■ Leased lines: Point-to-point connections indefinitely reserved for transmissions, rather than used only when transmission is required. The carrier establishes the connection either by dedicating a physical wire or by delegating a channel using frequency division multiplexing or time-division multiplexing (TDM). Leased-line connections usually use synchronous transmission.

■ Circuit-switched networks: A type of network that, for the duration of the connection, obtains and dedicates a physical path for a single connection between two network endpoints. Ordinary voice phone service over the PSTN is circuit-switched; the telephone company.

■ Packet-switched and cell-switched networks: A carrier creates permanent virtual circuits (PVC) or switched virtual circuits (SVC) that deliver packets of data among customer sites. Users share common carrier resources and can use different paths through the WAN (for example, when congestion or delay is encountered). This allows the carrier to use its infrastructure more efficiently than it can with leased point-to-point links. Examples of packet-switched networks include X.25, Frame Relay, and Switched Multimegabit Data Service.