 analog signal is first sampled at a 8-kHz sampling rate; then each sample is quantized into 1 of 256 levels and then encoded into digital eight-bit words. This encoding process is illustrated in Figure 3.7. The overall data rate of one speech signal becomes 8,000 X 8 = 64 Kbps. This same data rate is available for data transmission through each speech channel in the network. In the United States one bit of eight in every sixth frame is "robbed" for in-band signaling and the available transparent data capacity of a single speech channel in the network is reduced to 8,000 X 7 = 56 Kbps. Now we take a more detailed look at the three main processing phases of the PCM in the telecommunications network. Note that this principle is employed by all systems when there is a need to process analog signals with a digital system. Sampling rates and the number of quantizing levels vary from application to application, but the basic principle and phases of the process remain the same. 3.6.1 Sampling The amplitude of an analog signal is sampled first. The more samples per sec- ond there are, the more representative of the analog signal the set of samples Signals Carried over the Network Sampling Quantizing Encoding 10101111 01101101 Each quantized sample is encoded into an 8-bit code word Sample at twice Round off samples the highest voice to one of 8,000*8 bits = frequency 256 levels 64 Kbps 2*4,000 Hz = 8,000 Hz Figure 3.7 PCM. will be. After sampling, the signal value is known only at discrete points in time, called sampling instants. If these points have a sufficiently close spac- ing, a smooth curve drawn through them allows us to interpolate intermedi- ate values to any degree of accuracy. We can therefore say that a continuous curve can be adequately described by the sample values alone. In a similar fashion, an electrical signal can be reproduced from an appropriate set of instantaneous samples. The number of samples per second is called the sampling frequency or sampling rate, and it depends on the high- est frequency 