This read-only bitmap shows which DS0 timeslots on physical line 3 have already been claimed by existing logical ports, again with the most significant byte invalid for a DS1 and each set bit marking a used timeslot. An administrator checks this bitmap before building a new port on line 3 to avoid double-assigning a DS0 already in use by another port on that same physical line. If most bits are already set, line 3 is largely allocated and only a handful of DS0s remain available for new ports.