AX.25

AX.25 is a data link layer protocol derived from the X.25 protocol suite and designed for use by amateur radio operators. [citeweb|title=AX.25_Link_Access_Protocol_for_Amateur_Packet_Radio|url=http://www.tapr.org/pdf/AX25.2.2.pdf|publisher=tapr.org|accessdate=2008-03-05] 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 connection and connectionless modes of operation, the latter used to great effect by the Automatic Packet 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. [citeweb|title=Linux AX.25 Configuration|url=http://www.febo.com/packet/linux-ax25/index.html|publisher=febo.com|accessdate=2008-03-05]

Technically, AX.25 does not completely define a physical layer implementation, although it does define a physical-layer state machine. In practice AX.25 is almost exclusively used with 300 baud Bell 103 modems on HF and 1200 baud Bell 202 modems on VHF. These combinations are so ubiquitous that most users do not differentiate between the AX.25 data layer and the underlying physical layer.

AX.25 is often used a TNC that implements the KISS framing as an alternative to using an expensive HDLC controller card. The KISS framing is not part of the AX.25 protocol itself nor is it sent over the air. It merely serves to encapsulate the AX.25 frames in a way that can successfully pass over a serial link to the TNC. [citeweb|title=The KISS TNC: A simple Host-to-TNC communications protocol|publisher=ka9q.net|url=http://www.ka9q.net/papers/kiss.html|accessdate=2008-08-18]

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 Packet 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 valuein range 0 through 15.At ITU WARC2003 meeting earlier radio amateur station call-sign specification was amended so thatearlier maximum length of 6 character was raised to 7 characters. However AX.25 has a built in hard limitof 6 characters, which means a 7 character call-sign cannot be used in an AX.25 network.

In the AX.25 CONS protocol there is no "port" (or SAP) in the connection, thus there can be only one serviceper 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 the 430 MHz UHF band, to match existing legacy radio gear
* New radio level modulations would need different radio gear than what is currently in use and the resulting system would be incompatible with the existing one - thus requiring a large initial investment in new radio gear.
* Generating, and especially decoding, advanced modulations including "forward error correction" takes more effort than the 1,200 bit/s AFSK of Bell 202. Previously sufficient small 8 bit microprocessors with 128 bytes of RAM would not be enough, and new ones might cost USD 30 instead of USD 3. Phil Karn did demo decoding of this new modulation of his by running it on a Pentium II machine - some 10 years later, 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 (particularly packet radio) would make adoption marginal and impractical.

Small gadget transmitters do not need to know what is being transmitted. There is only a need to monitor channel occupation by radio receiver RSSI (Received Signal Strength Indication) to know when not to send.Transmitting interleaved Reed-Solomon FEC signal in some smart modulation needs a lot fewer resources than reception of the same signal, thus a sufficient microprocessor might cost just USD 5 instead of USD 3 and a system cost might stay below USD 50, transmitter included. However, in recent years, the ability to receive as well as send using cheap microcontrollers (such as the Atmel AVR or the Motorola 68HC08 families) have been demonstrated.

It seems, however, that any new system that is not compatible with the current Bell 202 modulation is unlikely to be widely adopted. The current modulation seems to fulfill sufficient need that little motivation exists to move to a superior design, especially if the new design requires significant hardware purchases.

References

Further reading

*cite book| author=Rich Roznoy, K1OF, Editor | publisher=American Radio Relay League, Inc | title=Packet: Speed, More Speed and Applications (2nd Edition) | year=1997 | isbn=0-87259-605-2 | location=Newington, CT
* [http://www.ampr.org AMPRNet] – a project to construct a global, radio-based network using TCP/IP over AX.25 links
* [http://www.linux-ax25.org Linux-AX25.org] – Site dedicated to packet radio on Linux
* [http://www.ax25.net/ AX.25 Layer 2] – This web site has been established to be a concise repository for AX.25 layer 2 design activities.
* [http://www.aprs.org/ APRS] - Bob Bruninga's official APRS website.