MGCP – What is it used for?

MGCP (Media Gateway Control Protocol) was developed from two Internet protocols known as SGCP (Simple Gateway Control Protocol) and IPDC (Internet Protocol Device Control). It is used by call control agents in VoIP systems to control the actions of a telephony gateway. MGCP was developed as part of the Internet Protocol Suite and the ITU-T had a standard that performed a similar function known as H.248. A working group known as MEGACO was formed to promote interoperability between the two standards.

Call control agents can be SIP servers, proprietary call agents such as Cisco Communications Manager, or an H.323 Gatekeeper. MGCP as a protocol uses a series of single-line commands and responses to communicate with a telephony gateway, and these commands are based on plain ASCII. When voice over IP became viable, most media gateways were based on TDM (Time Division Multiplexing) and VoIP system elements needed to communicate with these media gateways. This is why MGCP was developed.

In a telecommunications system using VoIP where there is a need to route calls to the PSTN, a media gateway provides the interface between the IP-based VoIP system and the traditional TDM-based PSTN that would normally use ISDN as its medium. of choice. Information from the PSTN had to be packed into IP packets in order to be transmitted on the VoIP side of the connection, and the VoIP media had to be decapsulated from the IP and RTP packets and assigned a channel in the TDM system.

Call control agents use MGCP to control and manage the actions of media gateways using simple 1-line commands, and these agents can communicate with each other using media gateways as interfaces to ensure that voice and signaling streams are translated. properly.

PSTN signaling information is sent to a signaling gateway and then to a call agent who ensures that the information is translated into the appropriate SIP messages for transmission within the VoIP network to SIP endpoints or other call agents.

There are 9 commands used by MGCP and MGCP packets are transmitted using UDP (User Datagram Protocol) as the transport method. These command messages are identified as:

AUEP – Audit End Points

AUCX – Audit Connection

CRCX – Create Connection

DLCX – Delete Connection

EPCF – Endpoint Configuration

MDCX – Modify Connection

NTFY – Notify

RQNT – Notification Request

RSIP – Reboot in Progress

The AUEP and AUCX messages are used by a call control agent to query the status of the media gateway. Media connections that invariably use RTP are managed through the use of CRCX, DLCX, and MDCX messages, which create, delete, and modify connections. A call control agent can request information about event notifications from a media gateway using the RQNT (notification request) message, to which the gateway must respond with an NTFY (notification) message. The types of events we refer to include:

Off-hook notification

Hangup notification

DTMF digits

fax tones

hook

continuity tones

Modem tones.

The encryption information can be changed at the gateway by sending an EPCF (Endpoint Configuration) message by the call agent and if successful, the gateway will respond with a RSIP (Reset In Progress) message. ).