Origin of SDH

             As seen from the previous post about PDH, PDH is a workable but flawed system.At the beginning it was the best available technology and was a giant leap forward in telecom transmission, As a result of growth in the field of silicon chips and integrated microprocessors, customer demand soon provided the need to introduce a new and better system.& it was expected  to solve the existing limitations of PDH.

        As a next step, Bellcore  introduced SYNTRAN (Synchronous Transmission) system. However this was only a development system. Soon it was replaced with SONET (Synchronous Optical Network).Initially SONET could only carry the ANSI (American National Standards Institute) bit rates i.e. 1.5, 6, 45 Mbit/s. Aim of the project was to provide easier international interconnection, Hence, SONET was modified to carry the European standard bit rates of 2, 8, 34 & 140 Mbit/s.

          In 1989 the ITU-T (International Telecommunications Union - Telecommunication's standardisation section), published recommendations which covered the standards for SDH. These were adopted in North America by ANSI (SONET is now thought of as a subset of SDH), making SDH a truly global standard