Journal Article

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[BibTex] [Abstract]

In the world where on-demand and trustworthy service delivery is one of the main preconditions for successful business, service and business process availability is of the paramount importance and cannot be compromised. For that reason service availability is coming into central focus of the IT operations and management research and practice. Still, our understanding of service and process availability is mostly empirical and at best, sketchy. Services are assessed using a mixture of qualitative, quantitative and analytical methods, with results of varying quality. We introduce a systematic model-based methodology and a tool for service and business process availability assessment. The main advantage of the proposed method is the ability to automatically generate availability models, based on the service/process description and technical infrastructure it is executing on. Therefore, service level agreements can be tested/simulated or return on investment calculation can be performed, without the need for costly experiments and/or actual investments.

[BibTex] [Abstract]

We present the results of analysis of Berlin's free multihop wireless network and provide topological, link reliability and traffic statistics we have captured from it. The network is relatively large compared with known wireless multihop networks and testbeds, and it had 316 participating nodes on the average. The observed properties are different from frequently used models: the network is connected but with low average node density, it has large number of bridges (some of them with very low quality of links) and the traffic distribution is highly asymmetrical.

[BibTex] [Abstract]

This paper describes the necessity of the third-party testing and monitoring in the field of e-business infrastructure, with the emphasis on expanding into the wireless environments for improvement of the quality of service. Outlining the purpose for monitoring and testing, this paper shows the methods of overwhelming the current problems in the wireless computing, giving statistical and anticipated references for devices that have active role in the mobile environments.

Book Section

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[BibTex]

Conference Paper

[BibTex] [Abstract]

Measurement noise and unreliability of communication in wireless sensor networks (WSNs) make event detection a non-trivial and error-prone task. In order to reduce errors in the event detection, collaborative event detection was introduced. This paper presents a stochastic model of collaborative event detection in presence of uncertainties caused by measurement noise at sensors and unreliable network. The model allows analysis of event detection accuracy in WSNs as well as synthesis of WSNs with set accuracy targets. An optimal stopping retry mechanism for the event detector is developed from the model. Simulation shows that the retry mechanism reduces probability of false alarms by 35 to 40\% with a minimal transport overhead.

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We analyze accuracy of hysteresis link status detectors in static and mobile wireless multi-hop networks (WMNs), with focus on errors in link status detection caused by the unreliable communication channel. The hysteresis link status detectors are of particular importance in WMNs due to their use in various protocols, such as AODV and OLSR. Stochastic models are derived that describe hysteresis link status detector behavior as a function of detector parameters and network characteristics. Models are applied to a real network and their analysis shows that the probability of errors in link status detection in WMNs is considerable. In order to reduce these errors, we optimize detector parameters so that the link detection error probability is minimized.

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Despite a considerable number of topology generation algorithms for simulation of wireless multihop networks it is difficult to find one with output similar to real networks. We propose NPART -- a Node Placement Algorithm for Realistic Topologies whose output topologies resemble networks encountered in reality. The algorithm is flexible since it is sufficient to provide it with different input data to obtain different topologies. To demonstrate its quality, we compare topologies generated by NPART algorithm with our measurements from open wireless multihop networks in Berlin and Leipzig. Compared with real topologies, the generated topologies have almost identical node degree distribution, similar number of cut-edges and vertices, and distribution of component sizes after bridge removal. The importance of node placement algorithm is demonstrated by comparing ns-2 simulation results for grid and uniform node placement with NPART generated topologies. Simulation results show that node placement model plays as important role in simulation outcome as the accuracy of wireless signal propagation model.

[BibTex] [Abstract]

The often neglected problem in the service availability analysis is mapping between ICT-infrastructure and service-level availability. We present an approach which allows to map ICT-infrastructure elements to services, and to analytically assess steady-state, interval and user-perceived service availability, based on failure distributions of ICT-elements that implement a composite service. In case that full topology or all failure distributions of ICT-infrastructure elements are unknown, we provide means to estimate upper and lower availability bounds.

[BibTex] [Abstract]

In the world where on-demand and trustworthy service delivery is one of the main preconditions for successful business, availability of the services and business processes is of the paramount importance and cannot be compromised. We present a framework for modeling business process availability that takes into account services, the underlying ICT-infrastructure and people. Based on a fault model, we develop the methodology to map dependencies between ICT-components, services and business processes. The mapping enables us to model and analytically assess steady-state, interval and user perceived availability at all levels, up to the level of the business process.

[BibTex] [Abstract]

We extend the Breadth First Search (BFS) algorithm to use it for cut-edge(bridge) detection in graphs. The changes in the algorithm are tailored such that the algorithm can be applied in wireless multihop networks: e.g., it fully utilizes the broadcasting nature of the wireless medium. The distributed BFS algorithm (flooding) is widely used for route discovery and information dissemination in wireless multihop networks (WMNs) so the overhead introduced by our bridge detection algorithm is limited -- the network is already performing the distributed BFS and we reuse the information from it to detect the bridges. We verify our detection algorithm on the data sampled from Berlin's free multi-hop wireless network. Detection precision varies depending on the algorithm parameters but for the representative algorithm configurations it stabilizes around 75%. Analysis of the data samples indicated that due to unreliability of wireless links and frequent occurrence of bridges the route discovery mechanism cannot find the route between two nodes although a valid route exists. We use our bridge detection algorithm to improve the route discovery success ratio from about 47\% to approximately 90% by utilizing unicast of route discovery messages over the bridges. We verified by using fault injection the robustness of our approach as precision and route discovery remained high even for frequent node failures in the network.

[BibTex] [Abstract]

We have developed a mathematical model for calculation of expected value of share of edges being dropped while constructing a Gabriel or a relative neighborhood graph starting from a random geometrical graph created by a homogeneous Poisson point process on a plane. The expressions for the expected values of number of faces and face size are derived as well. All expressions are given as functions of deployment configuration. The results are verified through detailed comparison with extensive simulation results. Since Gabriel and relative neighborhood graphs are used for various applications in ad-hoc networking we believe that our model simplifies theoretical analysis of functionality and performance of such networks.

[BibTex] [Abstract]

We propose an algorithm for detection of partitioning in location-aware mobile ad hoc networks. The partitioning occurs when movement pattern of nodes is such that they separate into groups that cannot communicate with each other. We use information about node position and speed in order to build a model that is able to predict when partitioning will occur and which link is critical, using properties of planar graphs that represent the network. Our algorithm is distributed and uses only local topology knowledge where nodes keep track of position and speed of their one-hop neighbors.

Report

[BibTex]