Synchronous Digital Hierarchy

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Synchronous Digital Hierarchy

(communications, standard)
(SDH) An international digital telecommunications network hierarchy which standardises transmission around the bit rate of 51.84 megabits per second, which is also called STS-1. Multiples of this bit rate comprise higher bit rate streams. Thus STS-3 is 3 times STS-1, STS-12 is 12 times STS-1, and so on. STS-3 is the lowest bit rate expected to carry ATM traffic, and is also referred to as STM-1 (Synchronous Transport Module-Level 1).

The SDH specifies how payload data is framed and transported synchronously across optical fibre transmission links without requiring all the links and nodes to have the same synchronized clock for data transmission and recovery (i.e. both the clock frequency and phase are allowed to have variations, or be plesiochronous).

SDH offers several advantages over the current multiplexing technology, which is known as Plesiochronous Digital Hierarchy. Where PDH lacks built-in facilities for automatic management and routing, and locks users into proprietary methods, SDH can improve network reliability and performance, offers much greater flexibility and lower operating and maintenance costs, and provides for a faster provision of new services.

Under SDH, incoming traffic is synchronized and enhanced with network management bits before being multiplexed into the STM-1 fixed rate frame.

The fundamental clock frequency around which the SDH or SONET framing is done is 8 KHz or 125 microseconds.

SONET (Synchronous Optical Network) is the American version of SDH.
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References in periodicals archive ?
Tellabs and Nokia Networks have integrated the Tellabs 8100 network manager into Nokia's NetAct radio access network management system to add Asynchronous Transfer Mode-layer (ATM) management to the existing Plesiosynchronous and Synchronous Digital Hierarchy (PDH and SDH) functions.
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Synchronous Digital Hierarchy products are boxes that are used for transmission purposes on telecoms channels and form part of Marconi's core Optical Networking portfolio.
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It will carry all types of telecommunications traffic, from conventional telephone calls to the Internet, using Marconi's SDH (synchronous digital hierarchy) and ring-based WDM (wave division multiplexing).
Since then it has built a 7,000-km high capacity network based on SDH (Synchronous Digital Hierarchy) and DWDM (Dense Wave Division Multiplexing) technology with breakout points at more than 2,000 UK towns and cities.
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Other telecommunications-related projects now underway within the French company include work on optical switching technology and synchronous digital hierarchy, as well as central switching, base stations, and mobile terminals for second- and third-generation digital mobile cellular communications networks.
The telecomm company will reportedly supply an optical network based on Synchronous Digital Hierarchy (SDH) and Dense Wavelength Division Multiplexing (DWDM) technologies.
In the Baltic states the new architecture of the GEANT2 network would be based on Wavelength and Synchronous Digital Hierarchy technologies at 2.5 gigabits per second.

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