In telecommunications technology, the abbreviation ISDN stands for the technical term "Integrated Services Digital Network" and refers to a digital standard for telephone networks. As the name implies, various types of communication services can be transmitted through this kind of network. In addition to voice calls, ISDN allows packet-switched or circuit-switched data transmission and fax transmission.

In some countries, ISDN technology has taken over from analogue telephone networks, integrating all types of services into a common network for the first time. This digitisation has also enabled digital ISDN phone lines to be made available to end users.

In comparison to an analogue connection, an ISDN subscriber line offers a whole range of advantages. The basic ISDN line allows multiple phones or communication devices, with different numbers (MSNs), to operate and use two parallel connections simultaneously. In addition, the ISDN connection also offers higher data transfer rates to an analogue connection using a modem, for example, when accessing the Internet.

To utilise the ISDN line, end users need special ISDN-capable phones or faxes. The use of so-called A/B converters, linking analogue devices to a digital

connection, is common. Meanwhile, other techniques such as DSL or cable connections are used for connecting to the internet. Nowadays, phone technology is evolving away from ISDN and towards IP telephony via Voice over IP (VoIP).