How Forward Error Correction (FEC) works and Reed-Solomon RS 255/239 definition in Optical transmission network.

Forward Error Correction (FEC) is one big advantage of OTH.

Over row 1 to 3824 and line 1 to 4 a special coded parity check is created and transmitted in the remaining bytes of the frame.

The Reed-Solomon (RS) coding is used to produce this redundant information. The termination of the signal is using the FEC bytes to identify and correct transmission errors. Up to 32 bit errors can be corrected in a frame. 

FEC works:-

The G.709 FEC separates the OTU frame into the 4 rows. Each row is divided into 16 sub-rows in a byte interleaving manner.

For the first sub-row the 1st, 17th, 33rd and so on bytes are multiplexed. This leads to 239 data bytes.

Over these data bytes the FEC parity check is calculated. Which leads to the length of 255 bytes of a sub-row, according to the Reed-Solomon RS 255/239 definition.

The same principle is applied for all 16 sub-rows.In each oft the 16 data streams, up to 8 faulty bytes can be corrected.

If no FEC is used, fixed stuff bytes (all-0s pattern) are transmitted inside the FEC data area. 

Some types of FECs used for improved coding gain.:-

RSFEC- standard Reed-Solomon G.709

UFEC- Appendix I.6 of ITU-T G.975.1.

EFEC- ITU-T G.975.1

AFEC- designed as a BCH code, and is compliant with Appendix I.9 of ITU-T G.975.1

SD FEC

Reference:- knowledge attain from online sources and ITU-T doc.