Media Gateway Control Protocol

MGCP (RFC 2705) is a relatively new protocol and as such, it is not as widely deployed as its H.323 and SIP predecessors. MGCP offers many key benefits and is growing in popularity, especially in Cisco CallManager deployments.

MGCP is a merger of the Simple Gateway Control Protocol (SGCP) and the Internet Protocol Device Control (IPDC). SGCP calls for a simplified design and a centralized intelligent call control. IPDC was designed to provide a medium to bridge VoIP networks and traditional telephony networks. MGCP is a media control protocol, suited for large-scale IP telephony deployment, and supports VoIP only.

MGCP incorporates media gateway controllers (MGCs) or call agents to perform all call connection and call control within an MGCP network. These MGCs signal to, and control, media gateways (MGs) to connect and control VoIP calls. All the information for making and completing a VoIP call is held in the MG.

MGs have very little intelligence and receive all their marching orders from the MGC; they cannot function without a controlling MGC. In a Cisco CallManager deployment, the MGC is often a CallManager server and the media gateway is a router used to connect to a dissimilar network. Examples of gateway applications are:

  • Trunking gateways Interfaces between the telephone network and a VoIP network. Manage a large number of digital circuits.

  • Voice over ATM gateways Interface to an ATM network.

  • Residential gateways Provide a traditional analog (RJ11) interface to a VoIP network. Examples of residential gateways include cable modem/cable set-top boxes, and broadband wireless devices.

  • Access gateways Provide a traditional analog (RJ11) or digital PBX interface to a VoIP network. Examples of access gateways include small-scale VoIP gateways.

  • Business gateways Provide a traditional digital PBX interface or an integrated "soft PBX" interface to a VoIP network.

  • Network access servers Can attach a modem to a telephone circuit and provide data access to the Internet. We expect that in the future, the same gateways will combine VoIP services and network access services.

When a gateway device detects that an end-user phone connection goes off-hook, it is directed by the MGC to provide a dial tone to the phone and receives the dialed digits and forwards them to the MGC for call processing.

MGCP Connections

In an MGCP connection, there are two basic types of logical devices: endpoints and connections. Endpoints are the physical, or logical interfaces that either initiate or terminate a VoIP connection. Endpoints are most often analog or digital ports in routers acting as gateway devices or digital interfaces into a PBX system.

Connections are temporary logical flows that are created to establish, maintain, and terminate a VoIP call. Once the call is complete, the connection is torn down and the resources that were allocated for that connection can be reused to support another connection. A one-to-one connection is really a point-to-point connection; a single endpoint signals to another single endpoint for the purposes of completing a single VoIP connection. Multipoint calls are used for conferencing and broadcast to multiple endpoints simultaneously.

MGCs manage connections in an MGCP network using the Session Description Protocol. SDP uses ASCII commands over IP/UDP to perform all call management functions. A series of eight connection messages is used by the MGC in order to control endpoints.

  • CreateConnection

  • ModifyConnection

  • DeleteConnection

  • NotificationRequest

  • Notify

  • AuditEndpoint

  • AuditConnection

  • RestartInProgress

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