Nortel 1000 Installation And Commissioning Manual page 166

166 Clock Controller description and installation
Figure 65
Example 6, Complex Isolated Private Network
Digital connections to the Central Office do not exist in this example. If it
does, the PBX connected to it will track off the CO and will in turn be used
as a clock source to other nodes.
When a second Digital loop from the Master system becomes available, it
can be used as a Secondary Clock Source in case the Primary Source fails.
Copyright © 2003-2007, Nortel Networks
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Nortel Communication Server 1000
ISDN Primary Rate Interface Installation and Commissioning
NN43001-301 02.03 Standard
Release 5.5 7 December 2007