ld~~ces in networkg tetiology and the mtabkhtnent of the Morrnation Superhighway have rendered the virtnd Xbrary a concrete possibfi~. ?iTeze currently investigating user &\Terience in _ through a large virtual environment in the contti% of bternet. This provid~users with the abiity to view t+ous virtual objects from tirent & t anca and angles, using common web browsers. To dtiver a good petiorrnance for such applications, we need to addr~s Several issues in Merent resemch discipbes.F&t., we must be able to modd virtual objects tiectivdy. The recently devdoped techniques for mdti-~olution object modting in computer graphics are of great Aue here, since they are capable of sintp~g the object mod-& and therefore reducing the time to render them. Secon& tith the Eted bandwidth constraint of the btemet, we need to reduce the response time by reducing the amount of data requwted over the network One dtemative is to cache object mod& of high -w.Prefetching object mod~by predicting those which~e &dy to be used in the near fnture and dotioading them in advance wiU lead to a Mar improvement. Third, the bt.ernet often tiers from disconnection. A caching mechanism hat dews objects to be cached n?th at least the-mminimum resolution m be wet o provide at least a coarse xtiewerfor improved~hd perception. k this paper, m propose a multi-T~*oZutionaching m~hrmtim wd invG tigate its tiectiveness in supporthg virtud w~u~ap-FEcations in the ktemet. environment. The caching mechanism is further complemented with severs object prefetching m echanLms for predicting future acc~ed objects. The performance of our proposed me~and their feasibfities are qnsmtXed via s"mtdat.ed~Terirnents.

CyberWalk is a distributed virtual walkthrough system that we have developed. It allows users at different geographical locations to share information and interact within a common virtual environment (VE) via a local network or through the Internet. In this paper, we illustrate that when the number of users exploring the VE increases, the server will quickly become the bottleneck. To enable good performance, CyberWalk utilizes multiple servers and employs an adaptive data partitioning techniques to dynamically partition the whole VE into regions. All objects within each region will be managed by one server. Under normal circumstances, when a viewer is exploring a region, the server of that region will be responsible for serving all requests from the viewer. When a viewer is crossing the boundary of two or more regions, the servers of all the regions involved will be serving requests from the viewer since the viewer might be able to view objects within all those regions. We evaluate the performance of this multi-server architecture of CyberWalk via a detail simulation model.

Distributedvirtual environments allow users at different geographical locations to share and interact within a common virtual environment via a local network or through the Internet.To deliver a good performance for such applications, we need to address several issues in different research disciplines.First, we must be able to model virtual objects effectively.The recently developed multi-resolution techniques for object modeling are of great value here, since they are capable of simplifying the object models and therefore reducing the time to render them. This may greatly reduce the demand for rendering performance on the client machines.Second, with the constraint of the limited bandwidth of the Internet, we need to reduce the response time by reducing the amount of data requested over the network. Caching of suitable object models of high affinity will reduce the amount of data requested over the network for a faster response time. Prefetching object models by predicting those which are likely to be used in the near future and downloading them in advance will lead to a similar improvement. Third, the Internet often suffers from disconnection. A caching mechanism that allows objects to be cached with at least their minimum resolutions will be useful to provide at least a coarse view of the objects to the viewer for improved visual perception.In this paper, we describe our implementation of a distributed walkthrough system. Two techniques are fundamental to our system, a mu&-resolzltzon caching mechanism and a set of object prefetching mechanisms. Towards the end of the paper, we quantify the performance of t,he proposed mechanisms.

Caching of remote data in a mobile client's local storage can improve data access performance and data availability. Traditional approaches are page-based, without taking advantage of the semantics of cached data. It is difficult for a client to determine if a query could be answered entirely based on locally cached data, forcing it to contact the database server for additional data. We propose a semantic caching mechanism which allows data to be cached as a collection of possibly related blocks, each of which is the result of a previously evaluated query. We investigate mechanisms for transforming projection-selection queries to reuse cached data blocks. This avoids transmitting unwanted data items over low bandwidth wireless channels. Cache replacement techniques based on the semantics of cached data are also proposed. We describe the design of our prototype and study its performance.

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