eel that the entire channel is entirely dedicated to them only. In TDM, capacity/channel allocation may be done either bitwise or wordwise. In bitwise allocation, each source is assigned a timeslot corresponding to a single bit, as Figure 3.3 shows for a four-signal multiplexing. In wordwise allocation, a time slot corresponds to some larger number of bits (normally 4 or 8 bits), referred to as a word. The TDM frame structure for the wordwise interleaving of a four-signal system is shown in Figure 3.4. With the advent of semiconductor circuits and the exponential growth of users of the telephone net- work, a new type of digital transmission method, pulse code modulation (PCM), was developed in the 1960s. With PCM, an analogue channel could be transmitted using the intermediate steps of sampling, quantisation and encoding. Thus an analogue telephone voice signal is first sampled at a rate more than the Nyquist criterion rate, i.e. at 3.1 kHz, then is quantised to set certain predefined levels for transmission and finally these predefined quantised levels are encoded, usually by the HDB3 (high density bipolar 3) encoding technique. The signal is then transmitted at the rate of 64 kbps. This is the basic rate of a single telephone voice channel more popularly/technically known as digital signal 0 (DSO). This basic voice channel is a very low bandwidth channel and thus a whole channel cannot be dedicated to it as it would mean a total waste of the bandwidth. There is a need of multiplexing these voice signals, 11112222333344441 Frame Figure 3.4 Four-bit word interleaving Digital Hierarchies - PDH and SDH which could make proper utilisation of the available bandwidth. The level/order of multiplexing them and the method of doing SO categorises the TDM technique into three major types, which are technologies in themselves. These types of multiplexing in the time domain are: Plesiochronous digital hierarchy (PDH) Synchronous digital hierarchy (SDH) Asynchronous transfer mode (ATM) 3.3 Digital Hierarchies - PDH and SDH 3.