g., Ethernet frames mapped within GFP) over
multiple SONET SPEs. Low order virtual concatenation (LOVCAT) virtually concatenates
VT1.5 payloads, while high order virtual concatenation (HOVCAT) virtually
concatenates STS-1 or STS-3c payloads. The notation for VCAT signals carries an additional
tag that identifies the number of virtually concatenated SPEs; for example, an
HOVCAT signal that combines five STS-3c SPEs would be designated an STS-3c-5v.
Table 11.2 shows how VCAT can improve EoS bandwidth efficiency.
All the VCAT intelligence resides at the endpoints of the virtually concatenated
SONET paths; the SONET network knows nothing of VCAT and treats the paths as independent
(e.g., a different pointer identifies the location of each virtually concatenated
STS signal, and each STS has its own POH). As a result, VCAT requires additional tools
for the VCAT endpoints to control the grouping of links within a VCAT group (VCG).
These tools allow VCAT to handle gracefully the addition and deletion of SONET paths
within a VCG, due either to provisioning or to network failure or restoration. The link
capacity adjustment scheme (LCAS), which ITU-T has standardized [10, 11] provides
these tools.
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