SESSION III
Howard BESSER Networking Multimedia Databases
What do we have to consider in building and distributing databases of visual materials in a multi-user environment? This presentation examines a variety of concerns that need to be addressed before a multimedia database can be set up in a networked environment.
In the past it has not been feasible to implement databases of visual materials in shared-user environments because of technological barriers. Each of the two basic models for multi-user multimedia databases has posed its own problem. The analog multimedia storage model (represented by Project Athena's parallel analog and digital networks) has required an incredibly complex (and expensive) infrastructure. The economies of scale that make multi-user setups cheaper per user served do not operate in an environment that requires a computer workstation, videodisc player, and two display devices for each user.
The digital multimedia storage model has required vast amounts of storage space (as much as one gigabyte per thirty still images). In the past the cost of such a large amount of storage space made this model a prohibitive choice as well. But plunging storage costs are finally making this second alternative viable.
If storage no longer poses such an impediment, what do we need to consider in building digitally stored multi-user databases of visual materials? This presentation will examine the networking and telecommunication constraints that must be overcome before such databases can become commonplace and useful to a large number of people.
The key problem is the vast size of multimedia documents, and how this affects not only storage but telecommunications transmission time. Anything slower than T-1 speed is impractical for files of 1 megabyte or larger (which is likely to be small for a multimedia document). For instance, even on a 56 Kb line it would take three minutes to transfer a 1-megabyte file. And these figures assume ideal circumstances, and do not take into consideration other users contending for network bandwidth, disk access time, or the time needed for remote display. Current common telephone transmission rates would be completely impractical; few users would be willing to wait the hour necessary to transmit a single image at 2400 baud.
This necessitates compression, which itself raises a number of other issues. In order to decrease file sizes significantly, we must employ lossy compression algorithms. But how much quality can we afford to lose? To date there has been only one significant study done of image-quality needs for a particular user group, and this study did not look at loss resulting from compression. Only after identifying image-quality needs can we begin to address storage and network bandwidth needs.
Experience with X-Windows-based applications (such as Imagequery, the University of California at Berkeley image database) demonstrates the utility of a client-server topology, but also points to the limitation of current software for a distributed environment. For example, applications like Imagequery can incorporate compression, but current X implementations do not permit decompression at the end user's workstation. Such decompression at the host computer alleviates storage capacity problems while doing nothing to address problems of telecommunications bandwidth.
We need to examine the effects on network through-put of moving multimedia documents around on a network. We need to examine various topologies that will help us avoid bottlenecks around servers and gateways. Experience with applications such as these raise still broader questions. How closely is the multimedia document tied to the software for viewing it? Can it be accessed and viewed from other applications? Experience with the MARC format (and more recently with the Z39.50 protocols) shows how useful it can be to store documents in a form in which they can be accessed by a variety of application software.