Network Working Group C. Adie Request for Comments: 1614 Edinburgh University Computing Service RARE Technical Report: 8 May 1994 Category: Informational Network Access to Multimedia Information Status of this Memo This memo provides information for the Internet community. This memo does not specify an Internet standard of any kind. Distribution of this memo is unlimited. Abstract This report summarises the requirements of research and academic network users for network access to multimedia information. It does this by investigating some of the projects planned or currently underway in the community. Existing information systems such as Gopher, WAIS and World-Wide Web are examined from the point of view of multimedia support, and some interesting hypermedia systems emerging from the research community are also studied. Relevant existing and developing standards in this area are discussed. The report identifies the gaps between the capabilities of currentlydeployed systems and the user requirements, and proposes further work centred on the World-Wide Web system to rectify this. The report is in some places very detailed, so it is preceded by an extended summary, which outlines the findings of the report. Publication History The first edition was released on 29 June 1993. This second edition contains minor changes, corrections and updates. Table of Contents Acknowledgements 2 Disclaimer 2 Availability 3 0. Extended Summary 3 1. Introduction 10 1.1. Background 10 1.2. Terminology 11 2. User Requirements 13 2.1. Applications 13 2.2. Data Characteristics 18
2.3. Requirements Definition 19 3. Existing Systems 24 3.1. Gopher 24 3.2. Wide Area Information Server 30 3.3. World-Wide Web 34 3.4. Evaluating Existing Tools 42 4. Research 47 4.1. Hyper-G 47 4.2. Microcosm 48 4.3. AthenaMuse 2 50 4.4. CEC Research Programmes 51 4.5. Other 53 5. Standards 55 5.1. Structuring Standards 55 5.2. Access Mechanisms 62 5.3. Other Standards 63 5.4. Trade Associations 66 6. Future Directions 68 6.1. General Comments on the State-of-the-Art 68 6.2. Quality of Service 70 6.3. Recommended Further Work 71 7. References 76 8. Security Considerations 79 9. Author's Address 79 Acknowledgements The following people have (knowingly or unknowingly) helped in the preparation of this report: Tim Berners-Lee, John Dyer, Aydin Edguer, Anton Eliens, Tony Gibbons, Stewart Granger, Wendy Hall, Gary Hill, Brian Marquardt, Gunnar Moan, Michael Neuman, Ari Ollikainen, David Pullinger, John Smith, Edward Vielmetti, and Jane Williams. The useful role which NCSA's XMosaic information browser tool played in assembling the information on which this report was based should also be acknowledged - many thanks to its developers. All trademarks are hereby acknowledged as being the property of their respective owners. Disclaimer This report is based on information supplied to or obtained by Edinburgh University Computing Service (EUCS) in good faith. Neither EUCS nor RARE nor any of their staff may be held liable for any inaccuracies or omissions, or any loss or damage arising from or out of the use of this report.
The opinions expressed in this report are personal opinions of the author. They do not necessarily represent the policy either of RARE or of ECUS. Mention of a product in this report does not constitute endorsement either by EUCS or by RARE. Availability This document is available in various forms (PostScript, text, Microsoft Word for Windows 2) by anonymous FTP through the following URL: ftp://ftp.edinburgh.ac.uk/pub/mmaccess/ ftp://ftp.rare.nl/rare/pub/rtr/rtr8-rfc.../ Paper copies are available from the RARE Secretariat. 0. Extended Summary Introduction This report is concerned with issues in the intersection of networked information retrieval, database and multimedia technologies. It aims to establish research and academic user requirements for network access to multimedia data, to look at existing systems which offer partial solutions, and to identify what needs to be done to satisfy the most pressing requirements. User Requirements There are a number of reasons why multimedia data may need to be accessed remotely (as opposed to physically distributing the data, e.g., on CD-ROM). These reasons centre on the cost of physical distribution, versus the timeliness of network distribution. Of course, there is a cost associated with network distribution, but this tends to be hidden from the end user. User requirements have been determined by studying existing and proposed projects involving networked multimedia data. It has proved convenient to divide the applications into four classes according to their requirements: multimedia database applications, academic (particularly scientific) publishing applications, cal (computeraided learning), and general multimedia information services.
