ic area. That area typically has a 3- or 4-mile (approximately 15,000- to 20,000-ft) radius. Figure 7.1 illustrates the typical service area of a telephone company central office. The mileage support radius is based on the transmission distance of an analog signal over copper twisted-pair wire. That radius varies because different wire gauge can be used to connect a telephone com- pany end office or central office to each subscriber. Because a thicker wire has less resistance to the flow of electrons, it is possible to extend the radius of support from a central office by using thick wire. To avoid any confusion, it should be noted that wire gauge in the United States is expressed in an inverse relationship based on the American Wire Gauge (AWG) standard; that is, the greater the AWG number, the thinner the wire. Thus, a lower AWG number reflects a thicker wire that has less resistance and extends the radius of support from a central office. Figure 7.1 The typical service area of a telephone Telephone three to four company central Company mile serving area office has a radius of End or 3 to 4 miles from the Central office. Office subscriber line or local loop Chapter Seven The Central Office As a phone call is routed through one or more telephone networks, it will eventually flow to a central office. That office directly serves sub- scribers and routes calls to each subscriber via a copper twisted-pair wire. Because the central office is the last or end office in the hierarchy of telephone company offices, it is also commonly referred to as an end office. The twisted-pair wire installed from the end office to each sub- scriber is referred to as a subscriber line or local loop. Each central or end office originally supported an individual three- digit exchange prefix, such as 477. Because four digits follow the prefix, this scheme resulted in an end office servicing a maximum of 10,000 subscribers. With advances in technology, switches installed in central offices in metropolitan locations may serve subscribers wi