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. The data link layer is level two of the seven-level OSI model. ... In computing, a protocol is a convention or standard that controls or enables the connection, communication, and data transfer between two computing endpoints. ... X.25 is an ITU-T standard protocol suite for WAN networks using the phone or ISDN system as the networking hardware. ... Amateur radio, commonly called ham radio, is a hobby enjoyed by many people throughout the world (as of 2004 about 3 million worldwide, 60,000 in UK, 70,000 in Germany, 5,000 in Norway, 57,000 in Canada, and 700,000 in the USA). ... Packet radio is a form of digital data transmission used in amateur radio to construct wireless computer networks. ... Computer network From Wikipedia, the free encyclopedia. ...

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. The physical layer is level one in the seven level OSI model of computer networking. ... The data link layer is level two of the seven-level OSI model. ... The Open Systems Interconnection Reference Model (OSI Model or OSI Reference Model for short) is a layered abstract description for communications and computer network protocol design, developed as part of the Open Systems Interconnect initiative. ... A packet is the fundamental unit of information carriage in all modern computer networks. ... A node is a device connected to a computer network. ... A Communications channel (or channel for short), models the medium through which information is transmitted from a sender (or transmitter) to a receiver. ... Ethernet (this name comes from the physical concept of ether) is a frame based computer networking technology for local area networks (LANs). ...

AX.25 supports both connected and connectionless modes of operation, the latter used to great effect by the Automatic Position Reporting System. Automatic Position Reporting System (APRS) is an amateur radio based automatic position reporting system for tracking and digital communications, and was developed by Bob Bruninga, callsign WB4APR, at the United States Naval Academy. ...

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. A terminal node controller (TNC) is a device used by amateur radio operators to participate in AX.25 packet radio networks. ... Microprocessors, including an Intel 80486DX2 and an Intel 80386 A microprocessor (abbreviated as µP or uP) is an electronic computer central processing unit (CPU) made from miniaturized transistors and other circuit elements on a single semiconductor integrated circuit (IC) (aka microchip or just chip). ... In computing, firmware is software that is embedded in a hardware device. ... A dumb terminal in computing consists of a computer screen and keyboard, but practically no processing ability. ... A transceiver is a device that has a transmitter and receiver which is combined into a one unit. ...

More recently, AX.25 implementations have appeared for personal computers. For example, the Linux kernel includes native support for AX.25 networking. The Linux mascot Tux created by Larry Ewing Linux kernel is a free unix-like operating system kernel created by Linus Torvalds in 1991 and subsequently improved with the assistance of developers around the world. ...

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 bulletin board system or BBS is a computer system running software that allows users to dial into the system over a phone line and, using a terminal program, perform functions such as downloading software and data, uploading data, playing games, reading news, and exchanging messages with other users. ...

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. For the album by the post-hardcore band Fugazi, see Repeater (album). ... A simplex communication system is one where all signals flow in one direction. ... For other meanings, see repeater (disambiguation). ...

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. The network layer is level three of the seven level OSI model. ... The network layer is level three of the seven level OSI model. ... The Internet Protocol (IP) is a data-oriented protocol used by source and destination hosts for communicating data across a packet-switched internetwork. ...

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. In telecommunications and computing, bit rate (sometimes written bitrate) is the frequency at which bits are passing a given (physical or metaphorical) point. It is quantified using the bit per second (bit/s) unit. ...

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. In telecommunication, forward error correction (FEC) is a system of error control for data transmission wherein the receiving device has the capability to detect and correct fewer than a predetermined number or fraction of bits or symbols corrupted by transmission errors. ... In computer science, data compression or source coding is the process of encoding information using fewer bits (or other information-bearing units) than a more obvious representation would use, through use of specific encoding schemes. ...

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