Publications

Conference Papers

Thomas Kohler, Frank Dürr, Kurt Rothermel: ZeroSDN: A Highly Flexible and Modular Architecture for Full-range Network Control Distribution. In Proceedings of the 13th ACM/IEEE Symposium on Architectures for Networking and Communications Systems (ANCS 2017), May 2017, DOI: 10.1109/ANCS.2017.13. [show abstract]

Recent years have seen an evolution of SDN control plane architectures, starting from simple monolithic controllers, over modular monolithic controllers, to distributed controllers. We observe, however, that today’s distributed controllers still exhibit inflexibility with respect to the distribution of control logic. Therefore, we propose a novel architecture of a distributed SDN controller in this paper, providing maximum flexibility with respect to distribution. Our architecture splits control logic into light-weight control modules, called controllets, based on a micro-kernel approach, reducing common controllet functionality to a bare minimum and factoring out all higher-level functionality. Light-weight controllets also allow for pushing control logic onto switches and enable local processing of data plane events to minimize latency and communication overhead. Controllets are interconnected through a message bus supporting the publish/subscribe communication paradigm with specific extensions for content-based OpenFlow message filtering. Publish/subscribe allows for complete decoupling of controllets to further facilitate control plane distribution. We evaluate both, the scalability and performance properties of our architecture, including its deployment on a White Box networking hardware switch.

Christoph Dibak, Andreas Schmidt, Frank Dürr, Bernard Haasdonk, Kurt Rothermel: Server-assisted Interactive Mobile Simulations for Pervasive Applications. In Proceedings of the 15th IEEE International Conference on Pervasive Computing and Communications (PerCom 2017), May 2017, best paper award, DOI: 10.1109/PERCOM.2017.7917857. [show abstract]

Currently, various hardware and software companies are developing augmented reality devices, most prominently Microsoft with its Hololens. Besides gaming, such devices can be used for serious pervasive applications, like interactive mobile simulations to support engineers in the field. Interactive simulations have high demands on resources, which the mobile device alone is unable to satisfy. Therefore, we propose a framework to support mobile simulations by distributing the computation between mobile device and a remote server. For the computation of parameter-dependent solutions of the simulation, we use the reduced basis method, which allows to drastically reduce the computation time and energy consumption. We present three approaches for the distributed execution of the reduced basis method between mobile device and server. Evaluations show that we can speed-up the numerical computation to over 131 times while using 73 times less energy compared to offloading everything to a server.

Ben Carabelli, Rainer Blind, Frank Dürr, Kurt Rothermel: State-dependent Priority Scheduling for Networked Control Systems. In Proceedings of the 2017 American Control Conference (ACC 2017), May 2017. [show abstract]

Networked control systems (NCS) have attracted considerable attention in recent years. While the stabilizability and optimal control of NCS for a given communication system has already been studied extensively, the design of the communication system for NCS has recently seen an increase in more thorough investigation. In this paper, we address an optimal scheduling problem for a set of NCS sharing a dedicated communication channel, providing performance bounds and asymptotic stability. We derive a suboptimal scheduling policy with dynamic state-based priorities calculated at the sensors, which are then used for stateless priority queuing in the network, making it both scalable and efficient to implement on routers or multi-layer switches. These properties are beneficial towards leveraging existing IP networks for control, which will be a crucial factor for the proliferation of wide-area NCS applications. By allowing for an arbitrary number of concurrent transmissions, we are able to investigate the relationship between available bandwidth, transmission rate, and delay. To demonstrate the feasibility of our approach, we provide a proof-of-concept implementation of the priority scheduler using real networking hardware.

Florian Berg, Frank Dürr, Kurt Rothermel: Increasing the Efficiency of Code Offloading in n-tier Environments with Code Bubbling. In Proceedings of the 13th Annual International Conference on Mobiquitous Systems: Computing, Networking and Services (Mobiquitous 2016), pp. 170–179, November 2016, best paper award, DOI: 10.1145/2994374.2994375. [show abstract]

Code offloading strives for increasing the energy efficiency and execution speed of mobile applications on resource-constrained mobile devices. First approaches considered only a code offloading between two (or three) tiers, executing code either locally on the mobile device or remotely on a powerful server in the vicinity or in a distant cloud. However, new execution environments comprise multiple tiers, containing highly distributed heterogeneous resources. We present in this paper our Code Bubbling Offload System (CoBOS). CoBOS targets n-tier environments containing highly distributed heterogeneous resources with different performance characteristics and cost implications. In such n-tier environments, it is very costly for a resource-constrained mobile device to gather a global view on available resources. As a result, we propose the novel concept of code bubbling. Code bubbling moves code dynamically and adaptively towards more powerful and more distant tiers, enabling an efficient and scalable code offloading in n-tier environments. Each tier makes autonomous decisions to execute code in the tier or forward it further to the next tier. To support such a recursive escalation of code along autonomous tiers, CoBOS offloads self-contained offload requests that possess all of the required information for the processing. Our real-world evaluation shows that CoBOS decreases the energy consumption by 77 % and the execution time by 83 % for code offloading in n-tier environments.

Naresh Nayak, Frank Dürr, Kurt Rothermel: Time-sensitive Software-defined Network (TSSDN) for Real-time Applications. In Proceedings of the 24th International Conference on Real-Time Networks and Systems (RTNS 2016), October 2016, DOI: 10.1145/2997465.2997487. [show abstract]

Cyber-physical systems (CPS), like the ones used in industrial automation systems, are highly time-sensitive applications demanding zero packet losses along with stringent real-time guarantees like bounded latency and jitter from the underlying network for communication. With the proliferation of IEEE 802.3 and IP networks, there is a desire to use these networks instead of the currently used field-buses for time-sensitive applications. However, these networking technologies, which originally were designed to provide best effort communication services, lack mechanisms for providing real-time guarantees. In this paper, we present Time-Sensitive Software-Defined Networks (TSSDN), which provide real-time guarantees for the time-triggered traffic in time-sensitive systems while also transporting non-time-sensitive traffic. TSSDN provides these guarantees by bounding the non-deterministic queuing delays for time-sensitive traffic. To this end, it exploits the logical centralization paradigm of software-defined networking to compute a transmission schedule for time-sensitive traffic initiated by the end systems based on a global view. In particular, we present various Integer Linear Program (ILP) formulations that solve the combined problem of routing and scheduling time-triggered traffic. Moreover, we show that end systems can comply with a given schedule with high precision using user-space packet processing frameworks. Our evaluations show that TSSDN has deterministic end-to-end delays (less than 14 micro-seconds on our benchmark topology) with low and bounded jitter (less than 7 mico-seconds).

Frank Dürr, Naresh Nayak: No-wait Packet Scheduling for IEEE Time-sensitive Networks (TSN). In Proceedings of the 24th International Conference on Real-Time Networks and Systems (RTNS 2016), October 2016, DOI: 10.1145/2997465.2997494. [show abstract]

The IEEE Time-sensitive Networking (TSN) Task Group has recently standardized enhancements for IEEE 802.3 networks for enabling it to transport time-triggered traffic (aka scheduled traffic) providing them with stringent bounds on network delay and jitter while also transporting best-effort traffic. These enhancements primarily include dedicating one queue per port of the switch for scheduled traffic along with a programmable gating mechanism that dictates which of the queues are to be considered for transmission. While the IEEE 802.1Qbv standards define these mechanisms to handle scheduled traffic, it stops short of specifying algorithms to compute fine-grained link schedules for the streams of scheduled traffic. Further, the mechanisms in TSN require creation of so-called guard bands to isolate scheduled traffic from the best-effort traffic. These guard bands may potentially result in bandwidth wastage, and hence schedules with lower number of guard bands are preferred. In this paper, we introduce the No-wait Packet Scheduling Problem (NW-PSP) for modelling the scheduling in IEEE Time-sensitive Networks and map it to the No-wait Job-shop Scheduling Problem (NW-JSP), a well-known problem from the field of operational research. In particular, we present a Tabu search algorithm for efficient computing of schedules and a schedule compression technique to reduce number of guard bands in schedule. Our evaluations show that our Tabu search algorithm can compute near-optimal schedules for over 1500 flows and the subsequent schedule compression reduces the number of guard bands on an average by 24 %.

Zohaib Riaz, Frank Dürr, Kurt Rothermel: On the Privacy of Frequently Visited User Locations. In Proceedings of the Seventeenth International Conference on Mobile Data Management (MDM 2016), June 2016, DOI: 10.1109/MDM.2016.49. [show abstract]

With the fast adoption of location-enabled devices, Location-based Applications (LBAs) have become widely popular. While LBAs enable highly useful concepts such as geo-social networking, their use also raises serious privacy concerns as it involves sharing of location data with non-trusted third parties. In this respect, we propose an approach that protects the frequently visited locations of users, e.g., a bar, against inferences from longterm monitoring of their location data. Such inferences equate a privacy leak as they reveal a user’s personal behavior and interests to possibly malicious non-trusted parties. To this end, we first present a study of a dataset of location check-ins to show the existence of this threat among users of LBAs. We then propose our approach to protect visit-frequency of the users to different locations by distributing their location data among multiple third-party Location Servers. This distribution not only serves to avoid a single point of failure for privacy in our system, it also allows the users to control which LBA accesses what information about them. We also describe a number of possible attacks against our privacy approach and evaluate them on real-data from the check-ins dataset. Our results show that our approach can effectively hide the frequent locations while supporting good quality-of-service for the LBAs.

Stephan Schnitzer, Simon Gansel, Frank Dürr, Kurt Rothermel: Real-time Scheduling for 3D GPU Rendering. In Proceedings of the Eleventh IEEE Symposium on Industrial Embedded Systems (SIES 2016), pp. 1–10, May 2016, DOI: 10.1109/SIES.2016.7509411. [show abstract]

3D graphical functions in cars enjoy growing popularity. For instance, analog instruments of the instrument cluster are replaced by digital 3D displays as shown by Mercedes-Benz in the F125 prototype car. The trend to use 3D applications expands into two directions: towards more safety-relevant applications such as the speedometer and towards third-party applications, e.g., from an app store. In order to save cost, energy, and installation space, all these applications should share a single GPU. GPU sharing brings up the problem of providing real-time guarantees for rendering content of time-sensitive applications like the speedometer. To solve this problem, we present a real-time GPU scheduling framework which provides strong guarantees for critical applications while still giving as much GPU resources to less important applications as possible, thus ensuring a high GPU utilization. Since current GPUs are not preemptible, we use the estimated execution time of each GPU rendering job to make the scheduling decisions. Our evaluations show that our scheduler guarantees given real-time constraints, while achieving a high GPU utilization of 97 %. Moreover, scheduling is performed highly efficient in real-time with less than 10 micro-seconds latency.

