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
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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.
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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.
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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.
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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.
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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".
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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
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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 [1]. 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.
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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.
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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 [1], 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.
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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 [2]. 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".
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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.
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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 wh