The Media Gateway Control Protocol (MGCP) is a signaling and call control

communication protocol A communication protocol is a system of rules that allows two or more entities of a communications system to transmit information via any kind of variation of a physical quantity. The protocol defines the rules, syntax, semantics and synchroniza ...

used in

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

(VoIP)

telecommunication Telecommunication is the transmission of information by various types of technologies over wire, radio, optical, or other electromagnetic systems. It has its origin in the desire of humans for communication over a distance greater than that fe ...

systems. It implements the media gateway control protocol architecture for controlling media gateways connected to the public switched telephone network (PSTN).RFC 2805, Media Gateway Control Protocol Architecture and Requirements, N. Greene, M. Ramalho, B. Rosen, IETF, April 2000 The media gateways provide conversion of traditional electronic media to the

Internet Protocol The Internet Protocol (IP) is the network layer communications protocol in the Internet protocol suite for relaying datagrams across network boundaries. Its routing function enables internetworking, and essentially establishes the Internet. ...

(IP) network. The protocol is a successor to the Simple Gateway Control Protocol (SGCP), which was developed by

Bellcore iconectiv is a supplier of network planning and network management services to telecommunications providers. Known as Bellcore after its establishment in the United States in 1983 as part of the break-up of the Bell System, the company's name ...

and

Cisco Cisco Systems, Inc., commonly known as Cisco, is an American-based multinational digital communications technology conglomerate corporation headquartered in San Jose, California. Cisco develops, manufactures, and sells networking hardware, ...

, and the Internet Protocol Device Control (IPDC). The methodology of MGCP reflects the structure of the PSTN with the power of the network residing in a call control center

softswitch A softswitch (''software switch'') is a call-switching node in a telecommunications network, based not on the specialized switching hardware of the traditional telephone exchange, but implemented in software running on a general-purpose computing ...

which is analogous to the central office in the telephone network. The endpoints are low-intelligence devices, mostly executing control commands from a call agent or media gateway controller in the softswitch and providing result indications in response. The protocol represents a decomposition of other VoIP models, such as H.323 and the Session Initiation Protocol (SIP), in which the endpoint devices of a call have higher levels of signaling intelligence. MGCP is a text-based protocol consisting of commands and responses. It uses the

Session Description Protocol The Session Description Protocol (SDP) is a format for describing multimedia communication sessions for the purposes of announcement and invitation. Its predominant use is in support of streaming media applications, such as voice over IP (VoIP) ...

(SDP) for specifying and negotiating the media streams to be transmitted in a call session and the

Real-time Transport Protocol The Real-time Transport Protocol (RTP) is a network protocol for delivering audio and video over IP networks. RTP is used in communication and entertainment systems that involve streaming media, such as telephony, video teleconference applicati ...

(RTP) for framing the media streams.

Architecture

The media gateway control protocol architecture and its methodologies and programming interfaces are described in RFC 2805. MGCP is a master-slave protocol in which media gateways (MGs) are controlled by a call control agent or softswitch. This controller is called a

media gateway controller The media gateway control protocol architecture is a methodology of providing telecommunication services using decomposed multimedia gateways for transmitting telephone calls between an Internet Protocol network and traditional analog facilities of ...

(MGC) or call agent (CA). With the network protocol it can control each specific port on a media gateway. This facilitates centralized gateway administration and provides scalable IP telephony solutions. The distributed system is composed of at least one call agent and one or usually, multiple media gateways, which performs the conversion of media signals between circuit-switched and packet-switched networks, and at least one signaling gateway (SG) when connected to the

PSTN The public switched telephone network (PSTN) provides infrastructure and services for public telecommunication. The PSTN is the aggregate of the world's circuit-switched telephone networks that are operated by national, regional, or local teleph ...

. MGCP presents a call control architecture with limited intelligence at the edge (endpoints, media gateways) and intelligence at the core controllers. The MGCP model assumes that call agents synchronize with each other to send coherent commands and responses to the gateways under their control. The call agent uses MGCP to request event notifications, reports, status, and configuration data from the media gateway, as well as to specify connection parameters and activation of signals toward the PSTN telephony interface. A softswitch is typically used in conjunction with signaling gateways, for access to Signalling System No. 7 (SS7) functionality, for example. The call agent does not use MGCP to control a signaling gateway; rather, SIGTRAN protocols are used to backhaul signaling between a signaling gateway and the call agents.

Multiple call agents

Typically, a media gateway may be configured with a list of call agents from which it may accept control commands. In principle, event notifications may be sent to different call agents for each endpoint on the gateway, according to the instructions received from the call agents by setting the NotifiedEntity parameter. In practice, however, it is usually desirable that all endpoints of a gateway are controlled by the same call agent; other call agents are available to provide redundancy in the event that the primary call agent fails, or loses contact with the media gateway. In the event of such a failure it is the backup call agent's responsibility to reconfigure the media gateway so that it reports to the backup call agent. The gateway may be audited to determine the controlling call agent, a query that may be used to resolve any conflicts. In case of multiple call agents, MGCP assumes that they maintain knowledge of device state among themselves. Such failover features take into account both planned and unplanned outages.

