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 bin is a closed, archived 24-hour interval, admins typically read it the next day to judge whether the previous day as a whole met SLA rather than to catch a fault as it happens. A reading of 5 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.