EXFO FTB-8510B Guia Del Usuario página 1321

In the event that the terminating equipment is unable to recover the
frame or the sequence, or if there is too much differential delay, the
system is equipped to generate alarms to the management system to
highlight these conditions.
VCAT Differential Delay
VCAT payload can be split and sent along different paths through the
network. Therefore, it is entirely possible that these different paths will not
cover the same distance and may contain a different number of network
elements along their route. This would mean that members of the VCG do
not reach the termination point (end PTE) at the same time. In order for
the terminating equipment to reassemble the payload, it must be able to
compensate for the difference in payload arrival times. This arrival time
difference is known as the differential delay.
Differential delay is the relative arrival time measurement between the
members of a VCG. This means that in a next-generation SONET/SDH
network, buffering is required at the terminating end of a VCG connection
in order to realign the data stream. For high-order VCAT paths, the
differential delay is measured by examining the multiframe indicator (MFI)
present in the path overhead of each VCG member. For low-order VCAT
paths, the frame-count information is used to determine differential delay.
The VCAT standards define the maximum differential delay between
members of a VCG to be 256 ms. However, given the amount of buffering
required at the terminating points, it is often the case that next-generation
SONET/SDH PTEs support less than this maximum, hence making VCAT
testing an important consideration when verifying the performance of
these network elements.
SONET/SDH Application
LOP Z7/K4 Bit 2 Multiframe Structure
Next-Gen - Virtual Concatenation (VCAT)
Glossary
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