Arbeitsgruppe Echtzeitsysteme und Kommunikation

Drahtlose Mesh-Netze erlauben die flexible Vernetzung großer Gelände und die Anbindung von Fabrikanlagen. Um die zur Anlagensteuerung erforderlichen Echtzeitgarantien durchsetzen zu können, muss das Netzwerk auf mehreren Schichten darauf abgestimmt sein. Dabei müssen Komponenten unterschiedlicher Schichten zusammenarbeiten und Informationen austauschen, um Ausfälle von Stationen und Veränderungen der Verbindungsqualität schnell zu erkennen und angemessen zu reagieren.

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Quality of Service for wireless mesh networks is an often requested feature for various kinds of applications. A common approach is the hop-by-hop reservation of bandwidth for individual routes. In this paper we address the problems of the reservation on a single hop. In previous works we used simulation studies to show that various existing approaches suffer from inconsistencies that lead to admission failures. In this paper, we discuss the reasons for these failures and present a protocol for preventing them. This allows to significantly increase the reliability of established communication links in WMNs.

@inproceedings{cite:Herms2008,
  author = {Andre Herms and Georg Lukas},
  title = {Preventing Admission Failures of Bandwidth Reservation in Wireless Mesh Networks},
  booktitle = {Proceedings of Workshop on Radio Resource Management in Wireless Mesh Networks in The 6th ACS/IEEE International Conference on Computer Systenms and Applications (AICCSA'08)},
  year = {2008},
  pages = {1094--1099},
  url = {http://ivs.cs.uni-magdeburg.de/EuK/forschung/publikationen/pdf/2008/RRMinMesh_2_Herms.pdf}
}

Wireless Mesh Networks (WMNs) are gaining popularity in many application areas because of their low cost and high flexibility. Technically, a WMN forms a distributed network-centric system which aims to provide communication services to the application layer, but has no built-in dependability provisions. To make the network dependable, many problems have to be solved on different layers. With this paper, we describe our ongoing work to provide an integrated solution to increase the dependability of WMNs. Our approach combines network coverage planning on the physical layer, bandwidth management on the link layer and live network monitoring to improve the reliability, availability and maintainability of a WMN. We provide fine-grained means to improve the predictability of the network components, thus making the WMN more dependable. In this paper, we present first results of our work, and describe how they are interleaved.

@inproceedings{cite:LHIN2008,
  author = {Georg Lukas and Andr\'e Herms and Svilen Ivanov and Edgar Nett},
  title = {An Integrated Approach for Reliability and Dependability of Wireless Mesh Networks},
  booktitle = {13th IEEE Workshop on Dependable Parallel, Distributed and Network-Centric Systems DPDNS '08},
  year = {2008},
  url = {http://ivs.cs.uni-magdeburg.de/EuK/forschung/publikationen/pdf/2008/dependable_wmn_camera_ready.pdf}
}

@inproceedings{cite:Lukas2008,
  author = {Georg Lukas AND Prof. Dr. Edgar Nett AND Svilen Ivanov},
  title = {{Distributed Event Synchronization for Diagnosing Wireless Mesh Networks}},
  booktitle = {{Proceedings of the International Workshop on Dependable Network Computing and Mobile Systems, DNCMS'08}},
  year = {2008},
  url = {http://ivs.cs.uni-magdeburg.de/EuK/forschung/publikationen/pdf/2008/dncms08-enett.pdf}
}

Wireless network research in the last years is often based on simulation.Ns-2 is a widely used wireless network simulation tool for this purpose.However, there are no published results about the accuracy of the ns-2 wireless model in the literature so far. In this paper we present a validation of the wireless model of ns-2 done by comparing the network characteristics of a simulated, an emulated, and a real wireless network. In order to show only the relevant differences, we have calibrated the radio propagation model of ns-2 to the real network and have used the same routing protocol implementation and the same application data traffic in all the compared networks. The results show that the packet delivery ratios, the connectivity graphs,and the packet latencies are represented in the model with an average error of 0.3 10 and 57% respectively. Based on these results we conclude that the packet delivery ratios, and network topologies are accurately represented in ns-2, once the simulation parameters are properly adjusted. The accuracy of the packet latencies is lower and therefore statements about latencies in the real network based on the simulation results have a lower validity. Based on these results we provide recommendations for future development of the ns-2.

@inproceedings{cite:Ivanov2007,
  author = {Svilen Ivanov and Andr\'e Herms and Georg Lukas},
  title = {Experimental Validation of the ns-2 Wireless Model using Simulation, Emulation, and Real Network},
  booktitle = {Proceedings of the 4th Workshop on Mobile Ad-Hoc Networks (WMAN'07)},
  publisher = {VDE Verlag},
  year = {2007},
  pages = {433 -- 444},
  url = {http://ivs.cs.uni-magdeburg.de/EuK/forschung/publikationen/pdf/2007/wman07magdeburg.pdf}
}

In this paper we present a QoS routing protocol for IEEE-802.11 based mesh networks.The main challenge for providing QoS in terms of bandwidth and latency is that the medium is shared between all nodes in close range, which complicates reservation of medium time. Furthermore, the use of standard compliant hardware components requires an integration of the existing medium access mechanism, which is designed for best-effort communication only. A cluster-based structure is used for representing the local domains of the shared medium and allowing reservation of medium time. On top of this reservation an optimistic reactive algorithm is used for discovery and reservation of routes that fulfil the application specified QoS requirements. Simulation results are presented that prove the correctness of this approach.

