Predictive continuous nearest neighbor queries are concerned with finding the nearest neighbor objects for some future time period according to the current object and query locations and their motion information. Existing continuous query processing algorithms are not efficient enough, requiring multiple dataset lookups to evaluate the query results throughout the duration of a continuous query. More importantly, the complete result for the whole query time interval is only available at the moment when all object motion updates have been examined, based on which adjustment of the query result is made. In this paper, we propose an algorithm which requires only one dataset lookup to deliver a complete predictive result. We then apply a differential update technique to maintain the query results incrementally in the presence of object location and motion updates.

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 shared virtual environment (VE) via a local network or through the Internet. In this paper, we illustrate that as 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 region partitioning technique 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 these regions. This is analogous to evaluating a database query using a parallel database server, which could improve the performance of serving a viewer's request tremendously. We evaluate the performance of this multiserver architecture of CyberWalk via a detail simulation model.

A distributed virtual walkthrough environment allows users connected to the geometry server to walk through a specific place of interest, without having to travel physically. This place of interest may be a virtual museum, virtual library or virtual university. There are two basic approaches to distribute the virtual environment from the geometry server to the clients, complete replication and on-demand transmission. Although the on-demand transmission approach saves waiting time and optimizes network usage, many technical issues need to be addressed in order for the system to be interactive. CyberWalk is a web-based distributed virtual walkthrough system developed based on the on-demand transmission approach. It achieves the necessary performance with a multiresolution caching mechanism. First, it reduces the model transmission and rendering times by employing a progressive multiresolution modeling technique. Second, it reduces the Internet response time by providing a caching and prefetching mechanism. Third, it allows a client to continue to operate, at least partially, when the Internet is disconnected. The caching mechanism of CyberWalk tries to maintain at least a minimum resolution of the object models in order to provide at least a coarse view of the objects to the viewer. All these features allow CyberWalk to provide sufficient interactivity to the user for virtual walkthrough over the Internet environment. In this paper, we demonstrate the design and implementation of CyberWalk. We investigate the effectiveness of the multiresolution caching mechanism of CyberWalk in supporting virtual walkthrough applications in the Internet environment through numerous experiments, both on the simulation system and on the prototype system.

The mobile environment is inherently asymmetric. To utilize the downstream bandwidth effectively, hot data items should be disseminated over the broadcast channel to the mobile clients. To equip clients with the ability to identify the nature of the broadcast and to determine the answerability of their queries, semantic descriptions are associated with information units in the broadcast, organized into data chunks. Based on the nature of data items received over the broadcast, clients can initiate appropriate requests to make up for the remaining items over the back channels in an on-demand basis. We investigation into several semantic-based algorithms to organize data chunks and compare their performance with traditional data item-based organization in the asymmetric environment.

A distributed virtual environment (DVE) 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. However, when the number of users exploring the VE increases, the server will quickly become the bottleneck. To enable good performance, we are currently developing a multi-server DVE prototype. In this paper, we describe an adaptive data partitioning technique to dynamically partition the whole VE into regions. All objects within each region will be managed by a single server. As the loading of the servers changes, we show how it can be redistributed while minimizing the communication cost. Our initial results show that the proposed adaptive partitioning technique significantly improves the performance of the overall system.

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.

Database broadcast is an effective and scalable approach to disseminate information of high affinity to a large collection of mobile clients. A common problem of existing broadcast approaches is the lack of knowledge for a client to determine if all data items satisfying its query could be obtained from the broadcast. We therefore propose a semantic-based broadcast approach. A semantic descriptor is attached to each broadcast unit, called a data chunk. This semantic descriptor allows a client to determine if a query can be answered entirely based on broadcast items and, if needed, identify the precise definition of the remaining items in the form of a "supplementary" query. Data chunks can be of static or dynamic sizes and organized hierarchically. Their boundary can be determined on-the-fly, adaptive to the nature of client queries. We investigate different ways of organizing the data chunks over a broadcast channel to improve access performance. We introduce the data affinity index metric, which more accurately reflects client-perceived performance. A simulation model is built to evaluate our semantic-based broadcast schemes.

A distributed virtual reality system allows remote users to share and to view a common virtual environment via connected networks. However, network latency and bandwidth are often the most crucial performance bottlenecks. We have recently developed a distributed virtual walkthrough environment that supports on-demand model transmission over the Internet through the use of, in addition to other techniques, a simple prefetching technique called EWMA. Although the prefetching technique has been shown to be effective in predicting 3D motion during our simulation experiments, it is less effective in our prototype experiments. The main reason is that most input devices used for navigation are 2D in nature (mostly 2D mice) and EWMA is not too effective in predicting user motion in moving a 2D mouse. To overcome this limitation, we propose in this paper a method for predicting the user motion in moving the mouse during a 3D navigation. To improve the accuracy of the prediction, we also propose a constrained navigation method. We will demonstrate the effectiveness of the new method through experimental results.

