This OID counts, in blocks, how many FEC blocks on this OTU2e (10G Ethernet-mapped 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 10G OTU2e tributary wavelength carrying a mapped 10GbE client signal 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.