This counter reports the cumulative number of one-second intervals in which the T1 line experienced at least one line-layer error, such as a bipolar violation, since the count began accumulating. Because it is scoped to the physical 'Line' layer rather than the framed 'Path' layer, a rising ESL count on this specific T1 port points to a problem on the copper/fiber facility itself (bad cabling, a connector, or the CSU) rather than a framing or payload issue further up the stack. For example, if this T1 circuit has a marginal connection at the demarc, ESL might climb by a few counts every hour even while the Path-layer ESP counter on the same port stays flat, telling the admin to check the physical line first.