Synchronous Optical Networking (SONET) and Synchronous Digital Hierarchy (SDH) are standardized protocols that transfer multiple digital bit streams synchronously over optical fiber using lasers or highly coherent light from light-emitting diodes (LEDs). What is Synchronous Digital Hierarchy? It's a high-speed, synchronous network technology that allows data streams to be efficiently conveyed over vast. This tutorial provides an overview of SDH/SONET, covering basics, HDLC framing, terminologies, rates, and the SONET STS-1 SDH Frame. Developed in the late 1980s by the International Telecommunication Union (ITU), SDH was designed to replace the.