This counts Available Seconds on this SONET/SDH port for this bin: the seconds during the interval that were not counted as unavailable, per the MIB's description. AS is essentially the complement of UAS for this bin on this port -- it's the 'good time' denominator admins use alongside Total Time to compute the actual uptime percentage for this specific interval, rather than working the number backward from the unavailable count alone; because this specific bin belongs to the completed-interval history table, its value is frozen the moment the interval closes, so re-polling it later will return the same number -- useful for pinpointing exactly which past 15 minutes a problem occurred in. For a 900-second bin, an AS value of 801 would correspond directly to 99 unavailable seconds in that same window on this port.