Administrator's Guide

Features and technical reference
555-233-5061678 Issue 5 October 2002
Facility and Non-Facility Associated
Signaling
Facility Associated Signaling (FAS) allows an ISDN-PRI T1/E1 interface
D-channel to carry signaling information for all the bearer (B) channels on its
associated span.
Non-Facility Associated Signaling (NFAS) allows 1 ISDN-PRI T1/E1 interface
D-channel to carry signaling information for up to 300 bearer (B) channels on its
associated spans. In other words, a D-channel can carry signaling information for
numerous B-channels located on different DS1 circuit packs.
NOTE:
NFAS is only valid for T1/E1 Country Protocol 1. Digital T1 service is also
sometimes called “DS1” to distinguish it from analog T1 service.
ISDN-BRI trunks don’t support Non-Facility Associated Signaling.
Brief Description
D-Channel Backup with NFAS
When NFAS is used, a backup D-channel can be administered to improve
reliability. The system switches to the backup D-channel if a signaling link failure
occurs on the primary D-channel span.
One D-channel is administered as the Primary D-channel and another D-channel
is administered as the Secondary D-channel. These assignments ensure that both
D-channels are in the same state at the same time and neither can be used to carry
B-channel traffic at any time. The Primary D-channel is given precedence over the
Secondary D-channel.
When D-Channel Backup is activated, all calls that have been answered are
preserved. However, some call-related information may be lost. Calls that are not
answered when D-channels are switched also may lose call-related information.
The figure below shows a possible configuration involving 3 ISDN-PRIs between
a DEFINITY Server and another DEFINITY Server or the public network.