Why H.323?

What is ITU-T Recommendation H.323? Why have the worlds largest carriers deployed and continue to deploy H.323? H.323 is the first and remains the most powerful international multimedia communications protocol standard, bringing the convergence of voice, video, and data. Built for the packet-based network, H.323 has found a strong home in IP networks, making it the leader in VoIP.

As with other carrier-grade communication protocols, H.323 is a standard published by the International Telecommunications Union. It was approved by the world governments as the international standard for voice, video, and data conferencing, defining how devices such as computers, telephones, mobile phones, PDAs, wireless phones, video conferencing systems, etc., communicate to bring a whole new experience to the user.

H.323 borrows from both the traditional PSTN protocols and the Internet-related standards. By leveraging from both circuit-switched and packet-switched protocol standards, H.323 is able to smoothly integrate with the PSTN, while at the same time send multimedia communications over such mediums as the Internet.

H.323 originated in the mid-'90s as a logical extension of the circuit-switched multimedia conferencing work being done within the ITU-T. Because of this heritage, H.323 interoperates well with a very large installed base of video conferencing equipment. However, H.323 introduced much more capability than was introduced by previous protocols. It brought with it the ability to integrate with the Internet and the World Wide Web (WWW), as well as interface with the PSTN, to provide a range of applications from wholesale transit of voice, pre-paid calling card services, residential voice/video services, enterprise voice/video services, and much more. With H.323, users at remote locations are able to hold a video call and edit a document together-in real-time over the Internet-using their personal computers. Not only that, but H.323 allows the users to customize their phones or phone services, locate users, transfer a call, or perform any number of other tasks by using an HTTP interface between the H.323 client and a server on the network. H.323 fully embraces the power of the Internet.

From the outset, designers of H.323 wanted to create a protocol that would serve well as the Next Generation Network protocol. H.323 significantly lowers the cost of communications and facilitates the rapid creation of many new kinds of services that were never before possible. In addition, H.323 enables endpoints to perform tasks that were previously only possible for centralized servers to perform. H.323 breaks away from the "old technology" model and introduces an intelligent endpoint capable of initiating and accepting calls without dependency on centralized network elements. However, recognizing the business requirements for centralized control in the service provider and enterprise markets, H.323 also allows for centralized control over the endpoint when desired. The level or extent to which a carrier or enterprise wishes to exert control is entirely up to them.

The Power of H.323

The brilliance of H.323 stems from the following characteristics:

Centralized and Distributed Control

H.323 pushes call control functionality to the endpoint, while still providing the service provider or enterprise with the option to control every aspect of a call. In the traditional circuit-switched network, centralized switches perform all call control functionality. With H.323, part or all of that functionality may be pushed to the edge. In some cases, such as a carrier backbone that is transiting voice or video traffic across the globe, it makes sense for H.323 endpoints to talk directly to each other without the encumbrance of centralized servers. However, there are also cases where such servers are necessary or highly desirable, such as when a service provider wants to carefully monitor telephone usage, provide certain mid-call services, perform lawful interception, or perform screening of telephone numbers (i.e., hiding the identity of a calling party). There are a number of reasons for desiring both modes of operation and H.323 provides the flexibility of centralized and decentralized control.


H.323 was designed with one clear objective: to provide users with voice, video, and data conferencing capabilities over a packet-switched network. The continued development of H.323 has stayed focused on this objective and, as a result, H.323 is the leader in this space. H.323 does not try to do more than it should and does not try to pretend to be something that it is not. The primary objective of any service provider or enterprise should be to find a way to reduce costs while enriching the users' experience. H.323, with the convergence of voice, video, and data conferencing, do precisely that.


The designers of H.323 knew that communications requirements differ from place to place, user to user, or company to company. Not only that, but it was well understood that communication requirements change with time. Given these facts, H.323 designers defined H.323 in such a way that equipment manufacturers can insert their own additions to the protocol and that the ITU-T or other standards bodies may define extensions to H.323 that will allow devices to acquire new kinds of features or capabilities, yet still remain fully backward-compatible with other systems and versions of H.323.

Integration with Internet Standards

H.323 fully embraces the Internet by integrating with existing Internet technologies, such as RTP/RTCP, URLs, and DNS. In fact, H.323 was the first standards-based protocol to adopt RTP/RTCP for multimedia transmission. H.323 allows users to place phone calls to other users by clicking on a URLs. H.323 endpoints may also perform DNS queries, much as a web browser would perform a DNS query to find a web site, in order to locate a user or services.

Because of its solid foundation and technical capabilities, H.323 has been highly successful. In fact, H.323 now carries billions of minutes of voice, video, and data conferencing traffic over IP networks every month. H.323 is the clear leader for voice, video, and data convergence.