Overview

As discussed in Chapter 29, "Synchronous Optical Network (SONET)," SONET is the high-speed optical-based architecture for carriers and users alike. This optical-based networking strategy was developed in North America by the American National Standards Institute (ANSI). While the ANSI committees were working on SONET though, another movement was underfoot. In Europe the standards committees were also wrestling with the logical replacement to the plesiochronous digital hierarchy (PDH), which is an asynchronous multiplexing plan to create high-speed communications channels. The Europeans came up with a separate multiplexing hierarchy called the Synchronous Digital Hierarchy (SDH) in support of the SONET standards.

Ever since the standards bodies approved the recommendations for the SDH (and SONET), the services have been effectively used to improve and revolutionize the industry. Significant cost efficiencies and performance improvements have been shown. SDH provides a means for the rest of the world to use the capabilities of fiber-based transport systems and multiplexing architectures to improve upon the older PDH, which was inefficient and expensive. The PDH evolved in response to the demand for plain old telephone services (POTS) and was not ideally suited to deliver the efficient use of bandwidth and high-speed services.

Digital networks continue to expand in complexity and penetration within the carriers' networks, now moving closer to the consumers' door. High-speed communications prior to the formulation of SDH in 1990 operated at speeds of up to 139.364 Mbps. However, the carriers implemented coaxial and radio-based systems operating at 140 Mbps to...