Nortel Circuit Card, Installation Manual, Page: 761 / 908

NTAK79 2.0 Mb PRI card Page 761 of 906
Circuit Card Description and Installation
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.