Nortel Networks Circuit Card - Manual (Page 732)

Page 732 of 906 NTAK10 2.0 Mb DTI card
553-3001-211 Standard 3.00 August 2005
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.
Reference clock selection through software
The 2MB DTI card 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 2MB DTI card as a primary reference source to the
clock controller. The secondary reference is obtained from another 2 Mbps
DTI card, which is designated by a craft person. No other clocks originating
from other 2MB DTI packs 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
2MB DTI references.