The Network in Software

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The Network
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Thankfully, work performed at Bell Laboratories at the beginning of the twentieth century helped network designers confront this challenge head-on. To understand it, let s take a tour of the telephone network. The typical network, as shown in Figure 3-69, is divided into several regions: the access plant; the switching, multiplexing, and circuit connectivity equipment (the central office); and the long-distance transport plant. The access and transport domains are often referred to as the out-
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Figure 3-69 Network regions. Access Plant
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Transport Plant
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Access Plant
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Telephony
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side plant, whereas the central office is, conversely, the inside plant. The outside plant has the responsibility of aggregating inbound traffic for switching and transport across the long haul as well as terminating traffic at a particular destination. The inside plant, on the other hand, has the responsibility of multiplexing incoming traffic streams, switching the streams, and selecting an outbound path for ultimate delivery to the next central office in the chain or the final destination. Switching therefore is centrally important to the development of the modern network.
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Multiplexing
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Equally important as the development of the central office switch is the concept of multiplexing, which enables 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, Figure 3-70 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 given 8,000 to 12,000 Hz, and
Figure 3-70 Frequency division multiplexing.
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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Telephony
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so on, up to the maximum range of frequencies available in the channelized system. In FDM, 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. FDM, while largely replaced today by more efficient systems that will be discussed later, is still used in analog cellular telephone and microwave systems, among others. This model worked well in early telephone systems. Because the lower regions of the 300 to 3,300 Hz voiceband carried the frequency components that enable 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 FDMed across shared physical facilities, thus enabling the telephone company to efficiently conserve network bandwidth. Time, of course, changes everything. As with any technology, FDM 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 longwire antenna, picking up noise along the length of the transmission path and very 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 signalbetter, 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
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