telephony Telephony ( ) is the field of technology involving the development, application, and deployment of telecommunication services for the purpose of electronic transmission of voice, fax, or data, between distant parties. The history of telephony is i ...

, call control refers to the

software Software is a set of computer programs and associated documentation and data. This is in contrast to hardware, from which the system is built and which actually performs the work. At the lowest programming level, executable code consists ...

within a

telephone switch telephone exchange, telephone switch, or central office is a telecommunications system used in the public switched telephone network (PSTN) or in large enterprises. It interconnects telephone subscriber lines or virtual circuits of digital syste ...

that supplies its central function. Call control decodes addressing information and routes

telephone call A telephone call is a connection over a telephone network A telephone network is a telecommunications network that connects telephones, which allows telephone calls between two or more parties, as well as newer features such as fax and interne ...

s from one end point to another. It also creates the features that can be used to adapt standard switch operation to the needs of users. These are called supplementary services and are commonly invoked by a

Vertical service code A vertical service code (VSC) is a sequence of digits and the signals star (*) and number sign (#) dialed on a telephone keypad or rotary dial to enable or disable certain telephone service features. Some vertical service codes require dialing of ...

. Examples include "

Call waiting Call waiting is a telephone service where a subscriber can accept a second incoming telephone call by placing an in-progress call on hold—and may also switch between calls. With some providers it can be combined with additional features such a ...

", " Call forward on busy" etc. Call control software, because of its central place in the operation of the telephone network, is marked by both complexity and reliability. Call control systems will typically require many thousands of person years in development. They will contain millions of lines of high level code. However they must and do meet reliability requirements that specify switch down time of only a few minutes in forty years. The required functionality and reliability of call control is a major challenge for

Voice over IP Voice over Internet Protocol (VoIP), also called IP telephony, is a method and group of technologies for the delivery of speech, voice communications and multimedia sessions over Internet Protocol (IP) networks, such as the Internet. The terms In ...

(VoIP) systems. VoIP systems are based on

Internet The Internet (or internet) is the global system of interconnected computer networks that uses the Internet protocol suite (TCP/IP) to communicate between networks and devices. It is a '' network of networks'' that consists of private, pub ...

standards and technology which have not previously attempted to satisfy such complex and demanding requirements as those that specify call control. An alternative name often used is call processing. In a VoIP network, call control is one of three major categories of communications traffic, the other two being call signaling and media communications. Call control uses Q.931, a connection protocol for digital networks, especially VoIP systems. Messages are transmitted as octets as specified in ITU H.245, which resolves the type of call media to be used (for example, conventional call, videoconferencing, or VoIP), and then manages the connection after it has been established. Call control functions include, but are not limited to, the determination of master/slave status for the endpoints, monitoring of the status of the endpoints, modification of the parameters of a connection, termination of a connection, and restarting a terminated or failed connection.

References

{{DEFAULTSORT:Call Control Teletraffic

See also

References