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An architecture for multimedia communication and real-time collaboration

IBM Systems Journal - September 1, 1995

Computers attached to networks are commonly used for collaborative activity; this is supported by a variety of mail, messaging, and database products. In most cases these applications are either person-to-person information exchanges where the parties are working together, but not simultaneously, or are person-to-machine interactions. The first category is illustrated by mail applications, where electronic documents are processed first by one party and then by the other; although the turnaround time can be very short, the essence of the application is alternate activity. The second category involves only one person directly; therefore there is no concept of a natural human dialog to be sustained.

In contrast, real-time collaboration has two essential elements: people are directly involved with each other, and simultaneous activity by these people is the essence of the interaction. Examples include desktop conferencing, distance learning, help desk operation, remote presentations, brainstorming, and shared document editing. Real-time collaboration requires multimedia communication, because the traditional data exchange between workstations needs to be enhanced with audio to allow conversation among the participants for effective human interaction. An alternative, equally valid perspective is to consider the exchanges as data enhancements to telephony. In addition to data and audio, live video can be justified for some, but not all, applications. In real-time collaborative activity, natural interaction between people requires low-latency transmission so that responses are not noticeably delayed. This aspect is much more demanding than is normal in existing messaging, mail, and related networked applications. Support for the audio and video streams also demands isochronous communications to prevent distortion, and this is not commonly available in data networks.

A current topic for research and development activity is the efficient provision of appropriate multimedia communication services. Much of the effort has focused either on the human factors of computer-supported collaborative work or on the associated computing science aspects. [1-3] Other activities have explored the social and organizational implications. [4,5] Rather less academic interest has been directed toward the design and specification of generic application programming interfaces, although this has been a topic for computing and telephony organizations. Many individual and collective developments are underway, although little has been published for competitive reasons.

From a standards viewpoint, the initiative by the International Telecommunication Union-Telecommunication Standardization Sector (ITU-T) through the draft recommendations of the various T.120 committees is of considerable interest, with the development of the Multipoint Communication Service (MCS)[6] and the associated Generic Conference Call (GCC).[7] Previous to this work the H.320 recommendation provided the basis for audio and video communication over the integrated services digital network (ISDN) and allowed the development of videoconferencing services. The requirements met by the H.320 recommendation were for an endpoint attached to the network to exchange a single audio stream, and optionally a single video stream, with other users; multipoint operation was being provided by multipoint control units within the network itself. Compatibility with the existing equipment was of primary importance and this dictated call setup and the audio formats. Also recognized in the H.320 recommendation was the need for data communication such as file transfer and document exchange, and therefore provision for multiplexing was included for data. The H.320 recommendation has been widely adopted for videoconferencing and provides interoperability between different manufacturers' equipment; furthermore it has encouraged the deployment by network operators of standard multipoint control units for multiway conferencing. However, the absence of any definition of the contents of the data channel has precluded interoperability for data services and this deficiency has given rise to the T.120 series of recommendations. Hence MCS provides multiple logical data channels to a user and allows multiparty operation, thus laying a foundation for collaborative data services. GCC builds on MCS to provide call management for conference setup and tear-down, while other T-series recommendations define application protocols for file transfer and shared whiteboards, with others to be added in the future.

The combination of the H.320 and T.120 recommendations will be important in allowing interoperability between desktop conferencing users over public switched networks. However, the real-time collaborative opportunity is much greater than the scope of H.320 and T.120 combined; some examples will illustrate the problems still to be addressed.

From a personal computer (PC) as opposed to a telephony perspective, audio and video support is rich and varied. Compact disk standards provide the basis of PC audio, and many video technologies are already in use, with quality approaching and moving beyond that of television. The G.711 audio and H.261 video recommendations of H.320 are not always acceptable substitutes, and the loss in quality will meet resistance from customers whose expectations are set by domestic television. PC networks are characterized by extreme diversity and do not normally have the isochronous capabilities of telephony networks; although their bandwidths are typically orders of magnitude greater, their high latency and jitter are problematic. New technologies, such as those based on asynchronous transfer mode (ATM), address these deficiencies and combine isochronous capabilities with high bandwidth. Such capacity allows video compression without loss of information, combining high quality with low latency.

Simple hierarchical topologies as envisaged by the authors of the ITU-T recommendations are not typical of installed corporate computer networks; complex meshes exist and the constituent links have widely varying, and often unpredictable, characteristics. Frequently the challenge is to exploit what exists while allowing new capabilities to be selectively integrated into the infrastructure. Multiple audio and video streams per user are required for some applications; for example, reviewing of television commercials, and synchronization between multiple audio, video, and data streams is desirable. An architecture that classifies streams into audio, video, or data fails to recognize that the true distinction is to be made on the basis of the communications requirements, or quality of service needed, for the stream, independent of its content.

From an application perspective, the requirement is not normally for multimedia communications but for distributed multimedia device connectivity. With few exceptions, most multimedia streams originate from devices, and the need is to transport the output of a source to one or more remote destination devices and provide end-to-end, device-to-device services. Many of the problems are to be found at the end points, where the devices are coupled to communication networks using the shared services of an operating system, processor, and bus.

Today the telephone is the natural choice for realtime collaboration and no viable alternative exists for most people; as demand increases for existing telephone calls to be enriched with data exchanges, and ultimately with video services, the personal computer is destined to become the instrument for personal communications. This can be seen either as an evolution of the telephone, or as an evolution of the personal computer, and although the result is the same in both cases, the process is very different. Equally interesting is the nature of the network to which the multimedia personal computer is attached, which can be the telephony network enhanced to supply the necessary bandwidth, or a computing data network enhanced to support low-latency isochronous communication.

This section introduced the need for real-time collaboration using multimedia communications. Following sections clarify the requirements and describe the IBM Lakes architecture designed to meet these requirements.

The real-time collaboration requirements

The detailed requirements for a real-time multimedia communications platform arise from three sources: the needs of the application programmer, the nature of the networks that form the collaborative infrastructure, and the demands of users and product developers. Requirements from each source follow.

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