In reference to equipment that is electrically powered by the telecommunications circuit to which it connects, thereby eliminating the need for local power. The analog single line local loop circuits in the public switched telephone network (PSTN) are powered from batteries in the central office (CO) at -48 volts DC (Direct Current), which is sufficient to power conversation over a simple analog telephone set. Ringing current is 110 volts AC (Alternating Current) at 20 Hz in North America. As the typical telephone company CO has an uninterruptible power supply (UPS) comprising multiple power utility circuits, substantial battery packs, and backup diesel generators, the typical analog residential telephone still works even if the lights go out.The telephone companies are said to be the largest power utilities in the world, although that is not the focus of their business. See also UPS.