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B-ISDN (Broadband ISDN)

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ISDN (Integrated Services Digital Network) development began in the 1980s as part of a plan by the carriers to develop a single integrated network transmitting voice, video, and data communications. B-ISDN development began in 1988 as developers saw a need for bandwidth above 155 Mbits/sec in anticipation of future video and multimedia services. ATM was developed from this work as the underlying transport, with B-ISDN being the overlying technology for controlling the network. Figure B-10 illustrates the B-ISDN architecture. ATM cells are delivered across a physical SONET network.

[Figure 10: See book]

In general, ISDN is based on 64-Kbit/sec channels. These channels are just wide enough to handle a digitized voice call. All other ISDN implementations are some multiple of this basic service. A typical user signs up for basic rate ISDN, which consists of two 64-Kbit/sec channels (called the B channels) and one 16-Kbit/sec D channel used for signaling. The B channels can be used individually for separate voice calls or combined to create a 128-Kbit/sec data channel. Primary rate ISDN consists of 23 B channels and one 64-Kbit/sec signaling channel. ISDN is useful for networking because the customer can add bandwidth in increments of 64 Kbits/sec as needed.

ISDN is a switched service and is designed around intelligent switching components in the carrier network. It allows users to dial any other ISDN point on the network and create a high-speed digital link that can mimic point-to-point T1 lines. However, T1 lines are usually permanently established between two points, while ISDN allows the switching of the line between many different points by the customer. This is due to the intelligent network components. The intelligent network also supports a variety of other services, including call forwarding, caller ID, and channel bonding.

B-ISDN provides the intelligent telecommunications services above ATM. It manages the establishment of point-to-point and point-to-multipoint connections through the switched network. It supports on-demand, reserved, and permanent services, as well as connection-oriented and connectionless services. The carriers had big plans to use B-ISDN for services like videotelephony, videoconferencing, electronic newspapers, and TV distribution. Now, B-ISDN is rarely discussed. All you hear about is ATM.

SONET is the physical transport backbone of B-ISDN. It is a fiber-optic-based networking standard that defines a hierarchy of transmission rates and data-framing formats. It is used as a transmission medium to interconnect carrier-switching offices worldwide, and so forms the structure of the communications network. SONET is now used as the medium between carrier-switching offices and customers. SONET transmission rates start at 51.4 Mbits/sec and increase in 52-Mbit/sec building blocks.

ATM is the switching technology for B-ISDN and provides B-ISDN users access to the SONET fiber-optic network. Information received at the ATM layer is placed in fixed-length cells, addressed, and transmitted over the SONET network. ATM provides very high-speed switching of these packets between the links attached to the SONET network. ATM takes full advantage of the transmission speeds available on fiber-optic cable.

Note that neither ISDN nor B-ISDN has gained the popularity envisioned by the carriers. IP networking has gained in popularity and the latest trend is to build converged networks where voice travels in packets. See "NPN (New Public Network)," "Voice/Data Networks," and "Voice over IP (VoIP)."

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