This reports whether this DS1 ESF port's Facility Data Link is currently in service or out of service. Because the FDL carries the performance report and path ID messaging described in the other pportds1FDL* objects, an out-of-service status here on this specific port means none of that overhead messaging is actually functioning, even if the DS1's main payload channel is passing traffic fine. If this port reports outOfService, you would expect pportds1FDLRcvPid and performance report exchange to be stale or absent even while the port's user data traffic continues normally.