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Cisco ONS 15454 Reference Manual, R7.0.1
OL-9217-01
Chapter 11 Circuits and Tunnels
11.17 11.17.3 Link Capacity Adjustment
CE-Series cards also support split fiber routing, which allows the individual members to be routed on
different fibers or each member to have different routing constraints. This mode offers the greatest
bandwidth efficiency and also the possibility of differential delay, which is handled by the buffers on the
terminating cards. Four protection options are available for split fiber routing: Fully Protected, PCA,
Unprotected, and DRI. Figure 11-15 shows an example of split fiber routing.
Figure 11-15 VCAT Split Fiber Routing
In both common fiber and split fiber routing, each member can use a different protection scheme;
however, for common fiber routing, CTC checks the combination to make sure that a valid route exists.
If it does not, the user must modify the protection type. In both common fiber and split fiber routing,
intermediate nodes treat the VCAT members as normal circuits that are independently routed and
protected by the SONET network. At the terminating nodes, these member circuits are multiplexed into
a contiguous stream of data.
11.17.3 Link Capacity Adjustment
The CE-100T-8 card supports the link capacity adjustment scheme (LCAS), which is a signaling
protocol that allows dynamic bandwidth adjustment of VCAT circuits. When a member fails, a brief
traffic hit occurs. LCAS temporarily removes the failed member from the VCAT circuit for the duration
of the failure, leaving the remaining members to carry the traffic. When the failure clears, the member
circuit is automatically added back into the VCAT circuit without affecting traffic. You can select LCAS
during VCAT circuit creation.
Note Although LCAS operations are errorless, a SONET error can affect one or more VCAT members. If this
occurs, the VCAT Group Degraded (VCG-DEG) alarm is raised. For information on clearing this alarm,
refer to the Cisco ONS 15454 Troubleshooting Guide.
Instead of LCAS, the FC_MR-4 (enhanced mode), CE-1000-4 card, and ML-Series cards support
software LCAS (SW-LCAS). SW-LCAS is a limited form of LCAS that allows the VCAT circuit to adapt
to member failures and keep traffic flowing at a reduced bandwidth. SW-LCAS uses legacy SONET
failure indicators like AIS-P and remote defect indication, path (RDI-P) to detect member failure.
SW-LCAS removes the failed member from the VCAT circuit, leaving the remaining members to carry
the traffic. When the failure clears, the member circuit is automatically added back into the VCAT
124065
VCAT
Function
Source VCAT at NE
Traffic Traffic
Virtually
Concatenated
Group
Member #1
Member #2
Member #3
Intermediate
NE
VCAT
Function
with
Differential
Delay Buffer
Destination VCAT at NE
Intermediate
NE
Intermediate
NE
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