Protocols: H.323 versus H.320

As described in the previous section, there are a number of videoconferencing technologies in use. In this presentation we shall concentrate on two that are in popular use at Northwestern University: H.320 and H.323-based videoconferencing technologies.

H.320
This is a protocol that defines how real-time multimedia communications and conferencing are handled over switched or dedicated ISDN telecommunication links. The protocol is an international standard of the International Telecommunications Union (ITU), and it was adopted in 1990. Multimedia refers to the fact that the standard covers voice, video, and data. The standard is an umbrella standard and includes many other protocols that describe, as an example, how to encode and decode voice and data, how to setup calls between terminals, and how to handle data connections.

The above definition of H.320 is rather complex. Lets see if we can describe it in simpler terms. Lets assume we have a H.320 compliant terminal that has a microphone, a speaker system, a display, a camera, an ISDN connection to the public telephone network, and the necessary electronics to implement the H.320 protocols. Assume another similar terminal exists at a remote site. The local user can dial the ISDN telephone number of the remote terminal, and the H.320 protocols handle the call setup between the terminals. The local terminalŐs microphone and camera picks up the audio and video from the local user, decodes and compresses the audio/video stream, and sends it to the remote site all using the protocols as defined in the H.320 standard. The digital stream is transmitted via the ISDN telecommunication lines to the remote site where it is uncompressed, decoded, and displayed and heard by the remote user. A similar audio/video stream is formed at the remote site and sent to the local site where the audio and video from the remote site can be heard and seen by the local user. A fully interactive videoconference can thus be held between the two sites.

H.320-based videoconferencing is considered to be "traditional", and has been around for a number of years. The problem is that H.320 terminals are expensive, and they normally are implemented in videoconferencing rooms that are, in themselves, very expensive to construct. Vendors have not made H.320 equipment that is applicable to desktop videoconferencing. In addition, ISDN telecommunications lines are expensive to install and incur rather large operating costs. However, there are a lot of H.320 rooms still in use, and thus H.320 is still an important part of the videoconferencing environment at Northwestern University.

H.323
With the advent of the Internet, a new low-cost communication medium became available for communication between sites both local and remote to Northwestern University. Perhaps I should qualify this and say that at least the incremental cost to the end user to use the Internet is low-cost. There are generally no usage costs for the Internet, the bandwidth, at least between major universities, is quite high, and the reliability is very good. This motivated developers to create a videoconferencing technology that uses the Internet to interconnect sites. H.323 is the resulting protocol they developed.

H.323 is an umbrella set of standards defining real-time multimedia communications and conferencing over packet switched networks. Version 1 was adopted by the ITU in 1996 and Version 2 was finalized in 1998. Like H.320, it is a part of a family of H.32X standards. H.323 borrowed many of the H.32X standards used by H.320, especially those for encoding/decoding the audio/video streams and the data sharing protocol (T.120), but it defined a new set for handling communication over the Internet.

Like H.320, vendors have produced a wide array of compatible H.323 compliant products. Unlike H.320, the H.323 products cover a much wider spectrum of capabilities and thus price ranges leading to products in the $500 range that are appropriate for desk top videoconferencing. Videoconferencing has thus moved into the "do it yourself" category and no longer requires expensive videoconferencing rooms. This low entry cost for equipment, the ease of interconnecting H.323 equipment over the Internet, and the fact that equipment exists that allows H.320 and H.323 terminals to interoperate in the same videoconference has led to a rapid deployment of H.323 equipment for videoconferencing in the educational environment.