This OID counts, in blocks, how many FEC blocks on this OTU4 (100G optical transport) interface contained more bit errors than forward error correction could fix, so the block was passed on still containing errors. Unlike the corrected bit-error counter, a nonzero value here means real, unrepaired data corruption reached the far end, a much more serious signal-quality problem than errors FEC successfully corrects. For example, if this counter on a 100G OTU4 wavelength spanning a metro DWDM link starts incrementing after sitting at zero for months, the operator should suspect a sudden change such as a bad connector or a failing optical amplifier rather than routine noise.