This counter accumulates the number of unavailable seconds (UAS) on the local DS1 link during the current 15-minute interval, marking periods when the line was considered out of service. Once a UAS condition is declared (typically 10+ consecutive severely errored seconds or signal loss), all subsequent seconds in that outage are marked as UAS until signal recovers, then a waiting period elapses. Rising UAS values indicate the line has experienced downtime or near-downtime conditions, and the link may need immediate carrier investigation.