all-digital high-speed network provided by telephone carriers that allows voice and data to be carried over existing telephone circuits.

In the early 1980s ISDN was developed as an offshoot of efforts to upgrade the telephone network from analog to digital using fibre optics. The expense of connecting every home with fibre-optic cables, however, led to changes in the ISDN standard. ISDN runs on ordinary copper wire, which lowers the cost but also lowers the speed. The ISDN standard divides a telephone line into separate data channels, which, along with a slower signaling channel, can be grouped together into “interfaces” for more speed. The two main interfaces are basic rate interface (BRI), which uses up to two data channels and is meant for home users, and primary rate interface (PRI), which uses up to 23 channels (up to 30 in Europe) and is meant for businesses that need more speed.

Despite its advantages over standard dial-up—faster speed and the ability to employ telephones and computers on the user's network at the same time—telephone companies had trouble selling ISDN services, which were much more expensive than dial-up. Because of this, there were only about one million ISDN users in the United States five years after it became available. Another problem for ISDN was the growth of digital subscriber line (DSL) and asymmetric digital subscriber line (ADSL) over copper telephone wires and the introduction of Internet services by cable television providers. These technologies were rolled out much faster and have since widely supplanted ISDN for home users, though some organizations still use ISDN. For example, many radio stations rely on ISDN lines to connect studios because of their clear sound quality.