Media Gateway Control Protocol - MGCP

Media Gateway Control Protocol (MGCP) is defined in RFC 3435 (which obsoletes RFC 2705), and it specifies an application programming interface and text-based master/slave protocol used by a media gateway controller (MGC) or Call Agent to control media gateways (MG). MGCP is based on two other (now obsolete) protocols, the Simple Gateway Control Protocol (SGCP) and Internet Protocol Device Control (IPDC).

MGCP MGCs and MGs can be described as follows:

• MGC/Call Agent: This element possesses call control intelligence and controls MGs. An MGC/Call Agent could,for example, be a Cisco CallManager.



• MG: This device provides translation between data packets and audio signals received over VoIP networks and other networks, such as the PSTN. A media gateway could, for example, be an IOS router with analog or digital voice ports.

Media gateways can be classified depending on the connectivity they provide. For example, a media gateway that terminates trunks connecting to the telephone network can be referred to as a trunking gateway, and a media gateway that provides analog connections to phones can be referred to as a residential gateway.

MGCP specifies a connection model involving endpoints and connections:

• Endpoint: MGCP media gateways contain endpoints,which are sources/destinations for data.Endpoints can be physical, such as interfaces terminating trunks connecting to a PSTN or interfaces terminating POTS connections to PBX, key systems, or telephones. Endpoints can also be virtual endpoints, such as audio content sourced from a server.



• Connection: This is an association between endpoints for the purpose of transmitting data, and can be either piont-to-point or multipoint in nature.

MGCs/Call Agents and MGs use several commands and responses (or verbs) to communicate with each other:

• EndpointConfiguration (EPCF): A Call Agent sends this message to a gateway to specify signal encoding that will be received by an endpoint. This message could, for example, be used to specify whether audio calls will be encoded
  using mu-law or a-law.


• CreateConnection (CRCX): This command creates a connection between two endpoints. The connection is created based on parameters included with the command, such as codec, allowable bandwidth, use of echo cancellation, silence suppression, gain control, and so on.


• ModifyConnection (MDCX): This is used to modify the parameters associated with a connection that was previously created.


• DeleteConnection (DLCX): The Call Agent can send this command to inform a gateway that it should terminate a connection, and a gateway can send this command to indicate that a connection can no longer be sustained. In response to a DeleteConnection command, a media gateway sends statistics associated with the connection.


• NotificationRequest (RQNT): This is sent by a Call Agent to instruct a gateway to inform it when specific events occur in an endpoint. These events could, for example, be on-hook/off-hook status changes or the reception of certain tones.


• Notify (NTFY): A media gateway uses this command to inform a Call Agent when requested events occur.


• AuditEndpoint (AUEP): The Call Agent sends the AuditEndpoint command to the gateway to audit the status of an endpoint. The Call Agent can, for example, find out signal status, event status, bearer information, and endpoint apabilities using this command.


• AuditConnection (AUCX): The Call Agent sends this command to the gateway to find out the status of a connection.Connection status information that can be retrieved using this command includes call ID, connection mode, and connection parameters


• RestartInProgress (RSIP): The gateway sends this command to the Call Agent to inform the Call Agent that it is taking an endpoint or group of endpoints out of service or is returning an endpoint or group of endpoints to service.

MGCP messages are sent on UDP port 2427. However, TCP port 2428 is used to exchange keepalives between an MG and CallManager, as well as for MGCP PRI/BRI backhaul between a media gateway and CallManager.

MGCP PRI/BRI backhaul is used to transport ISDN Q.931 (D channel) signalling information from the gateway to the Call Agent (CallManager). ISDN Q.921 signalling is terminated on the gateway and is not backhauled to the Call Agent.

MGCP is a popular gateway control protocol for several reasons, including the following:

• MG configuration is simplified because call control and dial plan are centralized on the MGC.
• Dial plan management and administration is simplified due to its centralization on the MGC.
• Cisco MGCP MG can be configured to switch over from a primary to backup CallManager in the event of a failure of the primary (there can be two backups in addition to the primary). Similarly, Cisco MGCP gateways can be configured to fall back to using H.323 for call control in case gateways are unable to communicate with any CallManagers.
• Active calls are preserved when an MGCP gateway switches over to a backup CallManager.
• Active analog and T1 CAS calls are preserved in the event of fallback to H.323. Any active PRI calls that are being backhauled to CallManager are not preserved during fallback.