96352
An Adaptive Direction-Based Location Update Scheme for Next Generation PCS Networks
This paper studies the location management problem in PCS network. An adaptive direction-based location update (ADBU) scheme is proposed in which a line-paging method is adopted to minimize the paging cost and an adaptive direction-based scheme is formulated for location update generation. Mobile terminals inspect their directions and a location update will be generated when the change in the direction of a mobile terminal is greater than the direction threshold. In the paper, we assume that the movement of a mobile terminal in a period of time can be divided into steps and each step has a destination. Then, we use a Gauss-Markov process to model the movement of a mobile terminal in each step. With the Gauss-Markov model and a feedback control mechanism, we estimate the optimal value for the direction threshold with the attempt to minimize the update cost and at the same time limit the paging cost using line-paging. Performance results have shown that the performance of ADBU is significantly better than another direction-based update scheme and the distance-based scheme a result of smaller paging cost and update processing cost.
16685984
A multi-version data model for executing real-time transactions in a mobile environment
With the significant advances in mobile computing technology, there is an increasing demand for various mobile applications to process transactions in a real-time fashion. When remote data access is considered in a mobile environment, data access delay becomes one of the most serious problems in meeting transaction deadlines. In this paper, we propose a multi-version data model and adopt the relative consistency as the correctness criterion for processing of real-time transactions in a mobile environment. The purpose is to reduce the impacts of unpredictable and unreliable mobile network on processing of the real-time transactions. Under the proposed model, the overheads for concurrency control can be significantly reduced, and the data availability is much enhanced even under network failures. A real-time transaction may access stale data, provided that they are relatively consistent with the data accessed by the transaction, and the staleness of the data is within the requirements. An image transaction model, which pre-fetches multiple data versions at fixed hosts, is proposed to reduce the data access delay and to simplify the management of the real-time transactions in a mobile environment. The image transaction model also helps in reducing the transaction-restart overheads and minimizing the impacts of the unpredictable performance of mobile network on transaction executions.
16283914
Approaches for broadcasting temporal data in mobile computing systems
Abstract Rapid advances in mobile communication technology have spawned many new mobile applications. A key element in many of these systems is the need to distribute real-time information from a database server to mobile clients. While data broadcast has been shown to be an efficient data dissemination technique, many issues such as selection of broadcast data and caching strategies at mobile clients are still active research areas. In this paper, we consider an important characteristic of many mobile computing systems which has often been ignored in the design of broadcast algorithms: the fact that many data items are associated with temporal constraints on their validity. We introduce the notion of absolute validity interval (AVI) to capture the temporal constraints of the data items, formulate a temporal data model and examine both static and dynamic approaches to select data items based on their access frequencies as well as their AVI. The reason for considering the AVI of the data items in broadcast selection is to increase the client cache hit probability so that the access delay for a data item will be much reduced. Based on the results from extensive simulation experiments, it is concluded the AVI-based approaches, by improving cache hit probability, can significantly improve the mean response time and reduce the number of deadline missing requests.
39491407
Mobile Distributed Real-Time Database Systems
8292420
Priority Scheduling of Transactions in Distributed Real-Time Databases
One of the most important issues in the design of distributed real-time database system (DRTDBS) is transaction scheduling which consists of two parts: priority scheduling and real-time concurrency control. In the past studies, mostly, these issues are studied separately although they have a very close interaction with each other. In this paper, we propose new priority assignment policies for DRTDBS and study their impact on two typical real-time concurrency control protocols (RT-CCPs), High Priority Two Phase Locking (HP-2PL) and Optimistic Concurrency Control with Broadcast Commit (OCC-BC). Our performance results show that many factors, such as data conflict resolution, degree of data contention and transaction restarts, that are unique to database systems, have significant impact on the performance of the policies which in turn affect the performance of the real-time concurrency control protocols. OCC-BC is more affected by the priority assignment policies than HP-2PL owing to the late detection of conflict. In the design of priority assignment policies, we have found that neither the purely deadline driven policies nor data contention driven policies are suitable for DRTDBS. Our proposed policy, the Mixed Method (MM), which considers both transaction timeliness and data contention, outperforms other policies over a wide range of system parameter settings.
