1. A specified period of time, during which bits are transferred from a source to a destination, where 30% of the blocks received are errored, or at least one severely disturbed period occurred. A severely disturbed period occurs when, over a period of time equivalent to four contiguous blocks or 1 ms, whichever is larger, all the contiguous blocks are affected by a high bit error density of 10-2. This definition applies for a specified block size. Note: Where a suitable block is not available, an alternate definition (#2 below) can be used. 2. A specified period of time, during which bits are transferred from a source to a destination, where a bit error ratio worse than 10-3 occurs.