A Media Gateway Controller (MGC) is a system used in certain Voice over IP telephony architectures. An MGC controls a number of dumb terminals, the Media Gateways. The MGC receives signalling information (like dialed digits) from the Media Gateway and can instruct it to alert the called party, to send and receive voice data etc. There are several protocols which can be used between MGC and Media Gateway: SGCP, IPDC, MGCP and Megaco (also known as H.248). The MGC also receives the term Call Agent when referred in the context of MGCP.
Some MGCs can interface with other signalling protocols, like SS7 (for interconnection with the traditional telephone system), H.323 and SIP.
A VoIP architecture using an MGC is sometimes called a softswitch architecture. Advantages of a softswitch architecture are that the Media Gateways are easy to maintain and that the softswitch operator retains full control. On the downside, softswitch architectures tend to be inflexible. Highly available MGCs are expensive to implement, as an MGC needs to be stateful.
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