What is an Integrated Services Digital Network (ISDN)?

Integrated services digital network (ISDN) is a telecommunications protocol for transmitting both voice and data over traditional phone lines, often simultaneously. ISDN's predecessor, analog, had capacity limitations that became apparent with the advent of the internet and services such as voice over IP (VoIP) and video conferencing. ISDN solved some of these problems by providing more bandwidth and allowing, for example, multiple devices to use the same phone line.

There are two main forms of ISDN services, distinguished by very different amounts of bandwidth. Basic Rate Interface (BRI) is the lower cost service and offers around 128 kbps transmission speed. Primary Rate Interface (PRI) is the premium ISDN option, with a data transfer rate of 2,048 kbps.

With the growing availability of significantly faster voice and data connections, such as DSL and cable, ISDN is being phased out in the United States. However, some companies use PRI ISDN as backup in the event their other methods of connectivity go down.