MGCP (Media Gateway Control Protocol) is an internal protocol used within a Voice over IP (VoIP) system, and specified in RFC 3435. This simple protocol was developed primarily to address carrier-based IP telephone network demands, and it has become the de facto standard for media gateway control worldwide. MGCP is a complementary protocol for both H.323 and SIP, which serve as IP signaling devices within an IP network. A Media Gateway Controller (MGC) uses MGCP to interface with the Media Gateway (MG), and handles all the processing by linking with an IP network.

How it Works

The MGCP system is comprised of a Call Agent (CA), one MG that performs the conversion of media signals between circuits and packets, and one signaling gateway (SG) when that SG is connected to the Public Switched Telephone Network (PSTN). MGCP, which utilizes SDP, is widely used between elements of a decomposed multimedia gateway. The gateway has a CA, which is comprised of the call control “intelligence” and a media gateway boasting the media functions, for example conversion from TDM voice to Voice over IP.

Media Gateways feature endpoints for the CA to create and manage media sessions with other multimedia endpoints. Endpoints are sources and/or sinks of data that can be physical or virtual. For creating physical endpoints, hardware installation is needed while virtual endpoint can be created using available software. Call Agents come with the capability of creating new connections or modify an existing connection.

Generally, a media gateway is a network element that provides conversion between the data packets carried over the Internet or other packet networks and the voice signals carried by telephone lines. The Call Agent provides instructions to the endpoints to check for any events and to create signals for existing events. The endpoints are designed in such a way as to automatically communicate changes in service state to the Call Agent. The Call Agent can audit endpoints and the connections on endpoints.

MGCP connections can be point-to-point or multipoint. Point-to-point connections can be created from a connection between two endpoints for transmitting data between these endpoints. Once the connection is setup between two endpoints, data transfer can take place between the endpoints. In a multipoint connection, the connection is set up between an endpoint and a multipoint session. In a multipoint connection, connections can be created over various types of bearer networks.

Basic Usage

MGCP is a popular VoIP application because the MGCP Call Agent works as seemingly complex software switch for a VoIP network; however, its simplicity is understated. It really does nothing more than direct the media gateways and signaling gateways that perform all the work.

Each and every command within MGCP architecture features a transaction ID, and it receives an acknowledgement and a response. These actions are often understood as a subscription architecture, as the CA informs the MG and signaling gateways as to the events that are attended and unattended.

MGCP packets usually are found wrapped in UDP port 2427. The MGCP datagrams are formatted with white space, and an MGCP packet can be either a command that begins with a four-letter verb or a three-number response code.

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