EXFO FTB-800 Series for FTB-1 - Manual (Page 559)

Glossary
NetBlazer 549
G.709 Optical Transport Network (OTN)
As depicted above, to create an OTU frame, a client signal rate is first
adapted at the OPU layer. The adaptation consists of adjusting the client
signal rate to the OPU rate. Its overhead contains information to support
the adaptation of the client signal. Once adapted, the OPU is mapped into
the ODU. The ODU maps the OPU and adds the overhead necessary to
ensure end-to-end supervision and tandem connection monitoring (up to
six levels). Finally, the ODU is mapped into an OTU, which provides
framing as well as section monitoring and FEC.
Following the OTN structure presented in figure Basic OTN Transport
Structure on page 548, OTUks (k = 1, 2, 3) are transported using the OCh;
each unit is assigned a specific wavelength of the ITU grid. Several
channels can be mapped into the OMS and then transported via the OTS
layer. The OCh, OMS and OTS layers each have their own overhead for
management purposes at the optical level. The overhead of these optical
layers is transported outside of the ITU grid in an out-of-band channel
called the optical supervisory channel (OSC).
When the OTU frame structure is complete (OPU, ODU and OTU), ITU
G.709 provides OAM&P functions that are supported by the overhead.