Database applications typically involve large collections of monomedia (non-text) data with associated textual and numeric fields. They require a range of search and retrieval techniques. Publishing applications require a range of media types, hyperlinking, and the capability to access the same data using different access paradigms (search, browse, hierarchical, links). Authentication and charging facilities are required. Cal applications require sophisticated presentation and synchronisation capabilities, of the type found in existing multimedia authoring tools. Authentication and monitoring facilities are required. General multimedia information services include on-line documentation, campus-wide information systems, and other systems which don't conveniently fall into the preceding categories. Hyperlinking is perhaps the most common requirement in this area. The analysis of these application areas allows a number of important user requirements to be identified: o Support for the Apple Macintosh, UNIX and PC/MS Windows environments. o Support for a wide range of media types - text, image, graphics and application-specific media being most important, followed by video and sound. o Support for hyperlinking, and for multiple access structures to be built on the same underlying data. o Support for sophisticated synchronisation and presentation facilities. o Support for a range of database searching techniques. o Support for user annotation of information, and for user- controlled display of sequenced media. o Adequate responsiveness - the maximum time taken to retrieve a node should not exceed 20s. o Support for user authentication, a charging mechanism, and monitoring facilities. o The ability to execute scripts.
o Support for mail-based access to multimedia documents, and (where appropriate) for printing multimedia documents. o Powerful, easy-to-use authoring tools. Existing Systems The main information retrieval systems in use on the Internet are Gopher, Wais, and the World-Wide Web. All work on a client-server paradigm, and all provide some degree of support for multimedia data. Gopher presents the user with a hierarchical arrangement of nodes which are either directories (menus), leaf nodes (documents containing text or other media types), or search nodes (allowing some set of documents to be searched using keywords, possibly using WAIS). A range of media types is supported. Extensions currently being developed for Gopher (Gopher+) provide better support for multimedia data. Gopher has a very high penetration (there are over 1000 Gopher servers on the Internet), but it does not provide hyperlinks and is inflexibly hierarchical. Wais (Wide Area Information Server) allows users to search for documents in remote databases. Full-text indexing of the databases allows all documents containing particular (combinations of) words to be identified and retrieved. Non-text data (principally image data) can be handled, but indexing such documents is only performed on the document file name, severely limiting its usefulness. However, WAIS is ideally suited to text search applications. World-Wide Web (WWW) is a large-scale distributed hypermedia system. The Web consists of nodes (also called documents) and links. Links are connections between documents: to follow a link, the user clicks on a highlighted word in the source document, which causes the linkedto document to be retrieved and displayed. A document can be one of a variety of media types, or it can be a search node in a similar sense to Gopher. The WWW addressing method means that WAIS and Gopher servers may also be accessed from (indeed, form part of) the Web. WWW has a smaller penetration than Gopher, but is growing faster. The Web technology is currently being revised to take better account of the needs of multimedia information. These systems all go some way to meet the user requirements. o Support for multiple platforms and for a wide range of media types (through "viewer" software external to the client program) is good. o Only WWW has hyperlinks.
o There is little or no support for sophisticated presentation and synchronisation requirements. o Support for database querying tends to be limited to "keyword" searches, but current developments in Gopher and WWW should make more sophisticated queries possible. o Some clients support user annotation of documents. o Response times for all three systems vary substantially depending on the network distance between client and server, and there is no support for isochronous data transfer. o There is little in the way of authentication, charging and monitoring facilities, although these are planned for WWW. o Scripting is not supported because of security issues o WWW supports a mail responder. o The only system sufficiently complex to warrant an authoring tool is WWW, which has editors to support its hypertext markup language. Research There are a number of research projects which are of significant interest. Hyper-G is an ambitious distributed hypermedia research project at the University of Graz. It combines concepts of hypermedia, information retrieval systems and documentation systems with aspects of communication and collaboration, and computer-supported teaching and learning. Automatic generation of hyperlinks is supported, and there is a concept of generic structures which can exist in parallel with the hyperlink structure. Hyper-G is based on UNIX, and is in use as a CWIS at Graz. Gateways between Hyper-G and WWW exist. Microcosm is a PC-based hypermedia system developed at the University of Southampton. It can be viewed as an integrating hypermedia framework - a layer on top of a range of existing applications which enables relationships between different documents to be established. Hyperlinks are maintained separately from the data. Networking support for Microcosm is currently under development, as are versions of Microcosm for the Apple Macintosh and for UNIX. Microcosm is currently being "commercialised".
