A Terminal Node Controller (TNC) is a device used in amateur radio and packet radio networks to interface a computer with a radio transceiver. It is a type of modem specifically designed to facilitate digital communication between computers over radio frequencies. The TNC is responsible for encoding digital data into audio signals that can be transmitted over the radio waves and decoding received audio signals back into digital data for the computer to process.

Key features and functions of a Terminal Node Controller include:

1. Packet Radio Communication: TNCs are primarily used for packet radio communication, which involves sending data in discrete packets over radio frequencies. This method allows for efficient and error-resistant data transmission, making it suitable for various applications such as email, file transfer, and remote control.
2. Data Encoding and Decoding: The TNC converts digital data from a computer into audio tones that can be transmitted over the radio. On the receiving end, it decodes the audio tones back into digital data for the computer to process.
3. Modulation and Demodulation: TNCs use modulation techniques to modulate digital data onto carrier signals for transmission and demodulation techniques to recover the digital data from received carrier signals.
4. AX.25 Protocol: TNCs often use the AX.25 protocol, a data link layer protocol derived from the X.25 protocol, for communication between nodes in a packet radio network. AX.25 defines packet formats, addressing, error checking, and flow control mechanisms.
5. Connection to Radio Transceiver: TNCs are connected to a radio transceiver, allowing them to transmit and receive data over the airwaves. They often connect to the transceiver’s microphone and speaker jacks to send and receive audio signals.
6. Dedicated Hardware or Software: TNCs can be standalone hardware devices or software-based implementations running on a computer. Software TNCs are often used in conjunction with a sound card for audio processing.
7. Packet Routing and Digipeating: TNCs can support features like packet routing and digipeating, allowing them to relay packets between different nodes in the packet radio network.
8. Error Correction: TNCs may incorporate error correction techniques to enhance the reliability of data transmission over potentially noisy radio channels.

TNCs have been widely used in amateur radio and emergency communication scenarios, where the ability to establish digital communication over long distances without the need for internet infrastructure is valuable. While TNCs were more prominent in the past, advancements in digital communication technologies have led to the development of alternative methods for connecting computers and radios, such as software-defined radios (SDRs) and direct computer-to-radio interfaces.