363-206-305
Transmission and Synchronization
5-14
Issue 3 June 2000
Synchronization Messaging
5
The DDM-2000 FiberReach Multiplexers provide a synchronization messaging
feature to ensure the integrity of network synchronization during both normal and
abnormal conditions. With synchronization messaging, the current quality of the
timing source can be conveyed from one DDM-2000 FiberReach Multiplexer to the
next. This capability allows the DDM-2000 FiberReach Multiplexers to
automatically change their timing reference in order to always maintain the highest
quality timing available.
Applications
5
The applications that are currently supported with the synchronization messaging
feature can be divided into two categories:
1. Automatic synchronization reconfiguration
2. Synchronization provisioning integrity.
Automatic Synchronization Reconfiguration
5
SONET was designed to operate optimally in a synchronous environment.
Although plesiosynchronous and asynchronous operation can be supported by
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. With
synchronization messages, the quality of the different timing references can be
made available at each DDM-2000 FiberReach Multiplexer. The DDM-2000
FiberReach Multiplexer shelf can always determine the best timing reference
available to it and switch to that reference. 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.
Consider the access ring network in Figure 5-7. Under normal operation, the ring
has one DDM-2000 Multiplexer externally timed and the other FiberReach
Multiplexers line-timed in the counterclockwise direction. If a fiber failure occurs
between the host DDM-2000 and the DDM-2000 FiberReach Multiplexers, the
synchronization auto-reconfiguration feature will cause the DDM-2000
FiberReach Multiplexers to change their line timing directions to clockwise. The
result is that the ring is again operating synchronously. The ring already provides
self-healing of the traffic; so it is especially important to maintain synchronous
operation during this type of failure to prevent service degradation due to
increased jitter and wander.
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