ISDN and B-ISDN

ISDN and B-ISDN

Integrated Services Digital Network (ISDN) is a group of ITU (CCITT) standards designed to provide voice, video, and data-transmission services on digital telephone networks. ISDN uses multiplexing to support multiple channels on high-bandwidth circuits. The relationship of the ISDN protocols to the OSI reference model is shown in Figure 7.10.

The original idea behind ISDN was to enable existing phone lines to carry digital communications. Thus, ISDN is more like traditional telephone service than some of the other WAN services discussed in this chapter. ISDN is intended as a dial-up service and not as a permanent, 24-hour connection.

ISDN separates the bandwidth into channels (see the following note for more information). Basic ISDN uses three channels. Two channels (called B channels) carry the digital data at 64 Kbps. A third channel (called the D channel) provides link and signaling information at 16 Kbps. Basic Rate ISDN thus is referred to as 2B+D. A single PC transmitting through ISDN can use both B channels simultaneously, providing a maximum data rate of 128 Kbps (or higher with compression). The larger-scale Primary Rate ISDN supports 23 64 Kbps B channels and one 64 Kbps D channel.

A variety of ISDN channel types are defined. These channel types, often called bit pipes, provide different types and levels of service. The following list details the various channels:

A channel.	Provides 4 KHz analog telephone service.
B channels.	Support 64 Kbps digital data.
C channels.	Support 8 or 16 Kbps digital data, generally for out-of-band signaling.
D channels.	Support 16 or 64 Kbps digital data, also for out-of-band signaling.

D channels support the following subchannels:

p subchannels support low-bandwidth packet data.
s subchannels are used for signaling (such as call setup).
t subchannels support telemetry data (such as utility meters).

E channels.	Provide 64 Kbps service used for internal ISDN signaling.
H channels.	Provide 384, 1,536, or 1,920 Kbps digital service.

ISDN functions as a data-transmission service only. Acknowledged, connectionless, full-duplex service is provided at the Data Link layer by the LAPD protocol, which operates on the D channel.

Broadband ISDN (B-ISDN) is a refinement of ISDN that is defined to support higher-bandwidth applications, such as video, imaging, and multimedia. Physical layer support for B-ISDN is provided by Asynchronous Transfer Mode (ATM) and the Synchronous Optical Network (SONET), discussed later in this chapter. Typical B-ISDN data rates are 51 Mbps, 155 Mbps, and 622 Mbps over fiber-optic media.