Meridian Nortel 1 Option 11C Technical Reference Manual page 476

Page 476 of 544
553-3011-100 Standard 14.00
NTAK10 2.0 Mb DTI card
• providing jitter filtering.
• 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 will be 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. 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.
January 2002