This read-only PerfTotalCount reports the lowest noise margin value observed on the DSL port, noting that this is always 0 for IDSL ports since noise margin isn't a measurable parameter there, scoped to one specific 15-minute bucket out of the 96 stored for that DSL port. Operationally, a low minimum noise margin means the line came dangerously close to losing sync at some point, an early warning sign before a line actually drops. For example, the noise margin dropping sharply in one 15-minute bucket, even if it recovers afterward, can be an early warning of a developing line fault before the customer notices any drop in service.