EXFO FTB-8510B Guia Del Usuario página 1311

Lower-rate virtual tributary signals have also been defined for both SONET
and SDH, as shown in the table below:
VT 1.5
VT 2
VT 6
Although contiguous concatenation has been successfully introduced and
deployed for years, it poses some major deficiencies when attempting to
transport packet-based signals. First, in contiguous concatenation, the
concatenated bandwidth requires the timeslots to be consecutive. Second,
it also requires that the network elements involved in the transport of the
traffic support this function from the source to the destination node,
including every intermediate node. Third, data-service rates are not well
matched to these defined containers, hence using GFP with current
contiguous concatenation schemes results in sub-optimal use of the
bandwidth, as Ethernet and Fibre Channel data rates are not properly
matched to these channels (e.g., 100M Ethernet service mapped over an
STS-3c or VC4 results in approximately 33% of wasted bandwidth).
To address these limitations, a complementary technology — virtual
concatenation (VCAT) — was developed and later defined in ANSI T1.105,
ITU G.707, and ITU G.783 recommendations. Two forms of virtual
concatenation were defined; i.e., high-order and low-order VCAT. This
introduced additional flexibility to SONET/SDH by allowing for the
non-contiguous concatenation of high-order or low-order payload frames
to better scale the requirements for incremental client-data streams. This
means that the concatenated payload does not need to be formed by
consecutive timeslots in the transport path. In addition, this new
concatenation capability allows the network element involved in the
SONET/SDH Application
SONET
VC-11
VC-12
VC-2
Next-Gen - Virtual Concatenation (VCAT)
Payload Capacity
SDH
1.6
2.17
6.78
Glossary
(Mb/s)
661