Showing papers by "Wang-Chien Lee published in 2005"

Supporting Complex Multi-Dimensional Queries in P2P Systems

Abstract: More and more applications require peer-to-peer (P2P) systems to support complex queries over multi-dimensional data. For example, a P2P auction network for real estate frequently needs to answer queries such as "select five available buildings closest to the airport". Such queries are not efficiently supported in current P2P systems. Towards an efficient and scalable P2P system capable of processing complex multi-dimensional queries, the authors first proposed a comprehensive framework for sharing, indexing, and querying multi-dimensional data, where (i) peers with more computational power coordinate indexing and query processing, and (ii) other peers participate in part of the computation in order to achieve scalability and load-balance. Based on this framework, Network-R-tree (NR-tree), a P2P adaptation of the dominant spatial index - R*-tree was proposed. NR-tree, indexing spatial data at clustered peers, is capable of processing complex queries such as range queries and k-nearest neighbor queries. The authors proposed query processing algorithms for range and k-nearest neighbor queries and experimentally prove the effectiveness of proposed techniques with real data

DSI: A Fully Distributed Spatial Index for Location-Based Wireless Broadcast Services

Abstract: Recent announcement of the MSN Direct Service has demonstrated the feasibility and industrial interest in utilizing wireless broadcast for pervasive information services. To support location-based services in wireless data broadcast systems, a distributed spatial index (called DSI) is proposed in this paper. DSI is highly efficient because it has a linear yet fully distributed structure that facilitates multiple search paths to be naturally mixed together by sharing links. Moreover, DSI is very resilient in error-prone wireless communication environments. Search algorithms for two classical location-based queries, window queries and kNN queries, based on DSI are presented. Performance evaluation of DSI shows that DSI significantly outperforms R-tree and Hilbert Curve Index, two state-of-the-art spatial indexing techniques for wireless data broadcast

Proactive caching for spatial queries in mobile environments

Abstract: Semantic caching enables mobile clients to answer spatial queries locally by storing the query descriptions together with the results. However, it supports only a limited number of query types, and sharing results among these types is difficult. To address these issues, we propose a proactive caching model which caches the result objects as well as the index that supports these objects as the results. The cached index enables the objects to be reused for all common types of queries. We also propose an adaptive scheme to cache such an index, which further optimizes the query response time for the best user experience. Simulation results show that proactive caching achieves a significant performance gain over page caching and semantic caching in mobile environments where wireless bandwidth and battery are precious resources.

PSGR: priority-based stateless geo-routing in wireless sensor networks

Abstract: Volunteer forwarding, as an emerging routing idea for large scale, location-aware wireless sensor networks, recently has attracted a significant amount of research attention. However, several critical research issues raised by volunteer forwarding, including priority assignment, acknowledgement collisions and communication voids, have not been well addressed by the existing work. In this paper, we propose a priority-based stateless geo-routing (PSGR) protocol to address these issues. Based on PSGR, sensor nodes are able to locally determine their priority to serve as the next relay node using dynamically estimated network density. This effectively suppresses potential communication collisions without prolonging routing delays. PSGR also overcomes the communication void problem using two alternative stateless schemes, rebroadcast and bypass. We analyze energy consumption and delivery rate of PSGR as functions of transmission range. An extensive performance evaluation has been conducted to compare PSGR with competing protocols, including GeRaf, IGF, GPSR and flooding. Simulation results show that PSGR exhibits superior performance in terms of energy consumption, routing latency and delivery rate, and soundly outperforms all of the compared protocols

Energy efficient processing of K nearest neighbor queries in location-aware sensor networks

Abstract: The k nearest neighbor (KNN) query, an essential query for information processing in sensor networks, has not received sufficient attention in the research community of sensor networks. In this paper, we examine in-network processing of KNN queries by proposing two alternative algorithms, namely the GeoRouting Tree (GRT) and the KNN Boundary Tree (KBT). The former is based on a distributed spatial index structure and prunes off the irrelevant nodes during query propagation. The latter is based upon ad-hoc geographic routing and first obtains a region within which at least k nearest sensor nodes are enclosed and then decides the k nearest nodes to the query point. We provide an extensive performance evaluation to study the impact of various system factors and protocol parameters. Our results show that GRT yields a good tradeoff between energy consumption and query accuracy in static scenarios. On the other hand, KBT achieves better energy efficiency while being more tolerant to network dynamics.

EASE: an energy-efficient in-network storage scheme for object tracking in sensor networks

Abstract: Energy efficiency is one of the most critical issues in the design of wireless sensor networks. Observing that many sensor applications for object tracking can tolerate a certain degree of imprecision in location data of tracked objects, this paper studies precision-constrained approximate queries that trade answer precision for energy efficiency. We develop an Energy-conserving Approximate StoragE (EASE) scheme to ef- ficiently answer approximate location queries by keeping error- bounded imprecise location data at some designated storage node. The data impreciseness is captured by a system parameter, i.e., approximation radius. We analyze the performance of EASE in terms of message complexity and derive the optimal setting of approximation radius. We show via extensive simulation experiments that, as compared to a conventional approach, the EASE scheme cuts down the network traffic by up to 96% and, in most cases, prolongs the network lifetime by a factor of 2�5. I. INTRODUCTION

