Nortel Networks Circuit Card - Manual (Page 737)

NTAK20 Clock Controller daughterboard Page 737 of 906
Circuit Card Description and Installation
• jitter filtering
• use of an algorithm to detect crystal aging and qualify clocking
information
Clocking modes
The clock controller can operate in one of two modes: tracking or
non-tracking (also known as free-run).
Tracking mode
In tracking mode, one or more DTI/PRI cards supply a clock reference to the
NTAK20 clock controller daughterboard. When operating in tracking mode,
one DTI/PRI card is defined as the Primary Reference Source (PREF) for
clock synchronization. The other DTI/PRI card is defined as the Secondary
Reference Source (SREF). PREF and SREF are defined in LD 73.
There are two stages to clock controller tracking:
• tracking a reference
• locking on to a reference
When tracking a reference, the clock controller uses an algorithm to match its
frequency to the frequency of the incoming clock. When the frequencies are
almost matched, the clock controller locks on to the reference. The clock
controller makes small adjustments to its own frequency until both the
incoming and system frequencies correspond.
If the incoming clock reference is stable, the internal clock controller tracks
it, locks on to it, and matches frequencies exactly. Occasionally,
environmental circumstances cause the external or internal clocks to vary.
When this happens, the internal clock controller briefly enters the tracking
stage. The green LED flashes until the clock controller is locked on to the
reference again.
If the incoming reference is unstable, the internal clock controller
continuously tracks, and the LED continuously flashes green. This condition
does not present a problem. It shows that the clock controller is continually
attempting to lock onto the signal. If slips occur, there is a problem with the
clock controller or the incoming line.