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MGCP is an implementation of the Media Gateway Control Protocol architecture[1] for controlling media gateways on Internet Protocol (IP) networks and the public switched telephone network (PSTN). The general base architecture and programming interface is described in RFC 2805 and the current specific MGCP definition is RFC 3435 (obsoleted RFC 2705). It is a successor to the Simple Gateway Control Protocol (SGCP).
MGCP is a signalling and call control protocol used within Voice over IP (VoIP) systems that typically interoperate with the public switched telephone network (PSTN). As such it implements a PSTN-over-IP model with the power of the network residing in a call control center (softswitch, similar to the central office of the PSTN) and the endpoints being "low-intelligence" devices, mostly simply executing control commands. The protocol represents a decomposition of other VoIP models, such as H.323, in which the media gateways (e.g., H.323's gatekeeper) have higher levels of signalling intelligence.
MGCP uses the Session Description Protocol (SDP) for specifying and negotiating the media streams to be transmitted in a call session and the Real-time Transport Protocol (RTP) for framing of the media streams.
Another implementation of the Media Gateway Control Protocol architecture exists in the similarly named Megaco protocol, a collaboration of the Internet Engineering Task Force (RFC 3525) and International Telecommunication Union (Recommendation H.248.1). Both protocols follow the guidelines of the API Media Gateway Control Protocol Architecture and Requirements in RFC 2805. However, the protocols are incompatible due to differences in protocol syntax and underlying connection model.
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The Media Gateway Control Protocol architecture and its methodologies and programming interfaces are described in RFC 2805.[2]
The distributed system is composed of a Call Agent (or Media Gateway Controller), at least one Media Gateway (MG) that performs the conversion of media signals between circuits and packets, and at least one Signaling gateway (SG) when connected to the PSTN.
The Call Agent uses MGCP to tell the Media Gateway:
MGCP also allows the Call Agent to audit the current state of endpoints on a Media Gateway.
The Media Gateway uses MGCP to report events (such as off-hook, or dialed digits) to the Call Agent.
(While any Signaling Gateway is usually on the same physical switch as a Media Gateway, this needn't be so. The Call Agent does not use MGCP to control the Signaling Gateway; rather, SIGTRAN protocols are used to backhaul signaling between the Signaling Gateway and Call Agent).
Typically, a Media Gateway is configured with a list of Call Agents from which it may accept programming (where that list normally comprises only one or two Call Agents).
In principle, event notifications may be sent to different Call Agents for each endpoint on the gateway (as programmed by the Call Agents, by setting the NotifiedEntity parameter). In practice, however, it is usually desirable that at any given moment all endpoints on a gateway should be controlled by the same Call Agent; other Call Agents are available only to provide redundancy in the event that the primary Call Agent fails, or loses contact with the Media Gateway. In the event of such a failure it is the backup Call Agent's responsibility to reprogram the MG so that the gateway comes under the control of the backup Call Agent. Care is needed in such cases; two Call Agents may know that they have lost contact with one another, but this does not guarantee that they are not both attempting to control the same gateway. The ability to audit the gateway to determine which Call Agent is currently controlling can be used to resolve such conflicts.
MGCP assumes that the multiple Call Agents will maintain knowledge of device state among themselves (presumably with an unspecified protocol) or rebuild it if necessary (in the face of catastrophic failure). Its failover features take into account both planned and unplanned outages.
MGCP packets are unlike those generated by many other protocols. Usually wrapped in UDP port 2427, the MGCP datagrams are formatted with whitespace, much like you would expect to find in TCP protocols.
An MGCP packet is either a command or a response. Every issued MGCP command has a transaction ID and receives a response. Commands begin with a four-letter verb. Responses begin with a three number response code.
There are nine (9) command verbs:
AUEP, AUCX, CRCX, DLCX, EPCF, MDCX, NTFY, RQNT, RSIP
Two verbs are used by a Call Agent to query (the state of) a Media Gateway:
AUEP - Audit Endpoint AUCX - Audit Connection
Three verbs are used by a Call Agent to manage an RTP connection on a Media Gateway (a Media Gateway can also send a DLCX when it needs to delete a connection for its self-management):
CRCX - Create Connection DLCX - Delete Connection MDCX - Modify Connection
One verb is used by a Call Agent to request notification of events on the Media Gateway, and to request a Media Gateway to apply signals:
RQNT - Request for Notification
One verb is used by a Call Agent to modify coding characteristics expected by the "line-side" on the Media Gateway:
EPCF - Endpoint Configuration
One verb is used by a Media Gateway to indicate to the Call Agent that it has detected an event for which the Call Agent had previously requested notification of (via the RQNT command verb):
NTFY - Notify
One verb is used by a Media Gateway to indicate to the Call Agent that it is in the process of restarting:
RSIP - Restart In Progress
Two implementations of the Media Gateway Control Protocol are in common use. The names of both are abbreviations of the protocol group:
Although similar in architecture, MGCP and Megaco are distinctly different protocols and are not interoperable.
This article was originally based on material from the Free On-line Dictionary of Computing, which is licensed under the GFDL.