heorem (i.e., the Nyquist theorem) that was developed stated that in order to reproduce voice in a toll-quality manner, you have to sample that voice at twice the rate of the highest frequency carried. The highest frequency being carried in the telephone channel was 4,000Hz, SO we needed a sampling rate of 8,000 samples per second. Remem- ber that a sample is a measurement of the amplitude, or voltage, of the signal at a particular point. Say that the amplitude of a signal is +1.25 volts. When we convert that amplitude value into a binary expression (an 8-bit word), we have 8,000 sam- ples per second and 8 bits per sample, resulting in 64Kbps required to carry voice in a digital manner. This is how the 64Kbps channel was derived as the basic build- ing block of PDH. ADPCM As networks become more digitized, fewer conversions take place, and voice can be carried at a higher quality over fewer bits per second. Another stan- dard used in the PSTN is Adaptive Differential PCM (ADPCM), which carries digi- tal voice at the compressed rates of 40Kbps, 32Kbps, 24Kbps, and 16Kbps. ADPCM does something wonderful for an end user. Say you have a traditional T-1 line with PCM channel banks. Over that one T-1, you can extract 24 channels, each of which carries 64Kbps. But your traffic increases, and you need more channels to carry voice traffic. You have two options: You can add another T-1 line, which means a substantial monthly investment, or you can put ADPCM channel banks Chapter 4 The PSTN on the T-1 that you already have, which gives you 48 channels of 32Kbps each. In essence, you can double the capacity of the network without having to add more subscriber lines. Needless to say, voice compression continues to be applied, and not just in the PSTN. For instance, wireless networks such as cellular networks, where spectrum is at a premium, compress voice down to 8Kbps SO that they can support more call- ers within each of the cells. T-Carrier and E-Carrier Signal Hierarchy Because competition has entered the marketplace, dif