Although using different technology and approach, some of the same component concepts that make up the PSTN also create a VOIP network. There are three major pieces to IP networks.

-a VOIP network
-media gateways/signalling controllers
-media gateways

Media gateways are responsible for call origination, call detection, analog-to-digital conversion of voice, and creation of voice packets (CODEC functions).

Typically, each conversation (call) is a single IP session transported by a Real-time Transport Protocol (RTP) that runs over UDP.Their features and services can include some or all of the following.
-Network access servers that can attach a modem to a telephone circuit and provide data access to the Internet.
-Business media gateways that provide a traditional digital PBX interface or an integrated soft PBX interface to a voice over IP network.
-Access media gateways that provide a traditional analog or digital PBX interface to a voice over IP network. Examples of residential gateways include cable modem/cable set-top boxes, xDSL devices, and broadband wireless devices.
-Trunking gateways that interface between the telephone network and a voice over IP network.
-Access media gateways that provide a traditional analog or digital PBX interface to a voice over IP network.
-Discreet IP telephones units.

-Network access servers that can attach a modem to a telephone circuit and provide data access to the Internet.
-Business media gateways that provide a traditional digital PBX interface or an integrated soft PBX interface to a voice over IP network.
-Access media gateways that provide a traditional analog or digital PBX interface to a voice over IP network. Examples of residential gateways include cable modem/cable set-top boxes, xDSL devices, and broadband wireless devices.
-Trunking gateways that interface between the telephone network and a voice over IP network.
-Access media gateways that provide a traditional analog or digital PBX interface to a voice over IP network.
-Discreet IP telephones units.
Media Gateway Controllers
The media gateway controller has the responsibility for some or all of the call signaling coordination, phone number translations, host lookup, resource management, and signaling gateway services to the PSTN (SS7 gateway). Media gateway controllers could be considered similar to that of H.323 gatekeepers. Media gateway controllers house the signaling and control services that coordinate the media gateway functions.The amount of functionality is based on the particular voice over IP enabling products used.

In a scalable voice over IP network, you can breakup the role of a controller into signaling gateway controller and media gateway controller. You could use a signaling gateway controller to directly connect to the SS7 network, while also interfacing to a VOIP network elements. For calls that originate and terminate within the domain of the voice over IP network, only a media gateway controller might be needed to complete calls.

The services of these devices are defined by the protocols and software they are running. There are several protocols and implementations that any number of vendors could deploy. Knowing the details of how the devices use their suite of protocols is important to designing the IP backbone that is to service the voice over IP elements.

Depending on the voice over IP protocols used, this system as a whole is sometimes referred to as a softswitch architecture.

Jim Francisto
business voip provider