VoIP and Voice over WLANs
Centralized and distributed VoIP architectures

One of the benefits of VoIP technology is that it allows networks to be built using either a centralized or a distributed architecture. In general, centralized architectures are associated with MGCP and H.248/Megaco protocols. These protocols were designed for a centralized device known as a media gateway controller, or a call agent that handles switching logic and call control. The centralized device communicates with the media gateways, which route and transmit the audio and media portion of the calls. In centralized architectures, the network intelligence is centralized and endpoints have relatively little intelligence.

Distributed architectures are associated with the H.323 and SIP protocols. These protocols allow network intelligence to be distributed between endpoints and call-control devices. In distributed architectures, intelligence refers to call state, calling features, call routing, provisioning, billing, or any other aspect of call handling.

The endpoints can be VoIP gateways, IP phones, media servers, or any device that can initiate and terminate a VoIP call. The call-control devices are called gatekeepers in an H.323 network, and proxy or redirect servers in a SIP network.

There are many protocols that are used for VoIP. The next several sections will describe the more important of these protocols, some of which are shown in Figure .