AX.25

AX.25. AX.25 is a data link layer protocol derived from the X.25 suite and HDLC, specifically designed for amateur radio communications. It was developed in the early 1980s by the ARRL and TAPR to enable reliable, connected data transfer over radio links. The protocol forms the backbone of traditional packet radio networks, facilitating everything from simple keyboard-to-keyboard chats to complex WAN systems like AMPRNet.

Overview

AX.25 was created to adapt the connection-oriented virtual circuit principles of commercial X.25 networks for the unique constraints of the amateur radio service. Its development was heavily influenced by the work of Doug Lockhart and the VADCG on earlier systems. The protocol operates primarily on VHF and UHF bands, using AFSK modulation commonly transmitted via Bell 202 compatible modems within a TNC. A key feature is its inclusion of amateur radio call signs as part of the address field, integrating station identification directly into the network layer addressing.

Protocol description

Functioning at the data link layer of the OSI model, AX.25 provides both connection-oriented (virtual circuit) and connectionless (datagram) services. The connection-oriented mode uses a balanced, asynchronous balanced mode link, similar to HDLC, involving frame exchange for establishing and terminating connections. It employs a sliding window protocol with a default window size of four for flow control and utilizes CRC for error detection, requesting retransmission of damaged frames. The protocol's design accommodates the half-duplex nature and longer latency of radio circuits compared to wired networks like Ethernet.

Frame structure

An AX.25 frame is bounded by flag bytes and contains distinct fields for addressing, control, and error checking. The address field can be exceptionally long, accommodating up to 256 octets to allow for multiple amateur radio call signs and SSID numbers in digipeated paths, such as via Kenwood or Kantronics stations. The control field defines the frame type, including Information, Supervisory, and Unnumbered frames, governing link management. Following a variable-length information field for higher-layer data, a frame check sequence using a 16-bit CRC-CCITT provides error detection before the closing flag.

Applications and usage

AX.25 is famously used in terrestrial packet radio networks, including global systems like AMPRNet and regional BBS networks that forward mail via protocols like Net/ROM. It is the standard for APRS, developed by Bob Bruninga, which provides real-time tactical digital communications for events like marathons or Field Day. The protocol has also been used for satellite communications, such as with the UoSAT and Fuji-OSCAR spacecraft, and for telemetry and control in amateur radio satellites and high-altitude balloon projects.

Implementation and variants

The primary implementation hardware is the TNC, with iconic early models like the TAPR TNC-1 and the VADCG VADCG Board. Software implementations, known as software TNCs or soundcard modems, such as Soundmodem and Dire Wolf, later became prevalent. Variants include Net/ROM and TexNet, which added network layer routing capabilities. For space use, the CCSDS developed a derivative known as CFDP. The KISS framing protocol was created to simplify the interface between the TNC and a host computer, leading to more flexible software development.

Technical specifications

AX.25 typically operates at standard data rates of 1200 baud over VHF using Bell 202 AFSK (1200 Hz/2200 Hz) and 9600 baud over UHF using G3RUH compatible MSK. The protocol specification, formalized in the ARRL publication "AX.25 Amateur Packet-Radio Link-Layer Protocol," details frame formats, connection states, and timers like T1 and T2. It uses modulo 8 sequence numbers by default, with an optional extension to modulo 128 for satellite links with long delay. Channel access is typically managed through CSMA persistence algorithms or deterministic methods in dedicated systems. Category:Amateur radio Category:Network protocols Category:Data link layer protocols