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.

Basic functioning of synchronous digital hierarchy

SDH combines n signals with a bit rate of b to form data streams with bit rates of n x b on a synchronously clocked network. Unlike PDH, the individual transmission paths have minimal clock discrepancies. The synchronous mode of operation allows low-order multiplex systems, such as communication links for telephone systems, to be inserted in higher hierarchy levels and then removed again via add and drop. In accordance with the levels defined by the standard, SDH recognises different hierarchies such as STM-1, STM-4, STM-16 or STM-64 (155 MBits, 622 MBits, 2,488 MBits, 9,953 MBits) and more. About five per cent of the gross data rate is reserved for OAM tasks (Operations, Administration and Maintenance). The data is transparently transported over the SDH network in containers.

The SDH components of the various layers

On the physical layer, digital synchronous hierarchy uses connections based on copper lines, fibre optics or satellite and directional radio links. Distorted or muted signals can be refreshed via regenerators. On the superior layer, multiplexers combine the signals into high-bit-rate data streams. Finally, virtual containers transport the individual containers of usable data and control mapping of the various signals of different bit rates.