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Primary Rate Interface equipment: overview
553-2901-200 Standard 11.00 August 1996
Clock operation
There are three types of clock operation: tracking mode, free run mode, and
holdover mode.
Tracking mode
In tracking mode, the PRI loop supplies an external clock reference to a Clock
Controller. Two PRI loops can operate in tracking mode, with one defined as
the primary reference source for clock synchronization, the other defined as
the secondary reference source. The secondary reference acts as a back-up to
the primary reference. Both the primary and secondary references are
assigned in LD 73. Each should have its own clocking source.
, a Meridian 1 system with dual CPUs can have two
Clock Controllers (CC-0 and CC-1). One Clock Controller acts as a back-up
to the other. The Clock Controllers should be completely locked to the
reference clock.
Free run (non-tracking) mode
The clock synchronization for a PRI loop can operate in free run mode if the
following conditions are true:
— The loop is not defined as the primary or secondary clock reference.
— The primary and secondary references are disabled.
— The primary and secondary references are in red alarm (local alarm).
Holdover mode
Holdover mode, available for vintages G and H, saves clock references,
remembering the last position and retaining it if system operation is
interrupted. Holdover mode maximizes synchronization accuracy.
Reference clock errors
Meridian 1 software checks every 15 minutes to see if a Clock Controller or
reference clock error has occurred.
In tracking mode, at any one time, there is one active Clock Controller, which
is tracking on one reference clock. If a Clock Controller error is detected, the
system switches to the back-up Clock Controller without affecting which
reference clock is being tracked.
