AT&T MERLIN LEGEND Release 4.0 Feature Reference page 436

Feature Reference
In the event of a maintenance failure of primary synchronization, backup
synchronization can be provided by secondary and tertiary clock
synchronization.
In addition, the source of synchronization is factory-set to Loop Clock Reference
Source (the clock is synchronized to the external endpoint) or with a 100D
module it can be set to Local Clock Reference Source (the clock is
free-running). However, this is not recommended for most permanent
installations and systems with Primary Rate Interface (PRI). This setting must be
made for the primary, secondary, and tertiary synchronization sources.
The following lists the options for primary, secondary, and tertiary clock
synchronization sources in order of preference:
1. The clock sources on BRI ports with DSLs in service. If at all possible, all
three clock sources should be on the same 800 NI-BRI module.
2. The loop clock source on any 100D module.
3. The loop clock source on any 100D module in T1 mode emulating
tie-trunks.
4. The local clock source on any 100D module.
NOTE:
Ports which are not in service should never be administered as clock sources.
Clock Switching
When the primary clock source is not able to provide the system clock, the
secondary clock source is used if it exists and is capable of providing the
system clock. If the secondary clock source is incapable of providing the
system clock, the tertiary clock source is used. If none of these is capable of
providing the system clock the communications system will select a system
clock in the following order:
The communications system searches 800 NI-BRI and 100D modules for a
clock source starting from the first module in the system and ending with the
last module. The clock is chosen with the following order of preference.
1. Loop clock source on an 800 NI-BRI or 100D module.
2. Local clock source on an 800 NI-BRI or 100D module.
3. Local clock source on the processor module.
Mode Differences
Key mode
Routing by Dial Plan for PRI or T1 data lines is not supported in Key mode.
436
Primary Rate Interface (PRI) and T1