Packet radio lets digital data travel over radio waves in structured packets, not just in a continuous stream. It relies on the AX.25 protocol at the data link layer, which lays out how stations address, send, and check information.
With this setup, you can send messages, position reports, or even network traffic—no need for the usual wired infrastructure.
AX.25 came from the X.25 protocol but added built‑in station identification and supports both connected and connectionless modes. You can run it with all sorts of gear, from classic terminal node controllers to slick software modems.
That flexibility keeps it useful for everything from amateur radio messaging to experimental IP networking over the air.
If you want to really get packet radio and AX.25, you need to look at how the protocol builds frames, manages connections, and talks to equipment. Once you break down the basics and see how the modes and uses work, you start to see why this tech still matters for reliable, independent radio data comms.
Fundamentals of Packet Radio
Packet radio sends digital data over radio frequencies, linking computers and stations without wires. It mixes radio communication with data networking, so amateur radio operators can send structured info reliably over all sorts of distances.
What Is Packet Radio
Packet radio is a way to send digital info in small, structured packets over amateur radio frequencies. Every packet carries addressing, error-checking, and control data, plus the actual payload.
It breaks data into separate packets, so the receiving station can put everything back together. Multiple stations can share the same frequency with less interference.
The AX.25 protocol is the go-to data link layer for amateur packet radio. It even sticks the sender’s callsign in every packet, which keeps regulators happy.
Depending on your setup, packet radio can carry text messages, position data, or even TCP/IP traffic.
History and Evolution
Packet-based radio communication grew out of early computer networking ideas. Amateur operators tweaked commercial packet protocols for radio links, which led to AX.25, a spin on X.25 made for the amateur service.
Early packet networks let operators connect to bulletin board systems (BBS), swap messages, and link up over long distances. These networks often mixed radio links with wired backbones to reach further.
The internet has replaced some packet radio uses, but modes like APRS (Automatic Packet Reporting System) are still going strong. APRS sends GPS data and short messages over packet radio, using a network of digipeaters and internet gateways.
Core Components and Stations
A basic amateur packet radio station usually has:
| Component | Purpose |
|---|---|
| Radio Transceiver | Sends and receives RF signals carrying packet data. |
| TNC (Terminal Node Controller) | Converts digital data to audio tones for transmission and decodes received signals. |
| Computer or Controller | Runs software to generate, route, and process packets. |
You might use a hardware TNC or a software modem like Direwolf. The TNC sits between the radio and computer, using protocols such as KISS to move data.
When stations connect directly or through digipeaters, they form networks. These setups let you do local, regional, or even global packet comms if you link up with internet gateways.
Overview of AX.25 Protocol
The AX.25 protocol is a data link layer standard built for reliable digital comms over radio. It brings error detection, structured addressing, and multiple modes, which makes it a natural fit for amateur packet radio networks.
People still use it for things like APRS, bulletin board systems, and TCP/IP over radio.
AX.25 Origins and Relation to X.25
AX.25 stands for Amateur X.25 and is based on Layer 2 (link layer) of the ITU-T X.25 protocol suite. X.25 was for wired packet networks, but AX.25 got tweaked for wireless use by amateur radio folks.
Instead of numeric node IDs, AX.25 uses callsign-based addressing. The protocol also takes into account the higher error rates and changing conditions you get with radio.
It keeps the frame structure and control fields from X.25, but tweaks some things for half-duplex, shared-frequency use. This way, stations can identify themselves by callsign and send data without a central manager.
People officially call it the AX.25 Link Access Protocol, and it’s the worldwide standard for amateur packet radio at the link layer.
Key Features and Modes
AX.25 checks for errors with a cyclic redundancy check (CRC) in every frame. It supports both connected mode (think TCP) and connectionless mode (think UDP), so you can pick the style that fits.
In connected mode, stations start a session before sending data, which keeps delivery in order and makes sure lost frames get resent. If you use connectionless mode, frames go out with no setup—handy for short messages or broadcasts.
The protocol uses HDLC-like framing with flags, address fields, control fields, and a frame check sequence. Address fields hold both source and destination callsigns, plus optional digipeater paths for multi-hop routes.
AX.25 works with different modulation methods thanks to the Terminal Node Controller (TNC) or software modem, so you can use it with lots of different radio gear.
Role in Amateur Radio Communications
AX.25 is the backbone for several digital modes in amateur radio. APRS relies on AX.25 frames to send position, weather, and short messages. TCP/IP over packet radio also uses AX.25 as its link layer.
The protocol lets stations act as digipeaters, retransmitting packets to extend range. It also supports igate operations, which bridge RF packets to the internet.
