This read-only PerfTotalCount reports the count of transmit underruns, where the DSL port's transmitter ran out of data to send before the next bit was due, scoped to the full previous calendar day's totals for the DSL port. Operationally, underruns point to the CPE or line card failing to keep the transmit pipeline fed, which can corrupt outgoing frames and hurt throughput. For example, an admin responding to a complaint filed this morning can check yesterday's total underrun count to see if the problem was already brewing the day before.