A read-only Counter32 giving the cumulative number of seconds the link spent out of synchronization since it was last administratively brought up, current as of this operational hour. Because sync loss represents a total, not merely degraded, connectivity interruption, tracking it hourly helps quantify how much real downtime, as opposed to just elevated error rate, the link experienced. For example, an hourly p2mpSu1HrSyncLossSeconds of several hundred seconds would represent meaningful cumulative downtime for that hour, distinct from an hour with plenty of errored seconds but zero sync loss.