Showing papers by "Christian S. Jensen published in 2007"

Continuous Clustering of Moving Objects

Abstract: This paper considers the problem of efficiently maintaining a clustering of a dynamic set of data points that move continuously in two-dimensional Euclidean space. This problem has received little attention and introduces new challenges to clustering. The paper proposes a new scheme that is capable of incrementally clustering moving objects. This proposal employs a notion of object dissimilarity that considers object movement across a period of time, and it employs clustering features that can be maintained efficiently in incremental fashion. In the proposed scheme, a quality measure for incremental clusters is used for identifying clusters that are not compact enough after certain insertions and deletions. An extensive experimental study shows that the new scheme performs significantly faster than traditional ones that frequently rebuild clusters. The study also shows that the new scheme is effective in preserving the quality of moving-object clusters.

Main-memory operation buffering for efficient R-tree update

Abstract: Emerging communication and sensor technologies enable new applications of database technology that require database systems to efficiently support very high rates of spatial-index updates. Previous works in this area require the availability of large amounts of main memory, do not exploit all the main memory that is indeed available, or do not support some of the standard index operations. Assuming a setting where the index updates need not be written to disk immediately, we propose an R-tree-based indexing technique that does not exhibit any of these drawbacks. This technique exploits the buffering of update operations in main memory as well as the grouping of operations to reduce disk I/O. In particular, operations are performed in bulk so that multiple operations are able to share I/O. The paper presents an analytical cost model that is shown to be accurate by empirical studies. The studies also show that, in terms of update I/O performance, the proposed technique improves on state of the art in settings with frequent updates.

S-GRID: a versatile approach to efficient query processing in spatial networks

Abstract: Mobile services is emerging as an important application area for spatio-temporal database management technologies. Service users are often constrained to a spatial network, e.g., a road network, through which points of interest, termed data points, are accessible. Queries that implement services will often concern data points of some specific type, e.g., Thai restaurants or art museums. As a result, the relatively few data points are relevant to a query in comparison to the number of network edges, meaning that queries, e.g., k nearest-neighbor queries, must access large portions of the network. Existing query processing techniques pre-compute distances between data points and network vertices for improving the performance. However, precomputation becomes problematic when the network or data points must be updated, possibly concurrently with the querying; and if the data points are moving, the existing techniques are inapplicable. In addition, multiple pre-computed structures must be maintained--one for each type of data point. We propose a versatile pre-computation approach for spatial network data. This approach uses a grid for pre-computing a simplified network. The above-mentioned shortcomings are avoided by making the pre-computed data independent of the data points. Empirical performance studies show that the structure is competitive with respect to the existing, more specialized techniques.

TRAX: real-world tracking of moving objects

Abstract: A range of mobile services rely on knowing the current positions of populations of so-called moving objects. In the ideal setting, the positions of all objects are known always and exactly. While this is not possible in practice, it is possible to know each object's position with a certain guaranteed accuracy. This paper presents the TRAX tracking system that supports several techniques capable of tracking the current positions of moving objects with guaranteed accuracies at low update and communication costs in real-world settings. The techniques are readily relevant for practical applications, but they also have implications for continued research. The tracking techniques offer a realistic setting for existing query processing techniques that assume that it is possible to always know the exact positions of moving objects. The techniques enable studies of trade-offs between querying and update, and the accuracy guarantees they offer may be exploited by query processing techniques to offer perfect recall.

Enabling Routes of Road Network Constrained Movements as Mobile Service Context

Abstract: With the continuing advances in wireless communications, geo-positioning, and portable electronics, an infrastructure is emerging that enables the delivery of on-line, location-enabled services to very large numbers of mobile users. A typical usage situation for mobile services is one characterized by a small screen and no keyboard, and by the service being only a secondary focus of the user. Under such circumstances, it is particularly important to deliver the "right" information and service at the right time, with as little user interaction as possible. This may be achieved by making services context aware. Mobile users frequently follow the same route to a destination as they did during previous trips to the destination, and the route and destination constitute important aspects of the context for a range of services. This paper presents key concepts underlying a software component that identifies and accumulates the routes of a user along with their usage patterns and that makes the routes available to services. The problems associated with of route recording are analyzed, and algorithms that solve the problems are presented. Experiences from using the component on logs of GPS positions acquired from vehicles traveling within a real road network are reported.

