AX.25 is a data link layer protocol derived from the X.25 protocol suite and designed for use by amateur radio operators. It is used extensively on amateur packet radio networks.

AX.25 occupies the first and second layers of the OSI networking model, and is responsible for transferring data (encapsulated in packets) between nodes and detecting errors introduced by the communications channel. It is thus comparable to Ethernet in the services it provides.

AX.25 supports both connected and connectionless modes of operation, the latter used to great effect by the Automatic Position Reporting System.

Implementations

Traditionally, amateur radio operators have connected to AX.25 networks through the use of a terminal node controller, which contains a microprocessor and an implementation of the protocol in firmware. These devices allow network resources to be accessed using only a dumb terminal and a transceiver.

More recently, AX.25 implementations have appeared for personal computers. For example, the Linux kernel includes native support for AX.25 networking.

Applications

AX.25 has most frequently been used to establish direct, point-to-point links between packet radio stations, without any additional network layers. This is sufficient for keyboard-to-keyboard contacts between stations and for accessing local bulletin board systems and DX clusters .

A simple routing mechanism using digipeaters is available at this level of operation. Digipeaters act as simplex repeaters, receiving and retransmitting packets from local stations. They allow multi-hop connections to be established between two stations unable to communicate directly.

The AX.25 specification defines a complete network layer protocol, but this has seen little use. NET/ROM , ROSE , and TexNet are more common protocols that provide routing between nodes. In principle, any layer 3 protocol can be used with AX.25, including the ubiquitous Internet protocol.

In recent years, the Automatic Position Reporting System has become a popular application.

Limitations

At the speeds commonly used to transmit packet radio data (rarely higher than 9,600 bit/s, and typically 1,200 bit/s), the use of additional network layers with AX.25 is impractical due to the data overhead involved. This is not a limitation of AX.25 per se, but places constraints on the sophistication of applications designed to use it.

Some amateurs, notably Phil Karn , have argued that AX.25 is not well-suited to operation over noisy, limited-bandwidth radio links, citing its lack of forward error correction and automatic data compression. However, a successor to AX.25 has yet to emerge, perhaps due to the legal restrictions imposed by some countries on the type of protocols amateur operators may use.

External links

• AX.25 Link Access Protocol for Amateur Packet Radio – the official specification, from Tucson Amateur Packet Radio
• AMPRNet – a project to construct a global, radio-based network using TCP/IP over AX.25 links