eaming media-voice and video). SONET is closely associated with ATM, but is not necessarily dependent upon it. Both require the speed that metallic media cannot reliably support over long distances. SONET can carry DS-0 and its multiples, and thereby provide a bridge between the older, slower digital signaling hierarchy and ATM's faster (minimum 155 Mbps) data delivery rates. With its smallest increment of provisioning VT-1.5 (the next increment, at 51.84 Mbps, is STS-1), SONET provides 1.7 Mbps of bandwidth using time division multiplexing to simultaneously transmit multiple dedicated data streams. Bellcore's GR-2837 standard maps ATM cells onto SONET, and turns a SONET network into a cell-switched (packet-switched) pipe that uses the full bandwidth and mitigates ATM's inherent waste. High-bandwidth (e.g., carrier, corporate WAN) lines are increasingly carrying IP traffic directly over SONET without ATM. DWDM fiber is projected to soon carry IP with no need for either ATM or SONET. The North American digital hierarchy specification was developed for copper media. The aggressive deployment of fiber optic cable makes the digital signaling hierarchy (based on DS-0 multiples) highly inefficient. Optical networks can transport data at speeds far in excess of 41.84 Mbps (DS-3). Synchronous optical network (SONET), an optical transmission interface originally proposed by Bellcore-and later standardized by ANSI-was conceived to replace the copper-oriented digital signaling (DS) interface over time, yet remain compatible with it throughout the upgrade process. SONET resumes where the digital signaling hierarchy concludes. The SONET and SDH standards establish a standard multiplexing format that uses 51.84 Mbps channel multiples as building blocks. Lower level ANSI and ITU-T digital signaling hierarchies used a 64 Kbps (DSO, EO) building block. The 51.84 Mbps rate was specified because the overhead involved in framing precluded higher speeds from being exact multiples of the 64Kbps base speed. A 51.84 Mbps signal can carr