EXFO FTB-800 Series for FTB-1 - Manual (Page 577)

Glossary
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SONET/SDH
The FTB-700G/800 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.
SDH Numbering Convention
As per ITU G.707, the high order paths are defined using a 2 to 5 level
convention E,D,C,B,A depending on the rate of the STM-n used.

E: the AUG-64 are numbered 1 to 4

D: the AUG-16 are numbered 1 to 4

C: the AUG-4 are numbered 1 to 4

B: the AUG-1 are numbered 1 to 4

A: the AU-3 are numbered 1 to 3
So for the naming is as follows for each of the following rate:

[E,D,C,B,A] for STM-256

[D,C,B,A] for STM-64

[C,B,A] for STM-16

[B,A] for STM-4