@inproceedings{cite:Herms2007a,
  author = {Andr\'e Herms and Georg Lukas},
  title = {A Pragmatic Approach for {QoS} in Wireless Multihop Networks},
  booktitle = {International Conference on Wireless Information Networks and Systems -- {WinsSys} '07, Proceedings of},
  year = {2007},
  url = {http://ivs.cs.uni-magdeburg.de/EuK/forschung/publikationen/pdf/2007/qos_clusters.pdf}
}

@inproceedings{cite:Herms2007b,
  author = {Andr{\'e} Herms and Stefan Schemmer and Georg Lukas},
  title = {{Echtzeitfähigkeit für Mesh-Netzwerke in der Automatisierung}},
  booktitle = {Proceedings of {SPS/IPC/DRIVES, Elektrische Automatisierung, Systeme und Komponenten} '07},
  year = {2007},
  url = {http://ivs.cs.uni-magdeburg.de/EuK/forschung/publikationen/pdf/2007/sps2007-schemmer_v1.pdf}
}

Communication in wireless mesh networks based on the IEEE~802.11 WLAN standard is mainly governed by the carrier sensing based medium access. Knowledge about, which nodes influence each other, can improve the performance and is essential for QoS provision in terms of bandwidth guarantees. However, until now only approximations for the determination of station in carrier sense range are used. We present an exact solution by measuring the carrier sense in static wireless mesh networks. Simulation studies and measurements are done that verify the correctness of the protocol and reveal some significant properties of the carrier sense. It is shown that the carrier sense relation is neither strict nor symmetric in the general case, in opposite to the assumptions normally found in literature. We further conclude that for evaluation better simulation models are required that match these properties.

@inproceedings{cite:Herms2007,
  author = {Andr\'e Herms and Georg Lukas and Svilen Ivanov},
  title = {Measurement-Based Detection of Interfering Neighbors for QoS in Wireless Mesh Networks},
  booktitle = {16th IST Mobile and Wireless Communications Summit 2007, Proceedings of},
  year = {2007},
  url = {http://ivs.cs.uni-magdeburg.de/EuK/forschung/publikationen/pdf/2007/interf-mobilesummit2007.pdf},
  doi = {10.1109/ISTMWC.2007.4299081}
}

Wireless mesh networks suffer from various problems like congestion or packet collisions. To identify and overcome these problems an exact global view of the communication is required. However, it is not possible to observe the whole network from a single location. Instead, a distributed monitoring is necessary, which has to include clock synchronization. We present a new interval-based algorithm for the off-line synchronization of passively monitored network events. It calculates the worst-case time interval for every event on a global clock, while considering inaccuracies caused by processing jitter and non-uniform clock drifts. The experimental evaluation on a live mesh network shows an accuracy of better than $130Âµs$ over a four-hop distance, which is below the minimum transmission time of data packets. Thereby, our algorithm creates a highly precise global view of the network, which allows a detailed diagnosis of wireless mesh networks.

@inproceedings{cite:Perf2007,
  author = {Georg Lukas AND Andr\'e Herms AND Daniel Mahrenholz},
  title = {Interval based Off-Line Clock Synchronization for Wireless Mesh Networks},
  booktitle = {Proceedings of the International Symposium on Computer Performance, Modeling, Measurements, and Evaluation},
  publisher = {ACM},
  year = {2007},
  volume = {35},
  url = {http://ivs.cs.uni-magdeburg.de/EuK/forschung/publikationen/pdf/2007/2007_performance-offlinesync.pdf}
}

In this paper we consider the problem of reliable communication in wireless routing protocols. Many routing protocols for wireless networks use assumptions, that are known to be invalid in this special kind of networks. These shortcomings are not detected in the protocol evaluation, because common simulation tools are also based on these assumptions. In this paper we present a more realistic simulation model and a simple way of enabling existing protocols to handle real conditions of wireless propagation. A proof-of-concept implementation is presented and evaluated using simulation and measurements in a real and emulated network.

@inproceedings{cite:awds06,
  author = {Andr\'e Herms and Georg Lukas and Svilen Ivanov},
  title = {Realism in Design and Evaluation of Wireless Routing Protocols},
  booktitle = {Proceedings of First international Workshop on Mobile Services and Personalized Environments (MSPE'06)},
  year = {2006},
  volume = {P-102},
  pages = {57--70},
  url = {http://ivs.cs.uni-magdeburg.de/EuK/forschung/publikationen/pdf/2006/awds-mspe.pdf}
}

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Projekte : Drahtlose Mesh-Infrastruktur für QoS-Anwendungen

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