Simon Gansel, Stephan Schnitzer, Ahmad Gilbeau-Hammoud, Viktor Friesen, Frank ürr, Kurt Rothermel, Christian Maihöfer, Ulrich Krämer: Context-aware Access Control in Novel Automotive HMI Systems. In Information Systems Security – Proceedings of 11th International Conference on Information Systems Security (ICISS 2015), Lecture Notes in Computer Science (LNCS) 9478, pp. 118–138, Springer International Publishing, December 2015, DOI: 10.1007/978-3-319-26961-0_8. [show abstract]

The growing relevance of vehicular applications like media player, navigation system, or speedometer using graphical presentation has lead to an increasing number of displays in modern cars. This effectuates the desire for flexible sharing of all the available displays between several applications. However, automotive requirements include many regulations to avoid driver distraction to ensure safety. To allow for safe sharing of the available screen surface between the many safety-critical and non-safety-critical applications, adequate access control systems are required. We use the notion of contexts to dynamically determine, which application is allowed to access which display area. A context can be derived from vehicle sensors (e.g., the current speed), or be an application-specific state (e.g., which menu item is selected). We propose an access control model that is inherently aware of the context of the car and the applications. It provides delegation of access rights to display areas by applications. We implemented a proof-of-concept implementation that demonstrates the feasibility of our concept and evaluated the latency introduced by access control. Our results show that the delay reacting on dynamic context changes is small enough for automotive scenarios.

Thomas Kohler, Frank Dürr, Kurt Rothermel: Update Consistency in Software-defined Networking based Multicast Networks. In Proceedings of the 2015 IEEE Conference on Network Function Virtualization and Software Defined Networks (NFV-SDN 2015), pp. 177–183, November 2015, DOI: 10.1109/NFV-SDN.2015.7387424. [show abstract]

When applying updates on distributed network elements with SDN, intra-update states may violate desired network properties, such as drop- and loop-freeness. Current stateless approaches cannot guarantee the constancy of arbitrary network invariants (correctness) in general, yet update procedures guaranteeing certain invariants do exist. In this paper, we investigate on update consistency for the case of multicast routing and show that there is no correct update procedure w.r.t. both drop- and duplicate-freeness. We show that certain updates of multicast routes inherently raise a concurrency issue, which necessarily results in the occurrence of either drops or duplicates. Furthermore, we present a generic update procedure for multicast routing updates that identifies concurrency-relevant update steps. This procedure allows for the selection of an update strategy, such that either drops or duplicates are avoided. These effects can severely degrade network performance or quality of experience. To investigate the implications of drops and duplicates, we evaluate their frequencies and impact for wide-area network scenarios both, analytically and empirically, through direct measurement in the data plane under update.

Naresh Nayak, Frank Dürr, Kurt Rothermel: Software-defined Environment for Reconfigurable Manufacturing Systems. In Proceedings of the 5th International Conference on Internet of Things (IoT 2015), pp. 122–129, October 2015, DOI: 10.1109/IOT.2015.7356556. [show abstract]

Conventional manufacturing systems like assembly lines cannot handle the constantly changing requirements of a modern-day manufacturer, which are driven by volatile market demands. In a bid to satisfy such requirements, modern manufacturing systems, comprising innumerable cyber-physical systems (CPS), aim to be reconfigurable. CPS implement production processes through an ICT infrastructure networked with sensors and actuators embedded in the shop floor. Reconfigurability, in context of manufacturing systems, must include the entire system of networked components and hence requires a flexible ICT infrastructure. Providing flexible ICT infrastructures, often, comes at the cost of diluted quality of service (QoS) guarantees. This, however, is not an option for manufacturing systems, most of which require strict QoS guarantees to function correctly. To overcome this obstacle, we propose a new software-defined environment (SDE) for reconfigurable manufacturing systems with real-time properties in this paper. Software-defined environment is an emerging technology that provides flexible ICT infrastructures modifiable using software. Our contributions include an SDE-based system architecture for dynamically configuring the underlying infrastructure for a manufacturing system. In particular, we focus on configuring the network for the time-sensitive communication flows essential for realising CPS. Moreover, we propose a pair of routing algorithms to calculate routes for these flows while configuring the network.

Christoph Dibak, Frank Dürr, Kurt Rothermel: Numerical Analysis of Complex Physical Systems on Networked Mobile Devices. In Proceedings of the 12th IEEE International Conference on Mobile Ad hoc and Sensor Systems (MASS 2015), pp. 280–288, October 2015, DOI: 10.1109/MASS.2015.12. [show abstract]

Recently, a new class of mobile applications has appeared that takes into account the behavior of physical phenomenon. Prominent examples of such applications include augmented reality applications visualizing physical processes on a mobile device or mobile cyber-physical systems like autonomous vehicles or robots. Typically, these applications need to solve partial differential equations (PDE) to simulate the behavior of a physical system. There are two basic strategies to numerically solve these PDEs: (1) offload all computations to a remote server; (2) solve the PDE on the resource-constrained mobile device. However, both strategies have severe drawbacks. Offloading will fail if the mobile device is disconnected, and resource constraints require to reduce the quality of the solution. Therefore, we propose a new approach for mobile simulations using a hybrid strategy that is robust to communication failures and can still benefit from powerful server resources. The basic idea of this approach is to dynamically decide on the placement of the PDE solver based on a prediction of the wireless link availability using Markov Chains. Our tests based on measurement in real cellular networks and real mobile devices show that this approach is able to keep deadline constraints in more than 61 % of the cases compared to a pure offloading approach, while saving up to 74 % of energy compared to a simplified approach.

Florian Berg, Frank Dürr, Kurt Rothermel: Increasing the Efficiency of Code Offloading through Remote-side Caching. In Proceedings of the 11th IEEE International Conference on Wireless and Mobile Computing, Networking and Communications (WiMob 2015), pp. 573–580, October 2015, DOI: 10.1109/WiMOB.2015.7348013. [show abstract]

End users execute today on their smart phones different kinds of mobile applications like calendar apps or high-end mobile games, differing in local resource usage. Utilizing local resources of a smart phone heavily, like playing high-end mobile games, drains its limited energy resource in few hours. To prevent the limited energy resource from a quick exhaustion, smart phones benefit from executing resource-intensive application parts on a remote server in the cloud (code offloading). During the remote execution on the remote server, a smart phone waits in idle mode until it receives a result. However, code offloading introduces computation and communication overhead, which decreases the energy efficiency and induces monetary cost. For instance, sending or receiving execution state information to or from a remote server consumes energy. Moreover, executing code on a remote server instance in a commercial cloud causes monetary cost. To keep consumed energy and monetary cost low, we present in this paper the concept of remote-side caching for code offloading, which increases the efficiency of code offloading. The remote-side cache serves as a collective storage of results for already executed application parts on remote servers, avoiding the repeated execution of previously run application parts. The smart phone queries the remote-side cache for corresponding results of resource-intensive application parts. In case of a cache hit, the smart phone gets immediately a result and continues the application execution. Otherwise, it migrates the application part and waits for a result of the remote execution. We show in our evaluation that the use of a remote-side cache decreases energy consumption and monetary cost for mobile applications by up to 97% and 99%, respectively.

Simon Gansel, Stephan Schnitzer, Riccardo Cecolin, Frank Dürr, Kurt Rothermel, Christian Maihöfer: Efficient Compositing Strategies for Automotive HMI Systems. In Proceedings of the 10th IEEE International Symposium on Industrial Embedded Systems (SIES 2015), pp. 1–10, June 2015, DOI: 10.1109/SIES.2015.7185036. [show abstract]

The relevance of graphical functions in vehicular applications has increased significantly during the last years. Modern cars are equipped with multiple displays used by different applications such as speedometer, navigation system, or media players. The rendered output of the applications is stored in so-called off-screen buffers and then bitblitted to the screen buffer at the respective window sizes and positions. To guarantee the visibility of the potentially overlapping windows, the compositing has to match the z-order of the windows. To this end, two common compositing strategies Tile compositing and Full compositing are used, each having performance issues depending on how windows overlap. Since automotive embedded platforms are restricted in power consumption, installation space, and hardware cost, their performance is limited which effectuates the need for highly efficient bitblitting. In order to increase the performance in compositing the windows, we propose Hybrid Compositing which predicts the required bitblitting time and chooses the most efficient strategy for each pair of overlapping windows. Using various scenarios we show that our approach is faster than the other strategies. In addition, we propose CacheHybrid Compositing which reduces the CPU execution time of our approach by up to 66 %. In case of an automotive scenario we show that our optimized approach saves up to 51 % bitblitting time compared to existing approaches.

Zohaib Riaz, Frank Dürr, Kurt Rothermel: Optimized Location Update Protocols for Secure and Efficient Position Sharing. In Proceedings of the 2nd International Conference on Networked Systems (NetSys 2015), pp. 1–8, March 2015, DOI: 10.1109/NetSys.2015.7089083. [show abstract]

Although location-based applications have seen fast growth in the last decade due to pervasive adoption of GPS enabled mobile devices, their use raises privacy concerns. To mitigate these concerns, a number of approaches have been proposed in literature, many of which rely on a trusted party to regulate user privacy. However, trusted parties are known to be prone to data breaches [1]. Consequently, a novel solution, called Position Sharing, was proposed in [2] to secure location privacy in fully non-trusted systems. In Position Sharing, obfuscated position shares of the actual user location are distributed among several location servers, each from a different provider, such that there is no single point of failure if the servers get breached. While Position Sharing can exhibit useful properties such as graceful degradation of privacy, it incurs significant communication overhead as position shares are sent to several location servers instead of one. To this end, we propose a set of location update protocols to minimize the communication overhead of Position Sharing while maintaining the privacy guarantees that it originally provided. As we consider the scenario of frequent location updates, i.e., movement trajectories, our protocols additionally add protection against an attack based on spatio-temporal correlation in published locations. By evaluating on a set of real-world GPS traces, we show that our protocols can reduce the communication overhead by 75% while significantly improving the security guarantees of the original Position Sharing algorithm.

Florian Berg, Frank Dürr, Kurt Rothermel: Optimal Predictive Code Offloading. In Proceedings of the 11th International Conference on Mobile and Ubiquitous Systems: Computing, Networking and Services (Mobiquitous 2014), pp. 1–10, December 2014, DOI: 10.4108/icst.mobiquitous.2014.258023. [show abstract]

Modern mobile devices like smart phones and tablets are equipped with powerful processing and memory resources, enabling resource-intensive mobile applications such as high-end mobile games. The main limitation, however, remains the energy resource. To improve the energy efficiency, code offloading has been proposed, which offloads code to remote servers and transfers the results back to the mobile device. Although several approaches have shown that code offloading improves energy efficiency significantly in general, they largely neglect the adverse effects of network disconnections. Therefore, we have proposed the concept of preemptive code offloading to improve energy efficiency also under link failures. It transmits so-called safe-points between server and mobile device during remote execution, enabling the re-use of partial remote results after link failures. In this paper, we improve our basic preemptive code offloading approach by optimizing the time when to generate and transmit safe-points to minimize the communication overhead and maximize energy efficiency. To find the optimal safe-point schedule, we use a predictive approach that predicts the mobile link quality in order to send safe-points before network disconnections. Moreover, we consider additional deadline constraints for code execution to ensure a minimal responsiveness of offloaded applications despite link failures. Our evaluation results show that energy efficiency can be improved significantly using our predictive offloading approach.

