Packet Radio — AX.25 and Amateur Digital Networking
Packet radio is amateur radio's original digital networking mode, using the AX.25 protocol to send data packets over VHF, UHF, and HF radio links. Developed in the 1980s from the X.25 data networking standard, packet radio created a nationwide amateur radio data network that predated the commercial internet. Today packet radio remains active primarily through APRS (Automatic Packet Reporting System) on VHF and through Winlink gateways on HF and VHF, as well as in some legacy bulletin board and messaging systems.
AX.25 protocol
AX.25 is a data link layer protocol derived from the X.25 standard, adapted for amateur radio use. It uses callsigns as network addresses — every station on a packet network is identified by its callsign. AX.25 packets contain the source callsign, destination callsign, up to eight digipeater addresses for routing, and a data payload. The protocol includes error checking to detect corrupted packets, and connected-mode operation (similar to TCP) can retransmit lost packets for reliable file transfer.
On VHF, packet radio uses 1200 baud Bell 202 AFSK audio — the same modulation used by APRS. On HF, packet uses 300 baud FSK. Higher speed modes like 9600 baud use direct FSK modulation and require radio modifications or specific hardware to bypass audio filtering.
TNCs and software TNCs
A TNC (Terminal Node Controller) is the device that modulates and demodulates AX.25 packets — the interface between a radio and a computer or terminal. Hardware TNCs like the Kantronics KAM, PacComm Tiny-2, and newer models from Mobilinkd and others are dedicated devices. Software TNCs like Direwolf run on a computer and use the sound card as the modem — providing equivalent functionality to a hardware TNC at no additional hardware cost. Direwolf is the standard software TNC for APRS and packet radio today and runs on Windows, Mac, Linux, and Raspberry Pi.
APRS
APRS (Automatic Packet Reporting System) is by far the most active application of packet radio today. Running on 144.390 MHz at 1200 baud, APRS uses AX.25 to broadcast position reports, weather data, short messages, and object information. A network of digipeaters relays packets across wide areas, and iGates upload received packets to the internet-based APRS-IS network for display on tracking sites like aprs.fi. APRS trackers in vehicles, weather stations, and portable devices all use standard AX.25 packet radio.
Winlink gateways
Winlink VHF gateways — RMS stations that accept email via radio — use AX.25 packet radio at 1200 or 9600 baud on VHF/UHF frequencies. When you connect to a Winlink VHF gateway with Winlink Express and a packet radio setup, you are using AX.25 packet to transfer email messages. This is one of the most practical current applications of packet radio outside of APRS, particularly for EmComm operators who want Winlink capability without the complexity of HF VARA or PACTOR.
Download Direwolf
Direwolf is available free from github.com/wb2osz/direwolf. Download the latest release for your operating system. Direwolf can function as an APRS tracker, digipeater, iGate, or Winlink TNC depending on configuration. The documentation is thorough — read the User Guide PDF included in the download.
Configure Direwolf
Edit the direwolf.conf configuration file to set your callsign, audio device (your sound card or radio's USB audio), and PTT method (serial port, USB, GPIO on Raspberry Pi). For basic APRS monitoring, a minimal configuration with your callsign and audio device is sufficient. For digipeating or iGate operation, additional configuration specifies your location and what packets to relay.
Connect your radio
Connect your VHF radio's audio output to your computer's sound card input and your computer's audio output to the radio's microphone input. For PTT, connect a serial port RTS line or VOX to your radio's PTT switch. A dedicated sound card interface handles this cleanly. Tune your radio to 144.390 MHz FM for APRS monitoring.
Monitor packet activity
Run Direwolf and watch the decoded packets scroll by — position reports, weather data, messages from nearby stations. For APRS display and mapping, connect Direwolf to YAAC or Xastir via the AGWPE or KISS interface. For Winlink VHF, connect Winlink Express to Direwolf using the Packet Winlink session type.
Is packet radio still worth learning?
For APRS and Winlink VHF gateway operation, packet radio knowledge is directly practical. For legacy bulletin board and message systems (BBS), activity has declined significantly. The core AX.25 protocol knowledge is also directly applicable to understanding APRS, which remains highly active for EmComm and vehicle tracking. Learning Direwolf as a software TNC is valuable for any operator interested in APRS or Winlink on VHF.
What is the difference between packet radio and APRS?
APRS is an application that runs over packet radio using the AX.25 protocol. Packet radio is the underlying transport layer — AX.25 protocol over radio. APRS defines the specific message formats, digipeater paths, and data types (position, weather, messages) that are carried inside AX.25 packets. All APRS is packet radio, but not all packet radio is APRS — packet can carry other data like email, bulletin board messages, and file transfers.
What speed should I use for VHF packet?
For APRS, 1200 baud is the standard and all equipment is compatible. For Winlink VHF gateways, both 1200 and 9600 baud are used — 9600 baud is faster and preferred when hardware supports it, but requires a radio that can handle the wider audio bandwidth (9600 baud bypasses the radio's audio filtering). Most basic setups use 1200 baud for maximum compatibility.
Can I do packet radio with just a radio and a computer?
Yes — Direwolf uses your computer's sound card as the TNC, so no dedicated TNC hardware is required. You just need a radio connected to your computer's audio input and output (through a sound card interface or direct cable), PTT switching, and Direwolf configured correctly. This makes packet radio accessible to anyone with an existing HF or VHF setup.