EMERGENCE OF SDH

SDH emerged in 1990 standards bodies around 1990. SDH has provided transmission networks with a vendor-independent and sophisticated signal structure that has a rich feature set. 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).
There are three primary market drivers that have accelerated the deployment of SONET/SDH technology:
A. Maturity of SONET/ SDH standards;
B. Emergence and growth of new network applications, resulting in an ever increasing need for higher bandwidth, and;
C. Most importantly, increased competition between various service providers resulting in the need for lower operating costs.

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