Amateur radio packet data communications has been done primarily with dedicated microprocessor systems called Terminal Node Controllers (TNCs). The TNC is typically connected to a host computer or terminal with an RS-232 DCE (Data Communications Equipment) connector. This allows any host with asynchronous serial communications capability (e.g. a personal computer or a mainframe) to use the TNC as if it were a conventional telephone mode. A TNC handles all necessary channel protocols, and since they are often difficult to implement on personal computers which do not provide real-time multitasking operating systems, this approach allows the use of packet radio in a much wider variety of applications.
A TNC is generally used to implement the chosen media access protocols (explained in previous section) in an amateur packet radio network. It may also implement higher layers protocols such as AX.25 and a simple PMS (Personal Messaging System), or running in KISS mode (see appendix A) may off-load these functions to another system.