Tandem or Class 4 switch is used for long distance communications in the PSTN, verses Class 5 switches which is primarily used to terminate local calls. Tandem is a telephony term meaning "to connect in series." Thus, a tandem switch connects one trunk to another. A tandem switch is an intermediate switch or connection between an originating telephone call or location and the final destination of the call.

A famous Class 4/Tandem Switch is called 4ESS, a Lucent Technology switch design for AT&T long distance communications. Currently, most class 5 switches can conduct both local and long distance phone services. In other words, the functions of a Tandem switch is integrated into a class 5 switch in many platforms.

Tandem Switch and Class 4 Switch

Related Terms：Class 5 Switch, PSTN, 4ESS