It’s often said that the nice thing about standards is there are so many to choose from. In the multimedia world, that saying certainly rings true. Fortunately, the standards that fuel the IP convergence world are grouped into several functional families. Here’s how they break down.
The T.120 series of recommendations, defined by the International Telecommunications Union’s telecommunications standards division (ITU-T), defines protocols and services for real-time, multipoint data communications. Applications of these protocols include desktop data conferencing, whiteboard image sharing and still image exchange. The protocols can be used with a number of network infrastructures and services, including circuit- and packet-switched nets, LANs and ISDN lines. Examples of standards in this family include:
• T.120: system model
• T.121: generic applications
• T.122: multipoint communication services
• T.123: audiographic and audiovisual applications
• T.124: generic conference control
• T.125: multipoint communication service protocol
• T.126: multipoint still image protocol
• T.127: multipoint binary file transfer protocol
All of the protocols above are in final form and have been implemented by a number of vendors.
The G.700 series, also from the ITU-T, addresses technical aspects of terminal equipment. Of greatest interest are the recommendations for coder/decoders, the devices that transform analogue voice signals into digital pulse streams. The standards in this family differ in the amount of bandwidth they consume and other characteristics, such as the ensuing processing delays that are required. The transmission speed specified by each is as follows:
• G.711: 64Kbps
• G.722: 48Kbps, 56Kbps and 64Kbps
• G.728: 16Kbps
• G.729: 8Kbps
• G.723.1: 5.3Kbps and 6.3Kbps
All of the protocols listed are in final form, however the breadth of the implementations varies widely by vendor. In addition, many vendors implement proprietary (non-standard) codecs.
The ITU-T’s H.320 series governs basic video telephony communications over point-to-point and multipoint connections. Like the T.120 series, the H.320 family includes recommendations for a number of specific applications and network types, including:
• H.320: narrowband (1.92Mbps or less) visual telephone systems
• H.323: packet-based multimedia systems
• H.324: audio and video compression over modem connections
The related standard H.225 specifies call control; H.245 addresses media stream procedures; and H.261 and H.263 specify video codec operation.
Each of these standards has been finalized, and a second version of H.323 was recently released. Many vendors have implemented H.323, but users should check to see if they use Version 1.0 or 2.0, and if the vendors have verified interoperability with other implementers.
Voice-over-IP protocol stack
Clearly, there are a lot of standards for vendors to deal with. But the International Multimedia Teleconferencing Consortium recently published its Voice-over-IP Forum Service Interoperability Implementation Agreement. This document recommends a protocol stack and shows how many of the protocols fit together within a consistent architecture. In addition to the ITU-T recommendations listed above, the Voice-over-IP Forum’s work incorporates several Internet protocols, including the Domain Name System, Real-time Transport Protocol, Real Time Control Protocol, the User Datagram Protocol (UDP), IP and TCP.
Mark Miller is president of DigiNet, a Denver-based data communications engineering firm. He has written 13 books on internetwork design and analysis, including Troubleshooting TCP/IP and Implementing IPv6.