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Combining Signaling Techniques for Higher Bit Rates
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Let s assume that we are operating in an analog network. Under the standard rules of the analog road, one signaling event represents one bit. For example, a high-amplitude signal represents a one, and a lowamplitude signal represents a zero. But what happens if we want to increase our bit rate One way is to simply signal faster. Unfortunately, the rules of physics limit the degree to which we can do that. In the 1920s, a senior researcher at Bell Laboratories, who has now become something of a legend in the field of communications, came to the realization that the bandwidth of the channel over which the information is being transmitted has a direct bearing on the speed at which signaling can be accomplished across that channel. According to Harry Nyquist, the broader the channel, the faster the signaling rate can be. In fact, put another way, the signaling rate can never be faster than two times the highest frequency that a given channel can accommodate. Unfortunately, the telephone local loop was historically engineered to support the limited bandwidth requirements of voice transmission. The traditional voice network was engineered to deliver 4 KHz of bandwidth to each local loop,2 which means that the fastest signaling rate achievable over a tele-
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One way in which this was done was through the use of load coils. Load coils are electrical traps that tune the local loop to a particular frequency range, only allowing certain frequencies to be carried. This created a problem later for digital technologies, as we will discuss.
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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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phony local loop is 8,000 baud. Yet, during the late 1980s and the early 1990s, it was common to see advertisements for 9,600 baud modems. This is where the confusion of terms becomes obvious: As it turns out, these were 9,600 bit-per-second modems a big difference. Modems with 9,600 baud were patently impossible. This, however, introduces a whole new problem: How do we create higher bit rates over signaling ratelimited (and therefore bandwidth limited) channels To achieve higher signaling rates, one of two things must be done: Either broaden the channel, which is not always feasible, or figure out a way to have a single signaling event convey more than a single bit. It s time to introduce a new word: Baud. Baud is the signaling rate. It may or may not be the same as the bit rate, depending on the scheme being used. Consider the following example. We know from our earlier discussion that we can represent two bits by sending a high-amplitude signal followed by a low-amplitude signal (high-amplitude signal represents a zero, low-amplitude signal represents a one). What would happen though, if we were to combine amplitude modulation with frequency modulation Consider the four waveforms shown in Figure 1-22. By combining the two possible values of each characteristic (high or low, frequency or amplitude), we create four possible states, each of which can actually represent two bits as shown in Figure 1-23. We can have a highbandwidth, high-frequency signal; a high-bandwidth, low-frequency signal; a low-bandwidth, low-frequency signal; and a low-bandwidth, high-frequency signal. Consider what we have just done: We have created a system in which each signaling event represents two bits, which means that our bit rate is twice our signaling rate. Let s take our concept one step farther. Figure 1-24 shows a system in which we are encoding four bits for each signal, a technique known as
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