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 one specific 15-minute bucket out of the 96 stored for that 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 scanning the 96 interval buckets for a port finds underruns concentrated in just one or two 15-minute windows, which narrows a transmit-starvation problem to a specific short period rather than the whole day.