Understanding H.323�Part I: History and Architecture
There are two significant VoIP architectures implemented today: H.323, developed by the International Telecommunications UnionTelecommunications Standard Sector (ITU-T), and the Session Initiation Protocol (SIP), developed by the Internet Engineering Task Force (IETF). This tutorial begins our examination of H.323 by looking at the development history and architecture of that protocol.
In our first tutorial (see Who Sets the Standards for VoIP?), we considered the philosophical and historical differences between the ITU-T and the IETF, and discussed that the ITU-T comes from a telephony and circuit-switched background, while the IETF comes from a data and packet-switched background. (Granted, these two distinctions get pretty blurry today with integrated voice/video/data networks, but the historical background is relevant, nevertheless.) The telephony history is quite evident within the architecture of the H.323 protocol, as it builds upon many circuit-switching constructs that have been telephony mainstays for decades.
This telephony history is also evident when examining other ITU-T standards that fit into their H-Series of standards, generally known as Audiovisual and Multimedia Systems. These include:
- H.320: Narrow-band visual telephone systems and terminal equipment (intended for use with Integrated Services Digital Network (ISDN) and Public Switched Telephone Network (PSTN) systems at data rates from 64-1,920 Kbps).
- H.321: Adaptation of H.320 visual telephone terminals to B-ISDN environments (intended for use with Broadband ISDN systems, such as Asynchronous Transfer Mode (ATM) terminals).
- H.322: Visual telephone systems and terminal equipment for local area networks which provide a guaranteed quality of service (intended for use with LANs such as those complying with the IEEE 802.9 standard).
- H.323: Packet-based multimedia communications systems (intended for use with LANs that do not provide a guaranteed quality of service, such as most LANs in service today.)
- H.324: Terminal for low bit-rate multimedia communication (intended for use with V.34 modems operating over the General Switched Telephone Network (GSTN), or what is called the PSTN here in the United States.)
Thus, the telephony theme clearly emerges from the above list, and with that come many of the associated complexities such as call setup, call disconnect, billing issues and so on.
If we focus specifically on the H.323 standard, we find that the multimedia part of the title provides quite a bit of latitudeallowing applications that support real-time audio, video and/or data communications. Support for audio applications is mandatory, while the video and data parts are optional. Thus, our first interoperability challenge is identified, as it would be possible for two productsone supporting audio only, and another supporting audio, video and datato both be H.323-compliant. However, they may not necessarily be compatible with each other.
In order to support such a variety of media types, an H.323 system may consist of several different components:
- Terminals: a network endpoint which may provide audio only, audio and video, audio and data, or audio, video, and data communications with another H.323 terminal.
- Gateways: a network function that provides access to terminals on a circuit switched network (such as the PSTN) or another H.323 network.
- Gatekeepers: a network function that provides address translation, access control, bandwidth management, and possibly other management operations for the network.
- Multipoint Control Units: a network function that allows three or more terminals to participate in a multipoint conference.
These components could be implemented individually or incorporated as a group within a single product. For example, a PC running an H.323 application, such as Microsoft's NetMeeting would be categorized simply as a terminal. Another device, such as a video conferencing system, could include the functions of the Terminal, Gateway, Gatekeeper and Multipoint Control Unit in a single box, which would be considerably more complex.
And since H.323 attempts to support such as wide variety of communications media, there are a number of protocols that are required to support all of the possible voice, video and data combinations. As such, H.323 has been described as an umbrella standard, under which a number of other protocols, supporting call setup and disconnect, audio encoding/decoding, video encoding/decoding, fit under. These protocols include the ITU-T H.225, H.245 protocols, plus the IETF's Real-Time Transport Protocol (RTP), and others. We will examine the details of these protocols in the next tutorial.
Copyright Acknowledgement: © 2005 DigiNet® Corporation, All Rights Reserved
Mark A. Miller, P.E. is President of DigiNet® Corporation, a Denver-based consulting engineering firm. He is the author of many books on networking technologies, including Voice over IP Technologies, and Internet Technologies Handbook, both published by John Wiley & Sons.