Telephone Exchange

In the field of telecommunications, a telephone exchange or telephone switch is a system of electronic components that connects telephone calls. A central office (CO) is the physical building used to house inside plant equipment including telephone switches, which make telephone calls "work" in the sense of making connections and relaying the speech information.

The term exchange area can be used to refer to an area served by a particular switch, but is typically known as a rate center or wire center in the US telecommunications industry. (Local calling areas, in which it is not necessary to pay a long-distance rate, typically cover more than one rate center even in micropolitan and small metropolitan areas.) The exchange code or central office code (often called the prefix by the public) refers to the first three digits of the local number (NXX), though most non-rural exchanges have more than one code or prefix.

Wireless rate centers for mobile phones typically cover a much larger area than landline rate centers in urban and suburban areas, and often have dozens of codes of their own, in addition to those introduced on individual telephone numbers by local number portability (due to customers porting a cancelled landline to a wireless). It is sometimes confused with the area code (NPA), which covers multiple exchanges, and which an exchange can have more than one of due to overlays of new area codes. In the United States, local exchange areas together make up a legal entity called local access and transport areas (LATA) under the Modification of Final Judgment (MFJ).