1) Synchronous digital hierarchy 
Synchronous Digital Hierarchy
(In topic `Network Architectures`) SDH (Synchronous Digital Hierarchy) is an international standard for the transmission of synchronous data over fibre-optic cables. SONET is the European equivalent of SONET used in North America (see SONET).
Synchronous Digital Hierarchy
Synchronous Digital Hierarchy, standards for high bandwidth fibre optic networks defined by European Telecommunications Standards Institute outside North America. SONET is the US equivalent defined by the American National Standards Institute (ANSI).
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