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Hybrid WWW and CD-ROM systems


When the Internet, and the World Wide Web (WWW) in particular, began to gather momentum, some forecasters saw the end of the CD-ROM. In fact, despite a dramatic increase in the Internet population, the number of CD-ROM titles available has continued to grow. This will report will look at the reasons for the continuing success of CD-ROMs and examine the recent trend to deliver hybrid WWW CD-ROM applications.


CD-ROMs were developed from the existing CD Audio product to provide large scale static storage. They have a number of advantages over other storage media:
  • Cost - per MB they are one of the cheapest forms of removable media.
  • Durability
  • Large data storage - 650MB, increasing to 4.7GB and more with the advent of DVD (Digital Versatile Disc).
  • Fast transfer speeds - most CD-ROM and DVD drives provide data transfer speeds of well over 1MB/s.
  • Random access allowing fast searching.
  • Cross platform delivery - provided the disk was produced using the ISO9660 standard.
  • Constant data stream allowing synchronized delivery of audio and video.

Over the last few years as computers have presented increasingly graphical interfaces, users have also been able to include more graphical and multimedia features within their own documents. Consequently the average program and document size has grown significantly. The large storage space provided by CD-ROMs, and in the near future their optical cousins, DVD discs, has been invaluable. You will seldom find a piece of commercial software delivered on floppy discs now. The high transfer speeds and constant data rates allow these media intensive programs to run smoothly at acceptable speeds.

The main disadvantage is the ROM - Read Only Memory - once the CD is produced material cannot be easily updated. While this has been overcome to a certain extent with the introduction of multi-session CDs, the vast majority of currently installed CD-ROM drives remain read only.


The WWW emerged from the physicist's networks to dominate the Internet in just a few years for several reasons:
  • Ease of use - developing HTML pages is something that can be done by anyone, although the quality of many pages is questionable. Developing simple text based pages is as easy and cheap as using a text editor.
  • Easily updated - the Web has proved particularly effective at delivering information where timeliness is the most important factor.
  • Cross platform - the WWW is a truly cross platform environment, and the development of browsers on multiple platforms has ensured that users can easily move between them.
  • Widely available - browsers are available (often free) for all platforms, allowing anyone with an Internet connection to view the Web. The web is also becoming available through the television, with the introduction of set-top boxes and browsers for games machines such as the SEGA.

Although originally designed for text based information, the WWW has developed into a full hypermedia system. New developments such as style sheets allow the author greater control over the final layout of a Web document, and an increasing number of 'plugins' allow many types of multimedia applications to be viewed from within a single browser.

The ease with which new Web pages can be created means the Internet is constantly changing, and the number of broken links on many pages highlights just how transient the online data can be. In this aspect, CD-ROMs provide more security, since the data is guaranteed to be present every time the disk is accessed.

The increasing amount of multimedia information available on the Web does, however, serve to highlight its main drawback - its speed.

Despite predicted increases in network speeds, it is unlikely that the Internet will every be fast enough to satisfy our multimedia needs, not least because as networks speed up, the demands we place on them also increase. The physical capability of a network to transmit a certain bandwidth is not the only limiting factor determining the actual bandwidth and speed of connections. For example, while a T1 connection can provide a similar data transfer rate to a single speed CD-ROM, the bandwidth is usually shared between a number of users. As the number of users increases, the effective speed of the link decreases. Additionally data transmitted across the Internet has to pass through the various routers and pipelines between the source server and the users, inevitably introducing delays.

The lack of bandwidth can often be more correctly described as a lack of instantaneous bandwidth, and this has been addressed with the development of streaming protocols. These allow media files to begin playing without needing to download the entire file first. However, even with streamed files, bandwidth still remains an issue, and the quality of such files, particularly video files, remains relatively poor as file size is traded against speed.

The result is that while local networks may well be suitable for delivering multimedia applications, particularly where Ethernet is replaced with fast Ethernet or similar, those distributing their multimedia applications on a larger scale are likely to continue to rely on CD-ROM or DVD based systems.

The ability of local networks to deliver multimedia applications at acceptable speeds may in the future be equally important to hybrid applications as fully networked applications. If network computers become more widespread, these diskless clients will be relying on CD-ROMs accessed from a central 'jute-box' or CD-ROM server.

Hybrid Systems

From the descriptions above it is obvious that CD-ROMs and the WWW are aimed at very different markets. CD-ROMs are particularly useful for large amounts of static, often multimedia, data. The WWW excels at delivering smaller amounts of regularly updated data to a wide audience. Indeed, far from being in competition with each other, the two media can offer a complementary service through the development of hybrid systems.

Hybrid systems, where part of the information is delivered over the WWW and part on CD, have been available for several years. One of the first was Microsoft's 'Complete Baseball' in 1994. The CD provided a multimedia encyclopaedia, and the WWW was to provide daily updates of scores and news. Unfortunately a strike among major league baseball players meant it did not go quite as planned, but the concept was certainly valid. The following year Bill Gates predicted

'…CD-ROM, online services and the Internet... will no longer be viewed as independent.'

Since then the market for such applications has continued to grow, including within education3, and is estimated to reach 25,000 hybrid titles by the year 2000, and encompass the bulk of CD-ROM titles.

Hybrid applications have a number of advantages over standalone CD-ROM applications and WWW sites, incorporating the best of both media, such as the timeliness and ease of update of the WWW, with the persistence and high bandwidth of the CD-ROM. It should be noted, however, that one of the keys to providing good quality hybrid applications will be their flexibility in dealing with demands when one part of the system is missing. For example, if the web site is unavailable, the application must be able to recover and provide local content, and likewise if the CD-ROM is not available a limited version should be available through the Web site.

