Synchronous Digital Hierarchy. Standardized by the ITU, SDH is a family of digital carrier rates. SDH is the term used by the ITU to refer to SONET OC rates, as they are called in the United States. The basic SDH building block is a rate of 155.52 Mbps, called STM-1. Multiples are at 622.08, 2488.32 and 9953.28Mbps.