Nikolas Mitrou

Bringing relational databases into the semantic web: a survey

Relational databases are considered one of the most popular storage solutions for various kinds of data and they have been recognized as a key factor in generating huge amounts of data for Semantic Web applications. Ontologies, on the other hand, are one of the key concepts and main vehicle of knowledge in the Semantic Web research area. The problem of bridging the gap between relational databases and ontologies has attracted the interest of the Semantic Web community, even from the early years of its existence and is commonly referred to as the database-to-ontology mapping problem. However, this term has been used interchangeably for referring to two distinct problems: namely, the creation of an ontology from an existing database instance and the discovery of mappings between an existing database instance and an existing ontology. In this paper, we clearly define these two problems and present the motivation, benefits, challenges and solutions for each one of them. We attempt to gather the most notable approaches proposed so far in the literature, present them concisely in tabular format and group them under a classification scheme. We finally explore the perspectives and future research steps for a seamless and meaningful integration of databases into the Semantic Web.

A Semantic-Based Architecture for Sensor Data Fusion

In this paper, we present a three-layer flexible architecture which intends to help developers and end users to take advantage of the full potential that modern sensor networks can offer. The proposed architecture deals with issues regarding data aggregation, data enrichment and finally, data management and querying using semantic Web techniques. Semantics are used in order to extract meaningful information from the sensorpsilas raw data and thus facilitate smart applications development over large-scale sensor networks. We describe an open, standards-based, modular architecture which is based on the current standardization efforts of the open geospatial consortium (OGC) and extends them to support semantic Web techniques comprising a core of reusable components and interfaces for supporting different types of services, through Web technologies.

Bursty traffic modeling and efficient analysis algorithms via fluid-flow models for ATM IBCN

In this paper fluid models for heterogeneous multiplexed traffic are considered. First, some extensions to the general theory applicable to superposed, time-reversible Markovian Rate Processes are given. These refer to the connection between performance metrics, the consideration for singular systems and the continuity of the solution, with respect to the system parameters. The general framework is then carried over to the heterogeneous multiplexing of ON/OFF sources. By combining the general theory with the special structure of the ON/OFF sources several important facets of this structure are highlighted. As a result, more powerful methods that improve computation speed, stability and ease of implementation are produced. More specifically, the numerical part of the method is reduced to a solution of a nonlinear equation per system eigenvalue. The solution is obtainable through a variant of the (locally quadratically convergent) Newton method. For this method, easily computable starting values that guarantee convergence are given. In addition, explicit expressions for the eigenvectors are provided with the potentially unstable quantities factored-out. The paper also provides explicit and stably computable formulae for upper bounds to the coefficients of the spectral components, present in the expressions for the performance measures of interest. Moreover, the paper proves a partial ordering property for the system eigenvalues and presents an algorithm that performs full ordering on-line. This, in many cases, results in a great reduction to the amount of computation, without any significant loss of precision. Lastly, the particular case of heterogeneity where the differences are only identified in the rates within bursts is seen to have features resembling homogeneous systems. The possibility to substitute an “equivalent” homogeneous system of reduced order, for the original heterogeneous one is addressed.

A context-aware middleware for real-time semantic enrichment of distributed multimedia metadata

This paper investigates the problem of the real-time integration and processing of multimedia metadata collected by a distributed sensor network. The discussed practical problem is the efficiency of the technologies used in creating a Knowledge Base in real-time. Specifically, an approach is proposed for the real-time, rule-based semantic enrichment of lower level context features with higher-level semantics. The distinguishing characteristic is the provision of an intelligent middleware-based architecture on which low level components such as sensors, feature extraction algorithms, data sources, and high level components such as application-specific ontologies can be plugged. Throughout the paper, Priamos, a middleware architecture based on Semantic Web technologies is presented, together with a stress-test of the system’s operation under two test case scenarios: A smart security surveillance application and a smart meeting room application. Performance measurements are conducted and corresponding results are exposed.

Statistical multiplexing, bandwidth allocation strategies and connection admission control in ATM networks

This paper gives a brief description and the main results of work done within the RACE 1022 project on bandwidth allocation and Connection Admission Control in ATM-based networks. Additionally, and before presenting these two main issues, some important facets of statistical multiplexing are highlighted. The key question which is answered by a bandwidth allocation algorithm is “how much bandwidth is required by a group of connections (e.g., within a VP), with certain traffic characteristics, in a certain multiplexing environment”. Answers to this question are given for specific traffic classes and mixing conditions. The advantages and the limitations of the proposed strategies are indicated. In cases where the proposed bandwidth allocation strategies fail to fulfil the specified objectives (e.g., in highly heterogeneous traffic mixes), sophisticated CAC algorithms are required. In particular, the hierarchically organised CAC strategy implemented within the RACE 1022 ATD Technology Testbed will be described in detail. This CAC scheme is based on a simple real-time processing algorithm which provides a quick acceptance decision. This decision will be refined by a precise but numerically complex background algorithm. Simulation studies confirm the potential of this concept.

