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 a closed, archived 24-hour interval, admins typically read it the next day to judge whether the previous day as a whole met SLA rather than to catch a fault as it happens. 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.