What is SDH (Synchronous Digital Hierarchy)? - Definition from Whatis.com

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SDH (Synchronous Digital Hierarchy)

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SDH (Synchronous Digital Hierarchy)

SDH (Synchronous Digital Hierarchy) is a standard technology for synchronous data transmission on optical media. It is the international equivalent of Synchronous Optical Network. Both technologies provide faster and less expensive network interconnection than traditional PDH (Plesiochronous Digital Hierarchy) equipment.

In digital telephone transmission, "synchronous" means the bits from one call are carried within one transmission frame. "Plesiochronous" means "almost (but not) synchronous," or a call that must be extracted from more than one transmission frame.

SDH uses the following Synchronous Transport Modules (STM) and rates: STM-1 (155 megabits per second), STM-4 (622 Mbps), STM-16 (2.5 gigabits per second), and STM-64 (10 Gbps).

Related glossary terms: virtual area network (VAN), 10-high-day busy period (10HD busy period), graceful degradation, online, softswitch, maximum transmission unit (MTU), traffic shaping (packet shaping), Dialed Number Identification Service (DNIS), WATS (wide-area telephone service), mail user agent (MUA)

This was last updated in November 1999

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