This counts, at the path layer, unavailable seconds, the count of seconds the circuit is formally considered down under the ANSI availability rules on this T3/E3 interface during the current 15-minute performance-monitoring interval, following the ANSI T1.231 performance-monitoring parameter set. It resets every 15 minutes, so an administrator polling it mid-interval sees a still-accumulating count for the current window, and an administrator watching it alongside its Path- or Line-layer sibling counters gets a standard, carrier-comparable picture of the circuit's error performance for that window.