Digital Switching Systems DMS-250 ISDN Reference Manual MWC14 (CSP14)
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Introduction
ISDN is a digital communications network that provides users with multiple
services through a single access point, and integrates voice, data, video, and
messaging across the network. ISDN provides a variety of circuit-switched
and packet-switched features.
An ISDN switch offers customers two methods of accessing the voice and
data networks: basic rate interface (BRI) for line service, and primary rate
interface (PRI) for trunk service. Both interfaces consist of time-division
multiplexed, bidirectional channels known as Bearer (B), and Data (D)
channels.
Primary rate interface
PRI is an internationally standardized architecture for the interface between
a customer’s equipment and public network switches or other PBXs. PRI
provides the interface within a corporate network for basic call and network
business services. PRI allows PBXs, interexchange carriers, host computers,
and subtending server computers to be connected to the ISDN switch over
digital trunks.
PRI is a common channel signaling system consisting of:
• B-channels—bearing voice or data at a transmission rate of 64 kbit/s
• at least one D-channel—bearing out-of-band signaling with a
transmission rate of 56 or 64 kbit/s (two D-channels may be used for
redundancy purposes)
PRI is transmitted over a DS-1 trunk. The DS-1 is a collection of 24
imbedded DS-0 channels.
Each DS-0 can be used as one of the following:
• PRI B-channel
• PRI D-channel
• provisioned channel
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