progress bar code in vb.net 2008 Figure 3-64 Role of the regenerator in Software

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Figure 3-64 Role of the regenerator
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Figure 3-65 Fourier series transform of analog wave to digital wave
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multiple of another wave), we see a rather remarkable thing happening: The waveform gets 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 have 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, highfrequency digital signals can be transmitted without restriction. Digitization brings with it 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 allows either 24 or 30 circuits to share a single, 4-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.
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The process of converting analog voice to a digital representation in the modern network is a logical and straight-forward process. It has 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.
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Pulse Code Modulation (PCM)
3
Thanks to the work performed by Harry Nyquist at Bell Laboratories in the 1920s, we know that to optimally represent an analog signal as a digitally encoded bitstream, the analog signal must be sampled at a rate that is equal to twice the bandwidth of the channel over which the signal is to be transmitted. Since each analog voice channel is allocated 4 KHz of bandwidth, it follows that each voice signal must be sampled at twice that rate, or 8,000 samples per second. In fact, that is precisely what happens in T-Carrier systems, which we now use to illustrate our example. The standard T-Carrier multiplexer accepts inputs from 24 analog channels, as shown in Figure 3-66. Each channel is sampled, in turn, every one eight-thousandth of a second in round-robin fashion, resulting in the generation of 8,000 pulse amplitude samples from each channel every second. The sampling rate is important. If the sampling rate is too high, too much information is transmitted and bandwidth is wasted; if the sampling rate is too low, then we run the risk of aliasing. Aliasing is the interpretation of the sample points as a false waveform, due to the paucity of samples. This PAM process represents the first stage of PCM, the process by which an analog baseband signal is converted to a digital signal for transmission across the T-Carrier network. This first step is shown in Figure 3-67. The second stage of PCM, shown in Figure 3-68, is called quantization. In quantization, we assign values to each sample within a constrained range. For illustration purposes, imagine what we now have before us. We have replaced the continuous analog waveform of the signal with a series of amplitude samples which are close enough together that we can
Figure 3-66 Time-division multiplexing (TDM)
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