Simon Gansel, Stephan Schitzer, Ahmad Gilbeau-Hammoudy, Viktor Friesen, Frank Dürr, Kurt Rothermel, Christian Maihöfer: An Access Control Concept for Noval Automotive HMI Systems. In Proceedings of the 19th ACM Symposium on Access Control Models and Technologies (SACMAT 2014), pp. 17–28, June 2014, DOI: 10.1145/2613087.2613104. [show abstract]

The relevance of graphical functions in vehicular applications has increased significantly during the few last years. Modern cars are equipped with multiple displays used by different applications such as speedometer or navigation system. However, so far applications are restricted to using dedicated displays. In order to increase flexibility, the requirement of sharing displays between applications has emerged. Sharing displays leads to safety and security concerns since safety-critical applications as the dashboard warning lights share the same displays with uncritical or untrusted applications like the navigation system or third-party applications. To guarantee the safe and secure sharing of displays, we present a formal model for defining and controlling the access to display areas in this paper. We prove the validity of this model, and present a proof-of-concept implementation to demonstrate the feasibility of our concept.

Stephan Schnitzer, Simon Gansel, Frank Dürr, Kurt Rothermel: Concepts for Execution Time Prediction of 3D GPU Rendering. In Proceedings of the 9th IEEE International Symposium on Industrial Embedded Systems (SIES 2014), pp. 160–169, June 2014, DOI: 10.1109/SIES.2014.6871200. [show abstract]

The relevance of graphical functions in vehicular applications has increased significantly during the last years. Modern cars are equipped with multiple displays used by different applications such as speedometer, navigation system, or media players. The recent trend towards hardware consolidation to reduce hardware cost, installation space, and energy consumption, causes graphical 3D applications of different safety-criticality to share a single GPU. This requires effective real-time GPU scheduling concepts to ensure safety and isolation for 3D rendering. Since current GPUs are not preemptible, a deadline-based scheduler must know the GPU execution time of GPU commands in advance. In this work, we present a novel framework to measure and predict the execution time of GPU commands using OpenGL ES 2.0. We present prediction models for the main GPU commands relevant for 3D rendering, namely, FLUSH, CLEAR, and DRAW. For the DRAW command we propose to use the 3D bounding box of the rendered model and apply the vertex shader projection to heuristically estimate the number of fragments rendered. We finally present the implementation and evaluation of our framework, which demonstrates its feasibility and shows that good prediction accuracy can be achieved. In our evaluation using realistic scenarios the absolute prediction error of the DRAW command did not exceed 260 microseconds.

Florian Berg, Frank Dürr, Kurt Rothermel: Increasing the Efficiency and Responsiveness of Mobile Applications with Preemptable Code Offloading. In Proceedings of the 3rd IEEE International Conference on Mobile Services (MS 2014), pp. 76–83, June 2014, DOI: 10.1109/MobServ.2014.20. [show abstract]

Mobile applications are getting more and more sophisticated and demanding. Although the processing, memory, and storage resources of mobile devices are constantly increasing to enable such resource-demanding mobile applications, battery capacity is still the main limiting factor. To solve this problem, mobile code offloading approaches can be used to offload parts of a mobile application to remote servers and utilize the resources of cloud services. In this paper, we propose a novel code offloading approach that makes code offloading robust against communication link failures, which are still a major problem of mobile systems. To this end, we propose preemptable code offloading. It allows for interrupting the offloading process and continuing the remote execution locally after a link failure, without abandoning the complete result calculated remotely so far. The basic idea of our approach is to create safe-points of the remote execution and transmit these intermediate results back to the mobile device. After a link failure, the mobile device can now continue execution from the last transmitted safe-point. Although safe-points induce communication and energy overhead, our evaluations show that using an optimized safe-point schedule this overhead quickly pays off under link failures. Besides reducing the overall energy consumption significantly, responsiveness also benefits from safe-points by meeting given execution deadlines after link failures.

Damian Philipp, Patrick Baier, Christoph Dibak, Frank Dürr, Kurt Rothermel, Susanne Becker, Michael Peter, Dieter Fritsch: MapGENIE: Grammar-enhanced Indoor Map Construction from Crowd-sourced Data. In Proceedings of the 12th IEEE International Conference on Pervasive Computing and Communications (PerCom 2014), pp. 139–147, March 2014, DOI: 10.1109/PerCom.2014.6813954. [show abstract]

While location-based services are already well established in outdoor scenarios, they are still not available in indoor environments. The reason for this can be found in two open problems: First, there is still no off-the-shelf indoor positioning system for mobile devices and, second, indoor maps are not publicly available for most buildings. While there is an extensive body of work on the first problem, the efficient creation of indoor maps remains an open challenge. We tackle the indoor mapping challenge in our MapGENIE approach that automatically derives indoor maps from traces collected by pedestrians moving around in a building. Since the trace data is collected in the background from the pedestrians’ mobile devices, MapGENIE avoids the labor-intensive task of traditional indoor map creation and increases the efficiency of indoor mapping. To enhance the map building process, MapGENIE leverages exterior information about the building and uses grammars to encode structural information about the building. Hence, in contrast to existing work, our approach works without any user interaction and only needs a small amount of traces to derive the indoor map of a building. To demonstrate the performance of MapGENIE, we implemented our system using Android and a foot-mounted IMU to collect traces from volunteers. We show that using our grammar approach, compared to a purely trace-based approach we can identify up to four times as many rooms in a building while at the same time achieving a consistently lower error in the size of detected rooms.

Marius Wernke, Frank Dürr, Kurt Rothermel: Protecting Movement Trajectories through Fragmentation. In Mobile and Ubiquitous Systems: Computing, Networking and Services – Proceedings of the 10th International Conference on Mobile and Mobiquitous Systems (Mobiquitous 2013), pp. 303–315, December 2013, DOI: 10.1007/978-3-319-11569-6_24. [show abstract]

Location-based applications (LBAs) like geo-social networks, points of interest finders, and real-time traffic monitoring applications have entered people’s daily life. Advanced LBAs rely on location services (LSs) managing movement trajectories of multiple users in a scalable fashion. However, exposing trajectory information raises user privacy concerns, in particular if LSs are non-trusted. For instance, an attacker compromising an LS can use the retrieved user trajectory for stalking, mugging, or to trace user movement. To limit the misuse of trajectory data, we present a new approach for the secure management of trajectories on non-trusted servers. Instead of providing the complete trajectory of a user to a single LS, we split up the trajectory into a set of fragments and distribute the fragments among LSs of different providers. By distributing fragments, we avoid a single point of failure in case of compromised LSs, while different LBAs can still reconstruct the trajectory based on user-defined access rights. In our evaluation, we show the effectiveness of our approach by using real world trajectories and realistic attackers using map knowledge and statistical information to predict and reconstruct the user’s movement.

Damian Philipp, Jaroslav Stachowiak, Frank Dürr, Kurt Rothermel: Model-Driven Public Sensing in Sparse Networks. In Mobile and Ubiquitous Systems: Computing, Networking and Services – Proceedings of the 10th International Conference on Mobile and Mobiquitous Systems (Mobiquitous 2013), pp. 17–29, December 2013, DOI: 10.1007/978-3-319-11569-6_2. [show abstract]

Public Sensing (PS) is a recent trend for building large-scale sensor data acquisition systems using commodity smartphones. Limiting the energy drain on participating devices is a major challenge for PS, as otherwise people will stop sharing their resources with the PS system. Existing solutions for limiting the energy drain through model-driven optimizations are limited to dense networks where there is a high probability for every point of interest to be covered by a smartphone. In this work, we present an adaptive model-driven PS system that deals with both dense and sparse networks. Our evaluations show that this approach improves data quality by up to 41 percentage points while enabling the system to run with a greatly reduced number of participating smartphones. Furthermore, we can save up to 81% of energy for communication and sensing while providing data matching an error bound of 1°C up to 96% of the time.

Marius Wernke, Frank Dürr, Kurt Rothermel: Speed Protection Algorithms for Privacy-aware Location Management. In Proceedings of the 9th IEEE International Conference on Wireless and Mobile Computing, Networking and Communications (WiMob 2013), pp. 344–351, October 2013, DOI: 10.1109/WiMOB.2013.6673383. [show abstract]

Nowadays, millions of users share their complete movement trajectory online when using real-time traffic monitoring applications, pay-as-you-drive insurances, or when sharing their last road trip with friends. However, many users still hesitate to use location-based applications as they are not willing to reveal, for instance, their driving behavior or the occurrence of a speeding violation. Therefore, we present novel speed protection algorithms protecting users from revealing a violation of given speed limits when using location-based applications. Our algorithms support time-based and distance-based position updates. To protect positions indicating a speeding violation, we either adjust temporal information by delaying position updates or adjust their spatial information. We evaluate our algorithms by using real world traces and show that the protected movement trajectory of the user is of high quality even after removing speeding violations.

Patrick Baier, Frank Dürr, Kurt Rothermel: Efficient Distribution of Sensing Queries in Public Sensing Systems. In Proceedings of the 10th IEEE International Conference on Mobile Ad-hoc and Sensor Systems (MASS 2013), pp. 272–280, October 2013, DOI: 10.1109/MASS.2013.11. [show abstract]

The advent of mobile phones paved the way for a new paradigm for gathering sensor data termed Public Sensing (PS). PS uses built-in sensors of mobile devices to opportunistically gather sensor data. For instance, the microphones of a crowd of mobile phones can be used to capture sound samples, which can be used to construct a city noise map. A great challenge of PS is to reduce the energy consumption of mobile devices since otherwise users might not be willing to participate. One crucial part in the overall power consumption is the energy required for the communication between the mobile devices and the infrastructure. In particular, the communication required for sending sensing queries to mobile devices has been largely neglected in the related work so far. Therefore, in this paper, we address the problem of minimizing communication costs for the distribution of sensing queries. While existing systems simply broadcast sensing queries to all devices, we use a selective strategy by addressing only a subset of devices. In order not to negatively affect the quality of sensing w.r.t. completeness, this subset is carefully chosen based on a probabilistic sensing model that defines the probability of mobile devices to successfully perform a given sensing query. Our evaluations show that with our optimized sensing query distribution, the energy consumption can be reduced by more than 70% without significantly reducing the quality of sensing.

