Referring to the internal communications functions that must take place within a network in order to ensure that it operates properly, signaling and control communications are in contrast to the communications of user data, or payload. The various elements of a network must have the capability to signal (i.e., alert and inform) each other, indicating their status and condition. Typical status indications include available (dial tone), unavailable (busy), and alerting (ringing signal).The endpoints (i.e., terminal devices) or end offices also must pass identification information and instructions, such as the originating address, the target address, and the pre-selected carrier. Switches and routers within the carrier network must pass information such as route preference and route availability. Signaling and control systems and networks also handle billing matters, perhaps querying centralized databases in the process. Network management information often is passed over signaling and control links. Such information is used for remote monitoring, diagnostics, fault isolation, and network control. There are two basic types of signaling and control: in-band and out-of-band.