EXFO FTB-800 Series for FTB-1 - Manual (Page 562)

Glossary
552 FTB-700G/800 Series
G.709 Optical Transport Network (OTN)

Optical Transport Unit (OTU)
The OTU overhead is comprised of the SM, GCC0 and RES bytes.
The section monitoring (SM) bytes are used for the trail trace
identifier (TTI), parity (BIP-8) and the backward error indicator
(BEI), or backward incoming alignment error (BIAE), backward
defect indicator (BDI), and incoming alignment error (IAE). The
TTI is distributed over the multiframe and is 64 bytes in length. It is
repeated four times over the multiframe.
General communication channel 0 (GCC0) is a clear channel used
for transmission of information between OTU termination points.
The reserved (RES) bytes are currently undefined in the standard.