This counts Line-layer Errored Seconds on this SONET/SDH port: one-second intervals with at least one Line-layer defect during the bin, per the MIB's description. ESL is the first-tier availability metric carriers quote for this specific SONET Line, distinct from the Section-layer ES counted lower in the stack -- a difference between ESL and Section ES on the same bin tells an admin whether trouble is isolated to the Line multiplex or shared with the raw Section signal; because this specific bin belongs to the completed-interval history table, its value is frozen the moment the interval closes, so re-polling it later will return the same number -- useful for pinpointing exactly which past 15 minutes a problem occurred in. A reading of 10 Line errored seconds for this bin would be checked against this port's Section-layer ES for the same interval to see whether the fault is Line-specific or shared.