Synchronous Optical Network (SONET) and Synchronous Digital Hierarchy (SDH) refer to a group of fiber-optic transmission rates that can transport digital signals with different capacities. SONET is an ANSI standard for high-speed transmission of digital signals using fiber-optic technology. Officially recognized by the telecommunications industry in the late 1980's, SONET quickly gained popularity in the 1990's due to the high-speed rates, "five-nines" reliability, OAM&P, Quality of Service and the standardization (interoperability). An almost equivalent standard called SDH, Synchronous Digital Hierarchy, was also established by ITU shortly after SONET to address the slightly different digital hierarchies found in Europe, Japan, and the rest of the world. For the most part, SONET and SDH are equivalent.
