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Definition: ISDN
(Integrated Services Digital Network) An earlier international standard for switched, digital dial-up voice and data service. Analog phones and fax machines are converted into digital by the ISDN terminal adapter (see below).
Bearer Channels (B) and Delta Channel (D)
ISDN uses 64 Kbps B channels to carry voice and data and one D channel 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 network, ISDN calls are dialed much faster than analog calls. See SS7.
Basic Rate (BRI) and Primary Rate (PRI)
Connecting to ISDN
Typical SOHO Installation
Typical LAN Installation
Bearer Channels (B) and Delta Channel (D)
ISDN uses 64 Kbps B channels to carry voice and data and one D channel 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 network, ISDN calls are dialed much faster than analog calls. See SS7.
BRI uses one wire pair for two B channels (2B+D). Channels are often "bonded," for example, six B channels provide 384 Kbps for video calls. PRI uses four wire pairs for 23B+D, equivalent to a T1 line. Europe's 30B+D is like an E1 line. See T1 and E1.
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 adapter. In the U.S., the NT1 and TA are combined in one unit. The terminal adapter is called an "ISDN modem" if it contains an modem for analog phones and fax machines. If the ISP supports Multilink PPP (MLPPP), the TA can bond channels for faster access.
The ISDN modem allows a phone, fax and PC to communicate via ISDN.
LANs typically connect to ISDN via an ISDN router, which enables multiple users to share the channels. For Internet access, the router supplies temporary IP addresses to each nodes, and it may provide analog phone support.