'ISDN' stands for Integrated Services Digital Network, a smart network technology intended to add new services to the PSTN by giving users direct access to end-to-end circuit-switched digital services.
There are two kinds of access to ISDN: 'Basic Rate' consisting of two 64 kbps digital data channels and a digital signalling channel, and 'Primary Rate' consisting of an E1 (or T-1 in the United States) containing a greater number of channels.
Calls are made over the data channels, with the signalling channels used for call setup and management. Once a call is set up, there is a simple 64 kbps synchronous bidirectional data channel between the users, lasting until the call is terminated. There can be as many calls as there are data channels, to the same or different end-points.
Amongst the kinds of data that can be moved over the 64 kbps channels are pulse-code modulated voice calls, providing access to the traditional voice PSTN. This information can be passed between the network and the user end-point at call set-up time.
ISDN is nowadays mostly used as an alternative to analog modems, most commonly for Internet access. The 'smart services' envisaged as being delivered over ISDN are now delivered over the Internet instead.