A read-only counter tracking the cumulative number of Unavailable Seconds this interface has experienced. Unavailable Seconds is a standard circuit-quality metric representing time the interface was considered out of service, so this counter gives an admin a running total of actual downtime accumulated on that interface, useful for SLA reporting. A steadily climbing pdnIfExtTotalUASs count on a WAN interface would indicate the underlying circuit is experiencing repeated outages worth escalating to the carrier.