versa. If the network is fully digital, analog voice is encoded into digital form at the transmitting end and decoded into analog form at the receiving end, as shown in Figure 3.6. This coding is performed in the subscriber interface of a digital telephone exchange and, in the case of ISDN service, in the subscrib- er's ISDN telephone or network terminal. This process has two main phases, as shown in Figure 3.6: 1. Analog-to-digital conversion (A/D): An analog signal is sampled at the sampling frequency and the sample values are then represented as numerical values by the encoder. These values, presented as Analog Digital signal, electrical Analog sample sample values pulses in serial form signal Encoder Connection Sample through the Microphone and hold network circuit Digital Ear piece Reconstructed telecommunications samples network (C) (E Decoder Filter Figure 3.6 Analog voice signal through a digital network. Introduction to Telecommunications Network Engineering binary words, are then transmitted within regular time periods through the digital channel. 2. Digital-to-analog conversion (D/A): At the other end of the channel, the decoder receives numerical values of the samples that indicate the values of the analog signal at sampling instants. The sample pulses that have amplitudes corresponding to the values of the origi- nal signal at sampling instants are reconstructed and the series they form is filtered to produce an analog signal close to the original one. The methods for these A/D and D/A conversions have to be specified in detail SO that the reproduction of the analog signal is compatible with the production of the digital signal that may have occurred on the other side of the world. In the next section we describe the method that is used in the tele- communications network and internationally standardized by the ITU. 3.6 PCM PCM is a standardized method that is used in the telephone network to change an analog signal to a digital one for transmission through the digital telecommunications network. The