Avaya Nortel Communication Server 1000 Reference page 509

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
Reference switchover
Switchover may occur in the case of reference degradation or reference failure. When
performance of the reference degrades to a point where the system clock is no longer allowed
to follow the timing signal, then the reference is said to be out of specification. If the reference
being used is out of specification and the other reference is still within specification, an
automatic switchover is initiated without software intervention. If both references are out of
specification, the clock controller provides holdover.
Autorecovery and chatter
If the software command "track to primary" is given, the clock controller tracks to the primary
reference and continuously monitors the quality of both primary and secondary references. If
the primary becomes out of specification, the clock controller automatically tracks to secondary
provided that it is within specifications. On failure (both out of specification), the clock controller
enters the HOLDOVER mode and continuously monitors both references. An automatic
switchover is initiated to the reference that recovers first. If the secondary recovers first, then
the clock controller tracks to the secondary, but switches over to the primary whenever the
primary recovers. If the primary recovers first, then the clock controller tracks to the primary.
If the software command "track to secondary" is given, the clock controller tracks to the
secondary reference and continuously monitors the quality of both primary and secondary
references. If the secondary becomes out of specification, the clock controller automatically
tracks to primary provided that it is within specifications. On failure (both out of specification),
the clock controller enters the HOLDOVER mode and continuously monitors both references.
Circuit Card Reference
Architecture
July 2011 509