Tandem exchange

A class-4, or tandem, telephone switch is a U.S. telephone company central office telephone exchange used to interconnect local exchange carrier offices for long distance communications in the public switched telephone network.

A class-4 switch doesn't connect directly to any telephones; instead, it connects to other class-4 switches and to class-5 telephone switches. The telephones of service subscribers are wired to class-5 switches. When a call is placed to a telephone that is not on the same class-5 switch as the subscriber, the call may be routed through one or more class-4 switches to reach its destination.

Tandem derives from the Latin adverb tandem meaning at length, and is used in English to mean a group of two people or machines working together, usually in series. A tandem switch is used to interconnect other switches via trunks. Thus, tandem switches are always part of a series of switches and lines that connect telephone callers to each other.

A sector tandem switch connects local telephone exchanges (class-5 switches) and carries traffic within the local access and transport area (LATA).

An access tandem switch connects local telephone exchanges to long-distance telephone companies (or interexchange carriers, "IXCs"). The point at which an access tandem connects to the IXC's switch is called the point of presence, or POP.

Modern tandem switches are often located at the center of the areas they serve, and may act as both sector tandems and access tandems.

Before the Bell System divestiture, class-4 switches in a telephone office that had operators present were called "toll centers;" if no operators were present, they were called "toll points." Either type of class-4 switch might be referred to as a "toll switch." These terms were used because long-distance, or "toll," calls had to pass through class-4 switches, where the billing for the calls would be handled.

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