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
.