CXR Larus 80-100-400 2-12
Issue
1,
July
2006
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2.231 (continued)
should be taken from different output cards of the Central Clock system. The
Central Clock may also provide primary and secondary timing signals, through
interoffice synchronization network paths specially selected for high availability,
to other Central Clocks in downstream offices.
2.24
SONET and SDH Network Elements
2.241 Special considerations for synchronizing SONET and SDH network elements
are still evolving. Although the details are beyond the scope of this document,
the most important are these:
a. External timing for SONET and SDH NEs from a Central Clock is the
preferred mode where a Central Clock is available.
b. When Central Clock timing is not available, other SONET/SDH timing
modes (line, through, or loop) should be chosen in such a way as to avoid
timing loops and to minimize the length of timing paths.
c. The use of secondary references should follow the rules in References 1
and 2 (paragraphs 2.25 and 2.26) to avoid timing loops. If a string of more
than one SONET/SDH add drop multiplexer (ADM) exists between Central
Clock timed offices, no secondary references should be used. This also
applies to ring configurations; timing should be passed primarily in one
direction only, between line timed ADMs. If Sync Status Messaging is used,
then they are sent primarily in one direction and secondarily in the opposite
direction.
d. It is recommended that payload DS1s carried on SONET and E1s on SDH
not be used for synchronization distribution. These DS1s/E1s are subject
to phase transients (pointer adjustments) that do not meet short-term
stability requirements.
e. SONET and SDH NEs may have the capability of deriving a DS1/E1 timing
signal that is not part of the payload but is locked to the incoming OC-N line
rate. This signal could be used for synchronization distribution. However, it
is recommended that synchronization not be passed in this way along a
route that has line or through timed ADMs until synchronization messages
are implemented. Otherwise, the downstream NE has no way of knowing
whether the intermediate ADM has a receive failure and is running on its
internal clock.
