EXFO FTB-800 Series for FTB-1 - Manual (Page 556)

Glossary
546 FTB-700G/800 Series
G.709 Optical Transport Network (OTN)
Each of these layers and their functions are distributed along the network
and activated when they reach their termination points, which are
illustrated in the following figure.
OTN Layer Termination Points
The termination of the OTS, OMS and OCh layers is performed at the
optical level of the OTN. It is at the termination of the OTU layer that further
functionality can be added. This layer is the digital layer—also known as
the “digital wrapper”—and offers specific overhead to manage the OTN’s
digital functions. The OTU also introduces a new dimension to optical
networking by adding forward error correction (FEC) to the network
elements, allowing operators to limit the number of required regenerators
used in the network which, in turn, lowers its cost.
FEC allows an increase in the optical link budget by providing a new
method to correct errors, thereby reducing the impact of network noise
and other optical phenomena experienced by the client signal traveling
through the network.
The OTU also encapsulates two additional layers—the ODU and the OPU—
which provide access to the payload (SONET, SDH, etc.). These layers are
normally terminated at the same location.