Everything You Should Know About FEC Function of Optical Modules

The meaning and principle of FEC

FEC (Forward Error Correction), also known as forward error correction, is a method to increase the reliability of data communication. When the optical signal is disturbed during transmission, the receiving end may misjudge the “1” signal as a “0” signal, or the “0” signal as a “1” signal.

Therefore, the FEC function combines the information code into a code with a certain error correction capability on the channel encoder at the transmitting end, and the channel decoder at the receiving end decodes the received code. If the number of errors generated during transmission is within the error correction capability range (discontinuous error), the decoder will locate the error and correct it to improve the quality of the signal.

Received signal processing methods of FEC

FEC can be divided into two categories: hard-decision decoding and soft-decision decoding. Hard-decision decoding is a decoding method based on the traditional error-correcting code viewpoint. The demodulator sends the judgment result to the decoder, and the decoder uses the algebraic structure of the codeword to correct the errors according to the judgment result.

Soft-decision decoding contains more channel information than hard-decision decoding, and the decoder can make full use of this information through probabilistic decoding, thereby obtaining greater coding gain than hard-decision decoding.

The development history of FEC

FEC has gone through three generations in terms of time and performance. The first generation of FEC uses hard-decision block codes, typically RS(255, 239), which has been written into ITU-T G.709 and ITU-T G.975 standards, and the code word overhead is 6.69%. When the output BER When =1E-13, its net coding gain is about 6dB. The second-generation FEC adopts hard-decision concatenated codes, and comprehensively applies technologies such as concatenation, interleaving, and iterative decoding.

The codeword overhead is still dominated by 6.69%. When the output BER=1E-15, the net coding gain is more than 8dB. , which can support long-distance transmission requirements of 10G and 40G systems. The third-generation FEC adopts soft decision, and the code word overhead is 15%~20%. When the output BER=1E-15, the net coding gain reaches about 11dB, which can support the long-distance transmission requirements of 100G and even beyond 100G systems.

FEC and 100G optical module application

The FEC function is used in high-speed optical modules such as 100G. Usually, after this function is turned on, the transmission distance of the high-speed optical module will be longer than that when the FEC function is not turned on. For example, a 100G QSFP28 ZR4 optical module can achieve a transmission distance of up to 80KM under normal circumstances.

When the FEC function is enabled in the system, the transmission distance through single-mode fiber can reach up to 90KM. However, due to the inevitable delay of some data packets in the process of correcting bit errors, it is not recommended to enable this function for all high-speed optical modules. For example, when using a 100G QSFP28 LR4 optical module, it is not recommended to enable the FEC function. At the same time, the switch needs to support the FEC function. If the FEC function is enabled on the A-side optical module, the B-side optical module must also enable this function, otherwise, the interface will not go up.

Summary

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