Collaborative spatial data sharing among mobile lightweight devices

Abstract: Mobile devices are increasingly being equipped with wireless peerto-peer (P2P) networking interfaces, rendering the sharing of data among mobile devices feasible and beneficial. In comparison to the traditional client/server wireless channel, the P2P channels have considerably higher bandwidth.Motivated by these observations, we propose a collaborative spatial data sharing scheme that exploits the P2P capabilities of mobile devices. Using carefully maintained routing tables, this scheme enables mobile devices not only to use their local storage for query processing, but also to collaborate with nearby mobile peers to exploit their data. This scheme is capable of reducing the cost of the communication between mobile clients and the server as well as the query response time. The paper details the design of the data sharing scheme, including its routing table maintenance, query processing and update handling. An analytical cost model sensitive to user mobility is proposed to guide the storage content replacement and routing table maintenance. The results of extensive simulation studies based on an implementation of the scheme demonstrate that the scheme is efficient in processing location dependent queries and is robust to data updates.

A Testbed for the Exploration of Novel Concepts in Mobile Service Delivery

Abstract: This paper describes an open, extendable, and scalable system that supports the delivery of context-dependent content to mobile users. The system enables users to receive content from multiple content providers that matches their demographic data, active profiles, and context such as location and time. The system also allows users to subscribe to specific services. In addition, it allows users to provide their own content and services, by either using the system's publicly available interface or by filling out one of the service-configuration templates.

Challenges in the Tracking and Prediction of Scheduled-Vehicle Journeys

Abstract: A number of applications in areas such as logistics, cargo delivery, and collective transport involve the management of fleets of vehicles that are expected to travel along known routes according to fixed schedules. Due to road construction, accidents, and other unanticipated conditions, the vehicles deviate from their schedules. At the same time, there is a need for the infrastructure surrounding the vehicles to continually know the actual status of the vehicles. For example, anticipated arrival times of buses may have to be displayed at bus stops. It is a fundamental challenge to maintain this type of knowledge with minimal cost. This paper characterizes the problem of real-time vehicle tracking using wireless communication, and of predicting the future status of the vehicles when their movements are restricted to given routes and when they follow schedules with the best effort. The paper discusses challenges related to tracking, to the prediction of future travel times, and to historical data analysis. It also suggests approaches to addressing the challenges

Integrating multiple calendars using τ Z AMAN

Abstract: Programmers are increasingly interested in developing applications that can be used internationally. Part of the internationalization effort is the ability to engineer applications to use dates and times that conform to local calendars yet can inter-operate with dates and times in other calendars, for instance between the Gregorian and Islamic calendars. $\tau$ Z AMAN is a system that provides a natural language- and calendar-independent framework for integrating multiple calendars. $\tau$ Z AMAN performs ‘runtime-binding’ of calendars and language support. A running $\tau$ Z AMAN system dynamically loads calendars and language support tables from XML-formatted files. Loading a calendar integrates it with other, already loaded calendars, enabling users of $\tau$ Z AMAN to add, compare, and convert times between multiple calendars. $\tau$ Z AMAN also provides a flexible, calendar-independent framework for parsing temporal literals. Literals can be input and output in XML or plain text, using user-defined formats, and in different languages and character sets. Finally, $\tau$ Z AMAN is a clientsserver system, enabling shared access to calendar servers spread throughout the Web. This paper describes the architecture of $\tau$ Z AMAN and experimentally quantifies the cost of using a calendar server to translate and manipulate dates. Copyright © 2006 John Wiley & Sons, Ltd.

Layered Implementation of Temporal DBMSs|Concepts and Techniques

Abstract: A wide range of database applications manage time-varying data. Examples include, e.g., accounting, personnel, schedule, and data warehousing applications. At the same time, it is well-known that querying and correctly updating time-varying data is di cult and error-prone when using standard SQL. As a result of a decade of intensive exploration, temporal extensions of SQL have reached a level of maturity and sophistication where it is clear that they o er substantial bene ts over SQL when managing time-varying data. The topic of this paper is the e ective implementation of temporally extended SQL's. Traditionally, it has been assumed that a temporal DBMS must be built from scratch, utilizing new technologies for storage, indexing, query optimization, concurrency control, and recovery. This paper adopts a quite di erent approach. Speci cally, it explores the concepts and techniques involved in implementing a temporally enhanced SQL while maximally reusing the facilities of an existing SQL implementation, e.g., Oracle or DB2. The topics covered span the choice of an adequate timestamp domain that include the time variable ow," a comparison of alternative query processing architectures including a partial parser approach, update processing, and transaction processing, the latter including how to ensure ACID properties and assign correct timestamps.

Distributed, concurrent range monitoring of spatial-network constrained mobile objects

Abstract: The ability to continuously monitor the positions of mobile objects is important in many applications. While most past work has been set in Euclidean spaces, the mobile objects relevant in many applications are constrained to spatial networks. This paper addresses the problem of range monitoring of mobile objects in this setting, in which network distance is concerned. An architecture is proposed where the mobile clients and a central server share computation, the objective being to obtain scalability by utilizing the capabilities of the clients. The clients issue location reports to the server, which is in charge of data storing and query processing. The server associates each range monitoring query with the network-edge portions it covers. This enables incremental maintenance of each query, and it also enables shared maintenance of concurrent queries by identifying the overlaps among such queries. The mobile clients contribute to the query processing by encapsulating their host edge portion identifiers in their reports to the server. Extensive empirical studies indicate that the paper's proposal is efficient and scalable, in terms of both query load and moving-object load.