AthenaMuse 2 is an ambitious distributed hypermedia authoring and presentation system under development by a university/industry consortium based at MIT. It will have good facilities for presentation and synchronisation of multimedia data, strong authoring support, and will include support for networking isochronous data. It will be a commercial product. Initial versions will support UNIX and X windows, with a PC/MS Windows version following. Apple Macintosh support has lower priority. The "Xanadu" project is designing and building an "open, social hypermedia" distributed environment, but shows no sign of delivering anything after several years of work. The European Commission sponsors a number of peripherally relevant projects through its Esprit and RACE research programmes. These programmes tend to be oriented towards commercial markets, and are thus not directly relevant. An exception is the Esprit IDOMENEUS project, which brings together workers in the database, information retrieval and multimedia fields. It is recommended that RARE establish a liaison with this project. There are a variety of other academic and commercial research projects which are also of interest. None of them are as directly relevant as those outlined above. Standards There are a number of existing and emerging standards for structuring hypermedia applications. Of these, the most important are SGML, HyTime, MHEG, ODA, PREMO and Acrobat. All bar the last are de jure standards, while Acrobat is a commercial product which is being proposed as a de facto standard. SGML (Standard Generalized Markup Language) is a markup language for delimiting the logical and semantic content of text documents. Because of its flexibility, it has become an important tool in hypermedia systems. HyTime is an ISO standardised infrastructure for representing integrated, open hypermedia documents, and is based on SGML. HyTime has great expressive power, but is not optimised for run-time efficiency. It is recommended that future RARE work on networked hypermedia should take account of the importance of SGML and HyTime. MHEG (Multimedia and Hypermedia information coding Experts Group) is a draft ISO standard for representing hypermedia applications in a platform-independent form. It uses an object-oriented approach, and is optimised for run-time efficiency. Full IS status for MHEG is expected in 1994. It is recommended that RARE keep a watching brief
on MHEG. The ODA (Open Document Architecture) standard is being enhanced to incorporate multimedia and hypermedia features. However, interest in ODA is perceived to be decreasing, and it is recommended that ODA should not form a basis for further RARE work in networked hypermedia. PREMO is a new work item in the ISO graphics standardisation community, which appears to overlap with MHEG and HyTime. It is not clear that the PREMO work, which is at a very early stage, is worthwhile in view of the existence of those standards. Acrobat PDF is a format for representing multimedia (printable) documents in a portable, revisable form. It is based on Postscript, and is being proposed by Adobe Inc (originators of Postscript) as an industry standard. RARE should maintain awareness of this technology in view of its potential impact on multimedia information systems. There are various standards which have relevance to the way multimedia data is accessed across the network. Many of these have been described in a previous report . Two further access protocols are the proposed multimedia extensions to SQL, and the Document Filing and Retrieval protocol. Neither of these are likely to have major significance for networked multimedia information systems. Other standards of importance include: o MIME, a multimedia email standard which defines a range of media types and encoding methods for those types which are useful in a wider context. o AVIs (Audio-Visual Interactive services) and the associated multimedia scripting language SMSL, which form a standardisation initiative within CCITT (now ITU-TSS) to specify interactive multimedia services which can be provided across telephone/ISDN networks. There are two important trade associations which are involved in standardisation work. The Interactive Multimedia Association (IMA) has a Compatibility Project which is developing a specification for platform-independent interactive multimedia systems, including networking aspects. A newly-formed group, the Multimedia Communications Forum (MMCF), plans to provide input to the standards bodies. It is recommended that RARE become an Observing Member of the MMCF. A third trade association - the Multimedia Communications Community of Interest - has also just been formed.
Future Directions Three common design approaches emerge from the variety of systems and standards analysed in this report. They can be described in terms of distinctions between different aspects of the system: o content is distinct from hyperstructure o media type is distinct from media encoding o data is distinct from protocol Distributed hypermedia systems are emerging from the research/development phase into the experimental deployment phase. However, the existing global information systems (Gopher, WAIS and WWW) are still largely limited to the use of external viewers for nontextual data. The most significant mismatches between the capabilities of currently-deployed systems and user requirements are in the areas of presentation and quality of service (i.e., responsiveness). Improving QOS is significantly more difficult than improving presentation capabilities, but there are a number of possible ways in which this could be addressed. Improving feedback to the user, greater multi-threading of applications, pre-fetching, caching, the use of alternative "views" of a node, and the use of isochronous data streams are all avenues which are worth exploring. In order to address these problems, it is recommended that RARE seek to adapt and enhance existing tools, rather than develop new ones. In particular, it is recommended that RARE select the World-Wide Web to concentrate its efforts on. The reasons for this choice revolve around the flexibility of the WWW design, the availability of hyperlinks, the existing effort which is already going into multimedia support in WWW, the fact that it is an integrating solution incorporating both WAIS and Gopher support, and its high rate of growth compared to Gopher (despite Gopher's wider deployment). Gopher is the main competitor to WWW, but its inflexibly hierarchical structure and the absence of hyperlinks make it difficult to use for highly-interactive multimedia applications. It is recommended that RARE should invite proposals for and subsequently commission work to: o Develop conversion tools from commercial multimedia authoring packages to WWW, and accompanying authoring guidelines.
