#Server busy switch to siemens step 7 code#
The NPA code and the central office code were used as prefixes in subscriber telephone numbers. Central office codes were unique in each numbering plan area. 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). 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 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. Such an area has also been referred to as the exchange or exchange area. 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. The term telephone exchange is often used synonymously with central office, a Bell System term. In historical perspective, telecommunication terms have been used with different semantics over time. It interconnects telephone subscriber lines or virtual circuits of digital systems to establish telephone calls between subscribers. A modern central office, equipped with voice communication and broadband data capabilitiesĪ telephone exchange, telephone switch, or central office is a telecommunications system used in the public switched telephone network (PSTN) or in large enterprises.
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