2. MGCP Protocol Summary

The Media Gateway Control Protocol (MGCP) is a control protocol used between elements of a decomposed gateway – specifically, between a central Call Agent (controller) and one or more Media Gateways (MGs). It allows the call control “intelligence” in the Call Agent to manipulate media channels on the distributed gateways [RFC 3435]. In MGCP’s model, the Call Agent instructs gateways to create, modify, and delete media connections on endpoints (ports or channels on the gateway) in order to establish and control voice sessions. The gateways, in turn, report back events (such as off-hook, dialed digits) and respond to commands from the Call Agent [RFC 3435]. MGCP is essentially a simple master/slave protocol: the Call Agent (also known as Media Gateway Controller, MGC) is the master, and the gateways execute the commands as slaves. This architecture centralizes call logic in the Call Agent while keeping the gateways relatively “dumb,” handling only media transformation (e.g. converting analog or TDM voice to IP packets) and event detection.

MGCP was defined in the late 1990s and documented as an informational RFC. Notably, it is not an IETF standard but an informational specification; the IETF/ITU-T jointly developed Megaco/H.248 as the official standards-track protocol addressing similar needs. MGCP 1.0 is described in [RFC 3435] (obsoleting RFC 2705), and a series of extension RFCs define its signal and event packages (e.g., RFC 3660 for packages). Despite its non-standard status, MGCP has been widely deployed in VoIP products and was a key protocol in early Voice over IP architectures. It provides a textual, transactional protocol for managing media gateways in a centralized call control paradigm, complementing higher-level signaling protocols like SIP or H.323 which operate between call agents or endpoints. In summary, MGCP offers a unified means for a call controller to command media gateways, bridging the gap between traditional telephony networks and IP networks in a distributed telephony system.