AX.25

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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.

Contents

1 Implementations
2 Applications
3 Limitations
4 External links

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.

The AX.25 protocol identifies each message by sender and destination station call-sign plus SSID value in range 0 thru 15. At ITU WARC2003 meeting earlier radio amateur station call-sign specification was amended so that earlier maximum length of 6 character was raised to 7 characters. However AX.25 has built in hard limit of 6 characters, which means one with such 7 character call-sign can not use theirs in AX.25 network.

In AX.25 CONS protocol there is no port (or SAP) in the connection, thus there can be only one service per AX.25 station SSID address - which is often kludged around with varying degrees of success.

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 (FEC) and automatic data compression. However, a successor to AX.25 has yet to emerge, likely reasons include:

Very large existing deployment of recycled narrowband FM radios and especially existing APRS applications.
Easy availability of cheap low-power FM transmitters especially for 430 MHz UHF band to match with existing legacy radio gear
New radio level modulations would need different radio gear than what is currently in use, and resulting system would be incompatible with old one - making huge inertial obstacle
Generating and especially decoding advanced modulations including forward error correction takes more effort, than 1,200 Bps AFSK of Bell 202. Previously sufficient small 8 bit microprocessor with 128 bytes of RAM would not be enough, and new one might cost USD 30 instead of USD 3. Phil Karn did demo decoding of this new modulation of his by running it on Pentium-II machine - some 10 years latter mid-level embedded microprocessors are capable enough to do the same with under USD 200 system cost.
A general decline in the use of Amateur Radio generally and packet radio in particular would make adoption marginal and impractical.

Small gadget transmitters do not need to know what somebody is transmitting at the frequency, only to monitor channel occupation by radio receiver chips analog RSSI (Received Signal Strength Indication) to know when not to send. Transmitting interleaved Reed-Solomon FEC signal in some smartish modulation needs a lot less resources than reception of the same signal, thus sufficient microprocessor might cost just USD 5 instead of USD 3 and system cost might stay below USD 50, transmitter included.

Nevertheless, new system that is incompatible with previous one does not easily gain deployment in an environment, where barely sufficient and cheap system already exists, especially if majority of existing radio gear would be incompatible with new modulations.

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
[Linux-AX25.org] – Site dedicated to packet radio on Linux
[ax25.net] – Site dedicated to the use of AX.25 at Layer 2

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