The key characteristic of ISDN is that it is a digital network. However, many of the devices and networks with which an ISDN user needs to communicate are not digital but analogue. In order for these two types of device to communicate, the information that they are exchanging must be converted from one form to the other.

In fact, except for data calls between computers to across the ISDN network, almost all other types of calls - voice, fax, modems - will all involve some kind of conversion from digital to analogue, or vice versa.

Much of this conversion takes place without the user's knowledge or intervention and is handled by the networks and devices involved. However, there are instances where an understanding of what is involved will assist in making successful connections and diagnosing problem areas.

You need to pay careful attention to the requirements of the ISDN device in use, particularly when sending and receiving faxes. This section provides the background to the various scenarios involved, and the practical implications for the different types of ISDN device that are available.