Patrick Baier, Frank Dürr, Kurt Rothermel: Opportunistic Position Update Protocols for Mobile Devices. In Proceedings of the 2013 ACM International Joint Conference on Pervasive and Ubiquitous Computing (UbiComp 2013), pp. 787–796, September 2013, DOI: 10.1145/2493432.2493439. [show abstract]

Many location-based applications such as geo-social networks rely on location services storing mobile object positions. To update positions on location servers, position update protocols are used. On the one hand, these protocols decide when an update has to be sent to ensure a certain quality of position information. On the other hand, they try to minimize the energy consumption of the mobile device by reducing communication to a minimum. In this paper, we show how to improve the energy efficiency of different update protocols by taking the energy characteristics of the mobile network interface into account. In particular, we show that the energy consumption can be reduced on average by 70% using an opportunistic update strategy sending position updates together with messages of other applications. We present a Markov model to predict the arrival of messages and an online optimization algorithm calculating an optimized schedule to send position updates.

Simon Gansel, Christian Maihöfer, Stephan Schnitzer, Frank Dürr, Kurt Rothermel: Towards Virtualization Concepts for Novel Automotive HMI Systems. In Embedded Systems: Design, Analysis and Verification – Proceedings of the 4th IFIP TC 10 International Embedded Systems Symposium (IESS 2013), pp. 193–204, Springer, June 2013, DOI: 10.1007/978-3-642-38853-8_18. [show abstract]

Many innovations in the automotive industry are based on electronics and software, which has led to a steady increase of electronic control units (ECU) in cars. This brought up serious scalability and complexity issues in terms of cost, installation space, and energy consumption. In order to tackle these problems, there is a strong interest to consolidate ECUs using virtualization technologies. However, current efforts largely neglect legal constraints and certification issues and the resulting technical requirements. In this paper, we focus on the consolidation of graphics hardware through virtualization, which received a lot of interest in the car industry due to the growing relevance of HMI systems such as head unit and instrument cluster in modern cars. First, we investigate relevant ISO standards and legal requirements and derive seven technical requirements for a virtualized automotive HMI system. Based on these requirements, we present the concept for a Virtualized Automotive Graphics System (VAGS) that allows for the consolidation of mixed-criticality graphics ECUs.

Frank Dürr: Improving the Efficiency of Cloud Infrastructures with Elastic Tandem Machines. In Proceedings of the 6th IEEE International Conference on Cloud Computing (Cloud 2013), pp. 91–98, June 2013, DOI: 10.1109/CLOUD.2013.85. [show abstract]

In this paper, we propose a concept for improving the energy efficiency and resource utilization of cloud infrastructures by combining the benefits of heterogeneous machine instances. The basic idea is to integrate low-power system on a chip (SoC) machines and high-power virtual machine instances into so-called Elastic Tandem Machine Instances (ETMI). The low-power machine serves low load and is always running to ensure the availability of the ETMI. When load rises, the ETMI scales up automatically by starting the high-power instance and handing over traffic to it. For the non-disruptive transition from low-power to high-power machines and vice versa, we present a handover mechanism based on software-defined networking technologies. Our evaluations show the applicability of low-power SoC machines to serve low load efficiently as well as the desired scalability properties of ETMIs.

Damian Philipp, Jaroslav Stachowiak, Patrick Alt, Frank Dürr, Kurt Rothermel: DrOPS: Model-Driven Optimization for Public Sensing Systems. In Proceedings of the 11th IEEE International Conference on Pervasive Computing and Communications (PerCom 2013), pp. 185–192, March 2013, DOI: 10.1109/PerCom.2013.6526731. [show abstract]

The proliferation of modern smartphones has given rise to Public Sensing, a new paradigm for data acquisition systems utilizing smartphones of mobile participants. In this paper, we present DrOPS, a system for improving the efficiency of data acquisition in Public Sensing systems. DrOPS utilizes a model-driven approach, where the number of required readings from mobile smartphones is reduced by inferring readings from the model. Furthermore, the model can be used to infer readings for positions where no sensor is available. The model is directly constructed from the observed phenomenon in an online fashion. Using such models together with a client-specified quality bound, we can significantly reduce the effort for data acquisition while still reporting data of required quality to the client. To this effect, we develop a set of online learning and control algorithms to create and validate the model of the observed phenomenon and present a sensing task execution system utilizing our algorithms in this paper. Our evaluations show that we obtain models in a matter of just hours or even minutes. Using the model-driven approach for optimizing the data acquisition, we can save up to 80% of energy for communication and provide inferred temperature readings for uncovered positions matching an error-bound of 1°C up to 100% of the time.

Marius Wernke, Frank Dürr, Kurt Rothermel: Efficient Position Sharing for Location Privacy using Binary Space Partitioning. In Mobile and Ubiquitous Systems: Computing, Networking, and Services (Proceedings of the Ninth Annual International Conference on Mobile and Ubiquitous Systems (MobiQuitous ’12)), Lecture Notes of the Institute for Computer Sciences, Social Informatics and Telecommunications Engineering 120, pp. 263–275, Springer Berlin Heidelberg, 2013, DOI: 10.1007/978-3-642-40238-8_22. [show abstract]

Millions of users use location-based applications (LBAs) to share their positions with friends, request information from points of interest finders, or get notifications from event finders, etc. Such LBAs are typically based on location servers (LSs) managing mobile object positions in a scalable fashion. However, storing precise user positions on LSs raises privacy concerns, in particular, if LS providers are non-trusted. To solve this problem, we present PShare-BSP, a novel approach for the secure management of private user positions on non-trusted LSs. PShare-BSP splits up precise user positions into position shares and distributes them to different LSs of different providers. Thus, a compromised provider only reveals user positions with degraded precision. Nevertheless, LBAs can combine several shares from different LSs to increase their precision. PShare-BSP improves on our previous position sharing approaches: It uses a deterministic share generation approach based on binary space partitioning to avoid probabilistic attacks based, for instance, on Monte Carlo simulations. Moreover, it significantly decreases the computational complexity and increases the efficiency by reducing the update costs for succeeding position updates.

Ben Carabelli, Andreas Benzing, Georg Seyboth, Rainer Blind, Mathias Bürger, Frank Dürr, Boris Koldehofe, Kurt Rothermel, Frank Allgöwer: Exact Convex Formulations of Network-Oriented Optimal Operator Placement. In Proceedings of the 51st IEEE Conference on Decision and Control (CDC 2012), December 2012, DOI: 10.1109/CDC.2012.6426790. [show abstract]

Data processing tasks are increasingly spread across the internet to account for the spatially distributed nature of many data sources. In order to use network resources efficiently, subtasks need to be distributed in the network so data can be filtered close to the data sources. Previous approaches to this operator placement problem relied on various heuristics to constrain the complexity of the problem. In this paper, we propose two generic integer constrained problem formulations: a topology aware version which provides a placement including the specific network links as well as an end-to-end delay aware version which relies on the routing capabilities of the network. A linear programming relaxation for both versions is provided which allows exact and efficient solution using common solvers.

Patrick Baier, Frank Dürr, Kurt Rothermel: TOMP: Opportunistic Traffic Offloading Using Movement Predictions. In Proceedings of the 37th IEEE Conference on Local Computer Networks (LCN 2012), pp. 50–58, October 2012, DOI: 10.1109/LCN.2012.6423668. [show abstract]

Recent forecasts predict that the amount of cellular data traffic will significantly increase within the next few years. The reason for this trend is on the one hand the high growth rate of mobile Internet users and on the other hand the growing popularity of high bandwidth streaming applications. Given the fact that cellular networks (e.g. UMTS) have only limited capacity, the existing network infrastructure will soon reach its limits. As a result, the concept of traffic offloading attracts more and more attention in research since it aims at the reduction of cellular traffic by shifting it to local-area networks like Wifi. Within the last few years, some first approaches for automatically offloading cellular traffic were proposed. These approaches either assume the wide availability of publicly accessible Wifi networks or knowledge about social relations of mobile users. However, these assumptions are usually not fulfilled. To face this issue, we developed the TOMP system. TOMP implements a system to distribute data from the infrastructure to a set of mobile devices by partly shifting traffic from the cellular network to the level of inter-device communication. In contrast to the prevailing approaches, TOMP does not rely on open Wifi networks and only uses information about the position and speed of mobile device. By using predictions about the future movement of mobile users, TOMP determines devices that are most suitable targets for traffic offloading. In this paper we show by simulation that TOMP can save up to 40% of cellular messages in comparison to a typical cellular network.

Marius Wernke, Frank Dürr, Kurt Rothermel: PShare: Position Sharing for Location Privacy based on Multi-Secret Sharing. In Proceedings of the 10th IEEE International Conference on Pervasive Computing and Communications (PerCom 2012), pp. 153–161, March 2012, DOI: 10.1109/PerCom.2012.6199862. [show abstract]

Location-based applications such as Facebook Places, Foursquare, or Loopt attract millions of users by implementing point of interest finders, friend finders, geosocial networking, etc. Typically, these applications act as clients to a location service such as Google Latitude or Yahoo Fire Eagle, which manage mobile object positions and ensure the scalability to provide various clients with mobile object positions. However, exposing precise user positions raises user privacy concerns, especially if location service providers are not fully trusted, and private position information could be lost, leaked, stolen, etc. To enable the secure management of private user positions on non-trusted location servers (LSs), we present novel position sharing approaches based on the concept of multi-secret sharing. Our approaches split up a precise user position into position shares, which are distributed to different LSs of different providers such that a compromised provider only reveals user positions with degraded precision. On the other hand, clients can combine several shares queried from different LSs to increase their provided precision without the need to store precise information at a single LS. We propose two position sharing approaches: PShare-SLM is the first position sharing approach presented so far for symbolic location models. For geometric location models, we present PShare-GLM, which improves existing geometric position sharing approaches by considering continuous position updates and by increasing the robustness against various attacks.

Patrick Baier, Frank Dürr, Kurt Rothermel: PSense: Reducing Energy Consumption in Public Sensing Systems. In Proceedings of the 26th IEEE International Conference on Advanced Information Networking and Applications (AINA 2012), pp. 136–143, March 2012, DOI: 10.1109/AINA.2012.33. [show abstract]

Utilizing peoples’ mobile devices for gathering sensor data has attracted a lot of attention within the last few years. As a result, a great variety of systems for sensing environmental phenomena like temperature or noise have been proposed. However, most of these systems do not take into account that mobile devices have only limited energy resources. For instance, an often assumed prerequisite is that mobile devices are always aware of their position. Given the fact that a position fix is a very energy consuming operation, continuous positioning would quickly drain a device’s battery. Since the owners of the mobile devices will not tolerate a significant reduction of the devices’ battery lifetime, such an approach is not suitable. To address this issue we present PSense, a flexible system for efficiently gathering sensor data with mobile devices. By avoiding unnecessary position fixes, PSense reduces the energy consumption of mobile devices by up to 70% compared to existing mobile sensing approaches. This is achieved by introducing an adaptive positioning mechanism and by utilizing energy efficient short-range communication to exchange position related information.

