8.2. SYNCHRONOUS DIGITAL HIERARCHY
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 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 led 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 ...