Specifications
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 setup 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 FXT1-PRI is a circuit card that you install on an FXT line board with special-purpose firmware to provide
ISDN-PRI access to the system. The FXT1-PRI card handles the D-channel signaling protocol and passes
messages to the FXT1-PRI board. A single FXT1-PRI card can control multiple FXT line 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 FXT1-PRI card. When installers do this, the FXT1-PRI 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 FXT1-PRI 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 FXT1-PRI 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 FXT1-PRI 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 FXT1-PRI 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.
6 – 58 General Description
GCA40–215 General Description