In contrast to the Automatic Repeat Request( ARQ), which involves the repetition of data packets containing errors, forward error correction involves reconstruction data being sent by the sender. However, the lower data throughput due to the high redundancy caused by the system is a disadvantage. Methods for forward error correction include Trellis coding and Reed-Solomon code.
One classification for the FEC methods is the code rate. This is the value that results from the additional bits inserted. The code rate is calculated from the ratio of bits from the original codeword to the transmitted bits. For example, if one additional redundancy bit is inserted for every three bits, then the code rate is 3/4.
The different variants of the FEC technique
The FEC technique has evolved over the years, from Generic FEC (G-FEC) to Enhanced FEC (E-FEC) and finally to the third generation, Soft Decision FEC( SD-FEC). The former methods are classified as Hard Decision FEC( HD-FEC). Generic FEC (G-FEC) is a procedure that ensures error correction and interoperability ofDWDM technology from different manufacturers. It is specified in the ITU-T recommendationG.709. Enhanced FEC (E-FEC) uses an algorithm with iterative decoding and has better coding gain than G-FEC. Manufacturers have different E-FEC implementations, so their products are incompatible. Soft Decision FEC (SD-FEC) takes advantage of advances in digitalsignal processing for use in 100- gigabit Ethernet and above. Depending on the implementation, net coding gain (NCG) increases by 11 dB or more.