Definition of: ISDN

(Integrated Services Digital Network) An international standard for switched, digital dial-up telephone service for voice and data. Analog telephones and fax machines are used over ISDN lines, but their signals are converted into digital by the ISDN terminal adapter (see below).

Although announced in the early 1980s, it took more than a decade before ISDN became widely available. It enjoyed a surge of growth in the early days of the Internet, because it provided the only higher-speed alternative to analog modems in many areas. Still working in many behind-the-scenes applications, ISDN is rarely used for Internet access today.

ISDN uses 64 Kbps "B" (bearer) channels to carry voice and data. A separate "D" (delta) channel is used for control. The D channel signals the carrier's voice switch to make calls, put them on hold and activate features such as conference calling and call forwarding. It also receives caller ID data. Because the D channel connects directly to the telephone system's SS7 signaling network, ISDN calls are dialed much faster than regular telephone calls.

Basic Service (BRI)
ISDN BRI (Basic Rate Interface) uses one wire pair to carry two 64 Kbps B channels and one 16 Kbps D channel (2B+D). Both B channels are often "bonded" into one, providing a total data rate of 128 Kbps.

Higher Capacity (PRI)
ISDN PRI (Primary Rate Interface) uses four wire pairs to provide 23 B channels and one 64 Kbps D channel (23B+D). A PRI line is equivalent to a 24-channel T1 line. Bonding channels is common; for example, six channels provide 384 Kbps for high-quality videoconferencing. In Europe, PRI includes 30 B channels and one D channel, equivalent to an E1 line. See SS7 and T1.

Connecting to ISDN
ISDN requires a network terminator (NT1) and terminal adapter (TA). The NT1 plugs into the two-wire line from the telephone company and provides four-wire output to the terminal adapter. In the U.S., the NT1 and TA are typically combined in one unit, but are separate in Europe and Japan.

The terminal adapter is called an "ISDN modem" if it has a built-in analog modem to hook up regular telephones and fax machines. If the ISP supports the Multilink PPP (MPPP) protocol, the TA can bond channels for faster Internet access.

The Terminal Adapter
TAs in the U.S. generally have a built-in NT1 and attach via a two-wire "U" interface. In Europe and Japan, the NT1 is installed by the telephone company and attaches to the TA via a four-wire "T" interface. The NT2 component, which is built into most devices, is a logical interface for multiple access and attaches via the "S" interface.

Typical SOHO Installation
An ISDN terminal adapter with phone support (ISDN modem) allows a telephone, fax machine and PC to communicate via the ISDN service.

Typical LAN Installation
LANs typically connect to ISDN via a router, which enables multiple users to share the available channels. For Internet access, the router supplies temporary IP addresses to each of the nodes. Routers may also provide analog phone support.