The acronym SDH stands for Synchronous Digital Hierarchy and refers to a multiplex technology used in telecommunications. SDH allows data streams with low bit rates to be combined into high-rate data streams. Since the entire network is synchronous, individual bit streams can be embedded into and extracted from high-rate data streams relatively easily.
Initial specifications were developed back in 1985 in the USA under the term SONET. As a new digital optic transmission system, SONET was to provide key advantages over plesiochronous digital hierarchy (PDH), while still being able to transport PDH. ITU-T standardised SDH based on SONET. SONET and SDH are interoperable. SDH is suitable as a transmission system for broadband ISDN and for transporting ATM cells, PDH signals, Ethernet aggregations, SAN signals and other communication signals.