Because AX.25 uses callsign-based addressing, it matches up with amateur radio licensing rules, so every transmission can be traced to a licensed operator.
You can run AX.25 over VHF, UHF, or HF bands, so it works for both local and long-distance packet networks. Even with newer tech out there, AX.25 stays reliable and familiar for digital comms.
Technical Structure of AX.25
AX.25 builds on high-level data link control (HDLC) framing and adapts it for amateur packet radio. It spells out how stations address each other, package data, and share the channel. The protocol separates physical signaling from link-layer stuff, so you can use different hardware and modulation.
Frame Structure and Addressing
AX.25 frames follow an HDLC-based format with a clear start and end flag, control fields, and a frame check sequence for error detection.
The address field breaks down into:
- Destination callsign
- Source callsign
- Optional repeater (digipeater) addresses
Each callsign gets a Secondary Station Identifier (SSID), a 4-bit value (0–15), so you can tell services or connections apart on the same station.
The protocol covers both connected mode (virtual circuit) and connectionless mode (UI frames). Address fields also carry control bits for repeater hops, which makes multi-hop routing possible without extra protocols.
For AX.25 v2.0, frame sizes usually max out at 256 bytes, but v2.2 lets you negotiate for bigger payloads. Extended sequence numbers in v2.2 mean you can have more unacknowledged frames outstanding, which helps at higher speeds.
Physical and Data Link Layers
AX.25 doesn’t define its own physical layer modulation. Instead, people usually run it over:
- 1200 baud Bell 202 AFSK for VHF/UHF
- 9600 baud G3RUH DFSK for faster VHF/UHF links
- 300 baud Bell 103 AFSK for HF bands
At the data link layer, AX.25 uses HDLC framing and NRZI encoding. It has timers for switching between transmit and receive, which really matters in half-duplex radio.
Because the physical layer is separate, AX.25 can run over various modulation schemes and hardware, from old-school TNCs to modern software modems like Dire Wolf. This separation even lets you run other network protocols, like IPv4, over radio.
Media Access and Transmission Methods
AX.25 uses Carrier Sense Multiple Access with Collision Recovery (CSMA/CR). Before transmitting, a station listens for activity. If the channel is busy, it waits; if it hears a collision, it retries after a random pause.
Digipeaters boost range by grabbing and retransmitting frames. They tweak the address field to show which hops have been repeated.
In connectionless operation, UI frames go out with no acknowledgments, which is perfect for things like APRS. Connected mode adds acknowledgments, sequence numbers, and retransmissions for reliable delivery.
Transmission speeds tend to be low, so protocol overhead and timing really affect performance. Using SSIDs smartly and keeping repeater hops to a minimum can help throughput.
AX.25 Operation and Modes
AX.25 lets amateur radio stations exchange data in a few different ways, supporting both structured and more casual communication.
It forwards packets over multiple stations, checks for errors, and tweaks transmission to make the most of limited bandwidth.
Connected and Connectionless Modes
AX.25 can run in connected mode, where two stations set up a virtual circuit before sending data.
This makes sure packets arrive in order and get acknowledged, so it acts a lot like a reliable wired link.
In connectionless mode (also called UI frames), stations just send packets—no session needed.
You’ll see this mode in APRS, where stations send out short bursts of info and don’t expect a reply.
Connected mode works well for longer exchanges, like keyboard-to-keyboard chats or file transfers.
Connectionless mode keeps things fast and simple, so it’s good when you care more about speed than guaranteed delivery.
The choice really depends on whether you need reliability or just want efficiency.
Source Routing and Digipeating
AX.25 lets a station use source routing, so it can tell a packet exactly which path to take through intermediate nodes.
Digipeaters handle this—they receive, decode, and retransmit packets.
A digipeater acts as a simplex relay, giving you more range between stations that can’t reach each other directly.
The address field in the packet lists the digipeater call signs, and these get updated as the packet travels.
You can chain several digipeaters for multi-hop connections.
Each hop adds some delay and raises the risk of collisions, so picking efficient paths really matters.
People use source routing a lot in local and regional packet networks, especially where there’s no automatic routing.
Error Handling and Efficiency
AX.25 frames use HDLC-style framing with checksums to spot errors.
If a frame has an error, the receiver just drops it, and in connected mode, the sender tries again.
Version 2.2 brought in selective reject, so only missing frames get resent instead of all the unacknowledged ones.
It also allows bigger payloads and more sequence numbers, which helps keep up on faster links.
In connectionless mode, there’s no automatic retransmission, so apps have to deal with lost packets themselves.
Efficiency depends on things like payload size, hop count, and channel access timing.
