A circuit-switched network is one that establishes a dedicated circuit (or channel) between nodes and terminals before the users may communicate.
As an example, when a subscriber makes a telephone call, the dialed number is used to set switches in the exchanges along the route of the call so that there is a continuous circuit from the caller to the called party. Because of the switching operation used to establish the circuit, the telephone system is called a circuit-switched network. If the telephones are replaced with modems, then the switched circuit is able to carry computer data.
The internal path taken by the circuit between exchanges is shared by a number of conversations. Time division multiplexing (TDM) gives each conversation a share of the connection in turn. TDM assures that a fixed capacity connection is made available to the subscriber.
If the circuit carries computer data, the usage of this fixed capacity may not be efficient. For example, if the circuit is used to access the Internet, there is a burst of activity on the circuit while a web page is transferred. This could be followed by no activity while the user reads the page, and then another burst of activity while the next page is transferred. This variation in usage between none and maximum is typical of computer network traffic. Because the subscriber has sole use of the fixed capacity allocation, switched circuits are generally an expensive way of moving data.
PSTN and ISDN are two types of circuit-switching technology that may be used to implement a WAN in an enterprise setting.