Posts with Tag ‘Wireless mesh networks’
Friday, March 20, 2015, 13:00
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Author:
Andreas Dittrich
Ph.D. Thesis, submitted to the Faculty of Mathematics and Natural Sciences, Humboldt University of Berlin, December 9, 2014. Oral defense March 20, 2015.
Grade: magna cum laudae.
Reviewers:
Prof. Dr. Miroslaw Malek, Lugano, Switzerland
Prof. Dr. Alexander Reinefeld, Berlin, Germany
Prof. Dr. Jörg Kaiser, Magdeburg, Germany
Download: published version, meta information
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Dependability of service provision is one of the primary goals in modern networks. Since providers and clients are part of a connecting Information and Communications Technology (ICT) infrastructure, service dependability varies with the position of actors as the ICT devices needed for service provision change. We present two approaches to quantify user-perceived service dependability. The first is a model-driven approach to calculate instantaneous service availability. Using input models of the service, the infrastructure and a mapping between the two to describe actors of service communication, availability models are automatically created by a series of model to model transformations. The feasibility of the approach is demonstrated using exemplary services in the network of University of Lugano, Switzerland. The second approach aims at the responsiveness of the service discovery layer, the probability to find service instances within a deadline even in the presence of faults, and is the main part of this thesis. We present a hierarchy of stochastic models to calculate user-perceived responsiveness based on monitoring data from the routing layer. Extensive series of experiments have been run on the Distributed Embedded Systems (DES) wireless testbed at Freie Universität Berlin. They serve both to demonstrate the shortcomings of current discovery protocols in modern dynamic networks and to validate the presented stochastic models. Both approaches demonstrate that the dependability of service provision indeed differs considerably depending on the position of service clients and providers, even in highly reliable wired networks. The two approaches enable optimization of service networks with respect to known or predicted usage patterns. Furthermore, they anticipate novel service dependability models which combine service discovery, timeliness, placement and usage, areas that until now have been treated to a large extent separately.
Categories: Publication, Research and Education
Tags: Availability, Client-server systems, Composition, Dependability, Dissertation, Distributed computing, Fault tolerance, IP, Modeling, Network, Quality of service, Responsiveness, Service Availability, Service dependability, Service discovery, Service network management, Service networks, SOA, UDP, User-Perceived Evaluation, Wireless mesh networks, Zeroconf
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Monday, June 30, 2014, 08:28
Service Discovery (SD) is an integral part of service networks. Before a service can be used, it needs to be discovered successfully. Thus, a comprehensive service dependability analysis needs to consider the dependability of the SD process. As a time-critical operation, an important property of SD is responsiveness: the probability of successful discovery within a deadline, even in the presence of faults. This is especially true for dynamic networks with complex fault behavior such as wireless networks. We present results of a comprehensive responsiveness evaluation of decentralized SD, specifically active SD using the Zeroconf protocol. The ExCovery experiment framework has been employed in the Distributed Embedded System (DES) wireless testbed at Freie Universität Berlin. We present and discuss the experiment results and show how SD responsiveness is affected by the position and number of requesters and providers as well as the load in the network. Results clearly demonstrate that in all but the most favorable conditions, the configurations of current SD protocols struggle to achieve a high responsiveness. We further discuss results reflecting the long-term behavior of the testbed and how its varying reliability impacts SD responsiveness.
Categories: Publication, Research and Education
Tags: Client-server systems, Dependability, Dissertation, Distributed computing, Embedded, Experiments, Fault tolerance, IP, Network, Quality of service, Responsiveness, Service dependability, Service discovery, Service network management, Service networks, UDP, Wireless mesh networks, Zeroconf
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Saturday, March 15, 2014, 00:00
Experiments are a fundamental part of science. They are needed when the system under evaluation is too complex to be analytically described and they serve to empirically validate hypotheses. This work presents the experimentation framework ExCovery for dependability analysis of distributed processes. It provides concepts that cover the description, execution, measurement and storage of experiments. These concepts foster transparency and repeatability of experiments for further sharing and comparison. ExCovery has been tried and refined in a manifold of dependability related experiments during the last two years. A case study is provided to describe service discovery as experiment process. A working prototype for IP networks runs on the Distributed Embedded System (DES) wireless testbed at the Freie Universität Berlin.
Categories: Publication, Research and Education
Tags: Client-server systems, Dependability, Distributed computing, Experiment Framework, IP, Network, Service networks, Tool Description, Wireless mesh networks, Zeroconf
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Tuesday, January 21, 2014, 10:46
In wireless mesh networks (WMNs), network-wide broadcasts (NWBs) are a fundamental operation, required by routing and other mechanisms that distribute information to all nodes in the network. However, due to the characteristics of wireless communication, NWBs are generally problematic. Optimizing them is thus a prime target when improving the overall performance and dependability of WMNs. Most of the existing optimizations neglect the real nature of WMNs and are based on simple graph models, which provide optimistic assumptions of NWB dissemination. On the other hand, models that fully consider the complex propagation characteristics of NWBs quickly become unsolvable due to their complexity. In this paper, we present the Monte Carlo method probabilistic breadth-first search (PBFS) to approximate the reachability of NWB protocols. PBFS simulates individual NWBs on graphs with probabilistic edge weights, which reflect link qualities of individual wireless links in the WMN, and estimates reachability over a configurable number of simulated runs. This approach is not only more efficient than existing ones, but further provides additional information such as the distribution of path lengths. Furthermore, it is easily extensible to NWB schemes other than flooding. The applicability of PBFS is validated both theoretically and empirically, in the latter by comparing reachability as calculated by PBFS and measured in a real-world WMN. Validation shows that PBFS quickly converges to the theoretically correct value and approximates the behaviour of real-life testbeds very well. The feasibility of PBFS to support research on NWB optimizations or higher level protocols that employ NWBs is demonstrated in two use cases.
Categories: Publication, Research and Education
Tags: Distributed computing, Monte Carlo methods, Network, Network-wide broadcasts, Probabilistic network graphs, Wireless mesh networks
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Monday, December 9, 2013, 16:11
In service networks, discovery plays a crucial role as a layer where providers can be published and enumerated. This work focuses on the responsiveness of the discovery layer, the probability to operate successfully within a deadline, even in the presence of faults. It proposes a hierarchy of stochastic models for decentralized discovery and uses it to describe the discovery of a single service using three popular protocols. A methodology to use the model hierarchy in wireless mesh networks is introduced. Given a pair requester and provider, a discovery protocol and a deadline, it generates specific model instances and calculates responsiveness. Furthermore, this paper introduces a new metric, the expected responsiveness distance der, to estimate the maximum distance from a provider where requesters can still discover it with a required responsiveness. Using monitoring data from the DES testbed at Freie Universität Berlin, it is shown how responsiveness and der of the protocols change depending on the position of nodes and the link qualities in the network.
Categories: Publication, Research and Education
Tags: Availability, Client-server systems, Dependability, Dissertation, Distributed computing, Fault tolerance, IP, Markov Models, Network, Real time systems, Responsiveness, Service discovery, SOA, UDP, Wireless mesh networks, Zeroconf
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