This is the Total Time in seconds for this bin on this SONET/SDH port: the full elapsed duration of the measurement interval, per the MIB's description. TT is the denominator every other second-based counter in this same row (ES, SES, UAS, AS) gets divided by to produce a percentage -- without it, none of those raw counts on this port can be turned into the availability figure an SLA actually cares about; 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 nominally 15-minute bin this would typically read 897 seconds (close to 900), and any noticeable deviation from that expected value on this port is itself a clue that the bin was clipped short by a clock adjustment or a mid-interval clear.