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Transmission and Synchronization Interfaces
Issue 1 December 1997 5-19
Automatic Synchronization Reconfiguration 5
SONET was designed to operate optimally in a synchronous environment.
Although plesiochronous and asynchronous operation can be supported through
the use of pointer adjustments, transmission quality is affected by the generation
of additional jitter and wander due to pointer adjustments. Because of this, it is
desirable to maintain synchronous operation whenever possible.
Through the use of synchronization messages, the quality of the different timing
references can be made available at each DDM-2000 Multiplexer. The DDM-2000
OC-12 Multiplexer shelf can always determine the best timing reference available
to it and switch to it. Through this mechanism, the synchronous operation of the
subnetwork can be maintained. The switching of timing references is hitless, and
the synchronization messages also allow it to be done without creating timing
loops in the process. Note that automatic synchronization reconfiguration applies
only to switching between available and line-timing references and not between
timing modes. For example, switching between line-timed to external timed or
external timed to line-timed would not be supported.
In the linear dual-homing network in Figure 5-7, normal operation includes an
external timing reference at each of the COs. The RT sites are each loop timed
from the CO DDM-2000 Multiplexers on the left. If a fiber failure occurs between
the first two DDM-2000 Multiplexers, the automatic synchronization
reconfiguration feature will cause the loop timed DDM-2000 Multiplexers to
change their direction of loop timing. This prevents any DDM-2000 Multiplexer
from operating in holdover for an extended period of time.
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