Gateway

A gateway is a device that is used to connect one type of network to another. For example, a gateway is used to connect the legacy PSTN network to the VoIP networks that are now being deployed. The gateway's responsibility is to provide signaling interworking, as well as transforming the bearer information (e.g., "what you hear on the phone") into a format that is suitable for the other network.

A gateway may exist as a single monolithic device or it may be decomposed into logical components. A gateway is viewed as a device that has a Signaling Gateway part, a Media Gateway part, and a Media Gateway Controller part.