Synchronous digital hierarchy (SDH) and synchronous optical network (SONET) refer to a group of fiber-optic transmission rates that can transport digital signals with different capacities. This tutorial discusses synchronous transmission standards in world public telecommunications. 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). It. This tutorial provides an overview of SDH/SONET, covering basics, HDLC framing, terminologies, rates, and the SONET STS-1 SDH Frame. They are physical layer. Cisco Services can help you build the right solution for your needs with the combined power of AI, automation, and human expertise. Developed in the late 1980s by the International Telecommunication Union (ITU), SDH was designed to replace the.