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Figure 3-13, or buried underground. We note that there may be multiple pairs in each drop wire because today the average household typically orders a second line for a home office, computer, fax machine, or teenager. Once it reaches the edge of the subscriber s property, the drop wire typically terminates on a terminal box, such as that shown in Figure 3-14. There, all of the pairs from the neighborhood are cross-connected to the main cable that runs up the center of the street. This architecture is used primarily to simplify network management. When a new
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Figure 3-13 Aerial drop wire
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Figure 3-14 Terminal box, sometimes known as a BBox
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neighborhood is built today, network planning engineers estimate the number of pairs of wire that will be required by the homes in that neighborhood. They then build the houses, install the network and crossconnect boxes along the street, and cross connect each house s drop wire to a single wire pair. Every pair in the cable has an appearance at every terminal box along the street, as shown in Figure 3-15. This allows a cable pair to be reassigned to another house elsewhere on the street, should the customer move. It also allows cable pairs to be easily replaced, should a pair go bad. This design dramatically simplifies the installation and maintenance process, particularly given the high demand for additional cable pairs today. This design also results in a challenge for network designers. These multiple appearances of the same cable pair in each junction box are called bridged taps. They create problems for digital services because electrical signal echoes can occur at the point where the wire terminates at a pair of unterminated terminal lugs. Suppose, for example, that cable pair number 117 is assigned to the house at #A2 Blanket Flower Circle. It is no longer necessary, therefore, for that cable pair to have an appearance at other terminal boxes because it is now assigned. Once the pair has been cross-connected to the customer s local loop drop wire, the technician should remove the appearances at other locations along the street by terminating the open wire appearances at each box. This eliminates the possibility of a signal echo occurring and creating errors on the line, particularly if the line is digital. ISDN is particularly susceptible to this phenomenon. When outside plant engineers first started designing their networks, they set them up so that each customer was given a cable pair from their house all the way to the central office. The problem with this model was cost. It was very expensive to provision a cable pair for each customer.
Figure 3-15 Along the street, terminal boxes (sometimes called B-Boxes) are installed. Each B-Box hosts an appearance of every pair of wires in the cable, which may contain as many as 500 pairs. With this design, every pair in the cable is potentially available to every home or business along the street, making installation extremely flexible.
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With the arrival of time-division multiplexing, however, the one dog, one bone solution was no longer the only option. Instead, engineers were able to design a system under which customers could share access to the network as shown in Figure 3-16. This technique, known as a subscriber loop carrier (SLC), uses a collection of T1 carriers to combine the traffic from a cluster of subscribers and, thus, reduce the amount of wire required to interconnect them to the central office. The best known carrier system is called the SLC-96 (pronounced SLICK), originally manufactured by Western Electric/Lucent, which transports traffic from 96 subscribers over four, four-wire T-Carriers (plus a spare). A remote SLC terminal is shown in Figure 3-17. Thus 96 subscribers require only 20 wires between their neighborhood and the central office instead of the 192 wires that would otherwise be required. The only problem with this model is that customers are by and large restricted to the 64 Kbps of bandwidth that loop carrier systems assign to each subscriber. That means that subscribers wishing to buy more than 64 Kbps such as those that want DSL are out of luck. And since it is estimated that as many as 70 percent of all subscribers in the United States are served from loop carriers, this poses a problem that service providers are scrambling to overcome. New versions of loop carrier standards and technologies such as GR-303 and optical remotes that use fiber instead of copper for the trunk line between the remote terminal and the central office terminal go a long way toward solving this problem by making bandwidth allocation far more flexible. There is still quite a ways to go, however. Typically, as long as a customer is within 12,000 feet from a central office (CO) they will be given a dedicated connection to the network, as shown in Figure 3-18. If they are farther than 12,000 feet from the CO, however, they will normally be connected to a subscriber loop carrier system of one type or another.
Figure 3-16 Subscriber loop carrier system Customers share access to the network via a collection of multiplexed facilities to reduce outside plant cost.
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