ISDN = Integrated Services Digital Network.

One of the fastest commercially available connections to the Internet, it is a set of communications standards that enables a single wire (or optical fibre) to carry voice, data, and video. It gives a user up to 56K of data bandwidth on a phone line (when also used for voice) or up to 128 Kbps if the line is only used for data. ISDN is rapidly becoming available to much of the USA and in most markets it is priced very comparably to standard analog phone circuits

ISDN is intended to replace eventually the "plain old telephone system" (POTS), and it mostly uses existing Public Switched Telephone Network (PSTN) switches and wiring, upgraded so that the basic "call" is carried on a 64 Kbps channel that's all digital, from end to end. Packet and frame modes are also provided in some places.

There are two types of ISDN:

• Basic Rate Interface (BRI) - consists of two 64-Kbps B-channels and one D-channel for transmitting control information.
• Primary Rate Interface (PRI) - consists of 23 B-channels and one D-channel (U.S.) or 30 B-channels and one D-channel (Europe).

The original version of ISDN employs baseband transmission. Another version, called B-ISDN, uses broadband transmission and is able to support transmission rates of 1.5 Mbps. B-ISDN requires fiber optic cables and is not widely available.