Media gateway control protocol architecture

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 the public switched telephone network (PSTN).^[1] The architecture was originally defined in RFC 2805 and has been used in several prominent voice over IP (VoIP) protocol implementations, such as the Media Gateway Control Protocol (MGCP) and Megaco (H.248), both successors to the obsolete Simple Gateway Control Protocol (SGCP).

The architecture divides the functions required for the integration of traditional telecommunication networks and modern packet networks into several physical and logical components, notably the media gateway, the media gateway controller, and signaling gateways. The interaction between the media gateway and its controller is defined in the media gateway control protocol.

Media gateway protocols were developed based on the Internet model of networking, the Internet Protocol Suite, and are referred to as device control protocols. A media gateway is a device that offers an IP interface and a legacy telephone interface and that converts media, such as audio and video streams, between them. The legacy telephone interface may be complex, such as an interface to a PSTN switch, or may be a simple interface to a traditional telephone. Depending on the size and purpose of the gateway, it may allow IP-originated calls to terminate to the PSTN or vice versa, or may simply provide a means to connect a telephone to a telecommunication system via an IP network.

Originally, gateways were viewed as monolithic devices that had call control, using protocols such as H.323 and the Session Initiation Protocol, and hardware required to control the PSTN interface. In 1998, the idea of splitting the gateway into two logical parts was proposed: one part, which contains the call control logic, is called the media gateway controller (MGC) or call agent (CA), and the other part, which interfaces with the PSTN, is called the media gateway (MG). With this functional split, a new interface existed between the MGC and the MG, requiring a framework for communication between the elements, resulting in the media gateway control protocol architecture.

SIP and H.323 are signaling protocols, while media gateway control protocols are device control protocols. The architectural difference between SIP and H.323, and the media gateway control protocols is that the relationships between entities in SIP and H.323 are peer-to-peer, while the relationships between entities in media gateway control protocols use the master/slave (technology) model. SIP and H.323 handle call setup, connection, management, and tear-down of calls between like interfaces, whereas media gateway control protocols define the mechanisms of setup of media paths and streams between IP and other networks.^[2]