o Implement and evaluate the most promising ways of overcoming the QOS problem. o Implement a specific user project using these tools, to validate that the facilities being developed are truly relevant to real applications. o Use the experience gained to inform and influence the development of the WWW technology. o Contribute to the development of PC/MS Windows and Apple Macintosh WWW clients, particularly in the multimedia data handling area. It is noted that the rapid growth of WWW may in the future lead to problems through the implementation of multiple, uncoordinated and mutually incompatible add-on features. To guard against this trend, it may be appropriate for RARE, in coordination with CERN and other interested parties such as NCSA, to: o Encourage the formation of a consortium to coordinate WWW technical development. 1. Introduction 1.1. Background This study was inspired by the realisation that while some aspects of distributed multimedia technology are being actively introduced into the European research community (for instance, audiovisual conferencing, through the MICE project), other aspects are receiving less attention. In particular, one category in which there seems to be relatively little activity is providing solutions to ease remote access to multimedia resources (for instance, accessing stored audio/video clips or images, or indeed entire multimedia applications, across the network). Few commercial products address this, and the relevance of existing standards in this area is unclear. Of the 50 or so research projects documented in the recent RARE distributed multimedia survey , only about six have a direct relevance to this application area. Where stated in the survey, the main research effort in these projects is often directed towards the "difficult" problems, such as the transfer of isochronous data and the design and implementation of object-oriented multimedia databases, rather than towards user-oriented issues.
This report is concerned with practical issues in the intersection of networked information retrieval, database and multimedia technologies. It aims to establish actual user requirements in this area, to look at existing systems which offer partial solutions, and to identify what additional work needs to be done to satisfy the most pressing requirements. 1.2. Terminology In order to discuss multimedia information systems, we need a consistent terminology. The vocabulary defined below embodies some of the concepts of the Dexter hypertext reference model . This model is sufficiently general to be useful for describing most of the facilities and requirements of the multimedia information systems described in this report. (However, the Dexter model does not describe searchable index objects - it is not a database reference model.) anchor An identified portion of a node. E.g., in a text node, an anchor might be a string of one or more adjacent characters, while in an image node it might be a rectangular area of the image. composite node A node containing data of multiple media types. document Often used loosely as a synonym for node. hyperdocument We refer to a collection of related nodes, linked internally with hyperlinks, as a "hyperdocument". Examples are a database of medical images and associated text; a module from a suite of teaching material; or an article in a scientific journal. A hyperdocument may contain hyperlinks to other data which exists in internally with hyperlinks, as a "hyperdocument". Examples are a other hyperdocuments, but can be viewed as largely self-contained. It is a highlevel "unit of authoring", but is not necessarily perceived as a distinct unit by a reader (although it may be so perceived, particularly if it contains few hyperlinks to outside entities). hyperlink Set of one or more source anchors and one or more target anchors. Also known simply as a "link".
isochronous (adjective) Describes a continuous flow of data which is required to be delivered by the network under critical time constraints. leaf node A node which contains no source anchors. media type An attribute of data which describes the general nature of its expected presentation. The value of this attribute could be one of the following (not exhaustive) list: o Text o Sound o Image (e.g., a "photograph") o Graphics (e.g., a "drawing") o Animation (i.e., moving graphics) o Movie (i.e., moving image) monomedia (adjective) Said of data which is all of the same media type. multimedia (adjective) Said of data which contains different media types. This definition is stricter than general usage, where "multimedia" is often used as a generic term for non-textual data, and where it may even be used as a noun. physical media Magnetic or optical storage. Not to be confused with media type! [simple] node A monomedia object which may be retrieved and displayed as a single unit. source anchor An anchor which may be "actioned" by the user, causing the node(s) containing the target anchor(s) in the same hyperlink to be retrieved and displayed. This process is called "traversing the link". target anchor an anchor forming part of a hyperlink, whose containing node is retrieved and displayed when the hyperlink is traversed.