Protocol overview

MGCP recognizes three essential elements of communication, the ''media gateway controller'' (call agent), the media gateway ''endpoint'', and ''connections'' between these entities. A media gateway may host multiple endpoints and each endpoint should be able to engage in multiple connections. Multiple connections on the endpoints support calling features such as

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 ...

and

three-way calling In human sexuality, a threesome is commonly understood as "a sexual interaction between three people whereby at least one engages in physical sexual behaviour with both the other individuals". Though ''threesome'' most commonly refers to sexua ...

. MGCP is a text-based protocol using a command and response model. Commands and responses are encoded in messages that are structured and formatted with the whitespace characters

space Space is the boundless three-dimensional extent in which objects and events have relative position and direction. In classical physics, physical space is often conceived in three linear dimensions, although modern physicists usually cons ...

horizontal tab The tab key (abbreviation of tabulator key or tabular key) on a keyboard is used to advance the cursor to the next tab stop. History The word ''tab'' derives from the word ''tabulate'', which means "to arrange data in a tabular, or table, fo ...

carriage return A carriage return, sometimes known as a cartridge return and often shortened to CR, or return, is a control character or mechanism used to reset a device's position to the beginning of a line of text. It is closely associated with the line feed ...

, linefeed, colon, and full stop. Messages are transmitted using the

User Datagram Protocol In computer networking, the User Datagram Protocol (UDP) is one of the core communication protocols of the Internet protocol suite used to send messages (transported as datagrams in packets) to other hosts on an Internet Protocol (IP) network ...

(UDP). Media gateways use the

port number In computer networking, a port is a number assigned to uniquely identify a connection endpoint and to direct data to a specific service. At the software level, within an operating system, a port is a logical construct that identifies a specific ...

2427, and call agents use 2727 by default. The message sequence of command (or request) and its response is known as a transaction, which is identified by the numerical Transaction Identifier exchanged in each transaction. The protocol specification defines nine standard commands that are distinguished by a four-letter command verb: AUEP, AUCX, CRCX, DLCX, EPCF, MDCX, NTFY, RQNT, and RSIP. Responses begin with a three-digit numerical response code that identifies the outcome or result of the transaction. Two verbs are used by a call agent to query the state of an endpoint and its associated connections. *AUEP: Audit Endpoint *AUCX: Audit Connection Three verbs are used by a call agent to manage the connection to a media gateway endpoint. *CRCX: Create Connection *DLCX: Delete Connection. This command may also be issued by an endpoint to terminate a connection. *MDCX: Modify Connection. This command is used to alter operating characteristics of the connection, e.g. speech encoders, muting, half-duplex/full-duplex state and others. One verb is used by a call agent to request notification of events occurring at the endpoint, and to apply signals to the connected PSTN network link, or to a connected telephony endpoint, e.g., a telephone. *RQNT: Request for Notification One verb is used by an endpoint to indicate to the call agent that it has detected an event for which the call agent had previously requested notification with the RQNT command: *NTFY: Notify One verb is used by a call agent to modify coding characteristics expected by the line side of the endpoint: *EPCF: Endpoint Configuration One verb is used by an endpoint to indicate to the call agent that it is in the process of restarting: *RSIP: Restart In Progress

Standards documents

* – Media Gateway Control Protocol (MGCP) Version 1.0 (this supersedes RFC 2705) * – Basic Media Gateway Control Protocol (MGCP) Packages (informational) * – Media Gateway Control Protocol (MGCP) Return Code Usage * – MGCP CAS Packages * – MGCP Business Phone Packages * – Media Gateway Control Protocol (MGCP) Redirect and Reset Package * – Media Gateway Control Protocol (MGCP) Lockstep State Reporting Mechanism (informational) * – Media Gateway Control Protocol Architecture and Requirements * – Proposal for an MGCP Advanced Audio Package

Megaco

Another implementation of the media gateway control protocol architecture is the H.248/Megaco protocol, a collaboration of the

Internet Engineering Task Force The Internet Engineering Task Force (IETF) is a standards organization for the Internet and is responsible for the technical standards that make up the Internet protocol suite (TCP/IP). It has no formal membership roster or requirements and a ...

(RFC 3525) and the

International Telecommunication Union The International Telecommunication Union is a specialized agency of the United Nations responsible for many matters related to information and communication technologies. It was established on 17 May 1865 as the International Telegraph Unio ...

(Recommendation H.248.1). Both protocols follow the guidelines of the overlying media gateway control protocol architecture, as described in RFC 2805. However, the protocols are incompatible due to differences in protocol syntax and underlying connection model.

References

{{reflist

External links

MGCP Information Site
Information related to MGCP
H.248 Information Site
Information related to H.248/Megaco, including pointers to standards and draft specifications VoIP protocols