ISDN controllers represent the cheapest and simplest way of achieving ISDN data communication. They can be obtained for most bus systems and hardware platforms. However, the most common form is as ISA insert cards for PCs. The ISDN speed is considerably less than that of today's bus systems. Consequently, PC insert cards are more than sufficient for what in today's terms is the relatively slow ISA bus. Cards for the PCI bus, on the other hand, are far easier to install.

How an ISDN card is used depends on whether the data communication is entirely digitised between the two ISDN end-devices, or whether one of them is connected to the analogue telephone network. Each of these possibilities is discussed below.

• Communication with Digital Remote Stations