Nortel Networks NN43001-301 manual Clocking modes

2001.5 Mb PRI implementation

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).

There are two stages to clock controller tracking, as follows:

•tracking a reference

•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 nearly matched, the clock controller locks onto the reference. The clock controller makes small adjustments to its own frequency until 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 locks onto the reference once again.

If the incoming reference is unstable, the internal clock controller is continuously in the tracking stage, with the LED flashing green all the time. This condition does not present a problem, instead it shows that the clock controller is continually attempting to lock onto the signal. However, if slips are occurring, there is a problem with the clock controller or the incoming line.

Nortel Communication Server 1000

ISDN Primary Rate Interface Installation and Commissioning

NN43001-301 02.03 Standard

Release 5.5 7 December 2007

Copyright © 2003-2007, Nortel Networks