Packet radio and AX.25

What it is

Packet radio chops data into small, self-contained packets, each with an address, the data, and a check value, and sends them over a radio link. It is how amateurs adopted the ideas behind computer networking: several stations can share one channel, messages are addressed to specific stations, and errors are detected. Amateur packet uses a protocol called AX.25, an adaptation of an early network standard with call signs as the addresses.

Anatomy of a packet

Every packet starts with a header, which holds the routing and handling information: who it is from, who it is for, and any relay stations to pass through. Then comes the data, and then a checksum. The receiver recomputes the checksum: if it does not match, the packet was damaged.

In connected mode, the receiver acknowledges every good packet and asks for a resend of a damaged one. This is ARQ: error detected, retransmission requested. If it keeps failing, the link gives up and the connection is dropped. In unconnected mode a packet is sent once with no acknowledgement, which is how broadcasts and APRS work.

Digipeaters and networks

VHF and UHF signals travel in straight lines, so two stations separated by hills or distance may not hear each other. A digipeater (digital repeater) on high ground hears the packet and sends it again. The relay path is written in the packet header, so the stations choose the route, and each hop is another chance for the channel to be busy.

Beyond single hops, nodes can route traffic over several links, and bulletin-board systems (BBSs) store messages for stations to read later. A newer relative of the idea, high-speed amateur data networks built from Wi-Fi hardware, is covered in AREDN and mesh networks.

In practice

• A TNC is the modem that converts packets to audio and back. Hardware boxes did this originally; software running on a computer's sound card can now do the same job.
• HF packet exists, but is slower and more fragile, since HF noise and fading cause many retries. Packet's role there has largely been taken by newer modes (see VARA, ARDOP and PACTOR).
• Today most packet traffic is not keyboard chat but APRS beacons and radio email access through Winlink and radio email nodes.

Related

• APRSPosition and message reporting.
• Winlink and radio emailEmail over HF and VHF.
• VARA, ARDOP and PACTORHigh-speed ARQ data modes.
• Digital modes overviewThe landscape of data modes and what each is for.
• Error detection and correctionParity, CRC and forward error correction.
• AREDN and mesh networksHigh-speed data on amateur spectrum.