When customers of the public switched telephone network make telephone calls, they commonly make use of a telecommunications network called a switched-circuit network. In a switched-circuit network, devices known as switches are used to connect the caller to the callee. Each switch has a number of inlets and outlets and by connecting a specific inlet to the correct outlet each switch helps to complete an end-to-end circuit between users [1].
In a modern circuit-switched network, switches can connect any inlet to any outlet; this is known as Full Availability.
[1] Kennedy I., Lost Call Theory, Lecture Notes, ELEN5007: Teletraffic Engineering, School of Electrical and Information Engineering, University of the Witwatersrand, 2005.
[2] Akimaru H., Kawashima K., Teletraffic: Theory and Applications, Springer-Verlag London, 2nd Ed., 1999, p 6.
[3] Farr R.E., Telecommunications Traffic, Tariffs and Costs: An Introduction For Managers, Peter Peregrinus, 1988, p 90.