Overview#Telephone exchange (Central Office) is a telecommunications system used in the Public Switched Telephone Network or in Private Branch Exchange which consists of electronic components and in older systems also human operators that interconnect (switch) 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 (CO), 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. Telephone exchange locations may also be identified in North America as wire centers, designating a facility from which a telephone obtains dial tone. For business and billing purposes, telephony carriers also define rate centers, which in larger cities may be clusters of central offices, to define specified geographical locations for determining distance measurements.