EXFO FTB-800 Series for FTB-1 - Manual (Page 566)

Glossary
556 FTB-700G/800 Series
G.709 Optical Transport Network (OTN)
Each of the six TCMi fields in the ODU overhead is assigned to a monitored
connection. There can be from zero to six connections that can be
configured for each connection. In the figure Tandem Connection
Monitoring on page 555, there are three different connections that are
actually monitored. Carrier C, due to its location, can monitor three TCM
levels as the ODU passes through its portion of the network.
In addition to monitoring maintenance signals, using the STAT field
associated with each TCM level, the TCM connection also monitors the
BIP-8 and BEI errors for each connection level. Maintenance signals are
used to advertise upstream maintenance conditions affecting the traffic
and errors provide an indication of the quality of service offered at each
segment of the network, which provides a valuable tool for the user and
carrier to isolate faulty sections of the network.
Forward Error Correction (FEC)
The ITU G.709 standard supports forward error correction (FEC) in the OTU
frame and is the last part added to the frame before the frame is
scrambled. FEC provides a method to significantly reduce the number of
transmitted errors due to noise, as well as other optical phenomena that
occur at high transmission speeds. This enables providers to support
longer spans in between optical repeaters.
An OTU frame is divided into four rows. Each row is broken down into 16
sub-rows comprised of 255 bytes each, as shown in figure Forward Error
Correction on page 557. A sub-row is composed of interleaved bytes. The
interleave is executed so that the first sub-row contains the first overhead
(OH) byte, the first payload byte and the first FEC byte, and so on for the
remaining sub-rows of each row in the frame. The first FEC byte starts at
position 240 for all sub-rows.