Nortel Networks Circuit Card 311 - Manual (Page 897)

Architecture 897
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 very near to being matched, the clock controller is locked
onto 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 onto it, and matches frequencies exactly. Occasionally, however,
environmental circumstances cause the external or internal clocks to drift.
When this happens, the internal clock controller briefly enters the tracking
stage. The green LED flashes momentarily until the clock controller is
locked onto the reference once again.
If the incoming reference is unstable, the internal clock controller remains
continuously in the tracking stage with the LED flashing green all the time.
This condition does not present a problem, rather, it shows that the clock
controller is continually attempting to lock onto the signal. If slips are
occurring, however, it means that there is a problem with the clock controller
or the incoming line.
Free-run (non-tracking) In free-run mode, the clock controller does not
synchronize on any source, it provides its own internal clock to the system.
This mode can be used when the Option 11C is used as a master clock
source for other systems in the network. Free-run mode is undesirable if
the Option 11C is intended to be a slave. It can occur, however, when both
the primary and secondary clock sources are lost due to hardware faults
or when invoked by using software commands.
Clock controller functions and features
The NTAK10 2MB DTI clock controller functions and features include:
• phase-locking to a reference, generating the 10.24 Mhz system clock,
and distributing it to the CPU through the backplane. Up to two
references at a time can be accepted.
• providing primary to secondary switchover and auto-recovery
• preventing chatter
• providing error burst detection and correction, holdover, and free running
capabilities
• complying with 2.0 Mb CCITT specifications
• communicating with software
• filtering jitter
• making use of an algorithm to aid in detecting crystal aging and to
qualify clocking information
Nortel Communication Server 1000
Circuit Card Reference
NN43001-311 01.04 Standard
Release 5.0 23 May 2008
Copyright © 2003-2008, Nortel Networks
.