Constantinos Djouvas

Applying adaptive QoS-aware medium access control in priority-based vehicular ad hoc networks

This paper proposes a novel, adaptive medium access control mechanism for vehicular ad hoc networks. A simple, effective, and efficient nonlinear control law is built, based on fuzzy logic control principles, which can be easily adopted in different network environments (e.g. vehicle-to-vehicle (V2V) communication and vehicle-to-infrastructure (V2I) communication). We demonstrate, via simulative evaluation, that the proposed fuzzy control methodology offers inherent robustness with effective control of the system under dense and dynamic conditions, without the need to (re)tune any parameters. The proposed approach offers distinct differentiation among differently prioritized traffic types, thus providing adequate Quality of Service (QoS) in terms of throughput performance, in contrast with the IEEE 802.11p MAC protocol we compared against.

Creating a maritime wireless mesh infrastructure for real-time applications

Advances in computing and networking technologies have revolutionised the way people communicate using interactive real-time applications such as instant messaging and audio and videoconferencing. This category of applications is nowadays readily available to users in terrestrial areas with high speed Internet connectivity. The aim of this work is to investigate the provision of IP-based network connectivity onboard sailing vessels in order to support real-time communication applications. More precisely, instead of the expensive and high-latency satellite communications solutions widely deployed in the maritime industry, we propose the use of long-range wireless networking technologies for creating a vessel-to-shore mesh network that can be used to form an infrastructure for the provision of telephony services based on Voice-over-IP (VoIP).

Improving quality of experience in wireless VoIP through novel call scheduling

In the last few years we have experienced a dramatic increase in the use of IP networks for voice applications (VoIP) over wireless networks due to increased bandwidth availability and enhanced device capabilities. Since demand often exceeds available capacity, Call Admission Control mechanisms are in place to prevent the uncontrolled usage of bandwidth. Through the use of an intermediary gateway, VoIP calls are in many cases terminated to a normal landline or cellphone; the capacity of such a gateway is also a finite resource since the number of users can vary significantly as many are mobile. In this article we propose an enhanced scheme that aims to manage access to the lines available so that they are used in a fair manner and utilized to the highest degree possible. This management is facilitated by enhancing a proxy implementation with a number of call scheduling policies. The ability to satisfy pending call requests as soon as lines become available, results in increased user service satisfaction. Moreover, it increases line utilization which is crucial from an economic viewpoint. The ultimate goal is to improve Quality of Experience which is deemed as highly important especially considering that wireless network users experience opportunistic and intermittent connectivity.

Testing a Network by Inferring Representative State Machines from Network Traces

This paper describes an innovative approach to network testing based on automatically generating and analyzing state machine models of network behavior. The models are generated by the network test tool AGATE (Automatic Generator of Automata for TEsting), which is also described in this paper. The proposed test approach mimics experimental method, requiring repeated cycles of observing the network, modeling the network, making predictions about network behavior, and evaluating predictions. This paper focusses on the modeling step, in which the test tool AGATE automatically generates representative state machines from observed network traces. The generated state machines closely approximate the behavior of components of the network under test. Faults in the system may be immediately apparent from the state machines, but more importantly the state machines can be used for formal analysis. We propose this as a cost-effective alternative to manually defining a state machine before beginning tests.

Applying delay tolerant networking routing algorithms in maritime communications

The availability of off-the-shelf equipment that facilitates long range wireless connectivity can prove to be useful in maritime communications. By establishing terrestrial wireless links, vessels may exchange data between them or with shore-based stations avoiding the use of costly satellite connectivity. On the other hand, vessel mobility and large distances between its nodes make a wireless maritime network hard to manage; frequent disconnections are expected to be the norm in this primarily low node density environment. As such, maritime communications appear as an ideal environment for delay tolerant networking. To this extent, we use actual vessel mobility data to evaluate some existing delay tolerant networking routing algorithms and identify where they fall short from the perspective of our particular domain. It appears that a hybrid approach in the form of a new routing algorithm that addresses the particular characteristics of a maritime network can prove beneficial.

