y this huge increase. The demand for greater channel capacity for 'bandwidth-hungry' applications and services, however, meant that more stages of multiplexing were needed throughout the world. This was one of the major reasons for looking out for another technology, which could not only provide more multiplexing stages and thus more channel bandwidth/capacity but also a synchronous service that PDH failed to give. Another major disadvantage was that standards already existed for electrical line interfaces at PDH rates, but there was no standard for optical line equipment at any PDH rate, which meant that it was specific Transmission Network Planning and Optimisation Table 3.4 SONET and SDH transmission speeds SONET signal SDH signal Bit rate [Mbps] STS-1 51.84 STS-3 STM-1 155.52 STS-12 STM-4 622.08 STS-24 1244.16 STS-48 STM-16 2488.32 STS-192 STM-64 9953.28 STS-768 STM-256 39814.32 to each manufacturer. This implied that fibre optic transmission equipment from one manufacturer might not be able to interact/interface with other manufacturers' equipment. As a result, service providers were often required to select a single vendor for deployment in areas of the network and were forced into using the network control and monitoring equipments of that particular vendor. The situation of reconfiguration of PDH networks was particularly difficult in North America, where a plesiochronous system (T-carrier) was in place. Understanding the scenario, Bellcore (the research affiliates of the Bell operating companies in the United States) decided to utilise the technological ad- vances and associated reductions in cost since plesiochronous systems had already been introduced. This made them propose a new transmission hierarchy in 1985. Bellcore's major goal was to create a synchronous system with an optical interface compatible with multiple vendors, but the standardisa- tion also included a flexible frame structure capable of handling either existing or new signals and also numerous facilities built into the signal overhead 