ISDN

ISDN, in full integrated services digital network, all-digital high-speed network provided by telephone carriers that allowed 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 ran on ordinary copper wire, which lowered the cost but also lowered the speed. The ISDN standard divided a telephone line into separate data channels, which, along with a slower signaling channel, could be grouped together into “interfaces” for more speed. The two main interfaces were basic rate interface (BRI), which used up to two data channels and was meant for home users, and primary rate interface (PRI), which used up to 23 channels (up to 30 in Europe) and was meant for businesses that needed 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 widely supplanted ISDN for home users, though some organizations still used ISDN. For example, many radio stations relied on ISDN lines to connect studios because of their clear sound quality. In many countries ISDN services were decommissioned in the 2010s and ’20s.