MGCP/Megaco Architecture

MGCP/Megaco exploded H.323's gatekeeper model and removed the signalling control from the gateway, putting it in a "media gateway controller" or "softswitch". This device would control multiple "media gateways". Effectively, this was a decomposition of the H.323 architecture into SS7 equivalents, creating signalling intelligence that could act as a peer to the SS7 entities.

In the MGCP/Megaco architecture, the intelligence (control) is unbundled from the media (data). It is a master-slave protocol where the master has absolute control and the slave simply executes commands. The master is the media gateway controller, or softswitch (or call agent) and the slave is the media gateway (this can be a VoIP gateway, a DSLAM, MPLS router, IP phone etc.). This is a contrast to the peer-to-peer nature of SIP and other Internet protocols where a client can establish a session with another client.

MGCP/Megaco instructs the media gateway to connect streams coming from outside a packet network on to a packet stream such as RTP. The softswitch issues commands to send and receive media from addresses, to generate tones, and to modify configuration. As can be seen from the diagram to the right, MGCP/Megaco is used for communication downward, to the media gateways. The architecture, however, requires a session initiation protocol for communication between gateway controllers.

In mirroring the SS7 architecture, MGCP/Megaco "soaks up" the complexity of the central offices. In this way, it can be used as a control protocol delivering services across the network via media gateways. By gathering intelligence (and service delivery) at the interconnect points of the network, MGCP/Megaco creates the "IP Central Office". This approach is seen as recreating the IN world or "PSTN over IP" thus precluding the potential and benefits of a new architecture. This is a contrast to the distributed service model of SIP and the world of the Internet.