Tandem switch

In this image, offices A, X, Y, and Z are local central offices, each serving telephone subscribers. Office "P" is a tandem office, which interconnects local offices A, X, Y, and Z. Note that the local offices can also have direct paths to each other, if necessary.

A tandem switch (or simply a tandem) is a type of telephone switch that connects exchanges to other exchanges. This is in contrast to a traditional class 5 exchange that connects subscribers to other subscribers. In a tandem office, there are no subscribers per se, since all of the incoming and outgoing paths to a tandem are connected to other telephone switches in the nearby network.

Usage

Tandems were usually used in metropolitan areas where the large number of central offices made it difficult to build direct trunks between each of them. Tandems reduced cost and complexity by providing a central point where inter-office calls could be routed, and then sent on their way to the terminating office. This was particularly useful for local offices that were distant from each other, as it saved having to run cables to and from physically distant points.

By the 1950s, tandems were increasingly being used for other purposes as well. In addition to simply being a hub for a metropolitan telephone network, they were used to link central offices that normally could not communicate with each other because they did not share any signaling systems in common. Tandems could natively accept pulsing in one form and easily convert it to a different form before sending it off to the receiving central office. Additionally, tandems were naturally suited as aggregation points for long distance calls. Since many calls to distant areas would have to pass through a tandem anyway, long distance billing equipment could be located there, instead of at each individual central office.