Carrier sense multiple access with collision recovery (CSMA/CR) helps everyone share the channel and recover from collisions.
Interfacing Hardware and Software
For packet radio to work well, your hardware and software need to play nicely together. The interface you pick affects how well things run, what gear you can use, and what features you’ll get for digital radio networking.
Terminal Node Controllers (TNCs)
A Terminal Node Controller (TNC) takes digital data from a computer and turns it into audio tones for radio transmission. It also converts received audio back into digital data.
The TNC runs the AX.25 protocol at the data link layer. It manages packet formatting, checks for errors, and handles retransmissions.
TNCs come in two main types: hardware-based and software-based.
| Type | Example Devices/Software | Key Features |
|---|---|---|
| Hardware TNC | Kantronics KPC, Mobilinkd | Standalone, reliable, often portable |
| Software TNC | Direwolf | Runs on a PC, flexible, low cost |
You’ll usually connect hardware TNCs directly between the radio and your computer, often with a serial or USB cable.
Software TNCs use a computer’s sound card instead. They can add features like APRS digipeating or iGate support, which is honestly pretty handy.
Computer Integration and Linux AX.25
The Linux operating system supports the AX.25 protocol out of the box. That makes Linux a solid choice for packet radio networking.
With Linux, your computer can treat a radio link almost like any other network interface.
You can use programs like kissattach or tncattach to link a TNC to the Linux network stack.
- kissattach works with kernel AX.25 modules to create a network device.
- tncattach sends Ethernet frames over a TNC without needing special kernel modules.
Linux tools like axcall, axlisten, and netromd let you control and monitor AX.25 links directly.
This flexibility means you can integrate routing, email gateways, or APRS servers into your setup.
BPQ and Ethernet Connectivity
BPQ software acts as a packet switch and node controller. It supports AX.25, NET/ROM, and TCP/IP over radio.
You can run BPQ on Windows, Linux, or even embedded systems.
BPQ nodes link multiple ports—VHF packet, HF packet, and Ethernet all at once.
This setup bridges radio networks with wired IP networks.
Ethernet connectivity lets packet nodes form high-speed backbones.
In lots of cases, BPQ connects a radio TNC port to an Ethernet interface. That way, remote users can get into the packet network over LAN or the internet.
This approach stretches coverage and lets you mix RF and IP routing.
Applications and Protocol Extensions
AX.25 gives amateur radio operators a ton of flexibility, from position tracking to full network routing.
You can use it for direct station-to-station communication or hook into bigger digital networks with protocol extensions and gateways.
It adapts well to local, regional, and even global data exchange.
APRS and Real-Time Packet Applications
APRS (Automatic Packet Reporting System) uses AX.25 to send out short bursts of data. These packets usually contain location, status, or messages.
Stations often pair a GPS receiver with a radio and TNC to broadcast position reports.
A network of digipeaters repeats these packets over wide areas.
Igate stations forward them to internet-based services like aprs.fi.
This setup lets you track and message in real time between mobile, portable, and fixed stations.
APRS handles more than just location—it supports weather reporting, telemetry, and short text messages.
Popular software like Xastir, YAAC, and PinPoint APRS provides mapping and messaging tools.
Since APRS shares a channel, it uses short packets and timed transmissions to keep collisions down.
Networking with NET/ROM and TCP/IP
NET/ROM builds on AX.25 by adding a network layer for routing between nodes.
It lets stations forward packets across multiple hops without manual path setup.
This makes it easier to create regional or national packet networks.
AX.25 can also carry TCP/IP traffic for internet-style networking.
Early setups used SLIP or KISS framing with a TNC to connect packet radio to IP systems.
That enabled things like email, file transfers, and remote access over RF links.
Modern software such as Direwolf and tncattach makes TCP/IP over AX.25 easier by emulating network interfaces.
You don’t need special kernel modules anymore.
Radio bandwidth still limits throughput, but these setups work well for low-rate, resilient networking.
Role of Standards and Organizations
The American Radio Relay League (ARRL) and Tucson Amateur Packet Radio (TAPR) have shaped and maintained AX.25 specifications for years. Their publications and reference docs help different equipment and software actually work together, which is honestly pretty important.
TAPR has built hardware and firmware designs that support AX.25 and its extensions. This kind of hands-on work really encourages people to experiment and try new things.
ARRL gets involved too, making sure AX.25 lines up with amateur radio regulations. They also push for more technical education, which is always a good thing in this hobby.
These organizations keep protocol standards on track. Thanks to their work, APRS, NET/ROM, and TCP/IP can all play nicely together in the amateur radio world.