363-206-285
Transmission and Synchronization Interfaces
Issue 3 June 2001
5-37
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 OC-3 Multiplexer. The DDM-2000 OC-3
Multiplexer shelf can be optioned to determine the best timing reference 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.
In the linear dual-homing network in Figure 5-20c, normal operation includes an
external timing reference at each of the COs. The RT sites are each line-timed
from the CO DDM-2000 OC-3 Multiplexers on the left. If a fiber failure occurs
between the first two DDM-2000 OC-3 Multiplexers, the automatic
synchronization reconfiguration feature will cause the line-timed DDM-2000 OC-3
Multiplexers to change their direction of line timing. This prevents any DDM-2000
OC-3 Multiplexer from operating in holdover for an extended period of time.
Consider the access ring network in Figure 5-21. Under normal operation, the ring
has one DDM-2000 OC-3 Multiplexer externally timed and the others line-timed in
the counterclockwise direction. If a fiber failure occurs between the first two
DDM-2000 OC-3 Multiplexers, the automatic synchronization reconfiguration
feature will cause the DDM-2000 OC-3 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 restoration 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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