Media Gateway (MG)
When a PSTN gateway is decomposed into separate call control and media control components, the component that is responsible for media is referred to as the Media Gateway.
The Media Gateway, in this case, would extract audio from a PSTN circuit and transport it over the IP network using protocols like RTP. It would do this under the control of a Gateway Control Protocol, such as H.248 or MGCP.
An MG may also serve as a component of a conference bridge, wherein all connections are IP. While much of the initial focus of the MG/MGC model was on PSTN gateways, it is possible for a MG to bridge one or more IP connections, performing audio or video mixing, for example.