2. User Requirements User requirements in an area such as networking, which is subject to rapid technological change, are sometimes difficult to identify. To an extent, technology leads applications, and users will exploit what is possible. 2.1. Applications Awareness of the range of networked multimedia applications which are currently being envisaged by computer users in the academic and research community leads to a better understanding of the technical requirements. This section outlines some projects which require remote access to multimedia information across research networks, and which are currently either at a preliminary stage or underway. The projects are divided into broad categories according to their characteristics. Multimedia Databases Here are several examples of multimedia projects which have a "database" character. The Peirce Telecommunity Project This project centres on the construction of a multimedia (text and image) database of the works of the American philosopher Peirce, together with tools to process the data and to make it available over the Internet. A sub-project at Brown University focuses on adapting existing client/server network tools for this purpose. The requirements for network access include facilities for structured viewing, intelligent retrieval, navigation, linking, and annotation, as well as for domainspecific processing. Museum Object Databases The RAMA (Remote Access to Museum Archives) project is funded under the EEC RACE II programme. Its objective is to develop a system which allows museums to make multimedia information about their exhibits and archived material available over an ISDN network. The requirements capture and technical architecture design phases are now complete, and a prototype system will be delivered in June 1993 to link the Ashmolean Museum (Oxford, GB), the Musee d'Orsay (Paris, FR) and the Museum Archeological National (Madrid, ES). Image data is the main media type of interest, although video and sound may also play a part.
The Bristol Biomedical Videodisk Project The Bristol Biomedical Videodisc is a collection of Medical, Veterinary and Dental images. The collection holds some 24,000 still images and is continuously growing. Textual information regarding the images is included as part of the database and this can be searched on any keyword, number or other data type, or a combination of any of these. The images are currently delivered in analogue form on a videodisc, but many institutions are unable to afford the cost of videodisc players. Investigations into making this image and text database available across the network are underway. ArchiGopher ArchiGopher is a Gopher server at the College of Architecture, University of Michigan, dedicated to the dissemination of architectural knowledge. Presently in its infancy, ArchiGopher is intended to become a multimedia resource for all architecture faculty and students world-wide. Some of the available or planned resources are: o The College's image bank. o The CAD group's collection of computer models (already started). o The Doctoral Program's recent dissertation proposals and abstracts. o Example archive of Kandinsky paintings. o Images of 3D CAD projects. The principal media type in ArchiGopher is image. Files are stored in both TIFF and GIF format. Vatican Library Exhibit In January 1993, the US Library of Congress mounted an electronic version of the exhibition ROME REBORN: THE VATICAN LIBRARY AND RENAISSANCE CULTURE. The exhibition was subsequently processed by the University of Virginia Library. The text files were broken into individual captions associated directly with each image and a WAIS-searchable version of the object index generated. This has been made available on Gopher by the University of Virginia Library.
This project is particularly interesting, as it demonstrates some limitations of the Gopher system. The principal media types are image and text, and it is difficult to associate a caption with its image - each must be fetched separately, and using the XMosaic or xgopher client software it is not possible to tell which menu entry is the image and which the caption. (This may be a consequence of how the data has been configured for the Gopher server; if so, a requirement for better publishing tools may be indicated.) Furthermore, searching the object index will result in a Gopher menu containing references to catalogue entries for relevant exhibits, but not to the online images of the exhibits themselves, which severely limits the usefulness of the index. It is interesting to note that during the preparation of this report, the Vatican Exhibition has been mounted on the WorldWide Web (WWW). The hypermedia presentation on the Web is very much more attractive to use than the Gopher version. Jukebox Jukebox is a project supported by the EEC libraries program. The project aims to evaluate a pilot service providing library users with on-line access to a database of digital sound recordings. The database will support multi-user access and use suitable storage media to make available sound recordings in a compressed format. Users will access the service with a personal computer connected to a telematic network. Scientific Publishing There are several refereed electronic academic journals presently distributed on the Internet. These tend to be text-only journals, and have not really addressed the issues of delivering and manipulating non-text data. Many scientific publishers have plans for electronic publishing of existing academic journals and conference proceedings, either on physical media or on the network. The Journal of Biological Chemistry is now published on CD-ROM, for instance. Some publishers view CD-ROM as an interim step to the ultimate goal of making journals available on-line on the Internet. The main types of non-text data which are envisaged are: o Images. In many cases, image data (a microphotograph, say) is central to an article. Software which recognises that the text may be of secondary importance to the image is required.
