A one-second period that contains >30% errored blocks or at least one severely disturbed period. A severely disturbed period occurs when, over a period of time equivalent to 1 ms, all the contiguous blocks are affected by a high bit error density. Note: A period of loss of signal or a bit error density of >10-2 is considered a period of errored blocks with high bit error density. It is not required to verify this bit error ratio (BER) by an actual in-service or out-of-service measurement.