Nortel Communication Server 1000S Installation And Configuration Manual page 248

Page 248 of 394
553-3001-201 Standard 2.00
1.5 Mb PRI implementation
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.
Free-run (non-tracking)
In free-run mode, the clock controller does not synchronize on any source, it
provides its own internal clock to the system. This mode can be used when
the system is used as a master clock source for other systems in the network.
Free-run mode is undesirable if the system is intended to be a slave. It can
occur, however, when both the primary and secondary clock sources are lost
due to hardware faults, or when invoked by using software commands.
Shelf slot assignment
For cabinet systems, the NTAK09 DTI/PRI card can be placed in main and
expansion cabinets in any single card slot given in Table 74 on
For CS 1000S, the NTAK20 can be placed in MG 1000S systems, in the slots
given in Table 74 on
September 2004
page 250.
page 250.