2.5 WAN Link Options
2.5.6 Circuit-switched connections

Circuit switching is a WAN switching method in which a dedicated physical circuit is established, maintained, and terminated through a carrier network for each communication session. Used extensively in telephone company networks, circuit switching operates much like a normal telephone call. ISDN is an example of a circuit-switched WAN technology. 

Circuit-switched connections from one site to another are brought up when needed and generally require low bandwidth. Basic telephone service connections are generally limited to 28.8 kbps without compression, and ISDN connections are limited to 64 or 128 kbps. Circuit-switched connections are used primarily to connect remote users and mobile users to corporate LANs. They are also used as backup lines for higher-speed circuits, such as Frame Relay and dedicated lines.

Web Links
Routing in Circuit-Switched Networks