A count of Severely Errored Seconds — seconds whose error or defect rate was high enough to be classed as severely degraded rather than just errored — recorded under the IETF DS1/E1 near-end performance-monitoring convention, for one historical 15-minute interval selected by the archive-interval index. Because SES weighs much worse than an ordinary errored second, an admin treats a nonzero count here as a stronger signal of real circuit trouble than the plain ES counter.