We consider the view maintenance problem in a web-based environment, in which c l i e n ts query information from databases, stored in the form of materialized data warehouses, without accessing the original data sources.In addition to base data, data w arehouses also con tain highly aggregated and summarized information suitable for decision support.As changes are made to the data sources, the warehouse views must be updated to re ect a consistent state of the data sources.Recomputation is often too expensive.We describe in this paper an incremental view maintenance method based on the net c hanges on base table, to maintain the views eciently in the presence of aggregation and summary information.An architecture of a web-based data w arehousing system is described to minimize the workload of the data warehouse.Finally, w e d e v elop a prototype application utilizing the new maintenance algorithm and conduct a performance study based on the prototype, comparing it with existing maintenance algorithm.

Data broadcast is an effective approach to disseminate information from a database server to numerous mobile clients in a mobile environment. Since a broadcast session contains only a subset of the database items, a client might not be able to obtain all its items from the broadcast and is forced to request additional ones from the server on demand. We describe a semantic-based broadcast approach which attaches a semantic description to each broadcast unit, called a chunk, which is a cluster of data items. This allows a client to determine if a query can be answered entirely using a broadcast as well as defining the precise nature of the remaining items in the form of a "supplementary" query. Chunks could be of different sizes and are hierarchically organized. We propose a heuristic to schedule the broadcast order of the chunks to improve the tuning time, access time, and a new metric called a data affinity index. The performances are evaluated via experiments based on a simulation model.

A mobile environment is weakly-connected, characterized by low communication bandwidth and poor connectivity. The conventional paradigm for surfing mobile Web documents is ineffective since portions of a document could be corrupted during transmission and it is expensive to re-transmit the whole document. It is important that the high content-bearing portions should be transmitted successfully so that a mobile client could at least obtain a high level content and determine if the corrupted portions need to be retransmitted. We have proposed a multi-resolution transmission paradigm which allows higher content-bearing portions of a Web document to be transmitted, by partitioning it into multiple organizational units and associating an information content with each unit. The client can explore the higher content-bearing portion earlier and terminate browsing an irrelevant document sooner. We extend our previous work and propose a fault-tolerant multi-resolution transmission scheme which allows units of higher information content to be recovered from transmission error. The client can obtain an overall content of a Web document and either terminate the transmission of the remaining portions or decide if the corrupted portions need to be retransmitted. We demonstrate its feasibility with a prototype and with simulation results.

Article Free Access Share on Incremental maintenance for dynamic database-derived HTML pages in digital libraries Authors: Ken C. K. Lee Department of Computing, the Hong Kong Polytechnic University, Hung Hom, Hong kong Department of Computing, the Hong Kong Polytechnic University, Hung Hom, Hong kongView Profile , Hong V. Leong Department of Computing, the Hong Kong Polytechnic University, Hung Hom, Hong kong Department of Computing, the Hong Kong Polytechnic University, Hung Hom, Hong kongView Profile , Antonio Si Sun Microsystems Inc., Palo Alto, CA Sun Microsystems Inc., Palo Alto, CAView Profile Authors Info & Claims CIKM '98: Proceedings of the seventh international conference on Information and knowledge managementNovember 1998 Pages 20–29https://doi.org/10.1145/288627.288637Online:01 November 1998Publication History 2citation619DownloadsMetricsTotal Citations2Total Downloads619Last 12 Months3Last 6 weeks0 Get Citation AlertsNew Citation Alert added!This alert has been successfully added and will be sent to:You will be notified whenever a record that you have chosen has been cited.To manage your alert preferences, click on the button below.Manage my Alerts New Citation Alert!Please log in to your account Save to BinderSave to BinderCreate a New BinderNameCancelCreateExport CitationPublisher SiteeReaderPDF

Communication between mobile clients and database servers in a mobile computing environment is via wireless channels with low bandwidth and low reliability. A mobile client could cache its frequently accessed database items into its local storage in order to improve performance of database queries and availability of database items for query processing during disconnection. We describe a mobile caching mechanism for a mobile environment utilizing point to point communication paradigm. In particular, we investigate issues on caching granularity, coherence strategy, and replacement policy of mobile caching. Via a detailed simulation model, we compare our proposed caching mechanism with conventional ones and discover that our mobile caching mechanism outperforms conventional ones in most situations.

