nbsCohpmHistoricAveCD is read-only and holds a finalized statistic from a completed past reporting interval for the average chromatic dispersion (CD) on this coherent optical interface, i.e. how much the fiber link has spread out the optical signal in time due to wavelength-dependent propagation delay; not-supported is encoded as 0x80000000. Unit measurement: 1 ps/nm. Because it belongs to a completed interval selected by nbsCohpmHistoricSample (1 = most recent), a sysadmin pulls a run of these to build a trend of this port's optical health over recent 15-minute or 24-hour periods rather than relying on a single live reading.