This chapter briefly describes the Synchronous Digital Hierarchy (SDH).
In 1988, the ITU-T (formerly CCITT) came to an agreement on the Synchronous Digital Hierarchy (SDH). The corresponding ITU-T Recommendation G.707 forms the basis of a global, uniform optical transmission network. SDH can operate with plesiochronous networks and therefore allows the continuous evolution of existing digital transmission networks.
The major features and advantages of SDH are:
Compatibility of transmission equipment and networks on a worldwide basis
Possibility of transmitting PDH (Plesiochronous Digital Hierarchy) tributary signals at bit rates commonly used at present
Simple adding and dropping of individual channels without special multiplexers (add/drop facility)
Due to the standardization of the network element functions SDH supports a superordinate network management and new monitoring functions and provides transport capacity and protocols (Telecommunication Management Network, TMN) for this purpose in the overheads of the multiplex signals.
High flexibility and user-friendly monitoring possibilities, e.g. end-to-end monitoring of the bit error ratio.
The basic purpose of SDH is to provide a standard synchronous optical hierarchy with sufficient flexibility to accommodate digital signals that currently exist in today’s network, as well as those planned for the future.
SDH currently defines standard rates and formats and optical interfaces. Today, mid-span meet is possible at the optical transmission level. These and other related issues continue to evolve through the ITU-T committees.
The set of ITU-T Recommendations defines
Multiplexing schemes to map existing digital signals (PDH) into SDH payload signals
Overhead channels to support standard operation, administration, maintenance, and provisioning (OAM&P) functions
For more detailed information on SDH, refer to
ITU-T Recommendation G.703, “Physical/electrical characteristics of hierarchical digital interfaces”, October 1996
ITU-T Recommendation G.707, “Network Node Interface For The Synchronous Digital Hierarchy (SDH)”, March 1996
ITU-T Recommendation G.780, “Vocabulary of terms for synchronous digital hierarchy (SDH) networks and equipment“ , November 1993
ITU-T Recommendation G.783, “Characteristics of Synchronous Digital Hierarchy (SDH) Multiplexing Equipment Functional Blocks “, April 1997
ITU-T Recommendation G.784, “Synchronous Digital Hierarchy (SDH) Management “, January 1994
ITU-T Recommendation G.785, “Characteristics of a flexible multiplexer in a synchronous digital hierarchy environment “, November 1996
ITU-T Recommendation G.813, “Timing characteristics of SDH equipment slave clocks (SEC)“, August 1996
ITU-T Recommendation G.823, “The control of jitter and wander within digital networks which are based on the 2048-kbit/s hierarchy“, March 1993
ITU-T Recommendation G.825, “The control of jitter and wander within digital networks which are based on the synchronous digital hierarchy (SDH)“, March 1993
ITU-T Recommendation G.826, “ Error performance Parameters and Objectives for International, Constant Bit Rate Digital Paths at or Above the Primary Rate”, February 1999
ITU-T Recommendation G.957, “Optical interfaces for equipments and systems relating to the synchronous digital hierarchy“, July 1995
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