363-206-285
Transmission and Synchronization Interfaces
5-32 Issue 3 June 2001
Figure 5-19a shows that the CO system is internally timed (free-running). At the
RT, the TG derives its timing from the incoming optical signal and uses it to time
itself and loop timing back to the CO.
Figure 5-19b and Figure 5-19c show the CO timed from an external stratum 3 or
better timing source. The RT derives its timing from the incoming optical line and
can send a DS1 output to a BITS clock.
The external timing configuration (Figure 5-19d) uses external DS1 timing to each
DDM-2000 Multiplexer in the network. Since this configuration requires local office
clocks at each site, it is most suited to interoffice applications. A DDM-2000
network may have all DS1 references traceable to a primary reference source
(PRS) called synchronous operation or multiple PRSs called plesiochronous
operation.
The PRS is equipment that provides a timing signal whose long-term accuracy is
maintained at 10
-11
or better with verification to universal coordinated time (time
and frequency standard maintained by the U.S. National Institute of Standards
and Technology), and whose timing signal is used as the basis of reference for the
control of other clocks in a network.
An interoffice ring should have each node externally timed if BITS clocks are
available. All other rings should have one node externally timed (two in some dual
homing architectures) and the rest of the nodes line-timed. See 363-206-200,
DDM-2000 Multiplexer Applications, Planning, and Ordering Guide,
for more
synchronization information.
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