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A terminal node controller (TNC) is a radio network device used to communicate with AX.25 packet radio networks. Typically this device consists of a dedicated microprocessor, a modem, flash memory and software that uses AX.25 protocol and provides a command line interface to the user. Typically TNC interfaces between a dumb computer terminal providing the data and a radio transceiver. The transceiver modulates and transmits the analogue radio signal containing the data provided by the TNC.
The TNC was originally developed by Doug Lockhart of Vancouver, British Columbia. TNC were popular devices used by amateur radio operators before personal computers had sufficient processing power and the sophistication needed to simultaneously manage a network connection and communicate with the user terminal.
Digital packet radio networks are composed of nodes that are connected to each other by radio link. The TNC manages data communication across the network. Data from the terminal (usually a PC) is formatted into AX.25 packets and modulated into audio signals for transmission by the radio. Received signals are demodulated, the data is unformatted and the output is 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. AX.25 is a data link layer protocol derived from the X.25 protocol suite and designed for use with amateur radio networks. AX.25 occupies the first, second and often the third layers of the OSI networking model, and is responsible for transferring data (encapsulated in packets) between nodes and detecting errors introduced by the communications channel.
TNC are still used in automatic packet reporting system (APRS) networks. These are amateur radio-based systems for real time communication of community alerts, news bulletins and other information of immediate value to the local area.