We could replace the internal modem with an internal ISDN adapter and a driver which appears to the application software to be a modem. In reality, this driver combines the functions of both the modem and the CODEC. This driver is known as a soft modem.

A soft modem requires a lot of processing power, since it has to operate in real time. The PC is therefore likely to appear a bit sluggish while the connection is active. The advantage of this solution is that it can be used with an inexpensive passive ISDN adapter.