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A circuit connection is a physical, permanent connection that lasts for the duration of a call. If a connection is made over a long distance, many circuits are dedicated to this one call, using substantial resources. Circuit calls are not shared by other traffic. (Replay animation.)

Circuits isolate communicating parties from one another. Although you may hear noise on a line, a circuit-switched connection is meant to be a one-to-one connection, without interference.

Voice communication requires a fixed-bit rate to transmit voice sound. Voice calls require fairly continuous transmission during the course of a connection, so a circuit switched, rather than a packet switched, connection is more effective.

Phone conversations originate with one person speaking into the phone at one end of a connection and another person listening and responding at the other end. But data transmissions originate in a computer environment long before they get to the public data network, and continue past the end point of the network into another computer environment. As such, data transmissions appear to be bursty, with apparent peaks and lulls in transmission. Voice calls, on the other hand, appear to be somewhat continuous.

Moving data in a packet-switched environment is more efficient than tying up a dedicated circuit, while voice calls benefit from a dedicated circuit. However, it is possible to transmit phone conversations over a packet switched network and packets over circuits.

(Also see Packet-switched service.)