ISDN is a communications standard for sending voice, video, and data over digital telephone lines or telephone wires. It supports data transfer rates of 64 Kbps and can support two lines at once (calles B channels), which allow you to use both lines to transfer data; this gives you data rates of 128 Kbps.
Technology to completely digitize the phone service that was abandoned after much work (it began in the early 1980s) in the early 1990s becayse it was too expensive. It is currently used for high-speed Internet access, slower than T1 but faster than a modem. It is widely used by phone networks in some areas.