Cognitive radio-aided wireless sensor networks for emergency response

A lot of research effort has been put into wireless sensor networks (WSNs) and several methods have been proposed to minimize the energy consumption and maximize the networks lifetime. However, little work has been carried out regarding WSNs deployed for emergency situations. We argue that such WSNs should function under a flexible channel allocation scheme when needed and be able to operate and adapt in dynamic, ever-changing environments coexisting with other interfering networks (IEEE 802.11b/g, 802.15.4, 802.15.1). In this paper, a simple and efficient method for the detection of a single operational frequency channel that guarantees satisfactory communication among all network nodes is proposed. Experimental measurements carried out in a real environment reveal the coexistence problem among networks in close proximity that operate in the same frequency band and prove the validity and efficiency of our approach.

NEURON: Enabling Autonomicity in Wireless Sensor Networks

Future Wireless Sensor Networks (WSNs) will be ubiquitous, large-scale networks interconnected with the existing IP infrastructure. Autonomic functionalities have to be designed in order to reduce the complexity of their operation and management, and support the dissemination of knowledge within a WSN. In this paper a novel protocol for energy efficient deployment, clustering and routing in WSNs is proposed that focuses on the incorporation of autonomic functionalities in the existing approaches. The design of the protocol facilitates the design of innovative applications and services that are based on overlay topologies created through cooperation among the sensor nodes.

Enforcing Effective Rates for Packet-Level QoS Control in IP Networks: Theory and Validation Based on Real Traffic Data

This paper presents experimental results aiming at underpinning a traffic engineering framework for traffic control and resource management in IP-based networks, which has significant theoretical qualities. Initially, the paper reviews the major components of a packet level traffic control framework:(a) a general traffic shaping algorithm for effective rate enforcement(b) mechanisms for aggregating, splitting and policing streams shaped with this algorithm, and(c) a calculus for quantitative end-to-end QoS.Following the illustration of the theoretical concepts the paper focuses on aspects pertaining to the applicability of the framework. Specifically, we study the impact of the shaping delay and we provide insight into the issue of enforcing effective rates to traffic streams. These studies take into account results from simulating the shaping algorithm, which are based on analyzing, processing and simulating real traffic traces collected from an HTTP server. Moreover, traffic modeling conclusions related to the proposed shaping algorithm, are derived and presented.

Traffic engineering using a class ofM/G/1models

The dominant-term approximation of theM/G/1queuing system is thoroughly investigated and its application in a number of traffic engineering problems is considered. The effective rate of traffic streams associated with theirM/G/1modelling is defined, the summability property of which is formally proven. Special attention is paid to theM/D/1model, the usage of which in ATM traffic control is demonstrated. An effective-rate shaping mechanism (based on this kind of modelling) is finally devised. The performance of the proposed shaper has been fully validated, both theoretically and experimentally.

A Flexible and Cost-Effecive ATM Traffic Generator

In this paper a flexible burst-level ATM traffic generator is presented. It is a PC-based system with a careful allocation of functions between hardware (PC ISA board) and software, in a way that allows it to work on-line at full-speed (155.52Mbps), on the one hand, and on the other, to be flexible enough to emulate a wide range of ATM traffic profiles. Up to 4 boards can be hosted by a single PC, each being able to generate up to 16 independent streams. Each stream consists of a continuous sequence of “traffic events” (burst-silence cycles), with each event being described by three parameters: the burst size, the silence duration and the inter-cell distance within the burst. The triplets, describing respective traffic events, are generated by software and downloaded on-line (through DMA) to the hardware; thus, in principle, any traffic model can be implemented, the limits being imposed only by the speed of the PC in relation to the total number of independent streams (up to 64) generated concurrently. For example, arbitrary distributions of the three traffic parameters (including experimental histograms) can be independently sampled; or correlated samples according to any law desired can be easily produced. Even further, real traffic can be emulated, provided that a monitoring device (e.g. a LAN traffic monitor) is connected to the PC feeding it with samples of the real traffic stream.