Synchronous Digital Systems Hierarchy (SDH) - definition
Synchronous Digital Systems Hierarchy (SDH) - definition
Synchronous Digital Hierarchy (SDH) is a telecommunications standard developed to unify and streamline the transmission of large amounts of data across optical and digital networks. It is based on the concept of synchronising all network elements to a common reference clock, which enables efficient, synchronised transport of information streams in a hierarchical structure. SDH was developed in the 1980s as a response to the limitations of earlier, non-synchronous transmission systems such as PDH (Plesiochronous Digital Hierarchy).
In the SDH structure, data is organised into Frames of fixed length and repetition period, allowing easy insertion, extraction and management of channels without the need for full signal reconversion. This enables the dynamic allocation of bandwidth and the integration of different types of data - voice, video, IP transmission - into a single transmission medium. The basic element in SDH is the STM (Synchronous Transport Module) transport unit, the successive levels of which correspond to different throughput values.
SDH offers advanced error detection, path reconfiguration and redundancy mechanisms, making it a highly reliable system, especially in critical applications such as telecom operators' backbone networks or industrial infrastructures. The SDH architecture is optimised for transmission over fibre, although it can also be implemented over the copper medium. The system also ensures international compatibility, facilitating global network integration, and forms the basis for modern transmission solutions, including integration with MPLS, ATM and WDM-class optical networks.
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