Distributed caching of multi-dimensional data in mobile environments

Abstract: Caching has been an important technique for saving network traffic and reducing response time, especially in mobile environments where bandwidth is often a scarce resource. In this paper, we propose a novel approach for caching multidimensional data in a cluster of mobile devices. In particular, we focus on the most common types of multi-dimensional queries, namely range and k-nearest neighbor queries, by computing a cacheable region for every query, caching the result at the client, and indexing it in an R*-tree at the cluster gateway. Subsequent queries are first issued to the R*-tree and only remainder queries or queries that cannot be guaranteed exact answers are sent to the remote data server. To the best of our knowledge, our work is the first to study caching results from complex multi-dimensional queries (e.g., kNN query) and propose to build an R*-tree on previously fetched query results in a cluster of mobile devices. Rigorous experiments show that our approach significantly reduces network traffic and response time.

Decentralizing query processing in sensor networks

Abstract: Recent research has led to the advent of software systems capable of performing query processing in sensor networks. They perform query processing in a sensor network by constructing a routing tree rooted at an access point (i.e., a base station), where the queries are submitted, parsed, and optimized. This approach to query processing is centralized in nature. Performing query optimization at a single node (base station) does not generate efficient query plans, and requires each node to report metadata to the access point. In addition, the routing tree infrastructure inefficiently aggregates data packets. To address these issues, this paper proposes several decentralized query processing systems that utilize sensor node's innate spatial and semantic characteristics. Experimental results conclude that decentralizing query processing significantly reduces energy costs. In addition, experimental results show that the spatial and semantic properties of nodes are influential in designing a decentralized query processing system.

A fully distributed spatial index for wireless data broadcast

Abstract: To support location-based services in wireless data broadcast systems, a distributed spatial index (called DSI) is proposed in this paper. DSI is highly efficient because it has a linear yet fully distributed structure that naturally facilitates multiple replications of the index by sharing links in different search trees. Search algorithms for point queries, window queries, and kNN queries, based on DSI are presented. Empirical evaluation of DSI are conducted. Result shows that DSI significantly out-performs R-tree and Hilbert curve index, two state-of-the-art spatial indexing techniques for wireless data broadcast.

Deep set operators for XQuery

Abstract: There are three set operators defined in XQuery, namely union, intersect and except. They take node sequences as operands, in which each node is identified by its node-ID and treated as an atomic entity. However, according to XML semantics, each node is “a set of set(s)”, which have descendants in a tree-structured hierarchy. Unfortunately, the regular set operators as described above ignored this structural feature of XML data. On the other hand, some XML applications can be benefited from set operators with different semantics considering ancestor-descendant relationships between nodes. In this extended semantics, the comparison during query processing are conducted not only on nodes of both operands, but also on their descendants, in a “deep” manner. In this paper, we identify the needs of such “deep” set operators and propose the deep-union, deep-intersect and deep-except operators. We further explore their properties as well as relationships to regular set operators, and present a preliminary experience on implementing them as user-defined functions of XQuery.

An efficient trajectory index structure for moving objects in location-based services

Abstract: Because moving objects usually moves on spatial networks, efficient trajectory index structures are required to gain good retrieval performance on their trajectories. However, there has been little research on trajectory index structure for spatial networks, like road networks. In this paper, we propose an efficient trajectory index structure for moving objects in Location-based Services (LBS). For this, we design our access scheme for efficiently dealing with the trajectories of moving objects on road networks. In addition, we provide both an insertion algorithm to store the initial information of moving object trajectories and one to store their segment information. We also provide a retrieval algorithm to find a set of moving objects whose trajectories match the segments of a query trajectory. Finally, we show that our trajectory access scheme achieves about one order of magnitude better retrieval performance than TB-tree.

KTR: an efficient key management scheme for air access control

Abstract: To provide secure access to data in wireless broadcast services, symmetric key-based encryption is used to ensure that only users who own the valid keys can decrypt the data. In this paper, we propose an efficient key management scheme (namely KTR) to handle key distribution with regarding to complex subscription options and user activities. KTR has the following advantages. First, it supports all subscription activities in wireless broadcast services. Second, in KTR, a user only needs to hold one set of keys for all subscribed programs, instead of separate sets of keys for each program. Third, KTR identifies the minimum set of keys that must be changed to ensure broadcast security and minimize the rekey cost. Our simulations show that KTR can save about 45% of communication overhead in the broadcast channel and about 50% of decryption cost for each user, compared with conventional approaches based on logical key hierarchy.

Balancing performance and confidentiality in air index

Abstract: Studies on the performance issues (i.e., access latency and energy conservation) of wireless data broadcast have appeared in the literature. However, the important security issues have not been well addressed. This paper investigates the tradeoff between performance and security of signature-based air index schemes in wireless data broadcast. From the performance perspective, keeping low false drop probability helps clients retrieve the information from a broadcast channel efficiently. Meanwhile, from the security perspective, achieving high false guess probability prevents the hacker from guessing the information easily. There is a tradeoff between these two aspects. An administrator of the wireless broadcast system may balance this tradeoff by carefully configuring the signatures used in broadcast. This study provides a guidance for parameter settings of the signature schemes in order to meet the performance and security requirements. Experiments are performed to validate the analytical results and to obtain optimal signature configuration corresponding to different application criteria.