Synchronous Digital Hierarchy (SDH)
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. It will cover their origins, features, applications, and advantages, as well as their impact on network design and synchronous signal structure.
This tutorial concentrates on the most common form of SDH, that defined by the European Telecommunications Standards Institute (ETSI) for Europe but now used everywhere outside of North America and Japan. The Japanese version of SDH differs only in details that are touched on here but are not significant for the purposes of this tutorial. SONET was defined by the American National Standards Institution (ANSI) and is used in North America. This tutorial refers to SONET where appropriate; a more detailed discussion is available in the International Engineering Consortium’s (IEC's) SONET Web ProForum tutorial. The reader is assumed to be comfortable with the basic concepts of a public telecommunications network, with its separate functions of transmission and switching, and is assumed to be aware of the context for the growth of broadband traffic. No specific prior knowledge is assumed about hardware or software technologies.
This material is based on an article first published in the IEE Electronics & Communication Engineering Journa l, June 1994, and the copyright to that article is owned by the (UK) Institution of Electrical Engineers.
1. Introduction: Emergence of SDH
2. SDH Standards
3. SDH Features and Management
4. Network Generic Applications: Evolutionary Pressures and Operations 5. Network Generic Applications: Equipment and Uses
6. Cross-Connect Types
7. Trends in Deployment
8. Network Design
9. SDH Frame Structure
10. Virtual Containers
11. Supporting Different Rates
1. Introduction: Emergence of SDH
Since their emergence from standards bodies around 1990, SDH and its variant, SONET, have helped revolutionize the performance and cost of telecommunications networks based on optical fibers.
SDH has provided transmission networks with a vendor-independent and sophisticated signal structure that has a rich feature set. This has resulted in new network applications, the deployment of new equipment in new network topologies, and management by operations systems of much greater power than previously seen in transmission networks.
As digital networks increased in complexity in the early 1980s, demand from network operators and their customers grew for features that could not be readily provided within the existing transmission standards. These features were based on high-order multiplexing through a hierarchy of increasing bit rates up to 140 Mbps or 565 Mbps in Europe and had been defined in the late 1960s and early 1970s along with the introduction of digital transmission over coaxial cables. Their features were constrained by the high costs of transmission bandwidth and digital devices. The multiplexing technique allowed for the combining of slightly nonsynchronous rates, referred to as plesiochronous, which lead to the term plesiochronous digital hierarchy (PDH).
The development of optical fiber transmission and large-scale integrated circuits made more complex standards possible. There were demands for improved and increasingly sophisticated services that required large bandwidth, better
Web ProForum Tutorials
The International Engineering Consortium
performance monitoring facilities, and greater network flexibility. Two main factors influenced the form of the new standard:
1. Proposals in Comité Consultif International de Telegraphique et Telephonique (CCITT) (now International Telecommunications...
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