3.1 Plesiochronous Digital Hierarchy (PDH) The plesiochronous digital hierarchy (PDH) is a technology used in telecommunications networks to transport large quantities of data over digital transport equipment such as fibre optic and microwave radio systems. The multiplexing of basic voice signals is just not enough for the high channels that are used these days. With the advent of optical fibres the capacity/bandwidth of the channel has considerably increased. It is thus required to multiplex the signals at different levels and not only once. Thus the type of application required for a service decides the level/order of multiplexing. For example, services such as the integrated services digital network (ISDN) require more bandwidth for running applications like voice, video and data and thus the bandwidth requirement is greater than required by an ordinary telephone/voice service. The more the order of this digital hierarchy, the greater is the bandwidth of the channel. The term "plesiochronous' is derived from the Greek word plesio, meaning near, and chronous, meaning time. This indicates that PDH networks run in a state where different parts of the network are almost, but not quite perfectly, synchronised. Simply put, the data streams, also called the tributaries, have the same nominal frequency but are not synchronised with each other; i.e. the rising and falling edges of the pulses in each tributary do not coincide. The European and American versions of PDH systems differ slightly in their data rates, but the basic principles of multiplexing are the same. In the North American hierarchy, four primary systems are combined/multiplexed to form an output having 96 channels. This is the second order of multiplexing. Seven 96-channel systems can be multiplexed to give an output of 672 channels (third order of multiplex- ing). Six 672-channel systems are multiplexed to give an output of 4032 channels (fourth order). Higher orders of multiplexing are also available but they employ another technique called the synchro