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Synchronous digital hierarchy (SDH) is an international technology standard that utilizes light-emitting diodes (LED) or lasers for synchronous optical fiber communication.
SDH was developed to eliminate synchronization issues and replace the plesiochronous digital hierarchy (PDH) system for bulk telephone and data exchange.
During the early 1990s, SDH and synchronous optical networking (SONET) ignited an optical fiber industry movement. SDH/SONET were engineered for widely sourced circuit-mode communication transport--for example, digital signal 1 (DS1)—and encoded voice support via pulse-code modulation (PCM).
SDH/SONET specifications are described in Telcordia Technologies Generic Requirements GR-253-CORE, which is respected by many global standards organizations.
Telcordia GR-499-CORE includes general criteria associated with SONET and other transmission systems, for example, asynchronous fiber optic and digital radio.