363-206-285
Maintenance Description
Issue 3 June 2001
9-27
Path Protection Switching (Path Switched Rings) 9
The path switched ring application is based on the Telcordia TR-TSY-000496/
GR-1400 path protection switching scheme. The DDM-2000 OC-3 Multiplexer
must be equipped with 22-type, 24-type (OC-12) or 27-type (OC-1) OLIUs in the
main slots for ring applications. The 22G-U, 22G2-U, 22F2-U, and 22D-U OLIUs
support path protection switching based on STS/VT unequipped and STS/VT
signal degrade, and are recommended for OC-3 ring applications.
The path protection switched ring has two single-fiber counter-rotating rings as
shown in Figure 9-13. 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 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 OC-3 can
provide both VT1.5 and/or STS-1 path protection, and DDM-2000 OC-12 can
provide STS-1 path protection. A single DDM-2000 OC-3 shelf can support a mix
of STS-1 and VT1.5 path switched rings. VT1.5 path protection is available at the
OC-12 rate by using DDM-2000 OC-3 systems colocated with a DDM-2000
OC-12 system.
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