The optical signal as defined in Synchronous Optical Network (SONET) for North America and Synchronous Digital Hierarchy (SDH) standards for international applications. All SONET levels are expressed in multiples of OC-1 (51.84 Mbps), which is a T3 signal (44.736 Mbps), or the equivalent, plus SONET signaling and control overhead, converted from electrical to optical format. The SONET digital hierarchy begins at OC-1 (51.84 Mbps) and the SDH hierarchy at OC-3 (155.52 Mbps). From OC-3, the progression of SONET and SDH levels are the same, culminating in OC-768 (39.813 Gbps). OC-1536 (79.626 Gbps) and OC-3072 (159.252 Gbps) have yet to be fully defined. See SONET for a table of OC-N levels. See also overhead, SDH, signaling and control, SONET, and T3.