What is the function of 100G optical module FEC?

Two important development trends of optical modules are to increase the transmission rate and extend the transmission distance. However, with the increase of the transmission rate, the transmission distance of the signal will be limited by many factors, such as chromaticity dispersion, nonlinear effect, polarization mode dispersion, etc. These factors will limit the transmission rate and transmission distance to increase at the same time. To reduce the impact of these adverse factors, industry experts have proposed the forward error correction function, or FEC (Forward Error Correction) for short. Next, we will introduce what the 100G optical module FEC function is.

FEC is an error code correction technology, which is used to solve the problem that part of the optical signals at the transmitter are disturbed in the transmission process, resulting in false judgment at the receiver. When the optical signal is disturbed during transmission, the receiver may misjudge the "1" signal as the "0" signal or the "0" signal as the "1" signal.

Therefore, the FEC function forms a code with a certain error correction capability on the channel encoder 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 to improve the signal quality. FEC function is used in 100G and other high-speed optical modules. Generally, when this function is enabled, the transmission distance of high-speed optical modules will be longer than when FEC function is not enabled.

100G optical modules generally have FEC function. Although FEC function has the advantages of forward error correction and increasing transmission distance, it will inevitably cause delay of some data packets in the process of error correction. Therefore, it is not recommended to enable this function for all high-speed optical modules. For example, when using 100G QSFP28 LR4 optical module, it is not recommended to enable FEC function.

The 100G optical module mainly relies on the FEC function configured on the device side to achieve error correction, so the switch also needs to support the FEC function. Generally, the switch that supports the FEC function is enabled by default. In addition, it should be noted that if the FEC function is enabled on the A-end optical module, the B-end optical module should also enable the function, otherwise the interface will not be Up.