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.
Main exchange in the core
A tandem switch interconnects with other PSTN switches. It is often called a â€œclass 4â€ switch, a reference to the original AT&T hierarchal network structure.