Nortel Communication Server 1000S Installation And Configuration Manual page 247

NTAK20 Clock Controller (CC) daughterboard
Digital Trunking requires synchronized clocking so a shift in one clock
source results in an equivalent shift of the same size and direction in all parts
of the network. Synchronization is accomplished with the NTAK20 clock
controller circuit card in each Main and IP expansion cabinet/MG 1000S.
Every Small System cabinet or Media Gateway that contains a digital
trunk must contain a clock controller.
If a Small System is equipped with digital trunks, it is recommended that
at least one digital trunk be placed in the main cabinet. For CS 1000S
systems, the digital trunk must be placed in the MG 1000S.
Note: Clocking slips can occur between systems that are clocked from
different COs, if the COs are not synchronized. The slips can degrade
voice quality.
The Clock Controller circuitry synchronizes the system to an external
reference clock, and generates and distributes the clock to the system. A
Small System can function either as a slave to an external clock or as a
clocking master.
The NTAK20AA version of the clock controller meets AT&T Stratum 3 and
Bell Canada Node Category D specifications. The NTAK20BA version
meets CCITT stratum 4 specifications.
Clocking modes
The system supports a single clock controller that can operate in one of two
modes - tracking or non-tracking (also known as free-run).
Tracking mode
In tracking mode, one or possibly two DTI/PRI cards supply a clock reference
to a clock controller daughterboard. One DTI/PRI is defined as the primary
reference source for clock synchronization, while the other is defined as the
secondary reference source (PREF and SREF in LD 73).
1.5 Mb PRI implementation
IMPORTANT!
ISDN Primary Rate Interface
Page 247 of 394
Installation and Configuration