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.
References in periodicals archive ?
Course aim: After completing this course a trainee will be able to describe the benefits, features and basic principles of the Synchronous Digital Hierarchy (SDH).
Mindspeed's fiber-optic portfolio spans the entire spectrum of optical networking transceiver solutions from OC-3 to OC-192 data rates, in support of synchronous optical network (SONET), synchronous digital hierarchy (SDH) and gigabit Ethernet/Fibre channel digital data transmission standards.
The migration from a legacy network infrastructure - Synchronous Optical Network (SONET) or Synchronous Digital Hierarchy (SDH) - to metro Ethernet is well under way," said Sterling Perrin, research manager for Optical Networks at IDC.
In the trial, NTT Com deployed GMPLS control plane including the Cisco12000 routers over its commercial wide-area synchronous digital hierarchy (SDH) optical network.
The Cisco ONS 15454 Series provides the functions of multiple traditional Synchronous Optical Network (SONET) or Synchronous Digital Hierarchy (SDH) network elements in a single platform.

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