First Things First
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signals weaken faster than lower frequencies and, therefore, don t travel as far without amplification and regeneration. Bandwidth is the last characteristic that we will discuss here, and the quest for more of it is one of the great challenges of telecommunications. Bandwidth is a measure of the number of bits that can be transmitted down a facility in any one-second period. In most cases it is a fixed characteristic of the facility and is the characteristic that most customers pay for. The measure of bandwidth is bits-per-second, although today the measure is more typically thousands (kilobits), millions (megabits), or billions (gigabits) per second. Facilities are often called channels because physical facilities are often used to carry multiple streams of user data through a process called multiplexing. Multiplexing is the process of allowing multiple users to share access to a transport facility, either by taking turns or using separate frequencies within the channel. If the users take turns, as shown in Figure 1-10, the multiplexing process is known as time-division multiplexing (TDM), because time is the variable that determines when each user gets to transmit through the channel. If the users share the channel by occupying different frequencies, as shown in Figure 1-11, the process is called frequency-division multiplexing (FDM), because frequency is the variable that determines who can use the channel. It is often said that in TDM, users of the facility are given all of the frequency some of the time, because they are the only one using the channel during their timeslot. In FDM, users are given some of the frequency all of the time, because they are the only one using their particular frequency band at any point.
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Figure 1-10 Time-division multiplexing (TDM)
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Subscriber 1: 0 4,000 Hz Subscriber 2: 4,000 8,000 Hz Subscriber 3: 8,000 12,000 Hz Subscriber 4: 12,000 16,000 Hz Subscriber 5: 16,000 0,000 Hz
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Figure 1-11 Frequencydivision multiplexing (FDM)
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Analog versus Digital Signaling: Dispensing with Myths
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FDM is normally considered to be an analog technology, while time-division multiplexing is a digital technology. The word analog means something that bears a similarity to something else, while the word digital means discrete. Analog data, for example, typically illustrated as some form of sine wave such as that shown in Figure 1-12, is an exact representation of the values of the data being transmitted. The process of using manipulable characteristics of a signal to represent data is called signaling.
Figure 1-12 Sine wave
First Things First
We should also introduce a few terms here just to keep things marginally confusing. When speaking of signaling, the proper term for digital is baseband, while the term for analog signaling is broadband. When talking about data (not signaling), the term broadband means big channel. The sine wave, undulating along in real time in response to changes in one or more parameters that control its shape, represents the exact value of each of those parameters at any point in time. The parameters are amplitude, frequency, and phase. We will discuss each in turn. Before we do, though, let s relate analog waves to the geometry of a circle. Trust me this helps. Consider the diagram shown in Figure 1-13. As the circle rolls along the flat surface, the dot will trace the shape shown by the dotted line. This shape is called a sine wave. If we examine this waveform carefully, we notice some interesting things about it. First of all, every time the circle completes a full revolution (360 degrees), it draws the shape shown in Figure 1-14. Thus, halfway through its path, indicated by the zero point on the graph, the circle has passed through 180 degrees of travel. This makes sense, since a circle circumscribes 360 degrees. The reason this is important is that we can manipulate the characteristics of the wave created in this fashion to cause it to carry varying amounts of information. Those characteristics amplitude, frequency, and phase can be manipulated as follows. Amplitude Modulation Amplitude is a measure of the loudness of a signal. A loud signal, such as that currently thumping through the window of my office from the subwoofer in the back window of the car that belongs to the kid across the street, has high-amplitude components (but very low-frequency components, as evidenced by the fact that I can hear him coming when he s still in southern Vermont), while lower volume signals are lower in amplitude. Examples are shown in Figure 1-15. The