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Advances in multimedia workstations, software systems, storage and user interfaces which allow the presentation of multimedia information have produced an environment in which interactive multimedia systems are developing at a rapid pace. Most current systems are local in their service range, but there will be an increasing demand for long distance multimedia communications. Multimedia communication issues are discussed in this paper. Present day networks go some way towards satisfying the service support requirements of multimedia, but the future Broadband Integrated Services Digital Network (B-ISDN) will ultimately have the capacity and flexibility required to ensure multimedia applications are completely freed from geographical restrictions.
Recently, there have been considerable advances in the components necessary to permit the development of powerful multimedia applications. These have occurred in the areas of hardware and software, and, to a lesser degree, standards.
Hardware developments have led to the ready availability of 24/32 bit colour display boards for PCs/workstations, allowing the presentation of high quality colour still and moving images, and graphics. "Video in a window" boards complement this by permitting real time video to be presented with other information in an integrated manner. The capability to reproduce high quality sound is also becoming more common. At a more fundamental level, the processing power and backplane speeds of computers permit the manipulation and transfer of large amounts of information, including motion video. Rapid, efficient image compression is possible in a single inexpensive chip. Plug in boards that implement standard motion video coding methods in real time on just a few chips are also now available.
Complementary to advances in PC/Workstation hardware are the software developments that allow these hardware capabilities to be exploited. These include the development of composite documents which combine media (eg. text and graphics) in the one document. This now allows desktop publishing in which one can actually see what is to be printed. It is also now possible to paste, not just line drawings, but high quality images into other documents, providing a new flexibility for desk top publishing and document preparation. Software systems are emerging that permit shared working by two or more users on the one document. Co-workers can therefore edit the same document interactively and simultaneously. Hypermedia systems offer the potential for information presentation in the most efficient and flexible manner possible in any given application by providing a hierarchical and cross indexed structure to complex multimedia documents. A recent advance has been the emergence of real time information handling software systems suited to the manipulation of real time information such as audio and video, providing the capability to manipulate (record, store, play, edit, copy, paste) these in the same way as ordinary documents.
Standards are also beginning to be developed to permit the widespread exchange of multimedia information. The ISO is studying stored audio and video systems, database access, multimedia/hypermedia representation, computer graphics systems and others. CCITT covers telecommunication service definition, network support, signalling and coding of communicative audio and video. Relevant activity in CCIR includes the television service definition and CMTT covers standards for coding of broadcast services for transmission.
While some of this standards activity concerns familiar applications, new and vastly different applications will emerge from some of the computer oriented developments. Initially, these will be confined to local applications, using local storage devices and proprietary interfaces and storage formats. At best, some of these could be shared over a Local Area Network (LAN). Increasingly, though, there will be pressure to extend the range of multimedia applications so that even sophisticated multimedia applications, like hypermedia documents, can be transferred across the country and around the world, and information in a variety of formats can be accessed from diverse and remote locations. There will, therefore, be increasing pressure on public telecommunications network providers to support this type of communications.
For some multimedia applications, standard transmission methods have been defined that will support combined data, audio and video over the ISDN. CCITT Rec. H.221 (CCITT, 1990) defines a multiplexing structure for several 64 kbit/s channels used to support audiovisual services defined by other CCITT Recommendations. However, this structure is not intended for universal support of multimedia services.
By providing transparent interconnection between LANs, these networks will provide the infrastructure to allow remote interactive multimedia applications to flourish. The remote systems will appear to be connected to one very large LAN, and the high capacity will ensure that reasonably fast response times are achieved. The MAN standard (IEEE 802.6) is based on fast packet switching methods in which LAN packets are split into a number of segments for transmission and switching. This means that a wide range of service rate demands can be accommodated and that connection control for different applications can be integrated. MANs are seen as a major stepping stone towards the B-ISDN.
The use of Asynchronous Transfer Mode (ATM) (again packet based, using short, fixed length packets called cells) as the transport mechanism for the B-ISDN is intended to provide flexibility for the carriage of all known, as well as unforeseen, services. The cell based transport allows adaptation to required service rate requirements and accommodates asymmetrical bandwidth needs easily. Furthermore, low level signalling capabilities permit multiplexing of the different components of a multimedia connection at the cell level. That is, each media has its own cell stream, permitting great flexibility in the routing of the individual media within the network, as well as matching of the cell stream attributes (such as quality of service, and therefore cost) to match the needs of each medium. For example, file transfer data may be much more tolerant of errors or cell losses, because of high level error control, than real time video services.
A discussion of the B-ISDN and its suitability for the support of multimedia is provided in (Biggar, 1991).