Article Free Access Share on On caching and prefetching of virtual objects in distributed virtual environments Authors: Jimmy H. P. Chim Department of Computing, The Hong Kong Polytechnic University, Hong Kong Department of Computing, The Hong Kong Polytechnic University, Hong KongView Profile , Mark Green Department of Computer Science, University of Alberta, Edmonton, Alberta, T6G 2H1, Canada Department of Computer Science, University of Alberta, Edmonton, Alberta, T6G 2H1, CanadaView Profile , Rynson W. H. Lau Department of Computer Science, City University of Hong Kong, Hong Kong Department of Computer Science, City University of Hong Kong, Hong KongView Profile , Hong Va Leong Department of Computer Science, City University of Hong Kong, Hong Kong Department of Computer Science, City University of Hong Kong, Hong KongView Profile , Antonio Si Sun Microsystems, 901 San Antonio Road, Palo Alto, CA Sun Microsystems, 901 San Antonio Road, Palo Alto, CAView Profile Authors Info & Claims MULTIMEDIA '98: Proceedings of the sixth ACM international conference on MultimediaSeptember 1998 Pages 171–180https://doi.org/10.1145/290747.290769Published:01 September 1998Publication History 51citation594DownloadsMetricsTotal Citations51Total Downloads594Last 12 Months27Last 6 weeks3 Get Citation AlertsNew Citation Alert added!This alert has been successfully added and will be sent to:You will be notified whenever a record that you have chosen has been cited.To manage your alert preferences, click on the button below.Manage my AlertsNew Citation Alert!Please log in to your account Save to BinderSave to BinderCreate a New BinderNameCancelCreateExport CitationPublisher SiteeReaderPDF

Distributed virtual 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 ...

Previous research on broadcast databases in a mobile computing environment utilizing wireless channels has been focused on mechanisms for a mobile client to selectively pick database items in which the client is interested from a broadcast channel. The fundamental issue of identifying the appropriate database items for broadcast or refrained from being broadcast have largely been ignored. In this paper, we consider the concept of ‘air-storage’, by treating the wireless broadcast media as a layer of cache storage. Broadcasting database items over the air-storage becomes similar in spirit to the caching of database items from the database server. Similarly, determining which database items need to be broadcast or refrained from being broadcast becomes similar in nature to cache management. Existing caching mechanisms are reviewed and management issues specific to the new air-storage are raised and discussed. In view of new issues in air-storage management, we propose and investigate several mechanisms in selecting the proper database items to be placed over this new layer of air-storage under a variety of data access patterns. Finally, the effectiveness of the mechanisms is evaluated by means of simulated experiments and the results are discussed.

To support database interoperability in federated databases systems, it is critical to be able to identify (potentially) equivalent data instances from individual autonomous database components. Since the components in a federation are autonomous, their data may be updated asynchronously, viz., modifications to a real world entity may be captured in different databases at different times; the authors term this effect update heterogeneity. Existing approaches largely base data instance similarity identification only on current attribute/property values; in the face of update heterogeneity, this is inadequate. They present an approach to address the problem of update heterogeneity in the federated databases context. They employ a probabilistic model, which utilizes historical database update information to estimate the degree of similarity between candidate data instances from different database components. They employ transaction history (log) information to this end, which is typically already available in the component database systems. They have experimentally implemented and tested this approach within the context of a prototype experimental federated databases system, FeXpress.

Article Free Access Share on Data broadcasting strategies over multiple unreliable wireless channels Authors: Hong V. Leong Department of Computing, The Hong Kong Polytechnic University, Hung Hom, Kowloon, Hong Kong Department of Computing, The Hong Kong Polytechnic University, Hung Hom, Kowloon, Hong KongView Profile , Antonio Si Department of Computing, The Hong Kong Polytechnic University, Hung Hom, Kowloon, Hong Kong Department of Computing, The Hong Kong Polytechnic University, Hung Hom, Kowloon, Hong KongView Profile Authors Info & Claims CIKM '95: Proceedings of the fourth international conference on Information and knowledge managementDecember 1995 Pages 96–104https://doi.org/10.1145/221270.221339Published:02 December 1995Publication History 30citation412DownloadsMetricsTotal Citations30Total Downloads412Last 12 Months3Last 6 weeks1 Get Citation AlertsNew Citation Alert added!This alert has been successfully added and will be sent to:You will be notified whenever a record that you have chosen has been cited.To manage your alert preferences, click on the button below.Manage my AlertsNew Citation Alert!Please log in to your account Save to BinderSave to BinderCreate a New BinderNameCancelCreateExport CitationPublisher SiteeReaderPDF

An approach and mechanism to support the dynamic discovery of information units within a collection of autonomous and heterogeneous database systems is described. The mechanism is based upon a core set of database constructs that characterizes object database systems, along with a set of self-adaptive heuristics employing techniques from machine learning. The approach provides an uniform framework for organizing, indexing, searching, and browsing database information units within an environment of multiple, autonomous, interconnected databases. The feasibility of the approach and mechanism is illustrated using a protein/genetics application environment. Metrics for measuring the performance of the discovery system are presented and the effectiveness of the system is thereby evaluated. Performance tradeoffs are examined and analyzed by experiments performed, employing a simulation model.>

An approach and mechanism to support the sharing of objects are described, an experimental implementation is presented, and the performance of the system is analyzed and evaluated. The mechanism is based on a core set of constructs that characterize object-based database systems. The approach provides a basis for controlled sharing in a heterogeneous database environment, using a kernel object-base model as an intercomponent exchange forum. A major goal is to make the importation of nonlocal information as transparent to a component as possible.>