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 backup clock
controller, without affecting which reference clock is being tracked.
A reference-clock error occurs when there is a problem with the clock driver or with the
reference system clock at the far end. If the clock controller detects a reference-clock error,
the reference clocks are switched.
Automatic clock recovery
An option for automatic clock recovery can be selected in LD 60 with the command EREF
.
A PRI loop is disabled when it enters a local alarm condition. If the local alarm is cleared, the
loop is enabled automatically. When the loop is enabled, clock tracking is restored in the
following conditions:
1. If the loop is assigned as the primary reference clock but the clock controller is
tracking on the secondary reference or in free run mode, it is restored to tracking
on primary.
2. If the loop is assigned as the secondary reference clock but the clock controller is
in free run mode, it is restored to tracking on secondary.
If the 15-minute clock check indicates the system is in free-run mode:
1. Tracking is restored to the primary reference clock, if defined.
2. If the primary reference is disabled or in local alarm, tracking is restored to the
secondary reference clock, if defined.
Note:
If the system was put into free-run mode intentionally by the craftsperson, it resumes tracking
on a reference clock at this time.
This occurs unless the clock-switching option has been
disabled (LD 60, command MREF), or the reference clock has been "undefined" in the
database.
Automatic clock switching
If the EREF option is selected in LD 60, tracking on the primary or secondary reference clock
is automatically switched in the following manner:
1.
If software is unable to track on the assigned primary reference clock, it switches
to the secondary reference clock and sends appropriate DTC maintenance
messages.
2. If software is unable to track on the assigned secondary reference clock, it switches
to free-run mode.
Clock operation
ISDN Primary Rate Interface Maintenance November 2011 79
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