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 bin is the still-open, currently-accumulating interval, its value is only a partial count until the bin's Duration elapses and it rolls into history -- polling it twice a few minutes apart is the fastest way to catch a fault building up in near real time. For a 900-second bin, an AS value of 877 would correspond directly to 23 unavailable seconds in that same window on this port.