Worksheet 4 - ISDN Networking

There are two types of ISDN service available throughout the world: Basic Rate Interface and Primary Rate Interface. BRI service is by far the most common and is typically found in homes and businesses alike. The higher capacity PRI service is a central-site solution for extending applications to large numbers of remote users, who communicate through their BRI connections.

In BRI service at an office or a home, the ISDN line consists of the same twisted pair of wires traditionally used for analog telephones. Numerous ISDN devices can connect to this single line.
The BRI line provides two types of ISDN communications channels: two "bearer service" B-channels, which carry data and services at 64 Kbps (or combine for 128 Kbps), and a single, 16 Kbps D-channel, which usually carries signaling and administrative information used to set up and terminate calls. Up to eight ISDN devices can be connected to a single BRI line, and can all share the B-channels and D-channel. Individual devices are distinguished through the use of multiple subscriber numbers, with a different ISDN number assigned to each device. D-channel signals automatically route communications to the appropriate ISDN device. Although only two B-channels are available at any point in time, numerous other calls may be put "on hold" via D-channel signaling, a feature referred to as "multiple call appearances".

ISDN PRI includes 23 B-channels in North America and Japan (30 B-channels in most of the rest of the world), plus one 64 Kbps D-channel. The number of B-channels is limited by the size of the standard trunk line, which is T1 in North America and Japan and E1 elsewhere. Unlike BRI, PRI does not support a bus configuration, and only one device can be connected to a PRI line. A PBX, however, can reallocate ISDN PRI resources onto multiple BRI buses.
A single PRI connection is usually much less expensive than obtaining the equivalent number of B-channels through multiple BRI connections. The primary benefit of PRI is that its bandwidth can be dynamically allocated among applications. For example, certain channels can be allocated for voice calls, but as those calls are terminated, the unused B-channels can be reallocated to such high-bandwidth applications as videoconferencing. This is usually accomplished via a PBX or a server capable of distributing the T1/E1 bandwidth on a PRI link.

With its BRI and PRI services, ISDN has the flexibility to meet the bandwidth needs of a home office, branch office, or company headquarters. A small office can use ISDN BRI to support all of its voice and data communications requirements. In a larger office, multiple ISDN BRI lines can be divided among multiple users and applications via a server or PBX. And users in a very large office can benefit from ISDN PRI's capacity to provide a large quantity of multiple B-channels.

Analog vs. digital ISDN Connections

Note: The Worksheet 5 contains detailed information on B-channels and D-channels. Thus is a logical continuation of this worksheet.

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