Despite hierarchical Distributed Hash Table (DHT) systems have addressed flat overlay system problems, most of existing solutions add a significant overhead to large scale systems. This not only... Show More

Despite hierarchical Distributed Hash Table (DHT) systems have addressed flat overlay system problems, most of existing solutions add a significant overhead to large scale systems. This not only increases the bandwidth consumption but also affect the routing efficiency. This paper deals with a resource discovery while minimizing maintenance overhead in hierarchical DHT systems. The considered resources are metadata describing data sources. In the solution, only one gateway in one overlay is attached to the superior level overlay. It aims to reduce both lookup and maintenance costs while minimizing the overhead added to the system. The authors present a cost analysis for a resource discovery process and discuss capabilities of the proposed protocol to reduce the overhead of maintaining the overlay network. The analysis result proved that our design decrease significantly the maintenance costs in such systems especially when nodes frequently join/leave the system.

One important aspect of ubiquitous environments is to provide users with the possibility to freely move about and continue to interact with the available applications through a variety of... Show More

One important aspect of ubiquitous environments is to provide users with the possibility to freely move about and continue to interact with the available applications through a variety of interactive devices such as cell phones, PDAs, desktop computers, intelligent watches, or digital television sets. Migratory applications are able to follow the user by sensing changes in the user’s context and adapting to available devices, ideally without interrupting the user experience. However, applications themselves must contain functions to monitor context information, coordinate a migration, handle application adaptation, and interact with the user during the migration process. To make life easier for developers and users of migratory applications, an integrated Migration Service Platform (MSP) is proposed, where all the common migration functions are centralised. The authors show how the platform is realised as middleware that contains a server for the central functions and lightweight client-side running on the end-user devices. The authors show how migratory applications can interact with the platform and thereby do not have to contain migration functions themselves. The authors describe the challenges following the centralisation of a migration platform that can support different types of applications, both games and business applications, implemented with either web-technologies or as component-based applications.

The Complete Automatic Public Turing test to tell Computers and Humans Apart (CAPTCHA) is a modern implementation of the Turing test. A CAPTCHA asks a series of questions which are supposed to be... Show More

The Complete Automatic Public Turing test to tell Computers and Humans Apart (CAPTCHA) is a modern implementation of the Turing test. A CAPTCHA asks a series of questions which are supposed to be relatively easy for mature human beings but hard for machines to answer. A CAPTCHA, as its name suggests, is generally used as a tool to distinguish human beings from machines when the corresponding party is invisible in a remote site. There are many CAPTCHAs proposed and widely used in many Internet applications, with varied strength and performance. In this article, a review of current CAPTCHAs is presented. Analysis and comparisons are provided for various CAPTCHAs. Two types of new CAPTCHAs, intelligent and multimedia CAPTCHAs, are proposed. Our experiments have shown that the newly proposed CAPTCHAs have stronger protection than most existing CAPTCHAs and they can be applied to various web applications, such as e-commerce.

This paper proposes a data management scheme which employs an energy constrained algorithm selecting between direct and multi-hop transmissions autonomously based on the residual energy level of the... Show More

This paper proposes a data management scheme which employs an energy constrained algorithm selecting between direct and multi-hop transmissions autonomously based on the residual energy level of the individual sensors. The proposed data management scheme rules out the selection of hotspot sensors, the sensors located closer to the base stations, as the intermediate sensors to avoid the dying of these sensors. In each data transmission, the scheme selects one of the neighborhood sensors having minimal Euclidean distance and maximum energy-level as the intermediate node from the neighboring set, without repeating the selection. The proposed data management scheme manages the data collection by utilizing two scheduling algorithms; as soon as possible (ASAP) and as late as possible (ALAP). As a measure of performance, the simulation results of the data management scheme have been compared with that of minimum connected dominating set algorithm (MCDS). The simulation results demonstrate that the data management scheme outperforms with respect to consume less energy; moreover, it can be observed that the scheme finishes an overall short waiting time of the selected sensors compared to the direct transmission in transmitting the data to the base station. The robustness of the proposed scheme is tested by varying the network sizes and varying the sensing radii.