Specifications

Reference Guide
2-23
Applications ISDN Primary Rate Interface (PRI) Server
•
Optional ISDN ‘U’ Interfaces
Three ISDN PRI Server Cards are supported:
•
8840A ISDN PRI Server Card with 1 D Channel
•
8840B ISDN PRI Server Card with 2 D Channels
•
8840C ISDN PRI Server Card with 8 D Channels
The cards provide flexible access to PRI-based ISDN services such as Switched 384, Switched
T1, and Switched 56/64. The PRI Server Card (PRI card) provides both local D channel
origination and termination and D channel consolidation. The PRI Server card is available in
three different software versions based on the number of D channels (1, 2, or 8) to be
supported. It offers a perfect alternative to standalone ISDN access devices when other
“non-ISDN” voice and data services must be consolidated in addition to ISDN services.
The 8840B and 8840C ISDN cards can be simultaneously connected to several network and
user side ISDN PRI facilities. The ISDN PRI card provides D channel support of both the
network side protocol and the user side protocol. Typically, the IAD with an ISDN PRI card,
supplies the network side protocol on a D channel connected to a PBX, and provides the user
side protocol on the D channel connected to a carrier switch.
The 8840C ISDN PRI server card, which supports eight (8) D-channels, can be configured to
route calls from a PBX, multiple PBXs, and DTE devices to multiple ISDN service providers.
services.
The 8840x ISDN PRI Server does not support BRI-to-PRI translation. This application is
supported by the 65100 ISDN BRI-PRI translation software running on an 8811xx ACS card.
2.6.1 NFAS (Non-Facility Associated Signaling)
All ISDN PRI cards can be configured to support NFAS (Non-Facility Associated Signaling).
The limit for NFAS is 191B+D in T1 environments (8 times 2 minus 1 D channel) and 239
B+D in E1 environments (8 times 30 time slots minus 1 D channel).
A basic ISDN PRI facility is a T1 link that consists of 23 B channels and 1 D channel
(23B+D), or an E1 link that consists of 30 B channels and 1 D channel (30B+D). Note that
one time slot on any E1 link is reserved for maintenance use and is neither a B channel nor a
D channel. The D channel provides signaling for all of the (23 or 30) B channels on the facility
carrying that D channel.
However, many ISDN applications have relatively low call rates (i.e., the D channel is not
very busy), but need more than 23 (or 30) B channels to carry user (bearer) traffic. In these
cases, a D channel can be set up to perform signaling not only for the B channels on its own
facility, but also for B channels on other facilities (i.e., other T1/E1 WAN links). When a D
channel is so provisioned, it is considered to be performing non-facility associated signaling