Pavel Skvortsov, Frank Dürr, Kurt Rothermel: Map-aware Position Sharing for Location Privacy in Non-trusted Systems. In Pervasive Computing (Proceedings of the 10th International Conference on Pervasive Computing (Pervasive 2012)), Lecture Notes in Computer Science (LNCS) 7319, pp. 388–405, Springer Berlin Heidelberg, 2012, DOI: 10.1007/978-3-642-31205-2_24. [show abstract]

Many current location-based applications (LBA) such as friend finder services use information about the positions of mobile users. So-called location services (LSs) have been proposed to manage these mobile user positions efficiently. However, managing user positions raises privacy issues, in particular, if the providers of LSs are only partially trusted. Therefore, we presented the concept of private position sharing for partially trusted systems in a previous paper [1]. The basic idea of position sharing is to split the precise user position into a set of position shares of well-defined limited precision and distribute these shares among LSs of different providers. The main contributions of this paper are two extended position sharing approaches that improve our previous approach in two ways: Firstly, we reduce the predictability of share generation that allows an attacker to gain further information from a sub-set of shares to further increase the position precision. Secondly, we present a position sharing algorithm for constrained movement scenarios whereas the existing approach was tailored to open space environments. However, open space approaches are vulnerable to map-based attacks. Therefore, we present a share generation algorithm that takes map knowledge into account.

Stamatia Rizou, Frank Dürr, Kurt Rothermel: Fulfilling End-to-End Latency Constraints in Large-scale Streaming Environments. In Proceedings of the 30th IEEE International Performance Computing and Communications Conference (IPCCC 2011), pp. 1–8, November 2011, DOI: 10.1109/PCCC.2011.6108086. [show abstract]

The online processing of high volume data streams is a prerequisite for many modern applications relying on real-time data such as global sensor networks or multimedia streaming. In order to achieve efficient data processing and scalability w.r.t. the number of distributed data sources and applications, in-network processing of data streams in an overlay network of data processing operators has been proposed. For such stream processing overlay networks, the placement of operators onto physical hosts plays an important role for the resulting quality of service—in particular, the endto- end latency—and network load. To this end, we present an enhanced placement algorithm that minimizes the network load put onto the system by a stream processing task under userdefined delay constraints in this paper. Our algorithm finds first the optimal solution in terms of network load and then degrades this solution to find a constrained optimum. In order to reduce the overhead of the placement algorithm, we included mechanisms to reduce the search space in terms of hosts that are considered during operator placement. Our evaluations show that this approach leads to an operator placement of high quality solution while inducing communication overhead proportional only to a small percentage of the total hosts.

Patrick Baier, Frank Dürr, Kurt Rothermel: MapCorrect: Automatic Correction and Validation of Road Maps Using Public Sensing. In Proceedings of the 36th Annual IEEE Conference on Local Computer Networks (LCN 2011), pp. 58–66, October 2011, DOI: 10.1109/LCN.2011.6115522. [show abstract]

With the increasing proliferation of small and cheap GPS receivers, a new way of generating road maps could be witnessed over the last few years. Participatory mapping approaches like OpenStreetMap introduced a way to generate road maps collaboratively from scratch. Moreover, automatic mapping algorithms were proposed, which automatically infer road maps from a set of given GPS traces. Nevertheless, one of the main problems of these maps is their unknown quality in terms of accuracy, which makes them unreliable and, therefore, not applicable for the use in critical scenarios. To address this issue, we propose MapCorrect: An automatic map correction and validation system. MapCorrect automatically collects GPS traces from people’s mobile devices to correct a given road map and validate it by identifying those parts of the map that are accurately mapped with respect to some user provided quality requirements. Since fixing a GPS position is a battery draining operation, the collection of GPS data raises concerns about the energy consumption of the participating mobile devices. We tackle this issue by introducing an optimized sensing mechanism that gives the mobile devices notifications indicating those parts of the map that are considered as sufficiently mapped and, therefore, require no further GPS data for their validation. Furthermore, we show by simulation that using this approach up to 50% of the mobile phones’ energy can be saved while not impairing the effectiveness of the map correction and validation process at all.

Lars Geiger, Frank Dürr, Kurt Rothermel: Adaptive Routing in a Contextcast Overlay Network. In Proceedings of the IEEE 7th International Conference on Wireless and Mobile Computing, Networking and Communications (WiMob 2011), pp. 97–104, October 2011, DOI: 10.1109/WiMOB.2011.6085340. [show abstract]

Context-based communication allows for the dissemination of messages to mobile users with a specified context, i.e., at a location and with certain attribute values. This enables, e.g., a message to students on campus attending a certain class, with information about a study group for an upcoming exam. An overlay network of context-aware routers efficiently disseminate the messages to all matching receivers. Directed forwarding of such messages requires that the routers maintain knowledge about the contexts of connected users. Global knowledge, i.e., each router knowing about every user, scales poorly, though, because of the necessary updates. To overcome this challenge, a router can selectively propagate context information that actually allows its neighbors to prune a message distribution tree. In this paper, we present an approach to adaptively propagate only those user contexts that offer a reduction in overall system load. The algorithm automatically and locally adapts to the observed messages and user contexts on each node. Our solution significantly improves the scalability of the system by reducing the overall load by almost 50%.

Damian Philipp, Frank Dürr, Kurt Rothermel: A Sensor Network Abstraction for Flexible Public Sensing Systems. In Proceedings of the 8th IEEE International Conference on Mobile Ad-Hoc and Sensor Systems (MASS 2011), pp. 460–469, October 2011, DOI: 10.1109/MASS.2011.52. [show abstract]

Public Sensing is a new paradigm for developing large-scale sensor networks at low cost by utilizing mobile phones that are already surrounding us in our everyday lives. In this paper we present a sensor network abstraction layer for creating flexible public sensing systems that can execute arbitrary queries. To this effect we develop several algorithms to select mobile nodes for executing a query. These algorithms allow a user to define a trade-off between quality and efficiency of query execution by choosing an appropriate algorithm. Our evaluations show that we can achieve a 99% increase in efficiency with the most efficient approaches and only about 10% decrease in result quality under worst conditions.

Bilal Hameed, Jorge Minguez, Michael Wörner, Philip Hollstein, Sema Zor, Stefan Silcher, Frank Dürr, Kurt Rothermel: The Smart Real-Time Factory as a Product Service System. In Functional Thinking for Value Creation: Proceedings of the 3rd CIRP International Conference on Industrial Product Service Systems, pp. 326–331, May 2011, DOI: 10.1007/978-3-642-19689-8_57. [show abstract]

In modern manufacturing landscape, companies are increasingly relying on product service systems i.e. bundling of products and services together in order to gain a competitive edge. In this article we present the Smart Real-Time Factory, a smart digital manufacturing environment that can transform the process of production into an informational service for the customers. The different components of the smart factory are discussed at length along with a discussion of the different services that can be offered by the smart factory.

Frank Dürr, Pavel Skvortsov, Kurt Rothermel: Position Sharing for Location Privacy in Non-trusted Systems. In Proceedings of the 9th Annual IEEE International Conference on Pervasive Computing and Communication (PerCom 2011), pp. 189–196, March 2011, DOI: 10.1109/PERCOM.2011.5767584. [show abstract]

Many novel location-based services (LBS) such as a friend finder service require knowledge about the positions of mobile users. Usually, location services are used to manage these positions, and for providing basic functionality like spatial range queries or spatial events to the LBS. Managing and using the positions of mobile users raises privacy issues, in particular, if the providers of LBS and location services are only partially trusted. Many different approaches for preserving a user’s privacy have been proposed in the literature, e.g. location obfuscation and the k-anonymity concept. However, most of them are not suitable if both LBS and location service providers are non-trusted. In contrast to these approaches, we present a novel approach for the secure management of private position information in partially trusted system environments. The main contribution in this paper is a position sharing concept which allows for the distribution of position information (shares) of strictly limited accuracy onto several location servers of different providers. With this approach, a compromised server will only reveal information of limited accuracy. Moreover, we will show how position shares of coarse granularity from multiple location servers can be fused into information of higher precision to satisfy the accuracy requirements of different LBS.

Harald Weinschrott, Julian Weißer, Frank Dürr, Kurt Rothermel: Participatory Sensing Algorithms for Mobile Object Discovery in Urban Areas. In Proceedings of the 9th Annual IEEE International Conference on Pervasive Computing and Communication (PerCom 2011), pp. 128–135, March 2011, DOI: 10.1109/PERCOM.2011.5767577. [show abstract]

This paper introduces mechanisms for the automated detection of mobile objects in urban areas. Widely available devices such as mobile phones with integrated proximity sensors such as RFID readers or Bluetooth cooperatively perform sensing operations to discover mobile objects. In this paper, we propose a coverage metric for assessing the completeness of sensing that considers spatial and temporal aspects. To maximize coverage while minimizing energy consumption of mobile nodes, we propose both a centralized and a distributed coordination algorithm for selecting nodes that need to sense. Moreover, we present strategies that allow selected nodes to perform efficient sense operations. By extensive simulations, we show that distributed coordination achieves drastic energy savings of up to 63%, while limiting the coverage loss to 13%. Moreover, we show that the centralized algorithm loses less than 1% coverage compared to the maximum possible coverage.

Faraz Memon, Daniel Tiebler, Frank Dürr, Kurt Rothermel: Optimized Information Discovery using Self-adapting Indices over Distributed Hash Tables. In Proceedings of the 29th International Performance Computing and Communications Conference (IPCCC 2010), pp. 105–113, December 2010, DOI: 10.1109/PCCC.2010.5682330. [show abstract]

Distributed Hash Table (DHT)-based peer-to-peer information discovery systems have emerged as highly scalable systems for information storage and discovery in massively distributed networks. Originally DHTs supported only point queries. However, recently they have been extended to support more complex queries, such as multiattribute range (MAR) queries. Generally, the support for MAR queries over DHTs has been provided either by creating an individual index for each data attribute or by creating a single index using the combination of all data attributes. In contrast to these approaches, we propose to create and modify indices using the attribute combinations that dynamically appear in MAR queries in the system. In this paper, we present an adaptive information discovery system that adapts the set of indices according to the dynamic set of MAR queries in the system. The main contribution of this paper is a four-phase index adaptation process. Our evaluations show that the adaptive information discovery system continuously optimizes the overall system performance for MAR queries. Moreover, compared to a non-adaptive system, our system achieves several orders of magnitude improved performance.

