(Integrated Services Digital Network) An international telecommunications standard for providing a digital service from the customer's premises to the dial-up telephone network. ISDN turns one existing wire pair into two channels and four wire pairs into 23 channels for the delivery of voice, data or video. Unlike an analog modem, which converts digital signals into an equivalency in audio frequencies, ISDN deals only with digital transmission. Analog telephones and fax machines are used over ISDN lines, but their signals are converted into digital by the ISDN modem.


ISDN uses 64 Kbps circuit-switched channels, called "B channels" (bearer channels), to carry voice and data. It uses a separate D channel (delta channel) for control signals. 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 information about incoming calls, such as the identity of the caller. Since the D channel connects directly to the telephone system's SS7 signaling network, ISDN calls are dialed much faster than regular telephone calls.


ISDN's basic service is BRI (Basic Rate Interface), which is made up of two 64 Kbps B channels and one 16 Kbps D channel (2B+D). If both channels are combined into one, called "bonding," the total data rate becomes 128 Kbps and is four and a half times the bandwidth of a V.34 modem (28.8 Kbps)
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ISDN's high-speed service is PRI (Primary Rate Interface). It provides 23 B channels and one 64 Kbps D channel (23B+D), which is equivalent to the 24 channels of a T1 line. When several channels are bonded together, high data rates can be achieved. For example, it is common to bond six channels for quality videoconferencing at 384 Kbps. In Europe, PRI includes 30 B channels and one D channel, equivalent to an E1 line. 
Connecting an ISDN Device
Connecting ISDN to a computer requires a network terminator (NT1) and ISDN terminal adapter (TA). The NT1 plugs into the two-wire line from the telephone company with an RJ-11 connector and provides four-wire output to the TA. Within the U.S., the NT1 is typically built into the TA; but in Europe and Japan, they are separate devices.


Often called an "ISDN modem" because the device may support an analog telephone or fax machine, the TA itself is technically not a modem, because it provides a digital to digital connection. External TAs plug into the serial port while internal TAs plug into an expansion slot. Some TAs hook into the parallel port for higher speed. The TA may also include an analog modem and automatically switch between analog and digital depending on the type of call.TAs support bonding for Internet operation, which links the channels together for higher speed, but the ISP must provide the Multilink PPP protocol (MPPP) to support this operation.


Although announced in the mid-1980s, it took more than a decade before ISDN usage became widespread. When people asked when would it arrive, the answer was ISDN: "I Still Don't Know."
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.