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 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 nominally 15-minute bin this would typically read 903 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.