Bilal Hameed, Imran Ahmed Khan, Frank Dürr, Kurt Rothermel: An RFID Based Consistency Management Framework for Production Monitoring in a Smart Real-Time Factory. In Proceedings of Internet of Things 2010 Conference (IOT 2010), pp. 1–8, November 2010, best paper award, DOI: 10.1109/IOT.2010.5678436. [show abstract]

The use of RFID devices for real-time production monitoring in modern factories is impeded by the inherent unreliability of RFID devices. In this paper we present a consistency stack that conceptually divides the different consistency issues in production monitoring into separate layers. In addition to this we have built a consistency management framework to ensure consistent real-time production monitoring, using unreliable RFID devices. In detail, we deal with the problem of detecting object sequences by a set of unreliable RFID readers that are installed along production lines. We propose a probabilistic sequence detection algorithm that assigns probabilities to objects detected by RFID devices and provides probabilistic guarantees regarding the real-time sequences of objects on the production lines.

Harald Weinschrott, Frank Dürr, Kurt Rothermel: StreamShaper: Coordination Algorithms for Participatory Mobile Urban Sensing. In Proceedings of the 7th IEEE International Conference on Mobile Ad-hoc and Sensor Systems (MASS 2010), pp. 195–204, IEEE Press, November 2010, DOI: 10.1109/MASS.2010.5663996. [show abstract]

In this paper we introduce mechanisms for automated mapping of urban areas that provide a virtual sensor abstraction to the applications. We envision a participatory system that exploits widely available devices as mobile phones to cooperatively read environmental conditions as air quality or noise pollution, and map these measurements to stationary virtual sensors. We propose spatial and temporal coverage metrics for measuring the quality of acquired sensor data that reflect the conditions of urban areas and the uncontrolled movement of nodes. To achieve quality requirements and efficiency in terms of energy consumption, this paper presents two algorithms for coordinating sensing. The first is based on a central control instance, which assigns sensing tasks to mobile nodes based on movement predictions. The second algorithm is based on coordination of mobile nodes in an ad-hoc network. By extensive simulations, we show that these algorithms achieve a high quality of readings, which is about 95% of the maximum possible. Moreover, the algorithms achieve a very high energy efficiency allowing for drastic savings compared to uncoordinated sensing.

Faraz Memon, Frank Dürr, Kurt Rothermel: Index Recommendation Tool for Optimized Information Discovery Over Distributed Hash Tables. In Proceedings of 35th International Conference on Local Computer Networks (LCN 2010), pp. 104–111, IEEE Press, October 2010. [show abstract]

Peer-to-peer (P2P) networks allow for efficient information discovery in large-scale distributed systems. Although point queries are well supported by current P2P systems – in particular systems based on distributed hash tables (DHTs) –, providing efficient support for more complex queries remains a challenge. Our research focuses on the efficient support for multi-attribute range (MAR) queries over DHT-based information discovery systems. Traditionally, the support for MAR queries over DHTs has been provided either by creating an individual index for each data attribute or by creating a single index using the combination of all data attributes. In contrast to these approaches, we propose to create a set of indices over selected attribute combinations. In order to limit the overhead induced by index maintenance, the total number of created indices has to be limited. Thus, the resulting problem is to create a limited number of indices such that the overall system performance is optimal for MAR queries. In this paper, we propose an index recommendation tool that implements heuristic solutions to this NP-hard problem. Our evaluations show that these heuristics lead to a close-to-optimal system performance for MAR queries.

Stamatia Rizou, Frank Dürr, Kurt Rothermel: Providing QoS Guarantees for Large-Scale Operator Networks. In Proceedings of the 12th IEEE International Conference on High Performance Computing and Communications (HPCC 2010), pp. 337–345, IEEE Press, September 2010, DOI: 10.1109/HPCC.2010.53. [show abstract]

Application areas like global sensor networks and data stream processing involve the on-line processing of large amounts of data in an overlay network of operators on top of the Internet infrastructure. Trying to fulfill QoS guarantees in such networks is a challenging task that should be realized under the requirement for optimal usage of common resources in the network. Therefore in this paper, we formalize a constrained optimization problem for the placement of operators in an overlay network which strives for satisfying user QoS constraints subject to latency, while minimizing the network load induced by the deployment of the operators in the network. Since the initial problem is NP-hard, we solve at a first step the problem in an intermediate continuous latency space and then we map the continuous solution to its discrete variant. Our evaluations provide an analysis about the inherent interdepedence between the two metrics, network usage and latency, subject to this paper and furthermore shows that our algorithm achieves a good balance between the user requirements and the usage of the network resources.

Ralph Lange, Frank Dürr, Kurt Rothermel: Indexing Source Descriptions based on Defined Classes. In Proceedings of the 14th International Database Engineering and Applications Symposium (IDEAS 2010), pp. 245–256, August 2010, DOI: 10.1145/1866480.1866514. [show abstract]

Scaling heterogeneous information systems (HIS) to thousands of sources poses particular challenges to source discovery. It requires a powerful formalism for describing the contents of the sources in a concise manner and for formulating compatible queries as well as a suitable structure for indexing and retrieving the source descriptions efficiently. We propose an extended logic-based description formalism for large-scale HIS with structured sources and a shared ontology. The formalism refines existing approaches that describe the sources by constraints on the attribute value ranges in several ways: It allows for complex, nested descriptions based on defined classes. It supports alternative descriptions to express that a source may be discovered by different combinations of constraints. Finally, it allows to adjust between positive matching, similar to keyword-based discovery, and negative matching, as used in existing logic-based approaches. We further propose the SDC-Tree for indexing such source descriptions. To allow for efficient discovery, the SDC-Tree features multidimensional indexing capabilities for the different attributes and the IS-A hierarchy of the shared ontology, but also incorporates the existence or absence of constraints. For this purpose, it supports three different types of node split operations which exploit the expressiveness of the description formalism. Therefore, we also propose a generic split algorithm which can be used with arbitrary ontologies.

Stamatia Rizou, Frank Dürr, Kurt Rothermel: Solving the Multi-operator Placement Problem in Large-Scale Operator Networks. In Proceedings of the 19th International Conference on Computer Communication Networks (ICCCN 2010), pp. 1–6, IEEE Press, August 2010, DOI: 10.1109/ICCCN.2010.5560127. [show abstract]

Processing streams of data in an overlay network of operators distributed over a wide-area network is a common idea shared by different applications such as distributed event correlation systems and large-scale sensor networks. In order to utilize network resources efficiently and allow for the parallel deployment of a large number of large-scale operator networks, suitable placement algorithms are vital that place operators on physical nodes. In this paper, we present a distributed placement algorithm that minimizes the bandwidth-delay product of data streams between operators of the network in order to reduce the induced network load. Since the fundamental optimization problem is NP-hard, we propose a heuristic solution. First, we calculate an optimal solution in an intermediate continuous search space, called latency space. Subsequently the continuous solution is mapped to the physical network. Our evaluations show that this algorithm reduces the resulting network load significantly compared to state of the art algorithms and achieves results close to the optimum.

Lars Geiger, Frank Dürr, Kurt Rothermel: Aggregation of User Contexts in Context-based Communication. In Proceedings of the 6th Euro-NF Conference on Next Generation Internet (NGI 2010), pp. 1–8, IEEE Press, June 2010, DOI: 10.1109/NGI.2010.5534466. [show abstract]

A context-based communication system enables the indirect addressing and routing of messages according to the users’ contexts. This provides, for example, the means to send a message to all students on campus who attend a certain class, with information about an upcoming exam. However, for a targeted forwarding of messages towards users, the routers need information about the context of connected users. Global knowledge, i.e., each router knowing about every user, is not scalable, though, because of the necessary update messages to keep this information up-to-date. To address this challenge, a router can aggregate similar contexts and only provide such an aggregated view to neighboring routers. In this paper, we present an approach to aggregate similar contexts, based on a similarity measure for user contexts. The algorithm can be adjusted according to the observed messages and user contexts in the system by specifying a similarity threshold to determine when contexts are aggregated. The aggregation of user contexts improves the scalability of our approach by significantly reducing the load of context updates by up to 30%, depending on the usage of the system. This improvement comes at the cost of a negligible increase in false positive messages due to the loss of information used for forwarding messages.

Harald Weinschrott, Frank Dürr, Kurt Rothermel: Symbolic Routing for Location-based Services in Wireless Mesh Networks. In Proceedings of the IEEE 24nd International Conference on Advanced Information Networking and Applications (AINA 2010), pp. 851–858, IEEE Press, April 2010, DOI: 10.1109/AINA.2010.71. [show abstract]

Wireless Mesh Networks are cost-efficient medium-scale networks that have the potential to serve as an infrastructure for advanced location-based services. As a basis for these services we present a routing algorithm that allows to address intuitive symbolic coordinates. This algorithm is based on a proactively maintained geographic routing structure that mimics the structure of a symbolic location model. Message forwarding is done greedily along short paths defined by a symbolic location model and if this fails, through an hierarchical overlay network built by selected mesh routers. We show how a geocast communication mechanism that allows to send messages to all hosts within a specific location can be implemented with this routing algorithm. In extensive evaluations we show that a low proactive routing overhead allows to achieve high message delivery rates even in case of mobility. Moreover, we show that the paths achieved are only 25% longer than the theoretic optimal paths for a wide range of simulation settings.

Stamatia Rizou, Kai Häussermann, Frank Dürr, Nazario Cipriani, Kurt Rothermel: A system for distributed context reasoning. In Proceedings of ICAS 2010: International Conference on Autonomous and Autonomic Systems, pp. 84–89, IEEE Press, March 2010, best paper award, DOI: 10.1109/ICAS.2010.21. [show abstract]

Context aware systems use context information to adapt their behaviour accordingly. In order to derive high level context information from low level context, such as sensor values, context reasoning methods that correlate observable context information, are necessary. Several context reasoning mechanisms have been proposed in the literature. Usually these mechanisms are centralized, leading to suboptimal utilization of network resources and poor system performance in case of large-scale scenarios. Therefore to increase the scalability of context reasoning systems the development of methods that distribute the reasoning process is necessary. Existing distributed approaches are method specific and do not provide a generic formalization for distributed reasoning. In this paper we introduce a novel system which enables distributed context reasoning in a generic way that is independent of the reasoning algorithm.

Lars Geiger, Ronald Schertle, Frank Dürr, Kurt Rothermel: Temporal Addressing for Mobile Context-Aware Communication. In Proceedings of the Sixth Annual International Conference on Mobile and Ubiquitous Systems: Computing, Networking and Services (MobiQuitous 2009), pp. 1–10, ICST, June 2009, DOI: 10.4108/ICST.MOBIQUITOUS2009.6836. [show abstract]

Mobile clients in context-aware systems benefit from the indirect addressing of users via their context (contextcast), such as addressing messages to all users in downtown Toronto whose age is below 35. There is, however, almost no support for a temporal decoupling in such a contextcast system, i.e., the addressing of users that were or will be in a certain context in the past or future, respectively. This could for instance be used to distribute the minutes of a meeting to all people who attended the meeting in room 1.138, 3 days ago, between 1 and 3 pm. To enable a context-aware communication system to address messages with temporal relations, especially those contexts in the past, the system needs to manage information about user context histories. This poses the risk that the system can be abused to profile users, which would most probably hinder acceptance. Therefore, privacy aspects need to be considered in the core design of such a system. We present an extension to our earlier work, which allows a temporal decoupling of messages and users and requires very little additional overhead to manage historic context information. The solution includes mechanisms to efficiently disseminate messages to both users with past and future contexts, while effectively preventing user profiling through the use of virtual identities.

