Nortel 1000 Installation And Commissioning Manual page 163

Figure 62
Example 3, Clocking Hierarchy referenced to a Public Network Master Clock
This is an example of a "STAR" arrangement— one Hub PBX is linked to
the Central Office and all other PBXs are connected as slaves. When a
second Digital loop from the system which forms the hub of this network
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
Description 163