Avaya Nortel Communication Server 1000 Reference page 533

the clock controller tracks to the secondary, but switches over to the primary when the primary
recovers. If the primary recovers first, 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 spec.), 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 primary recovers first,
then the clock controller tracks to the primary, but switches over to the secondary whenever
the secondary recovers. If the secondary recovers first, then the clock controller tracks to the
secondary.
A time-out mechanism prevents chatter due to repeated automatic switching between primary
and secondary reference sources.
Holdover and free-run
In the temporary absence of a synchronization reference signal, or when sudden changes
occur on the incoming reference due to error bursts, the clock controller provides a stable
holdover. The free-run mode is initiated when the clock controller has no record of the quality
of the incoming reference clock.
If the software command "free run" is given, the clock controller enters the free-run mode and
remains there until a new command is received. Note that the free-run mode of operation is
automatically initiated after the clock controller is enabled.
Reference clock selection through software
The NTAK79 has the necessary hardware for routing its reference to the appropriate line on
the backplane.
The software is responsible for the distribution of the secondary references and ensures that
no contention is present on the REFCLK1 backplane line. The software designates the
NTAK79 as the primary reference source to the clock controller. The secondary reference is
obtained from another NTAK79 card, which is designated by a technician. No other clocks
originating from other NTAK79 circuit cards are used.
The clock controller provides an external timing interface and is capable of accepting two
signals as timing references. In this case, an external reference refers to an auxiliary timing
source which is bridged from a traffic carrying signal. This is not intended to be a dedicated
non-traffic bearing timing signal. The clock controller uses either the two external/auxiliary
references or the NTAK79 references.
Circuit Card Reference
Architecture
July 2011 533