EXFO FTB-800 Series for FTB-1 - Manual (Page 560)

Glossary
550 FTB-700G/800 Series
G.709 Optical Transport Network (OTN)
OTU Frame Structure and Overhead
As shown in the figure below, the OTU frame is broken down into the
following components:

Framing

OTL, OTU, ODU, OPU overhead

OTU FEC
OTU Frame Description