EXFO FTB-800 Series for FTB-1 - Manual (Page 569)

Glossary
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G.709 Optical Transport Network (OTN)
The main attributes of the ODU multiplexer functionality are as follows:

The Asynchronous Mapping Procedure (AMP) is used for multiplexing
the tributary signals; this method uses a modified Justification Control
mechanism which has 2 positive Justification Control bytes and one
negative Justification Control byte.

The new multiplex method also supports the Generic Mapping
Procedure as the Justification Control mechanism is still using the OPU
OH JC bytes.

The Multiplex Structure Identifier (MSI) provides information that is
specific to each type of multiplexer provided.

Can handle multiplex signals with frequency offset of +/- 20 ppm on
every layer for the legacy architecture while the new architecture
(using GMP) can handle frequency offset of +/-100 ppm.