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). At low transmission rates, data can also be. 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 networks. Buy more and save up to 25% on eligible Cisco switching, routing, wireless, and software products. Get started with the right security solution for you. See more, move faster, go farther.