This PerfCounter64 tallies the number of Errored Seconds recorded on a SONET/SDH Line during the closed history bin, i.e. one-second intervals in which the line-layer overhead detected at least one anomaly. Because Line ES is a coarse first-tier quality indicator, an admin uses it to spot a line that is degrading before the more severe SES or UAS counters start climbing. Seeing ocnStmHistoryLineESs read 40 out of a 900-second 15-minute bin on an OC-12 line would prompt a check of the physical connectors and regenerator spans feeding that line.