Definition[]
Circuit switching is a specific type of communications in which a dedicated channel (or circuit) of constant bandwidth is established for the duration of the communications.
Overview[]
Voice telephony is based on circuit switching. A "circuit" is the particular path of physical wires over which the conversation will be carried. Typically there are many physical wires going among various locations. These wires will meet at various switching points across the country. When a call is made, it goes to a nearby switching station; there the switch must determine which outgoing wire of many the call should be routed on. That outgoing wire may in turn go to another switching station, which again must find an available outgoing wire; etc. This may happen at many different switching points.
Once the various switching points have chosen all the necessary wires, they form an end-to-end path or "circuit"; that path remains constant for the duration of a phone call, whether anyone is actually talking or not. The time delay necessary for the switches to find an available line and making the necessary connection, constitutes the delay one experiences after dialing — the time spent "waiting for a connection."
The disadvantage of this mechanism, compared to the Internet's "packet switching" mechanism, is that it "wastes" the resources of the circuit when there are pauses in the conversation. The advantage is that it ensures that the call will continue without interruption no matter how much the network may become congested. Indeed, congestion in the telephone network only affects those who are initially trying to place a call. If the network is highly congested, they will be unable to make a call at all and will instead receive a "fast busy" signal.