Two important developments in fibre optic communication are the increase in transmission rate and the extension of transmission distance. As the transmission rate increases, the factors that limit the transmission distance during signal transmission become more numerous, such as chromatic dispersion, non-linear effects, polarisation mode dispersion, etc., which also affect the simultaneous increase of both. In addition there is no ideal digital channel in the actual transmission process, the signal will always have aberrations and delays in the transmission process of various media, which means error codes and jitter. To reduce the impact of these adverse factors, industry experts have proposed Forward Error Correction, or FEC (Forward Error Correction) for short.

FEC is short for Forward Error Correction. Forward Error Correction is a type of error control, which refers to a technique where a signal is pre-processed according to a certain algorithm for coding before being sent into the transmission channel, adding redundant codes with the characteristics of the signal itself, and decoding the received signal at the receiving end according to the corresponding algorithm, so as to find out the error codes generated during transmission and correct them. To give a simple example, someone writes an article with many typos in it, then a very knowledgeable verifier who can read it, then corrects the typos and then shows it to a layman. Before the correction, the layman cannot read it, and only after the verifier has corrected the typos can he read it.

FEC is suitable for high speed communication (40G and 100G), the optical signal deteriorates during transmission due to various factors, resulting in mis-coding at the receiving end, which may misjudge the "1" signal as "0" signal or "0" signal as "1" signal, the FEC function consists of information code with certain error correction capability through the channel coder at the sending end, and the channel decoder at the receiving end decodes the received code. If the number of errors generated in the transmission is within the error correction capability (discontinuous errors), the decoder will locate and correct the errors in order to improve the quality of the signal.