Packet Guide to Voice over IP

Chapter 6. H.323 ITU-T Recommendation for Packet-Based Multimedia Communications Systems

The 1990s spawned many changes to the ways in which we communicate. Among them were videoconferencing and the idea that voice might actually be encapsulated in a data packet. The International Telecommunications Union–Telecom (ITU-T) H.323 recommendation dates from this time period. While it has been updated many times, most of the ideas have carried forward. The very basic problem addressed by the recommendation is the conversion of the signaling used on a traditional telecommunications network to Internet Protocol (IP) based techniques. Rather than reinvent the wheel, many of the ideas were retained, though the packaging was quite a bit different. Thus H.323 uses as its basis many of the protocols and standards associated with the public switched telephone network, or PSTN, encapsulated in protocols such as TCP (transmission control protocol), UDP (user datagram protocol), and IP. In addition, we see the recommendation leveraging messages from the Integrated Service Digital Network (ISDN), such as call signaling.

This recommendation was the standard for videoconferencing, which includes voice, video, and data. Because it addresses such things as call control and management for point-to-point and multipoint conferences, gateway administration of media traffic, bandwidth parameters, and user participation, it also became the defacto standard for Internet Telephony and VoIP. While this is no longer ...