rfc1614.txt

著名的RFC文档,其中有一些文档是已经翻译成中文的的.

           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".




Adie                                                            [Page 6]

RFC 1614        Network Access to Multimedia Information        May 1994


   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



Adie                                                            [Page 7]

RFC 1614        Network Access to Multimedia Information        May 1994


   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.



Adie                                                            [Page 8]

RFC 1614        Network Access to Multimedia Information        May 1994


   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.



Adie                                                            [Page 9]

RFC 1614        Network Access to Multimedia Information        May 1994


      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.





Adie                                                           [Page 10]

RFC 1614        Network Access to Multimedia Information        May 1994


   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".





Adie                                                           [Page 11]