Communication between existing applications is generally adapted to serial analogue Modems, which recognise the AT command set. To use ISDN, in such cases, the application of an ISDN terminal adapter with a serial interface is recommended. Since this is an external device with a serial RS-232 port, it can be used as a direct replacement for an analogue modem. Similarly to the case for analogue modems, control is performed using the extended AT instruction set.

For this reason, ISDN terminal adapters are suitable in principle, wherever data is to be transferred from the serial interface via ISDN. Examples of applications include workstations without an internal ISDN controller, as well as remote bridges and router with a serial interface. ISDN terminal adapters are also suitable for use with serial communication servers to implement high performance remote access applications via ISDN. Note that not every serial interface is appropriate for the high transmission rate of 64000 bit/s (up to 256000 bit/s or more with channel bundling/data compression) used in ISDN.
Refer to the technical specifications for particular interface for more detailed information.

In any case, a serial interface with an integrated data buffer (UART 16550 compatible interfaces) will prove beneficial. Furthermore, it should be mentioned that most ISDN terminal adapters can alternatively be operated with a CAPI interface in addition to the AT command set.