Terminal node controller

A terminal node controller (TNC) is a device used by amateur radio operators to participate in AX.25 packet radio networks. It is similar in function to the Packet Assembler/Disassemblers used on X.25 networks, with the addition of a modem to convert baseband digital signals to audio tones.

The TNC was originally developed by Doug Lockhart, VE7APU, of Vancouver, British Columbia, and popularized by the Tucson Amateur Packet Radio association with the TNC-1 and TNC-2.

A typical model consists of a microprocessor, a modem, and software (in EPROM) that implements the AX.25 protocol and provides a command line interface to the user. (Commonly, this software provides other functionality as well, such as a basic bulletin board system to receive messages while the operator is away.) Because the TNC contains all the intelligence needed to communicate over an AX.25 network, no external computer is required. All of the network's resources can be accessed using a dumb terminal.

The TNC connects to the terminal and a radio transceiver. Data from the terminal is formatted into AX.25 packets and modulated into audio signals (in traditional applications) for transmission by the radio. Received signals are demodulated, the data unformatted, and the output sent to the terminal for display. In addition to these functions, the TNC manages the radio channel according to guidelines in the AX.25 specification.

TNCs were uniquely necessary when home computers lacked the sophistication needed to simultaneously manage a network connection and communicate with the user. They are still used today throughout a very popular position reporting network known as Automatic Packet Reporting System (APRS) on 144.390 MHz (in Europe 144.800 MHz).

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