Clocking modes – Nortel Networks 1000 User Manual

Page 794

794

NTAK10 2.0 Mb DTI card

ATTENTION

IMPORTANT!

Each Media Gateway that has a digital trunk must have a clock controller
clocked to an external reference clock.

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 recovered clock from the external digital facility is provided to the
clock controller through the backplane-to-clock controller interface.
Depending upon the state of the clock controller (switched on or off), the
clock controller interface, in conjunction with software, enables or disables
the appropriate reference clock source.

The clock-controller circuitry on NTAK10 is identical to that of the NTAK20.
While several DTI/PRI packs can exist in one system, only one clock
controller can be activated. All other DTI/PRI clock controllers must be
switched off.

Clocking modes

The clock controller can operate in one of two modes: tracking or
non-tracking (also known as free-run).

The clock controller can operate in one of two modes: tracking or
non-tracking (also known as free-run). See

“Clocking modes” (page 806)

Tracking mode

There are two stages to clock controller tracking:

•

tracking a reference, and

•

locked onto a reference.

When tracking a reference, the clock controller uses an algorithm to
match its frequency to the frequency of the incoming clock. When the
frequencies are very near to being matched, the clock controller is locked
onto the reference. The clock controller makes small adjustments to its
own frequency until both the incoming and system frequencies correspond.

If the incoming clock reference is stable, the internal clock controller tracks
it, locks onto it, and matches frequencies exactly. Occasionally, however,
environmental circumstances cause the external or internal clocks to drift.
When this happens, the internal clock controller briefly enters the tracking
stage. The green LED flashes momentarily until the clock controller is
locked onto the reference once again.

Nortel Communication Server 1000

Circuit Card Reference

NN43001-311

02.06

Standard

27 August 2008