Antonio Si

CHECK: a document plagiarism detection system

Digital documents are vulnerable to being copied. Most existing copy detection prototypes employ an exhaustive sentence-based comparison method in comparing a potential plagiarized document against a repository of legal or original documents to identify plagiarism activities. This approach is not scalable due to the potentially large number of original documents and the large number of sentences in each document . Furthermore, the security level of existing mechanisms is quite weak; a plagiarized document could simply by-pass the detection mechanisms by performing a minor modification on each sentence. In this paper, we propose a copy detection mechanism that will el iminate unnecessary comparisons. This is based on the observation that comparisons between two documents addressing different subjects are not necessary. We describe the design and implementation of our exper imental proto type called CHECK. The results of some exploratory experiments will be illust rated and the security level of our mechanism will be discussed.

Data broadcasting strategies over multiple unreliable wireless channels

Data communication in a mobile envirorunent involves a stationary database server disseminating information to a large collection of “mobile clients” via wireless transmission media. One effective way to disseminate database information via narrow bandwidth wireless media to the mobile clients is to periodically broadcast the database over the “air”; individual clients will then “catch” their interested data on the fly selectively. Wireless transmission, however, is vulnerable to noise and signal distortion. Furthermore, a client might not aware of when its interested data will be broadcasted and might fail to catch the data at the appropriate moment. In either situation, it has to wait for the next broadcast cycle, severely affecting the performance of the database access. In this paper, we describe a spectrum of data broadcasting mechanisms in a mobile environment over noisy media. The data broadcasting schemes are baaed on replication and partition techniques that distribute the broadcasted data over multiple wireless media. We evaluate the relative performance of the broadcasting schemes analytically, as well as through simulation experiments. The simulation results are also studied and verified with the analytical results.

A Framework for Cache Management for Mobile Databases: Design and Evaluation

In a mobile computing environment, database servers disseminate information to multiple mobile clients via wireless channels. Due to the low bandwidth and low reliability of wireless channels, it is important for a mobile client to cache its frequently accessed database items into its local storage. This improves performance of database queries and improves availability of database items for query processing during disconnection. In this paper, we investigate issues on caching granularity, coherence strategy, and replacement policy of caching mechanisms for a mobile environment utilizing point-to-point communication paradigm.We first illustrate that page-based caching is not suitable in the mobile context due to the lack of locality among database items. We propose three different levels of caching granularity: attribute caching, object caching, and hybrid caching, a hybrid approach of attribute and object caching. Next, we show that existing coherence strategies are inappropriate due to frequent disconnection in a mobile environment, and propose a cache coherence strategy, based on the update patterns of database items. Via a detail simulation model, we examine the performance of various levels of caching granularity with our cache coherence strategy. We observe, in general, that hybrid caching could achieve a better performance. Finally, we propose several cache replacement policies that can adapt to the access patterns of database items. For each given caching granularity, we discover that our replacement policies outperform conventional ones in most situations.

On adaptive caching in mobile databases

We consider an environment in which a collection of mobile clients accesses a stationary database server via a wireless channel. Due to the limited bandwidth of a wireless channel and the instability of the wireless network, caching of frequently accessed data items in a clients local storage becomes especially important for improving the performance and data availability of data access queries. However, the network stability and high transmission bandwidth requirements of existing caching mechanisms for conventional clientserver and distributed database applications conflict with the mobility nature of mobile clients as well as the low-bandwidth of wireless channel. In this paper, we investigate issues that need to be addressed in caching mechanisms for a mobile environment and propose an adaptive caching mechanism that could cope with the nature of a mobile environment as well as the low-bandwidth wireless media. The results of some preliminary exploratory experiments will also be illustrated to demonstrate the feasibility of our mechanism.

Adaptive caching and refreshing in mobile databases

The mobile computing environment is receiving increasing attention recently. We consider a mobile environment in which a collection of mobile clients accesses a stationary database server via a wireless channel. Due to the limited bandwidth of a wireless channel and the instability of the wireless network, caching of frequently accessed data items in a clients local storage becomes especially important for improving the performance and data availability of data access queries. In this paper, we discuss the limitations of existing caching mechanisms in a mobile environment and investigate issues that need to be addressed. We propose an adaptive caching model that could cope with the nature of a mobile environment and the low-bandwidth wireless media, supporting fast data access. We describe the adaptive cache replacement and refresh mechanisms; explain the implementation in the context of object-oriented databases; and illustrate the results of some exploratory experiments to demonstrate the feasibility of the mechanisms.

