Introduction
Media Gateway Control Protocol (MGCP) is an emerging protocol used for controlling Media Gateways or VoIP Gateways on the Public Switched Telephone Network (PSTN) and Internet Protocol (IP). It is an original protocol, which evolved into Media Gateway Controller (MEGACO). It is utilized during a live session in multimedia conferencing.
It requires soft switched for call and has more resemblance to the circuit switched PSTN. The General base programming interface and architecture are based on RFC 2805 and defined in current RFC 3435. MGCP represents a decomposition of other VoIP models (SIP and H.323) and has higher levels of Signaling Intelligence. Media Gateway Control Protocol is used for controlling media gateways and to manage media establishment. It uses SDP (Session Description Protocol) for specifying the media stream and RTP (Real-time Transport Protocol) for framing the media stream. MGCP is a successor of Simple Gateway Control Protocol (SGCP).
Architecture
MGCP architecture (is) composed of a Media Gateway Controller (Call Agent) and a Media Gateway (MG). Function of Call Agent is to inform the Media Gateway –
• How endpoints should be connected to each other.
• What events should be reported to the Call Agent?
• What signals should be played on endpoints?
• To audit the current state of endpoints on a MG.
Media Gateway is used to report events such as dialed digits or off-hook to the Call Agent. It is also configured with a list of Call Agents from which it may accept programming. Media Gateway is controlled by Call Agent, if Call Agents losses contact with a MG then, it is the responsibility of Call Agent to reprogram the MG.
Commands
Media Gateway Control Protocol packets are wrapped in a UDP (User Datagram Protocol) port 2427 and formatted with whitespace. MGCP datagram’s are either a response or a command. There are total nine commands used by Call Agents. Out of nine –
Two commands are used by a Call Agent to query Media Gateway
• AUEP (AuditEndpoint) – It is used to retrieve the parameters attached to a connection.
• AUCX (AuditConnection) – It is used to find out the status of a given endpoint.
Three commands are used by a Call Agent to manage a RTP connection on a MG
• CRCX (CreateConnection) – It is used to create a connection between two endpoints.
• DLCX (DeleteConnection) – It is used to terminate a connection.
• MDCX (ModifyConnection) – It is used to modify the characteristics of a connection.
One command is used by a Call Agent to request notification of events on the MG
• RQNT (NotificationRequest) – It is used to request the gateway to send a notification upon the occurrence of specified events.
One command is used by a Call Agent to modify the coding characteristics on the MG.
• EPCF (EndpointConfiguration) – It is used to specify the encoding of the signals received at endpoint.
One command is used by a MG to notify the Call Agent that it has detected an event from the Call Agent
• NTFY (Notify) – It is used to notify.
Once command is used by a MG to indicate that it is in the process of restarting
• RSIP (RestartInProgress) – It is used to signal that an endpoint is out-of-service or put-in-service.
RFCs
RFCs (Request for Comments) related to Media Gateway Control Protocols are –
• RFC 3435 – It contains the information on MGCP version 1.0.
• RFC 3660 – It contains the information on MGCP Packages.
• RFC 3661 – It contains the information on MGCP Return Code Usage.
• RFC 3064 – It contains the information on MGCP CAS Packages.
• RFC 3149 – It contains the information on MGCP Business Phone Packages.
• RFC 3991 – It contains the information on MGCP Redirect and Reset Package.
• RFC 3992 – It contains the information on MGCP Lookstep State Reporting Mechanism.
• RFC 2805 – It contains the information on MGCP Architecture and Requirements.
• RFC 2897 – It contains the information on MGCP Advanced Audio Package.