controller (TNC), with a dumb terminal (or terminal emulator) used to input and display data. Increasingly, however, personal computers are taking over the functions of the TNC, with the modem either a standalone unit or implemented entirely in software.
The computer is responsible for managing network connections, formatting data as AX.25 packets, and controlling the radio channel. Frequently it provides other functionality as well, such as a simple bulletin board system to accept messages while the operator is away.
Following the OSI model, packet radio networks can be described in terms of the physical, data link, and network layer protocols on which they rely.
Physical layer: modem and radio channel
Modems used for packet radio vary in throughput and modulation technique, and are normally selected to match the capabilities of the radio equipment in use. The first amateur packet radio stations were constructed using surplus Bell 202 1,200 bit/s modems, and despite its low data rate, Bell 202 modulation has remained the standard for VHF operation in most areas. More recently, 9,600 bit/s has become a popular alternative. At HF frequencies, Bell 103 modulation is used, at a rate of 300 bit/s.
Custom modems have been developed which allow throughput rates of 19.2 kbit/s, 56 kbit/s, and even 1.2 Mbit/s over amateur radio links. However, special radio equipment is needed to carry data at these speeds, and their adoption has been limited.
Data link layer: AX.25
Packet radio networks rely on the AX.25 data link layer protocol, derived from the X.25 protocol suite and intended specifically for amateur radio use. Despite its name, AX.25 defines both the physical and data link layers of the OSI model. (It also defines a network layer protocol, though this is seldom used.)
Packet radio has most often been used for direct, keyboard-to-keyboard connections between stations, either between two live operators or between an operator and a bulletin board system. No network services above the data link layer are required for these applications.
To provide automated routing of data between stations (important for the delivery of electronic mail), several network layer protocols have been developed for use with AX.25. Most prominent among these are NET/ROM, ROSE, and TexNet.
In principle, any network layer protocol may be used, including the ubiquitous Internet protocol.
- AX.25 Link Access Protocol for Amateur Packet Radio: the official specification, from Tucson Amateur Packet Radio
- Automatic Position Reporting System