363-206-305
Maintenance Description
Issue 3 June 2000
9-15
Figure 9-5. Two-Fiber Unidirectional Ring
The signal that enters the ring is protected on a SONET path basis as switching is
performed independently for each path. Because of the ring's unidirectional
operation, time slots must be reserved all the way around the ring for all ring
traffic, limiting the capacity of the ring to the OC-N line rate. DDM-2000
FiberReach provides VT1.5 path protection.
Path protection switched rings feed a SONET payload from the ring entry point,
simultaneously in both rotations of the ring, to the signal's ring drop or exit point as
shown by traffic AC and CA in Figure 9-6a. This duplication of the signal that
enters the ring is called a "head-end bridge." The node that drops the signal from
the ring monitors both ring rotations and is responsible for selecting the signal that
has the highest quality based on LOS/LOF, path alarm indication signal (AIS), and
path bit error rate (BER) performance. This function at the ring exit point is called
a "tail-end switch." The system provides nonrevertive switching. All detected hard
failures (LOS, LOF, LOP, line AIS, STS-1 or VT1.5 path AIS, or STS-1 path signal
failure or signal degrade based on BER for path protected rings) result in AIS
insertion in the outgoing signals. This allows the drop node to detect path failures
and to select the good path.
Primary
Ring
Secondary
DDM-2000
Ring
DDM-2000
DDM-2000
DDM-2000
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