A media gateway is a device used in the core network of a telecom network operator to provide transformation and interworking between media streams that use different network standards, communication protocols, codecs and physical connections, so that phone calls work properly between networks using different technologies.
Network operators can use media gateways to provide seamless interworking between different parts of their network that use different technologies, or to enable a gradual transition from circuit switched Time-Division Multiplex (TDM) infrastructure to packet switched IP infrastructure. A media gateway is an essential component for IP network transformation.
Media gateways can typically convert and relay media streams for use in POTS, SS7, ISDN and 2G/3G mobile networks built on TDM infrastructure, and in Next Generation Networks (NGN) and IP Multimedia Subsystem (IMS) networks built on IP infrastructure.
Media gateways require physical connections of different types to connect to networks using different technologies. These connections can include T1/E1, DS3 and OC3/STM1 fiber optic connections for TDM networks, and Ethernet connections for IP networks.
Media gateways are controlled by a separate media gateway controller component that handles signalling and call control. In a NGN architecture this would be a call agent or call feature server, and in an IMS architecture it would be a Media Gateway Control Function. The controller typically communicates with the media gateway over the Media Gateway Control Protocol (MGCP), Megaco (H.248) or Session Initiation Protocol (SIP).
If a media gateways loses its connection with its controller because of a network outage, service can be lost and subscribers will suffer dropped calls or will be unable to make new calls. Some media gateways are designed to be resilient to lost connections, and can function independently as a standalone softswitch with basic call control functions to preserve service during a network outage.