SDH

Synchronous Digital Hierarchy, more commonly known as SDH, is a standard technology that enables synchronous data transmission over optical media. The optical media may include CD/DVD technology and similar storage mediums. SDH is internationally equivalent to SOD (Synchronous Optical Network). Both of these systems provide a faster, more affordable way to establish network interconnection as opposed to PDH (Plesiochronous Digital Hierarchy) technology. As it relates to digital phone transmissions, synchronous can be defined as the data in bits from a single call forwarded within a single transmission frame. In the same field, plesiochronous can be defined as almost but not quite synchronous. This means that a call is required to be extracted from more than one transmission frame. SDH uses various STMs (Synchronous Transport Modules) and transfer rates: STM-1 (155 megabits per seconds), STM-4 (622 megabytes per second), STM-16 (2.5 gigabits per second) and STM-64 (10 gigabytes per second).