EXFO FTB-800 Series for FTB-1 - Manual (Page 563)

Glossary
NetBlazer 553
G.709 Optical Transport Network (OTN)

Optical Data Unit (ODU)
The ODU overhead is broken into several fields: RES, PM, TCMi,
TCM ACT, FTFL, EXP, GCC1/GCC2 and APS/PCC.
The reserved (RES) bytes are undefined and are set aside for future
applications.
The path monitoring (PM) field is similar to the SM field described
above. It contains the TTI, BIP-8, BEI, BDI and Status (STAT) field.
There are six tandem connection monitoring (TCMi) fields, which
contain the BEI/BIAE, BDI and STAT fields. The STAT field is used in
the PM and TCMi fields to provide an indication of the presence or
absence of maintenance signals.
The tandem connection monitoring activation/deactivation (TCM
ACT) field is currently undefined in the standards.
The fault type and fault location reporting communication channel
(FTFL) is a message spread over a 256-byte multiframe that
provides the ability to send forward and backward path-level fault
indications.
The experimental (EXP) field is a field that is not subject to
standards and is available for network operator applications.
General communication channels 1 and 2 (GCC1/GCC2) fields are
very similar to the GCC0 field except that each channel is available
in the ODU.
The automatic protection switching and protection
communication channel (APS/PCC) supports up to eight levels of
nested APS/PCC signals, which are associated to a
dedicated-connection monitoring level depending on the value of
the multiframe.