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Figure 1-16 Square wave.
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Figure 1-17 Unipolar signaling scheme.
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First Things First
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Figure 1-18 Non-Return to Zero (NRZI) signaling scheme.
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Figure 1-19 Bipolar signaling scheme.
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There are other techniques in use, but these are among the most common. Clearly, both analog and digital signaling schemes can be used to represent digital data depending upon the nature of the underlying transmission system. It is important to keep the difference between data and signaling techniques clearly separate. Data is the information that is being transported, and it can be either analog or digital in nature. For example, music is a purely analog signal because its values constantly vary over time. It can be represented, however, using either analog or digital signaling techniques. The zeroes and ones that spew forth from a computer are clearly digital information, but they too can be represented either analogically or digitally. For example, the broadband access technology known as Digital Subscriber Line (DSL) is not digital at all: there are analog modems at each end of the line, which means that analog signaling techniques are used to represent the digital data that is being transmitted over the local loop.
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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 low ampli-
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First Things First
First Things First
tude 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 done 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 loop2, which means that the fastest signaling rate achievable over a telephony 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 bitper-second modems a big difference. This, however, introduces a whole new problem: How do we create higher bit rates over signal rate-limited (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. 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 1-20. By combining the two possible values of each characteristic (high and low frequency or amplitude), we create four possible states, each of which can actually represent two bits as shown in Figure 1-21. 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. It s time to introduce a new word: Baud.
2 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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