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Figure 1-13 Operators at early switchboard.
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Figure 1-14 A Strowger Telephone.
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Equally important as the development of the central office switch was the concept of multiplexing, which enabled multiple conversations to be carried simultaneously across a single shared physical circuit. The first such systems used frequency-division multiplexing (FDM), a technique made possible by the development of the vacuum tube, in which the range of available frequencies is divided into chunks that are then parceled out to subscribers. For example (and this is only an example), Figure 1-15 illustrates that subscriber 1 might be assigned the range of frequencies between 0 and 4,000 Hz, whereas subscriber 2 is assigned 4,000 to 8,000 Hz, 3 is assigned 8,000 to 12,000 Hz, and so on, up to the maximum range of frequencies available in the channelized system. In frequency-division multiplexing, we often observe that users are given some of the frequency all of the time, meaning that they are free to use their assigned frequency allocation at any time, but may not step outside the bounds given to them. Early FDM systems were capable of transporting 24-4 KHz channels, for an overall system bandwidth of 96 KHz. Frequency-division multiplexing, although largely replaced today by more efficient systems that we will discuss later, is still used in analog cellular telephone and microwave systems, among others.
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Figure 1-15 Frequency division multiplexing.
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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 20,000 Hz
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This model worked well in early telephone systems. Because the lower regions of the 300 3,300 Hz voiceband carry the frequency components that provide recognizability and intelligibility, telephony engineers concluded that, although the higher frequencies enrich the transmitted voice, they are not necessary for calling parties to recognize and understand each other. This understanding of the makeup of the human voice helped them create a network that was capable of faithfully reproducing the sounds of a conversation while keeping the cost of consumed bandwidth to a minimum. Instead of assigning the full complement of 10 KHz to each end of a conversation, they employed filters to bandwidth-limit each user to approximately 4,000 Hz, a resource savings of some 60 percent. Within the network, subscribers were frequency-division multiplexed across shared physical facilities, thus allowing the telephone company to efficiently conserve network bandwidth. Time, of course, changes everything. As with any technology, frequencydivision multiplexing has its downsides. It is an analog technology and therefore suffers from the shortcomings that have historically plagued all transmission systems. The wire over which information is transmitted behaves like a long-wire antenna, picking up noise along the length of the transmission path and effectively homogenizing it with the voice signal. Additionally, the power of the transmitted signal diminishes over distance, and if the distance is far enough, the signal will have to be amplified to make it intelligible at the receiving end. Unfortunately, the amplifiers used in the network are not particularly discriminating: they have no way of separating the voice noise. The result is that they convert a weak, noisy signal into a loud, noisy signal. This is better, but far from ideal. A better solution was needed. The better solution came about with the development of Time-Division Multiplexing (TDM), which became possible because of the transistor and integrated circuit electronics that arrived in the late 1950s and early 1960s. TDM is a digital transmission scheme, which implies a small number of discrete signal states, rather than the essentially infinite range of values employed in analog systems (the word digital literally means discrete). Although digital systems are just as susceptible to noise impairment as their analog counterparts, the discrete nature of their binary signaling makes it relatively easy to separate the noise from the transmitted signal. Digital carrier systems have only three valid signal values: one positive, one negative, and zero; anything else is construed to be noise. It is therefore a trivial exercise for digital repeaters to discern what is desirable and what
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