progress bar code in vb.net Figure 1-22 Di-bit encoding scheme in Software

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Figure 1-22 Di-bit encoding scheme
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Figure 1-23 Di-bit values
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11 10
Low Amplitude
01 00
High
Figure 1-24 Quadrature amplitude modulation (QAM)
0101 0011 0110 0100 0010 0000 1110 0001
1000 1010 1100
1011 1101
quad-bit encoding. This scheme, sometimes called quadrature amplitude modulation (QAM) (pronounced Kw am ), permits a single signal to represent four bits, which means that there is a 4:1 ratio between the bit rate and the signaling rate. Thus, it is possible to achieve higher bit rates in the bandwidth-limited telephony local loop by using multibit encoding techniques such as QAM. The first high bit rate modems (9,600 bitsper-second) used this technique or a variation of it to overcome the
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First Things First
First Things First
design limitations of the network. In fact, these multibit schemes are also used by the cable industry to achieve the high bit rates they need to operate their multimedia broadband networks. There is one other limitation that must be mentioned: noise. Look at Figure 1-25. Here we have a typical QAM graph, but now we have added noise in the form of additional points on the graph that have no implied value. When a receiver sees them, however, how does it know which points are noise and which are data Similarly, the oscilloscope trace shown in Figure 1-26 of a high-speed transmission would be difficult to interpret if there were noise spikes intermingled with the data. There is, therefore, a well-known relationship between the noise level in a circuit and the maximum bit rate that is achievable over that circuit, a relationship that was first described by Bell Labs researcher Claude Shannon, who is widely known as the father of information theory. In 1948, Shannon published A Mathematical Theory of Communication, which is now universally accepted as the framework for modern communications. We won t delve into the complex (but fascinating) mathematics that underlie Shannon s Theorem, but simply know that
Figure 1-25 QAM, this time with noise added to the constellation
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First Things First
Figure 1-26 Oscilloscope trace
1
his conclusions are seminal: The higher the noise level in a circuit, the lower the achievable bandwidth. The bottom line Noise matters. It matters so much, in fact, that network designers and engineers make its elimination the first order of business in their overall strategies for creating high-bandwidth networks. This is one of the reasons that optical fiber-based networks have become so critically important in modern transport systems they are far less subject (but not immune!) to noise, and absolutely immune to the electromagnetic interference that plagues copper-based networks. Cable companies that now offer data services have the same issues and concerns. Every time a customer decides to play installer by adding a cable spur for a new television set in his or her home and crimping the connector on the end of the cable with a pair of pliers instead of a tool specifically designed for the purpose, he or she creates a point where noise can leak into the system, causing problems for everyone. And it gets even more melodramatic than that: According to John Judson, a cable systems maintenance manager in the Midwest, unauthorized connection to the cable network can cause problems that go way beyond unauthorized access to service. He observes:
Cable networks are high-frequency systems. Some of the harmonics created in cable networks just happen to fall within the range of frequencies used in avionics, and therefore have the potential to affect aviation communications and navigation. So when you see the cable truck that looks like a commercial fishing boat cruising the neighborhood with
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