When the Internet Hits the Road.

Abstract: The Internet has recovered from the dot-com crash of the early 2000’s and now features an abundance of new, innovative technologies and services. We are also witnessing the emergence of a communication and computing infrastructure that encompasses millions of people with mobile devices, such as mobile phones, with Internet connectivity. This infrastructure will soon enable the Internet to go mobile. This paper describes the background and aspirations of a new research project that is concerned with data management aspects of innovative mobile Internet services. It is argued that mobile services will be context aware, and the project devotes particular attention to geographical context awareness. The project will adopt a prototyping approach where services are built and exposed to users, and where data management challenges are identified and addressed. The paper describes the evolving service platform that supports the approach chosen, it describes some of the data management techniques being integrated into the service platform, and it describes research guidelines that the project aims to follow.

Map Matching for Intelligent Speed Adaptation

Abstract: The availability of Global Navigation Satellite Systems (GNSS) enables sophisticated vehicle guidance and advisory systems such as intelligent speed adaptation (ISA) systems. In ISA systems, it is essential to be able to position vehicles within a road network. Because digital road networks as well as GNSS positioning are often inaccurate, a technique known as map matching is needed that aims to use this inaccurate data for determining a vehicle's real road-network position. Then, knowing this position, an ISA system can compare speed with the speed limit in effect and take measures against speeding. An on-line map-matching algorithm is presented with an extensive number of weighting parameters that allow better determination of a vehicle's road network position. The algorithm uses certainty value to express its belief in the correctness of its results. The algorithm was designed and implemented to be used in the large scale ISA project 'Spar pa' farten'. Using test data and data collected from project participants, the algorithm's performance is evaluated. It is shown that the algorithm performs correctly 95% of the time and is capable of handling GNSS positioning errors in a conservative manner.

Distributed, Concurrent Range Monitoring of Spatial-Network Constrained Mobile Objects

Abstract: The ability to continuously monitor the positions of mobile objects is important in many applications. While most past work has been set in Euclidean spaces, the mobile objects relevant in many applications are constrained to spatial networks. This paper addresses the problem of range monitoring of mobile objects in this setting, in which network distance is concerned. An architecture is proposed where the mobile clients and a central server share computation, the objective being to obtain scalability by utilizing the capabilities of the clients. The clients issue location reports to the server, which is in charge of data storing and query processing. The server associates each range monitoring query with the network-edge portions it covers. This enables incremental maintenance of each query, and it also enables shared maintenance of concurrent queries by identifying the overlaps among such queries. The mobile clients contribute to the query processing by encapsulating their host edge portion identifiers in their reports to the server. Extensive empirical studies indicate that the paper's proposal is efficient and scalable, in terms of both query load and moving-object load.

Collaborative Spatial Data Sharing Among Mobile Lightweight Devices

Abstract: Mobile devices are increasingly being equipped with wireless peerto-peer (P2P) networking interfaces, rendering the sharing of data among mobile devices feasible and beneficial. In comparison to the traditional client/server wireless channel, the P2P channels have considerably higher bandwidth.Motivated by these observations, we propose a collaborative spatial data sharing scheme that exploits the P2P capabilities of mobile devices. Using carefully maintained routing tables, this scheme enables mobile devices not only to use their local storage for query processing, but also to collaborate with nearby mobile peers to exploit their data. This scheme is capable of reducing the cost of the communication between mobile clients and the server as well as the query response time. The paper details the design of the data sharing scheme, including its routing table maintenance, query processing and update handling. An analytical cost model sensitive to user mobility is proposed to guide the storage content replacement and routing table maintenance. The results of extensive simulation studies based on an implementation of the scheme demonstrate that the scheme is efficient in processing location dependent queries and is robust to data updates.

An Open Platform for the Creation and Deployment of Transport-Related Mobile Data Services

Abstract: Advanced mobile computing devices with wireless communication and geo-positioning capabilities are finding increasingly widespread use in Europe and beyond. Example devices include smart phones, PDA phones, and navigation systems. It is thus becoming increasingly relevant and attractive to utilize these devices and the related communication infrastructure for the deployment of transport-related mobile services. This paper describes the architecture of a service platform that enables users to create their own mobile services. The platform is based on standard hardware and software technologies and offers integration of transportation-related services with other services.