H323 protocol is used, for example, by Microsoft Netmeeting to make VoIP calls.
This protocol allow a variety of elements talking each other: Terminals, clients that initialize VoIP connection. Although terminals could talk together without anyone else, we need some additional elements for a scalable vision.
1. Gatekeepers, that essentially operate:
2. address translation service, to use names instead IP addresses
3. admission control, to allow or deny some hosts or some users
4. bandwidth management
5. Gateways, points of reference for conversion TCP/IP − PSTN.
6. Multipoint Control Units (MCUs) to provide conference.
7. Proxies Server also are used. h323 allows not only VoIP but also video and data communications. Concerning VoIP, h323 can carry audio codecs G.711, G.722, G.723, G.728 and G.729 while for video it supports h261 and h263.