This read-only Integer32 reports the number of SONET Section-layer Byte Interleaved Parity errors detected on this OC-3 port during the previous, completed 15-minute period. It gives an administrator a finished measurement window to trend section-layer signal quality over time rather than the still-accumulating current period. For example, an operator building an hourly optical-quality report would poll this previous-period value rather than the constantly changing current-period counter.