EXFO IQS-8100 Series Transport Blazer for IQS-600 - Manual (Page 625)

Glossary
SONET/SDH Application 611
G.709 Optical Transport Network (OTN)
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 612. 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.