Lucent Technologies 8.2 User Manual

Issue 1

April 2000

Features and technical reference 

1332

Facility and Non-Facility Associated Signaling 

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 ECS and another DEFINITY ECS or the public network. 

With T1 (24 channel) interfaces, 2 of the ISDN-PRIs contain a D-channel and 23 
B-channels, while the other ISDN-PRI contains 24 B-channels. One of the 
D-channels is the Primary D-channel, and the other is the Secondary D-channel. 

1. DEFINITY ECS

2. Secondary D-Channel

3. Primary D-Channel

4. ISDN-PRI controlled by D-Channel

5. Network Far-End Switch DEFINITY ECS

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