SDHSONET

SDH/SONET is a standardized family of digital transport protocols for high-capacity communication over optical fiber. It provides a synchronous, multiplexed framework that carries diverse data streams with centralized timing and robust network management. The terms SDH (Synchronous Digital Hierarchy) and SONET (Synchronous Optical Networking) describe essentially the same technology, developed by ITU-T and ANSI respectively. In practice, equipment often supports both and uses mapping between the corresponding rates, enabling interoperability across regions.

The architecture centers on a synchronous frame with overhead and payload. Traffic is carried at hierarchical rates, starting from a basic rate (STS-1/OC-1 or STM-1 around 51.84 Mbps/155.52 Mbps) and building higher-rate signals by multiplexing. The payload is organized into virtual containers in the SDH sense (VC-4) and the SONET sense (VTs and STSs), nested within higher-rate containers. Overheads are divided into section, line, and path categories, providing maintenance signaling, framing, and network management. A pointer mechanism allows payloads to be flexibly located within the frame to accommodate clock differences and drift.

Protection and reliability are central features, with mechanisms for rapid protection switching and ring topologies. Network elements such as Add/Drop Multiplexers (ADMs) and cross-connects enable flexible grooming and restoration of traffic. SDH/SONET remains a foundational transport technology in many backbone and metro networks, often in conjunction with dense wavelength-division multiplexing and evolving toward Optical Transport Networks (OTN) and IP-enabled services.

The term SDH/SONET is sometimes merged as SDHSONET to denote the combined standard family; there is no separate standard by that name.