The Media Gateway Control Protocol
In late 1998, Cisco Systems and Telcordia Technologies jointly proposed MGCP to define the way communications occur between Call Agents, Media Gateways, and Telephony Gateways. MGCP combines the use of the SGCP and IPDC (Internet Protocol Device Control) protocols, the IPDC protocol being an IETF technical advisory committee (TAC) effort led by Level III Communications. Both protocols are concerned with the management of trunking gateways between the PSTN and an IP based network.
In 2000, MGCP was defined in RFC-3435, an informational document (i.e. non-standard} of the IETF. It supersedes SGCP and assumes a similar architecture of a Media Gateway Controller (Call Agent, a.k.a. softswitch), a Media Gateway, and a Signaling Gateway for call control between circuit switched and packet based networks. Once the call is set up, the endpoints control the actual RTP communications session.
The Media Gateway Control Protocol relies on a centralized master slave model where the call agent (MGC) handles call intelligence, leaving the Media Gateway to translate between networks. It allows for multiple call agents that communicate with gateways through MGCP over UDP, and amongst each other or other devices on the IP network through SIP, or H.323. As a multimedia point to point/point to multipoint protocol, MGCP can establish connection over an IP network, an ATM carrier, or a switched circuit network (SCN) such as the PSTN.
In an SCN, the MG is responsible for converting pulse modulated information into IP packets and back again, and handles the RTP stream across the IP network, say to another MG. The MG can exist as a telephony trunking gateway, a residential gateway to provide analog access to an IP network, or as an interface to an Asynchronous Transfer Mode (ATM) carrier network.
Anatomy of a Call using MGCP
MGCP provides for endpoints that reside on the gateway and connections between these endpoints. Endpoints can be physical (hardware) or virtual (software) and are used by the MGC to establish a connection.
For example, a user on an endpoint goes off hook, and gateway A notifies the controller of the incident. The MGC instructs the gateway to open a connection (dial tone) and to listen for the numbers being dialed. The MG sends the information back to the MGC along with a description of the session using SDP. The session description includes information such as IP address, encodings, media type, UDP port etc, useful for negotiating capabilities.
The MGC then decides how to route the call and instructs the appropriate gateway (B) to initiate the session (ring) who responds with an ack and its session description, which is passed on to gateway A.
Once the session is established, communications are handled by endpoints using RTP over UDP wrapped in an IP packet. During the call the MGC audits the connection and instructs the gateways to terminate the session at calls end. MGCs can also provide management and billing information.
The MGC is responsible for providing call control to the gateways. In an IP network, the MGC will get its call information externally by monitoring a signaling device, such as a SIP server or an H.323 Gatekeeper. On the PSTN side, it will provide the necessary translations of the signaling information between the SCN and the IP network through the Signaling Gateway.
The Media Gateway Control Protocol was designed to compliment SIP and H.323 and to leave the gateways to what they do best – namely translating protocols in a converged network. With one MGC capable of controlling 100 Media Gateways, scalability seems limited to only the number of IP addresses available to the network.
Redundancy and reliability are addressed by adding multiple Call Agents to the mix, providing for failover by programming the gateway to use a backup call agent in the event of a planned or unplanned outage.
Although MGCP never did become an adopted standard of the ITU or the IETF, it did nevertheless pave the way for the joint adoption of the H.248/Megaco protocol, which is the actual implementation of the Media Gateway Control Protocol.
The VoIP Signaling Protocols
H.323 | SGCP | MGCP | Megaco-H.248 | SIP
