Nortel Circuit Card 311 Reference page 899

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.
See
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.
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
Copyright © 2003-2008, Nortel Networks
.
"Reference switchover" (page
Nortel Communication Server 1000
Circuit Card Reference
NN43001-311 01.04 Standard
Release 5.0 23 May 2008
914).
Architecture 899