Faraz Memon, Daniel Tiebler, Frank Dürr, Kurt Rothermel, Marco Tomsu, Peter Domschitz: Scalable Spatial Information Discovery over Distributed Hash Tables. In Proceedings of 4th International Conference on COMmunication System softWAre and middlewaRE (COMSWARE 2009), pp. 1–12, ACM, June 2009, DOI: 10.1145/1621890.1621892. [show abstract]

In this paper, we present a Peer-to-Peer (P2P) spatial information discovery system that enables spatial range queries over Distributed Hash Tables (DHTs). Our system utilizes a less-distorting octahedral map projection in contrast to the quadrilateral projections used by majority of the previously proposed systems, to represent the spatial information. We also introduce a Space-Filling Curve (SFC)-based data placement strategy that reduces the probability of data hot-spots in the network. Moreover, we show that our system achieves scalable resolution of location-based range queries by utilizing a tree-based query optimization algorithm. Compared to the basic query resolution algorithm, the query optimization algorithm reduces the average number of parallel messages used to resolve a query, by a factor of 96%.

Lars Geiger, Frank Dürr, Kurt Rothermel: On Contextcast: A Context-aware Communication Mechanism. In Proceedings of the 2009 IEEE International Conference on Communications (ICC 2009), pp. 1505-1509, IEEE Press, June 2009, DOI: 10.1109/ICC.2009.5199239. [show abstract]

The dissemination of messages according to clients’ contexts (i.e., location and other attributes) opens up new possibilities in context-aware systems. While geocast or content-based publish/subscribe forward messages according to client location or attributes, respectively, neither uses a combination of the two. In this paper, we present this new communication paradigm and the challenges it poses. We also extend concepts from publish/subscribe networks to efficiently deal with highly dynamic user location to lower update rates by approximating the user’s location. This reduces update rates by between 25% and 90%, depending on the granularity of the approximation.

Harald Weinschrott, Frank Dürr, Kurt Rothermel: Efficient Capturing of Environmental Data with Mobile RFID Readers. In Proceedings of the 10th International Conference on Mobile Data Management (MDM 2009), pp. 41–51, IEEE Press, May 2009, DOI: 10.1109/MDM.2009.15. [show abstract]

In this paper we introduce a novel scenario for environmental sensing based on the combination of simple and cheap RFID-based sensors and mobile devices like mobile phones with integrated RFID readers. We envision a system, where the mobile nodes cooperatively read sensors installed in the environment as they pass by and transmit the data to a server infrastructure. To achieve quality requirements on the one hand, and on the other hand efficiency in terms of communication cost and energy consumption, this paper presents several algorithms for coordinating update operations. Mobile nodes form an ad-hoc network for the cooperative management of requested update times to meet the desired update interval and to avoid redundant sensor reading and collisions during read operations. Besides this decentralized coordination algorithm, we also show a complementary algorithm that exploits infrastructure based coordination. By extensive simulations we show that our algorithms achieve a high quality of sensor updates where nearly 100% of the possible updates are performed. Moreover, the algorithms achieve a very high energy efficiency allowing for several hundred hours of operation assuming a typical battery of a mobile phone.

Ralph Lange, Tobias Farrell, Frank Dürr, Kurt Rothermel: Remote Real-Time Trajectory Simplification. In Proceedings of the 7th Annual IEEE International Conference on Pervasive Computing and Communications (PerCom 2009), pp. 1–10, IEEE Press, March 2009, DOI: 10.1109/PERCOM.2009.4912767. [show abstract]

Moving objects databases (MODs) have been proposed for managing trajectory data, an important kind of information for pervasive applications. To save storage capacity, a MOD generally stores simplified trajectories only. A simplified trajectory approximates the actual trajectory of the mobile object according to a certain accuracy bound. In order to minimize the costs of communicating position information between mobile object and MOD, the trajectory simplification should be performed by the mobile object. To assure that the MOD always has a valid simplified trajectory of the remote object, we propose the generic remote trajectory simplification protocol (GRTS) allowing for computing and managing a simplified trajectory in such a system in real-time. We show how to combine GRTS with existing line simplification algorithms for computing the simplified trajectory and analyze trade-offs between the different algorithms. Our evaluations show that GRTS outperforms the two existing approaches by a factor of two and more in terms of reduction efficiency. Moreover, on average, the reduction efficiency of GRTS is only 12% worse compared to optimal offline simplification.

Faraz Memon, Daniel Tiebler, Frank Dürr, Kurt Rothermel, Marco Tomsu, Peter Domschitz: OID: Optimized Information Discovery using Space Filling Curves in P2P Overlay Networks. In Proceedings of 14th International Conference on Parallel and Distributed Systems (ICPADS 2008), pp. 311–319, IEEE Press, December 2008, DOI: 10.1109/ICPADS.2008.57. [show abstract]

In this paper, we present the system design and evaluation of a Space-Filling Curve (SFC)-based P2P information discovery system OID. The OID system uses multiple SFCs to significantly optimize the performance of multi-attribute range queries, particularly for applications with a large number of data attributes where a single big SFC-based index is inefficient. The basic idea is to have multiple SFCbased indices and select the best one to perform a query. We also introduce two tree-based query optimizations that increase the scalability of the system.

Ralph Lange, Frank Dürr, Kurt Rothermel: Scalable Processing of Trajectory-Based Queries in Space-Partitioned Moving Objects Databases. In Proceedings of the 16th ACM SIGSPATIAL International Conference on Advances in Geographic Information Systems (ACM GIS 2008), pp. 1–10, ACM, November 2008, DOI: 10.1145/1463434.1463474. [show abstract]

Space-partitioned Moving Objects Databases (SP-MODs) allow for the scalable, distributed management of large sets of mobile objects’ trajectories by partitioning the trajectory data to a network of database servers. Processing a spatio-temporal query q therefore requires efficiently routing q to the servers storing the affected trajectory segments. With a coordinate-based query – like a spatio-temporal range query – the relevant servers are directly determined by the queried range. However, with trajectory-based queries – like retrieving the distance covered by a certain object during a given time interval – the relevant servers depend on actual movement of the queried object. Therefore, efficient routing mechanisms for trajectory-based queries are an important challenge in SP-MODs. In this paper, we present the Distributed Trajectory Index (DTI) that allows for such efficient query routing by creating an overlay network for each trajectory. We further present an enhanced index called DTI+S. It accelerates the processing of queries on aggregates of dynamic attributes, like the maximum speed during a time interval, by augmenting DTI with summaries of trajectory segments. Our simulations with a network of 1000 database servers show that DTI+S can reduce the overall processing time by more than 98%.

Frank Dürr, Kurt Rothermel: Kommunikationsprotokolle zur Vermittlung kontextbezogener Informationen auf Basis räumlicher Umgebungsmodelle (invited paper). In Proceedings of VDE-Kongress 2008 – Zukunftstechnologien: Innovationen – Märkte – Nachwuchs, VDE-Verlag, November 2008. [show abstract]

In diesem Artikel stellen wir ein neuartiges kontextbezogenes Kommunikationsparadigma namens Contextcast vor. Contextcast ermöglicht das selektive Senden von Nachrichten an Teilnehmer mit einem bestimmten Kontext, wobei der Kontext eines Teilnehmers beispielsweise durch seinen Aufenthaltsort, seine Interessen oder seine Situation bestimmt ist. Typische Anwendungen für Contextcast sind die gezielte Verteilung von Warnmeldungen, Touristeninformationen oder Veranstaltungshinweise. Wir präsentieren zunächst Konzepte zur effizienten Vermittlung von ortsbezogenen Informationen (Geocast) und diskutieren anschließend notwendige Erweiterungen für die effiziente Realisierung der Contextcast-Kommunikation.

Ralph Lange, Frank Dürr, Kurt Rothermel: Online Trajectory Data Reduction using Connection-preserving Dead Reckoning. In Proceedings of the Fifth Annual International Conference on Mobile and Ubiquitous Systems: Computing, Networking and Services: MobiQuitous 2008, pp. 1–10, ICST, July 2008, DOI: 10.4108/ICST.MOBIQUITOUS2008.3460. [show abstract]

Moving objects databases (MODs) store objects’ trajectories by spatiotemporal polylines that approximate the actual movements given by sequences of sensed positions. Determining such a polyline with as few vertices as possible under the constraint that it does not deviate by more than a certain accuracy bound epsilon from the sensed positions is an algorithmic problem known as trajectory reduction. A specific challenge is online trajectory reduction, i.e. continuous reduction with position sensing in realtime. This particularly is required for moving objects with embedded position sensors whose movements are tracked and stored by a remote MOD. In this paper, we present Connection-preserving Dead Reckoning (CDR), a new approach for online trajectory reduction. It outperforms the existing approaches by 30 to 50%. CDR requires the moving objects to temporally store some of the previously sensed positions. Although the storage consumption of CDR generally is small, it is not bounded. We therefore further present CDRM whose storage allocation and execution time per position fix can be adjusted and limited. Even with very limited storage allocations of less than 1 kB CDRM outperforms the existing approach by 20 to 40%.

Frank Dürr, Kurt Rothermel: An Adaptive Overlay Network for World-wide Geographic Messaging. In Proceedings of the 22nd IEEE International Conference on Advanced Information Networking and Applications (AINA 2008), pp. 875–882, IEEE Press, March 2008, DOI: 10.1109/AINA.2008.67. [show abstract]

In this paper, we propose an overlay network supporting world-wide geographic messaging. Our approach is based on hierarchical symbolic coordinates like /usa/fl/miami/. Although hierarchical network topologies lend themselves to the implementation of such overlay networks, they may lead to bottlenecks at the root of the hierarchy, long message paths, and inefficient bandwidth utilization. To avoid these problems, we propose an overlay network that adapts its structure to the users’ communication patterns by dynamically adding shortcut links to the hierarchy leading to a routing mesh. We present an algorithm that carefully selects shortcuts based on their utility to assure short message paths on the one hand and to reduce the induced overhead on the other hand. Through simulations we show that this approach decreases the average path length significantly and reduces network load to about 50% compared to hierarchical routing.

