EXFO FTB-8510B Guia Del Usuario página 1256

Glossary
G.709 Optical Transport Network (OTN)
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.
Overhead
Each portion of the OTU frame has its own specific overhead functions.
They are displayed in figure OTU Frame Description on page 605, and
are briefly described below. Further details can be found about these
overhead fields in the ITU G.709 standard.
606
Optical Transport Unit (OTU)
The OTU overhead is comprised of the SM, GCC0 and RES bytes.
The section monitoring (SM) bytes are used for the trail trace
identifier (TTI), parity (BIP-8) and the backward error indicator
(BEI), or backward incoming alignment error (BIAE), backward
defect indicator (BDI), and incoming alignment error (IAE). The
TTI is distributed over the multiframe and is 64 bytes in length. It is
repeated four times over the multiframe.
General communication channel 0 (GCC0) is a clear channel used
for transmission of information between OTU termination points.
The reserved (RES) bytes are currently undefined in the standard.
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