First defined in 1988, ISDN is short for Integrated Services Digital Network, a collection of communications standards for the delivery of data, voice, video and other information and services over the circuits of the public switched telephone network. The key innovation of ISDN over previous communications protocols is that it allowed voice and data to be carried over the same lines, adding functionality and capabilities that were not present in the traditional telephonic system.

A circuit-switched telephonic network, ISDN also offers access to packet-switched networks, allowing digital transmission of data and voice over standard copper telephone lines, which offers in better sound quality than an analog phone system is capable of providing. ISDN offers connections for both voice and data in increments of 64 kilobits per second.

In some countries, ISDN remains a major source of Internet access, although in most developed nations, more sophisticated systems have largely supplanted ISDN, starting with the widespread adoption of Digital Subscriber Line (DSL) technology. ISDN remains in heavy use in the broadcast industry, however, where it is used as a reliable means of switching low latency, high quality, long distance audio circuits, and via satellite as a means of communication for field reporters operating in isolated areas.