EXFO FTB-8510B Guia Del Usuario página 1267

Glossary
SONET Numbering Convention
The FTB-8100 Series supports numbering SONET low order path using the
two-level "VTGroup#,VT#" convention for numbering VTs within an STS-1.
For example: VT1.5 [1,3], VT2 [3,2], VT6 [6,1].
The FTB-8100 Series supports numbering SONET high order path STS-nc
within an OC-N using the two-level "STS-3#,STS-1#". For example: STS-12c
[5,1].
Note: For STS-1e the numbering is limited to the A value as only one STS-1 exits.
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SONET/SDH Application