This counts OTN coding violations detected in the framing of this optical transport unit signal -- frame-level bit errors flagged by the OTU overhead's error-checking bytes. Because this reading comes from the still-filling, live 15-minute bin for this OTU signal, a rising CV count on this OTU signal, even without a service-affecting alarm yet, is usually the earliest measurable sign the link's error rate is creeping up before it turns into errored seconds. For example, an OTU signal normally logging single-digit CV counts per bin that jumps to several thousand is the kind of change that should trigger a look at the optical power and Q-factor readings for that same interval, so far within this still-open 15-minute interval.