Corresponds to DS-4 (Digital Signal Four), the fourth level in the North American T-carrier digital hierarchy. T4 was designed primarily as a metropolitan area transmission system, but could operate over distances of up to 500 miles in the backbone of the public switched telephone network (PSTN). Coaxial cable was originally specified, although optical fiber was later preferred. T4 operates at a signaling rate of 274.176 Mbps, which supports 4,032 DS-0 channels at 64 kbps. Little T4 was installed, and it is now considered obsolete, having been superseded by the SONET fiber optic transmission system (FOTS). See also backbone, channel, digital hierarchy, DS-0, DS-4, FOTS, PSTN, signaling rate, SONET, and T-carrier.