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A telephone exchange, telephone switch, or central office is a telecommunications system used in the public switched telephone network (PSTN) or in large enterprises. It interconnects telephone subscriber lines or virtual circuits of digital systems to establish telephone calls between subscribers.
In historical perspective, telecommunication terms have been used with different semantics over time. The term telephone exchange is often used synonymously with central office, a Bell System term. Often, a central office is defined as a building used to house the inside plant equipment of potentially several telephone exchanges, each serving a certain geographical area. Such an area has also been referred to as the exchange or exchange area. In North America, a central office location may also be identified as a wire center, designating a facility to which a telephone is connected and obtains dial tone. For business and billing purposes, telecommunication carriers define rate centers, which in larger cities may be clusters of central offices, to define specified geographical locations for determining distance measurements.
In the United States and Canada, the Bell System established in the 1940s a uniform nationwide numbering system of identifying central offices with a three-digit central office code and a three-digit numbering plan area code (NPA code, or area code). Central office codes were unique in each numbering plan area. The NPA code and the central office code were used as prefixes in subscriber telephone numbers. With the development of international and transoceanic telephone trunks, especially driven by direct customer dialing, similar efforts of systematic organization of the telephone networks occurred in many countries in the mid-20th century.
For corporate or enterprise use, a private telephone exchange is often referred to as a private branch exchange (PBX), when it has connections to the public switched telephone network. A PBX is installed in enterprise facilities, typically near large office spaces or within an organizational campus to serve the organization's telephones and any private leased line circuits. Smaller installations might deploy a PBX or key telephone system in the office of a receptionist.
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