62523912
Concurrency control strategies for ordered data broadcast in mobile computing systems
Although data broadcast has been shown to be an efficient method for disseminating data items in mobile computing systems, the issue on how to ensure consistency and currency of data items provided to mobile transactions (MT), which are generated by mobile clients, has not been examined adequately. While data items are being broadcast, update transactions may install new values for them. If the executions of update transactions and the broadcast of data items are interleaved without any control, mobile transactions may observe inconsistent data values. The problem will be more complex if the mobile clients maintain some cached data items for their mobile transactions. In this paper, we propose a concurrency control method, called ordered update first with order (OUFO), for the mobile computing systems where a mobile transaction consists of a sequence of read operations and each MT is associated with a time constraint on its completion time. Besides ensuring data consistency and maximizing currency of data to mobile transactions, OUFO also aims at reducing data access delay of mobile transactions using client caches. A hybrid re-broadcast/invalidation report (IR) mechanism is designed in OUFO for checking the validity of cached data items so as to improve cache consistency and minimize the overhead of transaction restarts due to data conflicts. This is highly important to the performance of the mobile computing systems where the mobile transactions are associated with a deadline constraint on their completion times. Extensive simulation experiments have been performed to compare the performance of OUFO with two other efficient schemes, the multi-version broadcast method and the periodic IR method. The performance results show that OUFO offers better performance in most aspects, even when network disconnection is common.
14189953
Adaptive record clustering
An algorithm for record clustering is presented. It is capable of detecting sudden changes in users' access patterns and then suggesting an appropriate assignment of records to blocks. It is conceptually simple, highly intuitive, does not need to classify queries into types, and avoids collecting individual query statistics. Experimental results indicate that it converges rapidly; its performance is about 50 percent better than that of the total sort method, and about 100 percent better than that of randomly assigning records to blocks.
8870380
Performance analysis of three related assignment problems
The relative placement of records in a database has a significant impact on the overall system performance. In this paper we study three related assignment problems. They are (1) the assignment of records to devices so as to minimize the expected completion time and to maximize the expected utilization of the devices, (2) the assignment of records to pages or blocks in secondary memory so as to minimize the number of blocks to be accessed, and (3) the analysis of the performance of a hashing scheme under the assumption that the record-to-bucket probability varies from one bucket to another. The solutions to these problems are obtained by studying an occupancy problem. In each case, an optimal solution is obtained.
33676247
Locating and extracting the eye in human face images
Facial feature extraction is an important step in automated visual interpretation and human face recognition. Among the facial features, the eye plays the most important part in the recognition process. The deformable template can be used in extracting the eye boundaries. However, the weaknesses of the deformable template are that the processing time is lengthy and that its success relies on the initial position of the template. In this paper, the head boundary is first located in a head-and-shoulders image. The approximate positions of the eyes are estimated by means of average anthropometric measures. Corners, the salient features of the eyes, are detected and used to set the initial parameters of the eye templates. The corner detection scheme introduced in this paper can provide accurate information about the corners. Based on the corner positions, we can accurately locate the templates in relation to the eye images and greatly reduce the processing time for the templates. The performance of the deformable template is assessed with and without using the information on corner positions. Experiments show that a saving in execution time of about 40% on average and a better eye boundary representation can be achieved by using the corner information.
28287084
Human face recognition based on spatially weighted Hausdorff distance
The edge map of a facial image contains abundant information about its shape and structure, which is useful for face recognition. To compare edge images, Hausdorff distance is an efficient measure that can determine the degree of their resemblance, and does not require a knowledge of correspondence among those points in the two edge maps. In this paper, a new modified Hansdorff distance measure is proposed, which has a better discriminant power. As different facial regions have different degrees of significance for face recognition, a new modified Hausdorff distance is proposed which is weighted according to a weighted function derived from the spatial information of the human face; hence crucial regions are emphasized for face identification. Experimental results show that the distance measure can achieve recognition rates of 80%, 87%, and 91% for the first, the first five, and the first seven likely matched faces, respectively.
30747843
Future Directions
decade she has taught cataloger training workshops, particularly in cataloging Internet resources. Before coming to Stanford, Greta was the Authority Control Librarian and head of the Quality Control Unit at WLN. She is also a silent film historian and serves as the librarian at the Niles Essanay Silent Film Museum, as well as maintaining a website on silent film actresses.