Nortel Communication Server 1000S Installation And Configuration Manual page 201

Clock Controller description and installation
Figure 61
Example 2, Isolated Private Network with Secondary Reference Clock
Small System
LD 60
TRCK FRUN
LD 73:
PREF <cr>
SREF <cr>
Master clock
Master Clock source:
Non-tracking (Free run)
In this example, there is no digital connection to the Central Office. For tie
lines between PBXs facilitated by a central office, clocking is derived from
the PBX, not the CO. When a second Digital loop is available, it can be used
as a Secondary Clock source in case the Primary Source fails.
ISDN Primary Rate Interface
loop X
loop Y
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Small System
LD 60
loop X
TRCK PCK
LD 73:
loop Y
PREF loop X
SREF loop Y
Slave
Clocking off Master:
Tracking on Primary
loop X and loop Y used
as a secondary clock
source
553-AAA0066
Installation and Configuration