Synchronous Digital Hierarchy (SDH) - ITU G.707 & G.708

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The Synchronous Digital Hierarchy (SDH) has been standardized by the Telecommunication Standardization Sector (ITU-T) of the "International Telecommunication Union (ITU)" based in Geneva, Switzerland.
SDH (G.707 & extensions G.708) is part of the "G-series" of standards, which cover "Transmission systems and media, digital systems and networks".
As the more powerful successor of PDH, SDH started at 155,52 Mbps (STM-1 = Synchronous Transport Module-level 1). It is a synchronous data transmission system with octet-aligned multiplexing.
Typical hierarchy stages are STM-1, STM-4, STM-16 etc. - a multiplexing factor of four.