This read-only integer (0..0xffffff) is a bitmap where each set bit represents a DS0 on physical line 1 that has already been claimed by some logical port defined so far on that DS1; the most significant byte is not meaningful for a DS1. An administrator reads this before creating a new channelized logical port on line 1 to see which DS0 timeslots remain free. For example, a portsUsedLine1 value with only the low 8 bits set shows the first 8 DS0s on line 1 are already assigned to logical ports, leaving the remaining DS0s free for a new port.