EXFO FTB-800 Series for FTB-1 - Manual (Page 561)

Glossary
NetBlazer 551
G.709 Optical Transport Network (OTN)
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Framing
The OTU framing is divided into two portions: FAS and MFAS.
The frame alignment signal (FAS) uses the first six bytes and, similarly
to SONET/SDH, it is used to provide framing for the entire signal. In
order to provide enough 1/0 transitions for synchronization, scrambling
is used over the entire OTU frame, except for the FAS bytes.
The multiframe alignment signal (MFAS) byte is used to extend
command and management functions over several frames. The MFAS
counts from 0 to 255, providing a 256 multiframe structure.
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Overhead
Each portion of the OTU frame has its own specific overhead functions.
They are displayed in figure OTU Frame Description on page 550, and
are briefly described below. Further details can be found about these
overhead fields in the ITU G.709 standard.

Optical channel Transport Lane (OTL)
The Optical channel Transport Lane (OTL) is an adaptation layer
whose purpose is to re-use the modules developed for Ethernet
40GBASE-R. These modules have a four-lane WDM interface to
and from a transmit/receive pair of G.652 optical fibers, and
connect to the host board via a 4-lane (OTL3.4) electrical interface.
The OTL layer is responsible for mapping the serial OTU signal
onto a parallel path designated lanes. In the case of OTU3 the
signal is distributed over 4 logical lanes.