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Maintenance Description
Issue 1 December 1997 9-21
Path Protection Switching (Path Switched Rings) 9
The path switched ring application is based on the Bellcore TR-TSY-000496/
GR-1400 path protection switching scheme.
The path protection switched ring has two single-fiber, counter-rotating rings as
shown in Figure 9-6. This architecture has distinct advantages over a linear
architecture. Each node on the ring terminates four fibers: a transmit and receive
fiber in each direction. Eight fibers are needed to connect the same NE in a
nonterminal position of a linear add/drop 1+1 protected arrangement: two transmit
and two receive in each direction, without providing the same level of protection
that the ring provides. The architecture of the ring is designed to protect against
any single point of failure, including a node failure, single fiber cut, or dual fiber
cut. Node failure or dual fiber failure in a linear network affects traffic to all
downstream nodes. A node failure in a ring only affects traffic dropped at the failed
node.
The DDM-2000 OC-12 ring provides STS-1 path protection switching to support
DS3, EC-1, and OC-3/IS-3 interfaces.
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.
Figure 9-6. Two-Fiber Unidirectional Ring
Primary
Ring
Secondary
DDM-2000
Ring
DDM-2000
DDM-2000
DDM-2000
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