Media Gateway Control Protocol

The methodology of MGCP reflects the structure of the PSTN with the control over the network residing in a call control center softswitch, which is analogous to the central office in the telephone network. The endpoints are low-intelligence devices, mostly executing control commands from a media gateway controller, also called call agent, 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 (SDP) for specifying and negotiating the media streams to be transmitted in a call session and the Real-time Transport Protocol (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 (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.

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.