o Application-specific data. The ChemLab and MoleculeLab applications are widely used, and the integration of corresponding data types with journal articles will enhance readers' ability to visualise molecular structures. Similarly, mathematics appearing in scientific papers could be represented in a form suitable for processing by applications such as Mathematica. Mathematical content could then become a much more interactive and dynamic aspect of research publications. o Tabular data. The ability for a reader to extract tabular data from a research paper, to produce a graphical representation, to subset the data, and to further process it in a number of different ways, is viewed as an essential part of scientific electronic publishing. o Movies. The American Astronomical Society regularly publishes videos to go with its academic journals. Electronic publishing can improve on this "hard copy" publishing by integrating video data much more closely with the source article. o Sound. There is perhaps slightly less demand for audio information in scientific publishing, but the requirement does exist in particular specialities (such as acoustics and zoology journals). Access to academic journals using at least four different paradigms is envisaged. Hierarchical access, perhaps using a traditional journal/volume/issue/article model, is perhaps the most obvious. Keyword searching (or full-text indexing) will be required. Browsing is another useful and often underestimated access model - to support browsing it is essential that "eye-catching" data (unlikely to be textual) is prominently accessible. The final method of access is perhaps the most important - the use of interactive viewing tools. Such tools would enable navigation of hypermedia links within and between articles, with gateways to special-purpose applications as described above. The use of these disparate access methods implies more than one structure being applied to the same underlying data. Standards, particularly SGML, are becoming important to publishers, and it is clear that the SGML-based HyTime standard will be a front runner in providing the kind of hypermedia facilities which are being envisaged. However, progress towards a common SGML Document Type Definition (DTD) for scientific articles, even within individual publishing houses and for text-only documents, is slow.
A specific initiative involving interested parties will be required to formalise detailed requirements and to pilot standards in this area. A preliminary demonstrator project, funded by publishers and by the British Library Research and Development Department, involves making about 30 sample scientific articles available over the SuperJANET network, using a range of different software products. The demonstrator project is being managed by IOP Publishing and is being carried out at Edinburgh University Computing Service. Existing tools, particularly WAIS and WWW, are relevant, but adequate security and charging mechanisms are required if commercial publishers are to use them. Many research groups are now making the text of preprints and published research papers available on Gopher servers. It is interesting to note that the proceedings of the Multimedia 93 conference run by the ACM will be published electronically (on CD ROM), using a multimedia document format designed specifically for the event. Computer-aided Learning The ready availability of user-friendly multimedia authoring tools such as AuthorWare Professional, Asymmetrix Multimedia Toolbook, Macromind Director and many more, has stimulated much interest in multimedia for computer-aided learning applications within the user community. Sophisticated interactive multimedia courseware applications are being developed in many disparate subjects throughout the European academic community. Users are now beginning to ask network technologists, "how can I make my multimedia application available to others across the network?". There is considerable interest in using the network to enhance delivery of multimedia teaching materials - for instance to allow students to take courses remotely (distance learning) and for their learning process to be supported, monitored and assessed remotely. The requirements which flow from this type of network application include the ability to identify and authenticate the students using the material, to monitor their progress, and to supply on-line assessment exercises for the student to complete. Multimedia authoring tools allow very attractive presentation environments to be created, which encourages learning; this is viewed as essential by course developers. Easy-to-use authoring tools (preferably existing commercial ones) are also essential. Finally, some learning applications involve simulations - examples include meteorological modelling and economic simulations. Network
delivery of teaching materials should cope with this requirement (perhaps by acknowledging that executable scripts are just another media type). General Information Services There are many other possible uses of multimedia data in networked information servers which don't conveniently fall into any of the above categories. Some examples are given below. o On-line documentation. Manuals and instruction books often rely heavily on pictorial information, and are enhanced by dynamic media types (sound, video). The ability to access centrally-held manuals across a network makes it much easier to keep the information up-to-date. o Campus-wide information systems (CWIS) are an important growth area. The opportunities for enhancing such a service with multimedia data (e.g., maps) is obvious. o Multimedia news bulletins (e.g., the Internet Talk Radio, which is sound only). o Product information (the multimedia equivalent of paper advertising matter). o Consumer systems - e.g., tourist information servers. The utility of such systems in an academic/research environment is perhaps questionable, but it is likely that such systems will address problems which will also be met in this environment. We should be prepared to learn from such projects. 2.2. Data Characteristics Some of the characteristics which make data more appropriate for network publication rather than publication on physical media are listed below. o The data may change frequently. o Implementing corrections and improvements to the data is very much easier. o It is more readily available to the data user - no purchase/delivery cycle need exist.
