 additional two service channels, for a total of 32 channels. By definition, this system must sample 8000 times per second because it is also optimized for voice operation; thus its frame period is 125 usec. To accommodate the 32 channels, the gate is open 125/32 usec, or 3.906 usec. 6.2.2 Quantization A PCM system simply transmits to the distant end the value of a voltage sample at a certain moment in time. Our goal is to assign a binary sequence to each of those voltage samples. For argument's sake we will contain the maximum excursion of the PAM wave to within +1 V to -1 V. In the PAM waveform there could be an infinite number of different values of voltage between +1 V and -1 V. For instance, one value could be -0.3875631 V. To assign a binary sequence to each voltage value, we would have to construct a code of infinite length. So we must limit the number of voltage values between +1 V and -1 V, and the values must be discrete. For example, we could set 20 discrete values between +1 V and 1 V, with each value at a discrete 0.1-V increment. Figure 6.1 A PAM wave as a result of sampling a single sinusoid. DIGITAL NETWORKS Analog speech signais Channel 1 adidas Sampler signals signals Channel 2 Quantizer encoder Channel 3 Transmitter Reconstructed analog speech signals Channel 1 Sampler signals signals Channel 2 Decoder Channel 3 Receiver Figure 6.2 A simplified analogy of the formation of a PAM wave. (Courtesy of GTE Lenkurt Demodulator, San Carlos, CA.) Because we are working in the binary domain, we select the total number of discrete values to be a binary number multiple (i.e., 2, 4, 8, 16, 32, 64, 128, 256, etc.). This facilitates binary coding. For instance, if there were four values, they would be as follows: 00, 01, 10, 11. This is a 2-bit code. A 3-bit code would yield eight different binary numbers. We find, then, that the number of total possible different binary combinations given a code of n binary symbols (bits) is 2nd. Thus a 7-bit code would have 128 different binary combinations (i.e., 27 = 128)