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is not, thus eliminating the problem of cumulative noise. The role of the regenerator, shown in Figure 1-16, is to receive a weak, noisy digital signal, remove the noise, reconstruct the original signal, and amplify it before transmitting the signal onto the next segment of the transmission facility. For this reason, repeaters are also called regenerators because that is precisely the function they perform. One observation: it is estimated that as much as 60 percent of the cost of building a transmission facility lies in the regenerator sections of the span. For this reason, optical networking, discussed a bit later, has various benefits, not the least of which is the capability to reduce the number of regenerators required on long transmission spans. In a typical network, these regenerators must be placed approximately every 6,000 feet along a span, which means that considerable expense is involved when providing regeneration along a long-haul network. Digital signals, often called square waves, comprise a very rich mixture of signal frequencies. Not to bring too much physics into the discussion, but we must at least mention the Fourier series, which describes the makeup of a digital signal. The Fourier series is a mathematical representation of the behavior of waveforms. Among other things, it notes the following fact. If we start with a fundamental signal such as that shown in Figure 1-17, and
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Figure 1-16 Regeneration of a weak, noisy signal.
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Figure 1-17 Analog to digital conversion.
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Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright 2004 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website.
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mathematically add to it its odd harmonics (a harmonic is defined as a wave whose frequency is a whole-number multiple of another wave), we see a rather remarkable thing happening: the waveform becomes steeper on the sides and flatter on top. As we add more and more of the odd harmonics (there is, after all, an infinite series of them), the wave begins to look like the typical square wave. Now of course, there is no such thing as a true square wave; for our purposes, though, we ll accept the fact. It should now be intuitive to the reader that digital signals comprise a mixture of low, medium, and high frequency components, which means that they cannot be transmitted across the bandwidth-limited 4 KHz channels of the traditional telephone network. In digital carrier facilities, the equipment that restricts the individual transmission channels to 4-KHz chunks is eliminated, thus giving each user access to the full breadth of available spectrum across the shared physical medium. In frequency division systems, we observed that we give users some of the frequency all of the time; in time-division systems, we turn that around and give users all of the frequency some of the time. As a result, high-frequency digital signals can be transmitted without restriction. Digitization brings a cadre of advantages, including improved voice and data transmission quality; better maintenance and troubleshooting capability, and therefore reliability; and dramatic improvements in configuration flexibility. In digital carrier systems, the time-division multiplexer is known as a channel bank; under normal circumstances, it enables either 24 or 30 circuits to share a single, four-wire facility. The 24-channel system is called T-Carrier; the 30-channel system, used in most of the world, is called E-Carrier. Originally designed in 1962 as a way to transport multiple channels of voice over expensive transmission facilities, they soon became useful as data transmission networks as well. That, however, came later. For now, we focus on voice.
Voice Digitization
The process of converting analog voice to a digital representation in the modern network is a logical and straightforward process. It comprises four distinct steps: Pulse Amplitude Modulation (PAM) sampling, in which the amplitude of the incoming analog wave is sampled every 125 microseconds; companding, during which the values are weighted toward those most receptive to the human ear; quantization, in which the weighted samples are given values on a nonlinear scale; and finally encoding, during which each value is assigned a distinct binary value. Each of these stages of Pulse Code Modulation (PCM) will now be discussed in detail.
Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright 2004 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website.
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