This counter counts the seconds during which the T1 receiver detected no usable signal energy at all on the line, the most severe line-layer defect this table tracks. Any non-zero value here on this port means the physical T1 circuit was completely down for that many seconds, not merely degraded, so this is the counter an admin checks first when a customer reports a total T1 outage. For example, LOSSL jumping to 900 in a bin would correspond to roughly 15 minutes of a fully dead line, consistent with a cut cable or a failed far-end line card.