H.323 is a recommendation from the ITU Telecommunication Standardization Sector (ITU-T) that defines the protocols to provide audio-visual communication sessions on any packet network.

[2] It is widely implemented[3] by voice and videoconferencing equipment manufacturers, is used within various Internet real-time applications such as GnuGK and NetMeeting and is widely deployed worldwide by service providers and enterprises for both voice and video services over IP networks.

It is a part of the ITU-T H.32x series of protocols, which also address multimedia communications over ISDN, the PSTN or SS7, and 3G mobile networks.

Within the context of H.323, an IP-based PBX might be a gatekeeper or other call control element which provides service to telephones or videophones.

The first version of H.323 was published by the ITU in November 1996[4] with an emphasis of enabling videoconferencing capabilities over a local area network (LAN), but was quickly adopted by the industry as a means of transmitting voice communication over a variety of IP networks, including WANs and the Internet (see VoIP).

[5] Recognizing that H.323 was being used for communication not only on LANs, but over WANs and within large carrier networks, the title of H.323 was changed when published in 1998.

[7] One strength of H.323 was the relatively early availability of a set of standards not only defining the basic call model, but also the supplementary services needed to address business communication expectations.

[7] The H.323 system defines several network elements that work together in order to deliver rich multimedia communication capabilities.

Inside an H.323 terminal is something referred to as a Protocol stack, which implements the functionality defined by the H.323 system.

The diagram, figure 1, depicts a complete, sophisticated stack that provides support for voice, video, and various forms of data communication.

In reality, most H.323 systems do not implement such a wide array of capabilities, but the logical arrangement is useful in understanding the relationships.

Gateways are widely used today in order to enable the legacy PSTN phones to interconnect with the large, international H.323 networks that are presently deployed by services providers.

A Gatekeeper is an optional component in the H.323 network that provides a number of services to terminals, gateways, and MCU devices.

Those services include endpoint registration, address resolution, admission control, user authentication, and so forth.

An administrative domain is the collection of all zones that are under the control of a single person or organization, such as a service provider.

It has support for both unicast and multicast communication, allowing the size of a conference to theoretically grow without bound.

H.245 enables rich multimedia capabilities, including audio, video, text, and data communication.

Codecs that are widely implemented by H.323 equipment include: H.245 also enables real-time data conferencing capability through protocols like T.120.

T.120-based applications generally operate in parallel with the H.323 system, but are integrated to provide the user with a seamless multimedia experience.

T.120 provides such capabilities as application sharing T.128, electronic whiteboard T.126, file transfer T.127, and text chat T.134 within the context of the conference.

After sending a TCS message, H.323 entities (through H.245 exchanges) will attempt to determine which device is the "master" and which is the "slave."

This process, referred to as Master/Slave Determination (MSD), is important, as the master in a call settles all "disputes" between the two devices.

Once capabilities are exchanged and master/slave determination steps have completed, devices may then open "logical channels" or media flows.

H.323 has been widely deployed by companies who wish to interconnect remote locations over IP using a number of various wired and wireless technologies.

A videoconference, or videoteleconference (VTC), is a set of telecommunication technologies allowing two or more locations to interact via two-way video and audio transmissions simultaneously.

Due to the price point and proliferation of the Internet, and broadband in particular, there has been a strong spurt of growth and use of H.323-based IP videoconferencing.

Videoconferencing is utilized in various situations, for example; distance education, telemedicine, Video Relay Service, and business.