o Publication on physical media may not be cost-effective for very large volumes of data. (Of course, there is a cost in networking the data as well, but the research/academic user is normally insulated from this.) o Access for large user communities can be established without requiring each user to purchase a potentially expensive physical media peripheral (such as a laser disk player). This is particularly helpful in classroom situations. o It may require less effort from the data publisher to make data available over a network, rather than set up a manual mechanism for distributing physical media. o If related data from many different sources is to be published, it may be more efficient to leave the data in situ, and simply publish the network addresses of the data. There are counter-reasons which may make physical media distribution more appropriate: o Easier to charge for. (However, charging mechanisms do exist in some network information systems. It may be that potential information providers need to be made more aware of this.) o Easier to deter or prevent copyright infringement, using traditional copy-protection techniques. 2.3. Requirements Definition From studying the applications described in the preceding section, and from discussions with the people involved with the applications, it is possible to draw up a list of general requirements which a distributed multimedia information system for the academic and research community should satisfy. These requirements are informally described in the following subsections. The descriptions are necessarily informal and incomplete: every individual application will have its own detailed requirements, which would take a great deal of effort to determine (and indeed some of the requirements may not become apparent until the application is into its development phase). Platforms It is clear that the European academic community, in common with other such communities, requires support for three main platforms: UNIX, Apple Macintosh, and PC/Windows. For multimedia client/server
systems, the latter two are less appropriate as server platforms, but client support for all three is vital. UNIX will be most often used as the server platform. There are other systems, such as VAX/VMS, which are also important in some sectors. Media Types Unsurprisingly, all applications require text data to be supported as a basic media type. Image and graphic media types are next in importance, followed by "application-specific" data (such as tabular scientific data, mathematical equations, chemical data types, etc). Sound and video media types are becoming more important as users discover how these can enhance applications. Many different encodings are possible for each media type (e.g., image data can be encoded as TIFF, PCX, GIF, PICT and many more). An information system should not constrain the type of encoding used, and should ideally offer either a range of alternative encodings, or conversion facilities between the stored encoding and an encoding suitable for display by the client workstation. Hyperlinks It is clear that many applications require their users to be able to navigate through the information base according to relationships determined by the information provider - in other words, hyperlinks. Academic publishing, CAL, on-line documentation and CWIS systems all require this capability. The user should be able, by some action such as clicking on a highlighted word in a text node or on a button, to cause another node or nodes to be retrieved and displayed. Some "hypermedia" systems are in fact simply hypertext, in that they require the source anchor of a hyperlink to be in a text node. A true hypermedia system allows hyperlinks to have their source anchors in nodes of any media type. This allows a user to click the mouse on a component of a diagram or on part of a video sequence to cause one or more related nodes to be retrieved and displayed. Some hypermedia systems allow target anchors of a hyperlinks to be finer-grained than a whole node - e.g., the target anchor could be a word or a paragraph within a text document. Without such a capability, it is necessary for target nodes to be quite small if precision is required in a hyperlink. This may be difficult to manage, and fine-grained target anchors are therefore better.
