Meridian Nortel 1 Option 11C Technical Reference Manual page 490

Page 490 of 544
553-3011-100 Standard 14.00
NTAK79 2.0 Mb PRI card
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 via software
The NTAK79 has the necessary hardware for routing its reference to the
appropriate line on the backplane
Software is responsible for the distribution of the secondary references and
ensures that no contention is present on the REFCLK1 backplane line.
Software designates the NTAK79 as a primary reference source to the clock
controller. The secondary reference is obtained from another NTAK79 card,
which is designated by a craft person. No other clocks originating from other
NTAK79 circuit cards are used.
January 2002