Although providing data updates may seem the most obvious use of the Internet in a hybrid application, there are many other potential benefits. For example, Nine Worlds from Palladium, an astronomy 'edutainment' CD-ROM, provides a Web site which include chat rooms, bulletin boards and a bi-weekly newsletter, all of which are accessible directly from the CD-ROM. Normal Web users can visit the site and see title pages for each section but are denied access to the actual content.

Creating a Hybrid Application

There are a number of issues involved in providing direct access to a Web site through a CD-ROM, including design and connection problems.

The application interface can be approached from one of two directions, either making the files on the CD-ROM accessible from a Web browser, or developing a CD-ROM based application to handle the Web based material.

Developers have in the past shied away from using standard Web browsers as the main interface. Bandwidth limitations have affected not only what material is available on the Web, but also how it is presented. 'Good Practice' guides for the Web will include recommendations to restrict the number of images, reduce colour depth and provide text alternatives. Coupled with the fact that different browsers interpret Web pages slightly differently, a web page design is restricted if the site is to be widely accessible. However with the advent of style sheets, XML and other Web developments this is likely to become less of problem.

The second issue revolves around providing seamless retrieval of the content. This poses a number of problems, the first of which is establishing an Internet connection. Where a CD-ROM is aimed at a particular audience, such as students working within a university, the producer can assume they already have Internet access. For wider audiences a simple method of setting up an account with an ISP (Internet Service Provider) must be provided. Ideally the application will be intelligent enough to determine if the user already has an installed Internet connection, or whether an initial setup is required. Although it is important to provide consistent interface when access both the Web and CD-ROM, the user may wish to know when data is being accessed remotely, particularly those paying for ISP services by the minute.

Payment for online services is an important issue that has yet to be satisfactorily resolved, although much work is being done in this area. While a CD-ROM is discrete product, for which the user pays once at the point of sale, charging for continually updated Web services is more difficult. This can be accomplished in several ways, through subscription services, advertising, and in the future E-Cash. For more information about Electronic Commerce, see the EC World online magazine.

Once the user is connected, determining whether to use data from the CD-ROM or WWW can be achieved in a number of ways. The simplest way is to always use the same media for the same files. This is very inflexible however, and most applications have some method of determining which is the most recent copy of a file and using that version. Some examples of hybrid tools and how they accomplish this follow.

Example Tools

SiteBooster from NetZepplin ( and WebCD from MarketScape ( both use a similar approach, accessing the files on CD-ROM through the use of browser specific metatags. The browser checks the HTTP headers of all files to determine which is the newest version before downloading them from the server.

SiteBooster is a Windows application which uses the LiveCache technology of Netscape Navigator. An external cache of an existing WWW site is created and then written to CD-ROM. Elements of the cache may then be modified and new files added, for example images replaced by larger, full colour versions and video clips added.

To make use of the CD-ROM, a 'jump page' containing the metatag:
<META HTTP-EQUIV="Ext-cache" CONTENT="name=XXX;instructions=insert the CD-ROM and locate the XXX file">
This will bring up a dialog asking for the CD-ROM. If the user does not have the CD-ROM, they can click cancel, and all information will be taken from the Web server. If the user is not using the Netscape browser, they will simply see a standard web site and will not be able to use the cached files on CD-ROM.

The main problem with this system is that if the date/time stamp is changed accidentally, for example by moving a file, that file will then always be accessed in preference to the CD-ROM version (when connected to the Internet).

WebCD Publisher also creates a cache and it includes CD-ROM mastering software to complete the process. The CD-ROM produced will automatically include standard helper applications, a full index of text documents and the WebCD browser. Like SiteBooster it allows additional files to be added through a simple drag and drop interface. Server side image maps, CGI output and virtual paths are all automatically processed and converted by the WebCD viewer, which provides more functionality than standard browsers, acting in part as a single-user web server.

Push Technology

When a user access a Web server, it is their client that is requesting, or pulling, information from the server. Push technologies, such as Marimba's Castanet, Microsoft's Webcasting and Netscape's Netcaster, allow the server to automatically deliver data to a user at a specified time. The user subscribes to the service, providing details for a user profile and schedule. The server then uses this information to download relevant files to the user's machine at the requested time. This allows information to be delivered in the background or at off-peak times, e.g., a catalogue application might download a new price list every morning. In actual fact, most push technology is not truly server initiated push, but rather an automated pull, where the client asks for updates, and the server fulfils the request according to the user profile and schedule.

The InfiniteCD from Intel provides an infrastructure for creating a Microsoft Windows client and a server designed to use push technology to update a hybrid CD. Clients can be created using any tool that supports ActiveX or Macromedia Directors Xtras. It is designed to work with a variety of push servers.

NewAlloy's LiveCD also uses push technology. Using LiveCD Publisher developers can produce their own client applications, which can also make use of any Netscape Plugin. The LiveServer uses 'Persistent Cache Update Management ' to automatically inform clients of an updated material. LiveCD Publisher is available on Windows 3.x, Win95, Win NT, and PowerMac platforms. LiveServer is available on Win95, Win NT and Solaris. For more details see:


While recent advances in WWW technology have made delivering multimedia over the Internet much more feasible, bandwidth is still a limiting factor and is likely to remain so. CD-ROMs provide the bandwidth, but lack the timeliness that users familiar with the WWW now demand.

Hybrid CD-ROM/WWW applications have the potential to deliver the best of both worlds, providing care is taken to provide a seamless interface between them.


The Interactive Learning Connection - University Space Network. A Hybrid Internet/CD-ROM course in Spacecraft Systems Design. Dr. William Brimley, P. Eng. NAU/Web 97

Nine Worlds, Palladium. Web site to accompany the CD-ROM.

EC World. Online magazine about electronic commerce.

Hybrid Application Cookbooks from the Intel Developer Relations Group.

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