Forward Error Correction (FEC)

What is Forward Error Correction(FEC)?

    Forward error correction is a major feature of the OTN. Already SDH has a FEC defined. It uses undefined SOH bytes to transport the FEC check information and is therefore called us in band FEC. It allows only a limited number of FEC check information,which limits the performance of the FEC. For the OTN a ReedSolomon 16 byte interleaved FEC scheme is defined, which uses 4x256 bytes of check information per ODU frame. In addition enhanced (proprietary) FEC schemes are explicitly allowed and widely used. FEC has been proven to be effective in OSNR limited systems as well as in dispersion limited systems. 

   As for non linear effects, reducing the output power leads to OSNR limitations, against which FEC is useful. FEC is less effective against PMD, however. G.709 defines a stronger Forward Error Correction for OTN that can result in up to 6.2 dB improvement in Signal to Noise Ratio (SNR). Another way of looking at this, is that to transmit a signal at a certain Bit Error Rate (BER) with 6.2 dB less power than without such an FEC. The coding gain provided by the FEC can be used to:Increase the maximum span length and/or the number of spans, resulting in an extended reach. (Note that this assumes that other impairments like chromatic and polarization mode dispersion are notbecoming limiting factors.) Increase the number of DWDM channels in a DWDM system which is limited by the output power of the amplifiers by decreasing the power per channel and increasing the number of channels.

   The FEC is an enabler for transparent optical networks: Transparent optical network elements like OADMs and OXCs introduce significant optical impairments (e.g. attenuation). The number of transparent optical network elements that can be crossed by an optical path before 3R regeneration is needed is therefore strongly limited. With FEC a optical path can cross more transparent optical network elements.This allows to evolve from today’s point to point links to transparent, meshed optical networks withsufficient functionality.