Maintaining page coherence for dynamic HTML pages

Fueled la.rgely by the gaining popularity of World Wide Web (web) servers and browsers, more and more sites have published their databases to the Internet/Intranet. This allows users to retrieve and explore database items as dynamic HTML pages. Existing dynamic HTML pages are passive. When the content of a database is changed, the corresponding dynamic HTML page does not get reflected. We term this, the page coherence problem. In this paper, we illustrate the limitations of current web browsers in addressing the page coherence problem. We next illustrate that since a dynamic HTML page is maintained in the storage cache of a web client, maintaining the coherence of a dynamic HTML page is similar in spirit to cache coherence problem. However, since a web server is both stateless and connectionless, conventional push-based cache coherence approaches that require a server to invalidate and propagate the updates to the cache are not feasible. We propose a page coherence mechanism which requires a web client to take an active role in invalidating and updating its cache and thus, a dynamic HTML page. We illustrate our implementation on Netscape Navigator using Netscape Plugins, The effectweness of our mechanism is also studied

4DIS: a temporal framework for unifying meta-data and data evolution

Time is all important aspect of all rval-world phenomena. Events occur at specific poims in time: objects and their relationships exist and change over ,ime. [radii iomdly, temporal database systems allow different snapshots of data units to be modeted and queried, ltowever, it has been quite difficult to model evolution of meta-data: this is primarily due to the lack of proper logical data independence supporI in existing database systems. In this paper, we introduce a temporal object database model called F o u r Dimensional Information Space (,{DIS). [n 4DIS, both recta-data and data Ulfits are modeled uniformly as objects. The databa~se is modeled as a set of relationships, each of which relates two individual objects. Each relation is also ~ussociated with a temporal dimension which supports valid and transaction time. This allows time to be naturally integrated with objects of arbitrary granularity and provides flexible logical data independence support for the database. Meta-data aud data evolution could thus be unified in a standard framework. I~mporal operations can also be evaluated against both recta-data and data units in a uniform maturer.

Incremental view update for a mobile data warehouse

In a mobile environment, querying the database at a stationary server by a mobile client is expensive due to the limited bandwidth of a wireless channel and the instability of the wireless network. We address this problem by maintaining a materialized view in a mobile clients local storage. The materialized view contains results of common queries in which the mobile client is interested. We term such a materialized view, a mobile data warehouse. In this paper, we address the view update problem for maintaining a mobile data warehouse, viz., changes to the databaze should be reflected to the derived materialized view in a consistent manner. Existing view update mechanisms are push-based. The server is responsible for notifying all clients whose views might be affected by the database changes. This is not appropriate in a mobile environment due to the frequent disconnection of a wireless channel. We propose a pull-based approach that allows a materialized view to be updated at clients incrementally. Our approach requires a client to request changes to its view from the server when the client needs it. We will demonstrate the feasibility of our approach with preliminary experiments via simulation.

Modeling video objects in 4DIS temporal database system

Video objects are temporal in nature~ A video object is composed of a set of video frames which are related in a total time ordering. By imposing additional timing constraints among video frames, various presentation operators on video objects could be defined. A core set of presentation operators include Play, Pause, Resume, Fast Forward, Fast Backward, Slow Motion, and Stop. A video database system must be able to support the temporal ordering of video frames and the temporal constraints required by any presentation operators. In this paper, we demonstrate how our Four Dimensional Information Space (4DIS) temporal database system is used to model video objects and the presentation operators. Since rimeis defined as a first class object in 4DIS, querying operators and constraints for time objects are supported, These temporal querying operators and constraints a r e used to describe the timing requirements of video objects and the presentation operators. This allows users to describe video objects and operators declaratively, without worrying about their low level representation. Furthermore, a user could specify arbitrary operations on video objects using the query language supported by 4D1S. This is in contrast with existing video database systems in which operators on video objects are limited and are defined in advance.

Caching Data Over a Broadcast Channel

We consider an environment in which a collection of mobile clients interacts with a stationary database server. Due to the low bandwidth of wireless communication channels, it is necessary to broadcast highly popular data items and deliver other data items on a demand basis to the mobile clients. This broadcast wireless media can be considered as an extra layer of cache storage, which we term air-storage. We investigate several mechanisms in selecting the data items to be placed over this new layer of air-storage, and illustrate their effectiveness by means of simulation.