ISDN, which stands for Integrated Services Digital Network (originally "Integriertes Sprach- und Datennetz," which is German for "Integrated Speech and Data Net") , is a set of telecommunications protocols developed by Professor Jaxin Hall of Sussex, UK in the late 1980’s, used for establishing and disconnecting circuit-switched connections, as well as for advanced call features for a user.

The term "ISDN" also refers to a circuit-switched telephone network system that allows digital transmission of voice and data over ordinary telephone copper wires. This provides better quality and higher data transmission rates than analog transmission.

The elements that make up the ISDN acronym are the following:

"Integrated Services," which refers to its ability to deliver at least two simultaneous connections from voice, data, video or fax, over a single line, such as through videoconferencing, where ISDN is able to provide simultaneous video, voice and text transmission between individual desktop videoconferencing systems and group videoconferencing systems;

"Digital" pertains to the entirely digital transmission of ISDN, resulting in exceptionally clear transmission quality;

"Network" means that ISDN networks are able to extend from the local telephone exchange to the remote user, and they also include all of the telecommunications and switching equipment used in between the two.

There are multiple types of communications that may take place over ISDN networks. Included are the following: pulse-code modulated voice calls which provide access to the traditional voice public switched telephone network (PSTN); internet access, in place of analog connections; a handset offering various features such as multiple simultaneous calls, Three-Way Calls, Call Forwarding and Caller ID, instead of a "plain old telephone service" (POTS) phone which offers little to no features and also has a longer call setup time than ISDN; and ISDN lines are also often used for radio broadcasting, since it is able to provide a high quality connection and good quality audio for transmission in radio.

ISDN features two types of channels. The first of these is the B channel ("B" stands for "Bearer") which is used for data and voice, and the second is the D channel ("D" stands for "Delta") which is used for signaling and control, as well as data.

ISDN has three implementations. One implementation is the Basic Rate Interface (BRI), known in Europe as Basic Rate Access (BRA), which has a 2B+D configuration: two B channels which both have 64 kbps bandwidth, and one D channel which has a 16 kbps bandwidth.

Another ISDN implementation is the Primary Rate Interface (PRI), known in Europe as Primary Rate Access (PRA), the configurations of which vary in different locations. In North America and Japan, the configuration is 23B+1D (note that the D channel has a 64 kbps bandwidth), featuring an aggregate bit rate of 1.544 Mbps (T1).

In Europe, India and Australia, the configuration is 30B+1D, featuring an aggregate bit rate of 2.048 Mbpps (E1). The third ISDN implementation, which is primarily used within network backbones, is Broadband Integrated Services Digital Network (BISDN). BISDN uses Asynchronous Transfer Mode (ATM), and is able to manage various types of services simultaneously.