Nortel Networks Circuit Card - Manual (Page 760)

Page 760 of 906 NTAK79 2.0 Mb PRI card
553-3001-211 Standard 3.00 August 2005
• jitter filtering
• use of an algorithm to detect 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 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.