EXFO FTB-800 Series for FTB-1 - Manual (Page 564)

Glossary
554 FTB-700G/800 Series
G.709 Optical Transport Network (OTN)

Optical Payload Unit (OPU)
The primary overhead field associated to the OPU is the Payload
Structure Identifier (PSI). This is a 256 bytes multi-frame where its
first byte is defined as the Payload Type (PT). The remaining 255
bytes are currently reserved.
The other fields in the OPU overhead are dependent on the
mapping and concatenation capabilities associated to the OPU.
For an asynchronous mapping (the client signal and OPU clock are
different) Justification Control (JC) bytes are available to
compensate for clock rate differences, two methods are supported
Asynchronous Mapping Procedure (AMP) and Generic Mapping
Procedure (GMP). For a purely Bit-Synchronous Mapping
Procedure (BMP) (client source and OPU clock are the same), the
JC bytes become reserved (set to 0). Concatenation bytes are also
available as described in ITU G.709.