Optics Pre-FEC BER Rate

Pre-forward error correction (pre-FEC) bit error rate (BER) provides an insight into the link quality. Typically, the device uses FEC-corrected code words to calculate the pre-FEC BER. Certain devices with an ASIC serializer/deserializer (SerDes) support the FEC histogram, which provides a detailed view of link quality in terms of the number of symbol error corrections taking place per FEC code word.

With four-level pulse-amplitude modulation (PAM4) and a high-speed SerDes such as the 56G or 112G, the eye spacing is reduced to one third of the nonreturn to zero (NRZ). Due to less spacing between the voltage levels, the signal-to-noise ratio (SNR) is typically low, making the link very susceptible to noise.

To compensate for that, devices can use stricter FEC algorithms. When these algorithms are used, the FEC-corrected code words counter usually increments. In these conditions, you can use the pre-FEC BER and FEC histogram to forecast the link quality.

On devices that support this feature, it is enabled by default. To display the pre-FEC BER and the FEC histogram, use the show interfaces interface-name command. This command displays FEC corrected and un-corrected code words counters, the pre-FEC BER, and the FEC histogram.

Use Feature Explorer to confirm platform and release support for specific features.