The need for a system design methodology to address challenges in wireless access in vehicular ad hoc networks

Vehicular networking is nowadays considered as one of the most important enabling technologies as it allows vehicles to communicate with each other (vehicle-to-vehicle (V2V) communication) and/or with roadside infrastructure (vehicle-to-infrastructure (V2I) communication). Vehicular ad hoc networks (VANETs) are ideal candidates for supporting various types of traffic applications, such as traffic safety, traffic efficiency and management applications, and infotainment type of applications. As a result, there is ongoing research concerning the efficient medium access control and multichannel coordination, separately. In this paper, we review a number of schemes proposed and stimulate the discussion on the need for a generic system design methodology to address efficiently the challenges in the wireless access in vehicular ad hoc networks. Specifically, the aim shall be to design an efficient and robust system architecture that deals with the wireless medium access and multichannel coordination in all possible environments (V2V and V2I in urban, suburban, and highway) and allows differently prioritized types of traffic applications to coexist.

Methodological Challenges in the Analysis of Voting Advice Application Generated Data

Voting advice applications (VAA) have become an increasingly popular feature of electoral campaigns. VAAs are online tools that use survey techniques to measure the degree to which the policy preferences of citizens match those of political parties or candidates. In some cases, such as The Netherlands, VAAs can attract millions of respondents providing an incredibly rich source of mass public opinion data. As a result political scientists have begun to exploit such datasets and this is fuelling a burgeoning literature on the topic. To date, however, there has been surprisingly little research on the cleaning techniques used to filter out the many rogues entries that are known to be present in VAA generated datasets. This paper presents the various methods used for cleaning VAA generated datasets that have been used for empirical research. Two main techniques are used based on item response timers and pattern recognition techniques. We show why cleaning matters and the problems that flow from not establishing rigorous cleaning techniques. The problem as such is not exclusive to VAA data but is common to all web based research involving self-administered surveys. To that end the techniques we present could be generalisable beyond the specific case of VAA-generated datasets.

A View Behind the Scene: Data Structures and Software Architecture of a VAA

In todays rapidly evolving and growing online community many different applications are proposed and implemented. One category of such applications that drew high attention during the last few years are the so-called Voting Advice Applications (VAAs). VAAs are online systems used during elections that allow voters to create a political profile, the comparison of this profile with the profiles of political parties and candidates provides the voter with an estimation of his/her proximity to parties and candidates. In this paper, the data structure and the software architecture used for implementing a VAA platform along with its technical requirements are presented. Furthermore, a novel approach used for supporting multi-lingual content is described.

Contention window adaptation for broadcast beaconing in vehicular ad hoc networks

Vehicular networking major concern is to effectively support intelligent transportation system applications. For that, robust, adaptive techniques for efficient communication and control between vehicles need to be developed. In this paper, the key issue under investigation is the efficient, intelligent provision of medium access control in vehicular ad hoc networks in the case of broadcast beaconing. A well established option is proposed, for controlling nonlinear systems using linguistic information, by utilizing fuzzy logic control. In particular, an intelligent adaptive controller, based on the traffic density, is built to regulate dynamically the contention window during message broadcasting. Simulation results, on dense and dynamic scenarios, reveal the inbuilt robustness and proficient control of the system provided by the proposed controller. This is in contrast with the conventional, IEEE 802.11p standard, solution we compared against, in terms of channel utilization, latency, collisions, beacon reception rate, and beacon inter-arrival time.

Testing Self-Similar Networks

Abstract A hard problem in network testing is verifying the correctness of a class of networks, as well as the actual networks under test. In practice, at most a few networks (sometimes only one) are actually tested. Thus an important question is how to select one or more networks that are sufficiently representative to apply the results to a class of networks. We present a model-based technique for selecting a representative network. The central theorem establishes that the representative network displays any faults present in any network of the class. This paper introduces the concept of “self-similarity,” which is used to select the network, and presents the results of an experiment in testing one class of networks.