This counter reports the number of bit errors that Forward Error Correction was able to correct on the OTU2 signal during the current one-second interval. Because FEC quietly repairs a certain amount of line noise before it becomes a service-affecting error, this value gives an early signal of degrading optical signal quality on the fiber. A rising trend here, even with zero uncorrectable errors, would prompt an admin to check for fiber attenuation or connector issues before the link actually fails.