Integrated Services Digital Network.
ISDN lines are connections that use ordinary phone lines to transmit digital instead of analog signals, allowing data to be transmitted at a much faster rate than with a traditional modem.

ISDN converts audio signals - your voice for instance - into digital bits. Since bits can be transmitted very quickly, you can get much faster speed out of the same telephone line - four times faster than a 14.4 kbps modem. In addition, ISDN connections are made up of two different channels, allowing two simultaneous "conversations" so you can speak on one channel and send a fax or connect to the Internet over another channel. All of these transactions occur on the same twisted-pair phone line currently plugged into your telephone. To find out if you can get ISDN, contact your local phone company or call around to a few local Internet service providers.

ISDN is a powerful tool for Internet communications, but it is not available everywhere. Traditionally, it has been used in urban business zones and large corporate settings with special digital switching equipment, but residential ISDN service is expanding rapidly. If you are shopping for an Internet access provider that offers you ISDN, be sure to thoroughly evaluate the equipment costs. An ISDN line can offer you inexpensive, high-bandwidth connections, but you may have to buy special equipment (like routers and switchers) that allow ISDN to communicate with your internal networks.