SdH

SDH, or Synchronous Digital Hierarchy, is a standardized multiplexing scheme for transporting multiple digital bit streams over optical fiber. Developed by the ITU-T, it was designed to replace older PDH systems, enabling easy interconnection of equipment from different vendors and providing robust synchronization, management, and protection mechanisms. In North America, a closely related standard called SONET serves a similar role, but SDH is the international counterpart.

At the heart of SDH is the transmission frame. The basic transport rate is STM-1, which carries a payload at 155.52 Mbps. An STM-1 frame consists of nine rows and 270 columns of bytes transmitted every 125 microseconds. The frame carries overhead for section, line, and path signals, and a synchronous payload envelope that carries one or more virtual containers (VCs).

Virtual containers are multiplexed into various levels and form the synchronous transport module structure (STM-n). Pointers inside the overhead indicate the position of the payload within the frame, allowing payload alignment despite clock differences. This design supports flexible multiplexing and easy addition or dropping of tributaries at network nodes.

SDH provides hierarchical scaling: STM-1 is the basic unit; multiple STM-1s can be multiplexed to higher levels such as STM-4 (622 Mbps), STM-16 (2.488 Gbps), and STM-64 (9.953 Gbps). The architecture supports rapid protection switching, performance monitoring, and network management, enabling ring and mesh topologies and fault restoration.

Although overtaken in some new deployments by IP and dense wavelength division multiplexing (DWDM), SDH remains widely used for backbone transport, ensuring interoperability across international networks and continuing to transport legacy PDH and other data traffic alongside modern services.