Протокол AX.25: Обзор и особенности применения в любительском радиосвязи

Amateur radio data communications protocol
AX.25 (Amateur X.25) is a data link layer protocol originally derived from layer 2 of the X.25 protocol suite and designed for use by amateur radio operators. It is used extensively on amateur packet radio networks. AX.25 v2.0 is responsible for establishing link layer connections, transferring data encapsulated in frames between nodes, and detecting errors introduced by the communications channel. AX.25 v2.2 Stations can automatically negotiate payload sizes larger than the previous limitation of 256 bytes. Extended sequence numbers (7 vs. 3 bits) allow a larger window size, the number of frames that can be sent before waiting for acknowledgement. "Selective Reject" allows only the missing frames to be resent, rather than having to wastefully resend frames that have already been received successfully. Despite all these advantages, few implementations have been updated to include these improvements published more than 20 years ago. The only known complete implementation of v2.2, at this time (2020), is the Dire Wolf software TNC. AX.25 is commonly used as the data link layer for network layer such as IPv4, with TCP used on top of that. AX.25 supports a limited form of source routing. Although it is possible to build AX.25 switches similar to the way Ethernet switches work, this has not yet been accomplished.

AX.25 does not define a physical layer implementation. In practice 1200 baud Bell 202 tones and 9600 baud G3RUH DFSK are almost exclusively used on VHF and UHF. On HF the standard transmission mode is 300 baud Bell 103 tones. At the physical layer, AX.25 defines only a "physical layer state machine" and some timers related to transmitter and receiver switching delays. At the link layer, AX.25 uses HDLC frame syntax and procedures. (ISO 3309) frames are transmitted with NRZI encoding. HDLC specifies the syntax, but not the semantics, of the variable length address field of the frame. AX.25 specifies that this field is subdivided into multiple addresses: a source address, zero or more repeater addresses, and a destination address, with embedded control fields for use by the repeaters. To simplify compliance with amateur radio rules, these addresses derive from the station call signs of the source, destination and repeater stations. Media access control follows the Carrier sense multiple access approach with collision recovery (CSMA/CR). AX.25 supports both virtual circuit connected and datagram style connectionless modes of operation. The latter is used to great effect by the Automatic Packet Reporting System (APRS). A simple source routing mechanism using digipeaters is available at the datalink level. Digipeaters act as simplex repeaters, receiving, decoding and retransmitting packets from local stations. They allow multi hop connections to be established between two stations unable to communicate directly. The digipeaters use and modify the information in the frame's address field to perform this function. The AX.25 specification defines a complete, albeit point to point only network layer protocol, but this has seen little use outside of keyboard to keyboard or keyboard to BBS connections. NET/ROM, ROSE, and TexNet exist to provide routing between nodes. In principle, a variety of layer 3 protocols can be used with AX.25, including the ubiquitous Internet Protocol (IP). This approach is used by AMPRNet, which is an amateur radio TCP/IP network using AX.25 UI frames at the datalink layer.

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. AX.25 has also been implemented on personal computers. For example, the Linux kernel includes native support for AX.25. The computer connects to a transceiver via its audio interface or via a simple modem. The computers can also interconnect to other computers or be bridged or routed to TNCs and transceivers located elsewhere using BPQ over Ethernet framing, which is also natively supported by the Linux kernel to facilitate more modern setups with the actual transceivers directly placed under or in the antenna mast, creating a 'low loss', shorter RF wiring need, and replacing expensive and long and thick coax cables and amplifiers with cheap fiber (RFI (both ways)/EMP/lightning resistant) or copper Ethernet wiring. BPQ Ethernet framing allows connecting entire stacks of TNC+transceiver pairs to any existing network of computers which then can all access all radio links offered simultaneously (transparently bridged), communicate with each other internally over AX.25, or with filtered routing select specific TNCs/radio frequencies. Dire Wolf is a free open source replacement for the 1980s style TNC. It contains DSP software modems and a complete implementation of AX25 v2.2 plus FX.25 forward error correction. It can function as a digital repeater, GPS tracker, and APRS Internet Gateway (IGate) without any additional software.

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. In recent years, APRS has become a popular application. For tunneling of AX.25 packets over IP, AXIP and AXUDP are used to encapsulate AX.25 into IP or UDP packets.