Examples of multimedia applications in which synchronisation will be most important include audio/video synchronisation for both distributive and communicative video applications, shared workspaces in which it is essential that the same workspace is seen simultaneously by each participant.
A range of issues arise when considering multipoint, multimedia service capabilities. Some of these are:
The most obvious application of this concept is in the most demanding service type - video. Work is already well advanced in the definition of hierarchical, or layered, video coding architectures that allow interworking across a broad range of services.
This will mean that one terminal can accept and present a video signal almost independently of the original source and transmitted resolution, frame rate, etc. (Scott, 1990). Complementary studies are required on integration of the various other service components.
These issues include: how to provide control of the connections, which may be to different sites, in a manner that is "user friendly"; what tariffing feedback should be provided to allow the user to monitor and "control" the price paid for a particular multimedia connection; and how control of a multimedia connection is to be divided between the user and network operator, and whether the arrangement should vary between media.
An example where this capability might be required is the interrogation of a text based electronic mailbox via an audio only telephone. If this capability is to be permitted, then text to speech conversion will be required somewhere in the link. Another example might be the inclusion of a participant with videophone capability into a multimedia conference where computer graphics files are being exchanged. In this situation, the means to convert the graphics image to standard video signal format would be required.
Investigations are required to determine the range of media conversions which may be required, the feasibility and importance of providing them and the important question of where the conversion should occur. Must it be provided within the network or is it a terminal/database issue?
While interworking across networks is often considered to be solely a network consideration, many service issues also arise, such as how much of the multimedia multiplex should be passed through a network constrained by bit rate or functionality. How much of the decision making process should be given to the user, and for how much will the network intelligence be responsible? How should a multipoint connection be implemented if one of the participants can only be contacted over a low rate network or on a terminal of limited capability (single media)?
Integration of each of the various media is also important in this situation since, for example, the layered "Universal Video Coding" architecture would permit lower rate (and quality) subsets of a video signal to be passed over a rate constrained connection (Tan, 1991).
At the same time, switched network capacities are increasing (and developments in other areas such as signal processing occurring) to the point where we can consider support of more traditional media, such as television and high quality audio distribution, that has not been possible on the public network in the past.
These developments indicate the need for an overall framework for the support of these services under the banner of "multimedia" (which can include single media as special cases). Identification of the special requirements of these services, how these can be satisfied, the impact on the network and the most effective means of presenting the service to the customer are important areas for further investigation.
This paper has provided an initial view of the impact that multimedia services may have on communications networks, what the network options and evolutions may be, and a more detailed look at some of the issues that must be considered as the B-ISDN evolves to support this important service category. The B-ISDN has the transmission capacity and flexibility suited to the support of all the components of a multimedia connection, and should be able to deliver very fast response times and a high perceived performance (potentially indistinguishable from operation with local peripherals). However, there remain several important areas to be studied to ensure that this vision can be realised.
CCITT (1990). Frame Structure for a 64 to 1920 kbit/s channel in audiovisual teleservices. Recommendation H.221.
Day, A. and Dorman, D. (1990). Towards an Australian Broadband Network Infrastructure, Telecommunication Journal of Australia, 40(2), 3-14.
Montgomery, M. J. (1992). FASTPAC: The Commercial Reality. Proc. Aust. Broadband Switching and Services Symposium. Melbourne, 15-17 July 1992, pp.193-199.
Scott, C., Biggar, M. and Dorman, D. (1990). Getting the Picture - Integrated Video Services in B-ISDN. Telecommunication Journal of Australia, 40(2), 21-25.
Tan, W. (1991). Advanced Video Service Features through the UVC Architecture. Proc. Aust. Broadband Switching and Services Symposium. Sydney, 3-5 July 1991, pp.8-14.
| Michael Biggar has BE and MEngSc degrees from the University of Melbourne, and a PhD from the University of London. He has worked in many areas of video, image and multimedia communications at Telecom Research Laboratories, particularly the delivery of these services over broadband networks. He is active in international video communications standards bodies including the CCITT and ISO/IEC Moving Picture Experts Group, and is the Australian Coordinating member for the CCITT SGXV Experts Group on Video Coding for ATM Networks. Contact made be made through Michael Biggar, Customer Services and Systems Branch, Telecom Australia Research Laboratories, PO Box 249, Clayton 3168, Victoria, Australia.
Please cite as: Biggar, M. J. and Scott, C. J. (1992). Telecommunications impacts on interactive multimedia. In J. G. Hedberg and J. Steele (eds), Educational Technology for the Clever Country: Selected papers from EdTech'92, 156-163. Canberra: AJET Publications. http://www.aset.org.au/confs/edtech92/biggar.html |