Additional structure above or orthogonal to the underlying hyperlinked data is required in some applications. This allows the same (generally non-textual) data to be used in several different applications, or the implementation of different access paradigms. Presentation Related information of different media types must be capable of synchronised display. Commercial multimedia authoring packages provide many different ways of presenting, synchronising and interacting with media elements. Some of these are summarised below. o Backdrops. An application may present all its visual information against a single background bitmap - e.g., a CAL application might use a background image of an open textbook, with graphics, text and video data all presented on the open pages of the book. o Buttons. A "button" can be defined as an explicitly- delimited area of the display, within which a mouse click will cause an action to occur. Typically, the action will be (or can be modelled as) a hyperlink traversal. Applications use different styles of button - some may use "tabs" as in a notebook, or perhaps "bookmarks" in conjunction with the open textbook backdrop mentioned above. Others may use plain buttons in a style conforming to the conventions of the host platform, or may simply highlight a word or phrase in a text display to indicate it is "active". o Synchronisation in space. When two or more nodes are presented together (e.g., because a link with more than one target anchor has been traversed), the author of the hyperdocument may wish to specify that they be presented in a spatially-related way. This may involve: x/y synchronisation - e.g., a video node being displayed immediately above its text caption; it may involve contextual synchronisation - e.g., an image being displayed in a specific location within a text node; or it may involve z- axis synchronisation as well - for instance a text node containing a simple title being displayed on top of an image, with the text background being transparent so that the image shows through. o Synchronisation in time. Isochronous data may require synchronisation - the obvious case being audio and video tracks (where these are held separately). Other examples are: the synchronisation of an automatically-scrolling text panel to a video clip (for subtitling); or to an audio clip
(e.g., a translation); or synchronising an animation to an explanatory audio track. Searching Database-type applications require varying degrees of sophistication in retrieval techniques. For applications addressed in this report, non-text nodes form the major data of interest. Such nodes have associated descriptions, which may be plain text, or may be structured into fields. Users need to be able to search the descriptions, obtain a list of "hits", and select nodes from that list to display. Searching requirements vary from simple keyword searching, via full-text indexing (with or without Boolean combinations of search words), to full SQL-style database retrieval languages. Interaction The user must be able to annotate documents retrieved from the information server. The annotations may be stored locally. Similarly, the user may wish to add his own (locally-held) hyperlinks to documents. (Actual modification of documents in the information system itself, or shared annotations to documents - i.e., the information system as a CSCW environment - is viewed as separate issue which this report does not address.) If an information provider has included contact details (such as a mail address) in a document, it should be possible for the reader to invoke a program (such as a mailer) which initiates communication with the author. In some applications, it may make sense for a user to be able to specify a region of interest in an image or movie clip, and to request a more detailed view of (or other information about) that region. Some applications require a sequence of images to be presented under control of the user. For instance, a three-dimensional microscopic structure could be represented as a sequence of images taken with the microscope focused on a different plane for each image. For display, the user could control which image was displayed using some kind of slider control, giving the illusion of focusing a microscope. (This particular example has been taken from the Theseus project at John Moore's University, Liverpool, GB.)
Quality of Service Research has shown  that user toleration of delay in computer systems depends on user perception of the nature of the requested action. If the user believes that no computation is required, tolerable delays are of the order of 0.2s. If the user believes the action he or she has requested the computer to perform is "difficult" - for instance a computation of some form - then a tolerable delay is of the order of 2s. Users tend to give up waiting for a response after about 20s. Networked multimedia information systems must be able to provide this level of responsiveness. Management In order to support applications involving real-money information services (e.g., academic publishing) and learning/assessment applications, there must be a reliable and secure access control mechanism. A simple password is unlikely to suffice - Kerberos authentication procedures are a possibility. Users must be able to determine the charge for an item before retrieving it (assuming that pay-per-item will be a common paradigm alternatives such as pay-per-call, pay-per-duration are also possible). Access records must be kept by the information server for charging purposes. Learning applications have similar requirements, except that the purpose here is not to charge for information retrieved, but to monitor and perhaps assess a student's progress. Scripting Many authoring packages provide scripting languages. In most cases, these languages are used to manage the presentation environment and control navigation within the hypermedia document. There are other, declarative rather than procedural, methods for achieving this, so scripting of this type is not necessarily a requirement. However, some application areas require executable scripts for other purposes (e.g., simulations in CAL applications). Care in providing such a facility is required, because of the potential for abuse (the possibility of "trojan" scripts). However, there is work going on to produce "safe" scripting languages - an example is "safe tcl", being developed by Borenstein and Ousterhout (contact email@example.com).
Bytestream Format For the easy transfer and handling of a hyperdocument, it must be capable of being encoded into a bytestream form, in such a way that the structure of the document is preserved and it can be decoded without loss of information. This facility makes it possible for such documents to be supplied to a user over electronic mail, in such a way that he or she can browse them at his or her own site. This may be appropriate where the user does not have a direct connection to the Internet. It will also be useful for printing the hyperdocument. Authoring It is essential that a multimedia information system should have adequate authoring tools which make it easy to prepare and publish hypermedia information. Such tools need similar power to existing commercial multimedia authoring software for stand-alone multimedia applications.