Nortel 1000 Installation And Commissioning Manual page 165

Figure 64
Example 5, digital connection to the CO
In this example, digital connections to the Central Office do exist. When a
second Digital loop from the CO becomes available, it can be used as a
Secondary Clock Source in case the Primary Source fails.
Slaves can track on each other as a secondary source since the chances of
both links to the Central Offices going down at the same time are minimal.
All Central Offices must have a path back to the same stratum 1 source.
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
Description 165