Information Technology

Gartner Glossary


SDH/SONET technology differs by region. SONET is sold only in North America. SDH is sold outside North America and comes in two types: SDH (Japan) is sold only in Japan, while SDH (ETSI) is sold in the rest of the world. SDH/SONET supports ring and mesh topologies, and protection of circuits through automatic protection switching (APS) is standard, via various protection programs.


SDH/SONET comes in standardized bit rates, with SDH circuits referred to as synchronous transfer mode (STM)-x (where x = 1, 4, 16, 64 and 256) connections and SONET circuits known as Optical Carrier Rate (OC)-n (where n = 3, 12, 48, 192, 768) connections. The standardized bit rates are:


  • STM-1/OC-3 (155 Mbps).
  • STM-4/OC-12 (622 Mbps).
  • STM-16/OC-48 (2.5 Gbps).
  • STM-64/OC-192 (10 Gbps).
  • STM-256/OC-768 (40 Gbps).


Originally, the SDH/SONET standards were defined as a rigid hierarchy optimized specifically to handle circuit-based voice traffic. With the requirement to support data services becoming more evident, additions were made to the SDH/SONET standards to allow for more flexible traffic handling, especially with regard to data services in general and Ethernet services in particular. New features such as the Generic Framing Procedure (GFP) and LCAS were added to the standards, as well as support for increased control plane functionality as defined in the standards for Automatic Switched Transport Network (ASTN)/Automatic Switched Optical Network (ASON).


In addition, vendors have added cross-connect functionality to SDH/SONET equipment to support add/drop functionality. Also, convergence between SDH/SONET and WDM products has become the norm in the optical transport market.

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