Browsing by Author "Hähner, Jörg"

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Open Access
Consistent data replication in mobile ad hoc networks
(2007) Hähner, Jörg; Rothermel, Kurt (Prof. Dr.)
Mobile ad-hoc networks (MANETs) are used in situations where networks need to be deployed immediately but no network infrastructure is available. If MANET nodes have sensing capabilities, they can capture and communicate the state of their surroundings, including environmental conditions or objects in their proximity. If the sensed state information is propagated to a database to build a consistent model of the real world, a variety of promising context aware applications becomes possible. The models and concepts proposed in this dissertation can be applied to cooperatively maintain a model of the state of physical world objects on devices in MANETs. State information may be updated by independent observers either sequentially or concurrently. Applications that read the state of any object from the model multiple times can rely on the guarantee that every successive read operation will read either the same state information or newer state information that has been reported by an observer after the previously read information. The first contribution of this dissertation formalizes these requirements and defines a novel consistency model called update-linearizability. Secondly, it introduces a new class of data replication algorithms that provably guarantees update-linearizability in MANETs without using synchronized clocks on any pair of nodes in the system. The presented algorithms allow executing read and write operations at any time, which provides high availability of data. These properties are even maintained in networks that are temporarily partitioned and where nodes are highly mobile. Finally the dissertation provides a proof that all replicas held in the system eventually converge towards the most recent state information of the physical world objects which they represent.
Open Access
Consistent update diffusion in mobile ad hoc networks
(2002) Rothermel, Kurt; Becker, Christian; Hähner, Jörg
Applications of mobile ad hoc networks (MANETs) occur in situations, where networks need to be deployed immediately but network infrastructures are not available. If MANET nodes have sensing capabilities, they can capture and communicate state of their surroundings, including environmental conditions or other nodes in proximity. If the sensed state information is propagated and collected in a database, this allows for a variety of promising automatic monitoring, tracking and navigation applications, using global state information to built up models of reality. Since state changes represent events happening in reality, applications are typically interested to see the most recent state. Also the order of state changes should be consistent with the corresponding order of events in reality. In particular, preserving consistency becomes a challenging research problem if there are multiple nodes sensing the same object, either subsequently or even concurrently. In this paper, we introduce a generic model of state propagation in MANETs and propose two consistency levels for this model. For each of these consistency levels, we define a state propagation algorithm based on information diffusion. Our simulations show, that for typical scenarios the additional synchronization overhead for achieving the proposed consistency levels is low. In terms of communication overhead and state propagation latency one of the proposed algorithms shows a similar performance as the underlying flooding mechanism.