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ISDN-PRI Interface

Integrated Services Digital Network (ISDN) provides integrated voice and data over a common

telephone company facility. There are two methods of ISDN access currently available: ISDN–BRI

(basic) and ISDN–PRI (primary). ISDN–BRI provides a way for subscribers to get digital service

from the telephone company supplier for two telephones or a telephone and a data device over one

special pair of wires. ISDN–PRI provides a way for subscribers to get digital service from the

telephone company supplier for 24 lines over two special pairs of wires. This latter method is the

concern of this publication.

ISDN–PRI access directly connects a multiple-station, digital communications system to the

telephone company’s ISDN network through a single high-speed line. This high-speed line is

commonly known as T1 in North America and consists of 24 digitized channels. ISDN–PRI access

consists of a grouping of 23 voice channels (B–channels) and one signaling or control channel

(D–channel). B–channels are clear channels suitable for voice or data transmission while the

D–channel is a packet-switched link for call set-up and signaling. When you use ISDN–PRI access

with T1, one of the T1 channels serves as the signaling channel. This arrangement is sometimes

referred to as 23B+D ISDN service.

The DXPRI is a circuit card that you install on an FXT1 line board with special-purpose firmware to

provide ISDN–PRI access to the system. The DXPRI card handles the D–channel signaling protocol

and passes messages to the FXT1 board. A single DXPRI card can control multiple FXT1 boards

when you tie the boards together through board programming action. This arrangement (known as

NFAS or Network Facility Associated Signaling) allows the system to derive 24 B–channels from all

boards except the one where installers have added the DXPRI card. When installers do this, the

FXT1/DXPRI combination that provides the control is designated as FXT1-PRI; and the boards

being controlled are designated as FXT1-NFAS. The NFAS boards do not require a DXPRI card as

they are slaves to the master –PRI board. Throughout the remainder of this discussion, the term

T1/PRI means a T1 board specifically configured for ISDN–PRI operation be it either an FXT1-PRI

or an FXT1-NFAS board.

Since central office protocols differ, the DXPRI card supports several protocol arrangements and

provides interface support for several signaling types. The installer and the end user can select the

interface support that the system needs for a particular application. The support they choose

determines how the telephone company treats the signal.

The DXPRI card provides a calling number identification feature that provides the following

services: delivery of network provided calling numbers, calling line identification presentation

(CLIP), or automatic number identification (ANI). On originated calls, the system automatically

provides the calling number if available; otherwise, it provides the default directory number. For

incoming calls, the called station displays the calling number if available; otherwise, the display

shows information determined by station class of service programming. The system installer can

customize the caller ID parameters for outgoing calls as site requirements dictate.

Also see the discussion titled Channel Negotiations.

Introducing the T1/PRI Line Board

The FXT1 board serves as host for the DXPRI card and gives the digital communications system the

capability to handle up to 24 channels of voice and/or data transmissions over a single four-wire

cable using multiplexing techniques. The FXT1 board includes a customer service unit (CSU) to

eliminate the need for an external CSU.

Understanding the Features – 111

General Description GCA40–242

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Comdial FX Series Especificaciones Pagina 115

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Comdial FX Series Especificaciones Pagina 115

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