Field Guide · protocol

Also known as: packet radio, AX.25 packet

Packet radio is the amateur-radio method of sending digital data over the air using the AX.25 link-layer protocol. Most commonly it uses AFSK at 1200 bps (Bell 202 tones through an FM radio) or direct GFSK at 9600 bps, framing data into addressed, error-checked packets that digipeaters can relay across a network.12

flag dest source ctrl PID information FCS flag AX.25 frame · addressed, CRC-protected · AFSK 1200 or GFSK 9600 bps
Packet radio wraps data in AX.25 frames carrying callsign addresses and a frame-check sequence between HDLC flags.

Overview

Packet radio borrows the HDLC framing of X.25 and adapts it, as AX.25, to carry amateur callsigns as source and destination addresses. A terminal node controller (TNC) handles the modem and framing, presenting the computer with clean packets. Channel access is unslotted CSMA: stations listen before transmitting and back off on collision. Digipeaters extend range by decoding a packet and re-transmitting it toward its destination.

Technical characteristics

Property Value
Link layer AX.25 (HDLC-derived)
Modulation (VHF) AFSK 1200 bps, Bell 202 tones
Modulation (higher rate) GFSK 9600 bps (G3RUH)
Error check 16-bit CRC frame-check sequence
Access CSMA, carrier-sense with back-off
Addressing Amateur callsigns + SSID

The 1200 bps AFSK mode passes audio tones through an ordinary FM transceiver, so any voice radio can send packet. The 9600 bps mode instead feeds the modulator directly, needing a radio with a flat baseband (“9600 ready”) port.

History

Amateur packet radio took off in the 1980s after TAPR produced affordable TNC designs and the AX.25 specification was standardised. It powered bulletin-board systems, keyboard-to- keyboard chat, and store-and-forward mail networks before the internet, and later became the transport for APRS position and telemetry reporting.

Deployment

Packet radio remains active on VHF/UHF amateur bands, most visibly as the AX.25 layer under APRS on 144.390 MHz (North America) and regional equivalents, plus Winlink email gateways and emergency-communications nets.

Decoding it with GopherTrunk

Packet radio is not decoded by GopherTrunk, whose scope is land-mobile trunking and a few paging codes. AX.25 packet is well served by dedicated software such as Direwolf, a software TNC that demodulates AFSK/GFSK and emits KISS frames. It is documented here to place the amateur data mode in the broader signal landscape.

Sources

  1. Packet radio — Wikipedia, for the amateur packet-radio system, its AFSK 1200 / GFSK 9600 modes, and the TNC/digipeater architecture. ↩

  2. AX.25 — Wikipedia, for the AX.25 link-layer framing, callsign addressing, and frame-check sequence. ↩

See also