SONET (Synchronous Optical Network) is a standard for connecting fiber-optic transmission systems sold in North America only. SONET was proposed by Bellcore in the mid-1980s and is now an ANSI standard. SONET defines interface standards at the physical layer of the Open Systems Interconnection (OSI) seven-layer model. The standard defines a hierarchy of interface rates that allow data streams at different rates to be multiplexed. SONET establishes Optical Carrier (OC) levels from 51.8 Mbps (OC-1) to 9.95 Gbps (OC-192). The international equivalent of SONET, standardized by the International Telecommunication Union (ITU), is called Synchronous Digital Hierarchy (SDH).