Also known as: AX.25, AX25

AX.25 is the data-link-layer protocol used in amateur packet radio. It adapts the telecom HDLC frame format with amateur callsign addressing, and is the link layer beneath APRS and traditional packet networks.¹

AX.25 is the amateur packet link layer: an HDLC frame delimited by flags, with addresses, payload, and an FCS.

Overview

AX.25 frames are delimited by flag bytes, bit-stuffed, and protected by a frame check sequence (CRC). They carry source and destination callsigns (with SSIDs) plus an optional digipeater path. APRS uses connectionless “UI” (unnumbered information) frames; classic packet BBS use connected modes.

Technical characteristics

Property	Value
Frame format	HDLC-derived, flag-delimited
Addressing	Callsign + SSID, digipeater path
Error check	16-bit FCS
Line coding	NRZI over AFSK/FSK

History

Developed by the amateur community in the 1980s as a radio adaptation of X.25/HDLC for packet networks and bulletin-board systems.¹

Deployment

Amateur packet radio, APRS, and store-and-forward networks.

Decoding it with GopherTrunk

GopherTrunk recovers AX.25 frames as part of its APRS pipeline. See the APRS / AX.25 page.

Sources

1. AX.25 — Wikipedia, for the amateur packet data-link protocol, its HDLC-derived framing, callsign addressing, and FCS. ↩ ↩²

See also

• APRS
• AFSK
• Cyclic redundancy check (CRC)

Learn it step by step

• Other signals you'll meet

From the blog

• SDR Internals, Part 10: Protocol decoders & state machines