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ISDN (Integrated Services Digital Network)

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ISDN is an all-digital telephone system that was originally designed by the world's telephone companies and service providers as a replacement for the aging analog telephone system. What ISDN does is extend the telephone company's digital network into the local loop all the way to the subscriber. Traditionally, the local loop is an analog voice circuit.

ISDN is based on 64-Kbit/sec channels. These channels are just wide enough to handle a non-compressed digitized voice call. The three ISDN implementations listed here are based on rate.

  • BRI (Basic Rate ISDN)    The version of ISDN that is of most interest to consumers because it operates over the existing copper wire local loop to provide digital voice and data channels.

  • PRI (Primary Rate ISDN)    Organizations with a need for ISDN at higher data rates (for network connections) get involved with PRI. It basically provides additional channels as required up to a total of 23 B channels and one 64-Kbit/sec D channel for a total bandwidth that is equivalent to T1 (1.544 Mbits/sec).

  • B-ISDN (Broadband-ISDN)    The broadband version of ISDN designed to handle multimedia services in the carrier backbones. The B-ISDN architecture defines ATM (Asynchronous Transfer Mode) at the data link layer and so implements cell-based virtual circuits across the ATM network. SONET (Synchronous Optical Network) is the underlying physical network. See "B-ISDN (Broadband ISDN)."

The remainder of this section covers the consumer-oriented Basic Rate ISDN and Primary Rate ISDN. ISDN was one of the most well-funded projects in the history of the telecommunication industry, but it was slow to catch on. Obtaining ISDN service is still problematic in most areas, and the service is sometimes more expensive than competing services that offer better bandwidth. Competing services include DSL (Digital Subscriber Line) and cable modems.

ISDN Services and Connections

For businesses, ISDN can provide switched-circuit connections that are more practical than analog modems and more cost effective than leased lines if the bandwidth requirements vary. ISDN supports bandwidth on demand, as discussed later. The services take advantage of existing local loop cables, but give consumers direct access to the ISDN digital network created by the carriers. The ISDN circuit supports multiple devices at the same time via TDM (time division multiplexing).

In North America, the carrier installs an NT (network termination) device at the customer site. An NT1 device connects the customer with the telephone company's local loop. For home or small office connections (BRI), ISDN connects directly to the NT1. An NT2 supports the attachment of other devices, such as network switches, multiplexers, and PBXs.

The NT device provides a connection for TE (terminal equipment) and TA (terminal adapter) equipment to the local loop. TE devices are ISDN compatible, while TAs are devices that provide a connection point for non-ISDN equipment such as analog phones. A TA looks like an external modem, and usually has a jack for a telephone and a jack for a PC.

ISDN supports bandwidth on demand, an inverse multiplexing technique that combines multiple lines into a higher-bandwidth link when more bandwidth is required. For example, two 64K channels can be combined to provide a 128-Kbit/sec channel. With PRI, up to 23 channels can be combined as needed to provide a throughput rate up to 1,536 Mbits/sec (in the U.S.). This process can be automatic or programmed for specific times of the day. By switching circuits in and out, you only pay for the bandwidth you need. See "Inverse Multiplexing."

The ISDN D channel provides the signaling to set up calls. This signaling operates in the physical, data link, and network layers relative to the OSI protocol model. The protocols define message types that are exchanged between the customer equipment and the local exchange for setting up and maintaining calls.

Copyright (c) 2001 Tom Sheldon and Big Sur Multimedia.
All rights reserved under Pan American and International copyright conventions.