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CXR Larus 80-100-400 Issue 2, July 2006 2-11
.224 (continued)
b.
ction
switching, and the number of repeaters or multiplexers on the path.
c.
avoiding loops in a T1 interoffice network are given in References 1 and
2.
d.
C. The
objective should always be to maximize the overall availability.
e.
too complex, then consider
the concept of “GPS Stratum 1 everywhere.”
.23 Intraoffice
Distribution
2.231
. Many NEs have primary and
secondary timing ports; the signals to these
2
For a given Central Clock, a DS1/E1 facility with the highest availability
should be selected for the primary reference input from an upstream clock.
A facility with the next highest availability should be selected for the
secondary reference, preferably on a route from a different upstream clock.
The availability of a path is determined by its historical failure record,
installation or rearrangement activity, facility length and type, prote
No timing loops are allowed in the synchronization network for any
combination of primary and secondary facilities. The potential for loops
exists when either primary or secondary reference signals are passed
between clocks of the same level. Loops are avoided within an office by
distributing timing from the Central Clock in a star topology. Detailed rules
for
There are no fixed rules for the number of Central Clocks that can be used
in a cascade. Any clock can track any other clock of equal or higher level
and will filter out any jitter and short-term wander introduced along the
timing reference path. However, long-term wander (over periods of hours)
will accumulate along the cascaded paths from the PRS to a given clock.
Depending on the nature and length of the facilities, this may limit the total
path length. Furthermore, the failure of a reference path may affect all
downstream clocks. For these reasons, the number of cascaded clocks
should be minimized as far as possible, consistent with the use of whatever
highly reliable reference paths are available in the synchronization network.
As an example, it would be preferable to time a Stratum 3 clock or LNC
from another Stratum 3 clock or LNC upstream, which is in turn timed from
a Stratum 2 clock or TNC through reliable facilities, rather than use a direct
path of questionable reliability to that Stratum 2 clock or TN
If interoffice distribution engineering becomes
2
The Central Clock system, described in Reference 3, is the preferred method of
distributing timing within an office. Redundant hardware and automatic
switching between primary and secondary reference inputs provide a high
degree of availability. The Central Clock supplies timing directly to all digital
equipment in the office requiring synchronization, usually by means of DS1 or
E1 framed ones or a 64 kb composite clock