Martin Saternus, Mirko Knoll, Frank Dürr, Torben Weis: Symstry: Ein P2P-System für Ortsbezogene Anwendungen. In Proceedings of 15. ITG/GI-Fachtagung (KiVS 2007), pp. 99–104, VDE-Verlag, February 2007. [show abstract]

Bern, Switzerland

Frank Dürr, Kurt Rothermel: An Overlay Network for Forwarding Symbolically Addressed Geocast Messages. In Proceedings of the 15th International Conference on Computer Communications and Networks (ICCCN 2006), pp. 427–434, IEEE Press, October 2006, DOI: 10.1109/ICCCN.2006.286314. [show abstract]

Geocast, which allows for forwarding messages to hosts residing at specified geographic areas, is a promising communication paradigm with a wide range of applications. Geocast target areas can be specified either by geometric figures or symbolic addresses, such as /usa/fl/miami/market-street. In this paper, we present a novel geocast routing protocol for symbolically addressed messages. Compared to geocast protocols based on geometric information, our protocol can operate on simple symbolic location models, and message forwarding does not require costly geometric operations. The proposed protocol is based on an overlay network that is mapped to an IP-based network infrastructure. The overlay network is structured in a hierarchical fashion, to ensure a scalable global geocast service supporting also large target areas. Although our protocol does not rely on a layer 3 multicast protocol, we also show how to improve the performance of message forwarding by integrating a light-weight layer 3 multicast protocol. Our evaluations of the protocol underline the scalability of our approach and show good routing quality leading to short message paths.

Matthias Wieland, Frank Leymann, Lamine Jendoubi, Daniela Nicklas, Frank Dürr: Task-orientierte Anwendungen in einer Smart Factory. In Mobile Informationssysteme – Potentiale, Hindernisse, Einsatz. (1. Fachtagung Mobilität und Mobile Informationssysteme: MMS 2006), Lecture Notes in Informatics (LNI) P-76, pp. 139–143, Gesellschaft für Informatik, February 2006. [show abstract]

In diesem Beitrag wird aufgezeigt, welche Möglichkeiten sich durch den Einsatz task-orientierter, explorativer Anwendungen im Umfeld einer Smart Factory ergeben. Nach einer kurzen Charakterisierung dieser Anwendungsklasse wird ein Szenario geschildert, das Wartungsaufgaben in einer Fabrik mit diesen Konzepten löst. Daraus ergibt sich die Vision einer Smart Factory, in der die Geschäftsprozesse über ein Kontextmodell mit den technischen Prozessen der Produktionsstraße gekoppelt werden.

Frank Dürr, Christian Becker, Kurt Rothermel: Efficient Forwarding of Symbolically Addressed Geocast Messages. In Proceedings of the 14th International Conference on Computer Communications and Networks (ICCCN 2005), pp. 77–85, IEEE Press, October 2005, DOI: 10.1109/ICCCN.2005.1523812. [show abstract]

Geocast is used to send messages to all hosts located in a geographic area. This target area can be defined either by geometric figures like polygons or by symbolic addresses like city names or room numbers. Geographic routing algorithms, which forward messages based on geographic information, can be used to forward geocast messages. If routing of symbolically addressed messages is based on geometric coordinates, complex mappings between symbolic addresses and their geometric extent as well as complex geometric operations are required. Therefore, we propose a routing algorithm for symbolically addressed geocast messages that operates directly on a symbolic location model. This approach does not require any geometric information for message forwarding, and forwarding decisions can be realized efficiently by comparably simple operations.

Kurt Rothermel, Frank Dürr: Location-based Services: Auf dem Weg zu kontextbezogenen Informations- und Kommunikationssystemen. In Zukunft durch Informationstechnik – Schnell – Mobil – Intelligent – Jubiläumsfachtagung 50 Jahre ITG, pp. 121–128, VDE-Verlag, April 2004. [show abstract]

Die rasant fortschreitende Entwicklung von drahtloser Kommunikation, Positionierungssystemen und Rechnertechnologie sowie deren Integration in multifunktionale Endgeräte unterstützt die vielversprechende neue Anwendungsklasse der ortsbezogenen Systeme. Ortsinformation kann von solchen Anwendungen genutzt werden, um ihr Verhalten dem aktuellen Standort der Benutzer oder auch anderer Objekte anzupassen. Heute schon bieten Netzbetreiber so genannte “Location-based Services” an, die primär Navigationsfunktionen und eine ortsbezogene Selektion von Informationen unterstützen. Ortsinformation kann aber auch auf der Systemebene gewinnbringend verwendet werden, etwa zur Bereitstellung neuer ortsbezogener Kommunikationskonzepte oder zur Optimierung der Kommunikation. In diesem Beitrag wird der Begriff der ortsbezogene Anwendungen erläutert und Anwendungen dieser Art am Beispiel existierender Systeme kurz skizziert. Außerdem werden zwei Beispiele für die Nutzung von Ortsinformation auf Systemebene gegeben, die Geocast-Kommunikation und ein Hoarding-Verfahren.

Frank Dürr, Kurt Rothermel: On a Location Model for Fine-Grained Geocast. In UbiComp 2003: Ubiquitous Computing (Proceedings of the Fifth International Conference on Ubiquitous Computing 2003), Lecture Notes in Computer Science (LNCS) 2864, pp. 18–35, Springer-Verlag, October 2003, DOI: 10.1007/b93949. [show abstract]

Geographic communication (geocast) is used to send messages to geographic areas, e.g. to distribute warning messages or other information within these areas. It is based on a location model which is used to determine a message’s target area and the receivers’ positions and therefore has strong influence on the achievable granularity of geographic addressing. A hybrid location model and and a fine-grained addressing scheme for geocast based on this model are presented in this paper which support two- and three-dimensional geometric locations as well as symbolic locations like room numbers, embedded local coordinate systems, and mobile target areas like trains.

Kurt Rothermel, Dominique Dudkowski, Frank Dürr, Martin Bauer, Christian Becker: Ubiquitous Computing – More than Computing Anytime Anyplace? (invited paper). In Proceedings of Photogrammetric Week 2003, Wichmann-Verlag, September 2003. [show abstract]

The rapid development of mobile communication networks and the availability of miniaturized and inexpensive sensor technology allow for the development of multifunctional mobile devices that are able to collect and communicate context information. We also currently see a clear trend towards the integration of embedded systems including sensors into ßmart“ everyday things, which is also referred to as Ubiquitous Computing. The deployment of sensor technology allows for so-called context-aware applications, going far beyond of providing mobile access to information and computing resources. Context-aware applications exploit captured sensor information to adapt to the current situation of mobile users. For instance, the information offered to a user may depend on his current location, and the way how it is presented to him may depend on the speed he moves, whether he is on its own or with others, and so on. In this paper, we characterize context-aware applications and present the vision of a digital world model as the foundation for this class of applications. Further, the paper discusses the benefits and reseach challenges associated with digital world models, which allow applications to take into account the state of the real world and react to state changes.

Peter Coschurba, Kurt Rothermel, Frank Dürr: A fine-grained addressing concept for GeoCast. In Trends in Network and Pervasive Computing – ARCS 2002 (Proceedings of the International Conference on Architecture of Computing Systems 2002), Lecture Notes in Computer Science (LNCS) 2299, pp. 101–113, Springer-Verlag, April 2002, DOI: 10.1007/3-540-45997-9. [show abstract]

GeoCast provides the functionality of sending messages to everyone in a specific area. So far, only the addressing of larger two-dimensional areas was possible. For the use in an urban environment it is crucial that small and three-dimensional areas can be addressed. For example, GeoCast can then be used to send lecture notes to all in a classroom. In this paper we describe a fine-grained addressing concept for GeoCast that supports such areas. In addition we present an architecture that allows the use of that addressing concept together with the GeoRouting-approach developed by Navas and Imielinski. We also present some modifications necessary to enhance the scalability of GeoCast.

Journal Articles

Sukanya Bhowmik, Muhammad Adnan Tariq, Boris Koldehofe, Frank Dürr, Thomas Kohler, Kurt Rothermel: High Performance Publish/Subscribe Middleware in Software-Defined Networks. IEEE/ACM Transactions on Networking, 25(3), pp. 1501–1516, December 2016, DOI: 10.1109/TNET.2016.2632970. [show abstract]

With the increasing popularity of software-defined networking (SDN), ternary content-addressable memory of switches can be directly accessed by a publish/subscribe middleware to perform filtering operations at low latency. In this way, three important requirements for a publish/subscribe middleware can be fulfilled, namely, bandwidth efficiency, line-rate performance, and low latency in forwarding messages between producers and consumers. Nevertheless, it is challenging to sustain line-rate performance in the presence of dynamically changing interests of producers and consumers. In this paper, we realize a scalable, SDN-based publish/subscribe middleware, called PLEROMA, that performs efficient forwarding at line-rate. Moreover, PLEROMA offers methods to efficiently reconfigure a deployed topology in the presence of dynamic subscriptions and advertisements. We evaluate the performance of both the data plane and the control plane of PLEROMA to support our claim. Furthermore, we evaluate and benchmark the performances of SDN-compliant hardware and software switches in the context of our middleware.

Thomas Kohler, Frank Dürr, Kurt Rothermel: Consistent Network Management for Software-defined Networking based Multicast. IEEE Transactions on Network and Service Management, 13(3), pp. 447–461, June 2016, DOI: 10.1109/TNSM.2016.2585672. [show abstract]

Updating a network is an essential and continual task in the management of today’s softwarized networks. When applying updates on distributed network elements, desired network properties, such as drop- and loop-freeness, might be transiently violated. Although being crucial, update consistency has yet been less considered in network management. In this article, we argue for incorporating the particularities of update consistency into the reconfiguration process of continuous network management. We present a generic management architecture allowing for an appropriate selection of an update mechanism and its parameters based on expected inconsistency effects. We investigate update consistency for the case of multicast routing and show in an extensive analysis why simultaneous drop- and duplicate-freeness is not possible. We present an update procedure for multicast routing updates that identifies critical update steps, which are fed back into the reconfiguration process, along with a lightweight approach that allows for the selection of an update strategy, preventing either drops or duplicates. Furthermore, we present an optimization of an existing powerful, but resource-intensive update approach as well as an approach for in-network filtering of duplicates.

Marius Wernke, Pavel Skvortsov, Frank Dürr, Kurt Rothermel: A Classification of Location Privacy Attacks and Approaches. Personal and Ubiquitous Computing, 18(1), pp. 163–175, January 2014, DOI: 10.1007/s00779-012-0633-z. [show abstract]

In recent years, location-based services have become very popular, mainly driven by the availability of modern mobile devices with integrated position sensors. Prominent examples are points of interest finders or geo-social networks like Facebook Places, Qype, Loopt. Because these services access private position information, location privacy concepts become mandatory in order to ensure that users accept these services. Many different concepts and approaches for the protection of location privacy have been described in the literature. These approaches differ with respect to the protected information and their effectiveness against different attacks. The goal of this paper is to assess the applicability and effectiveness of location privacy approaches systematically. We first identify different protection goals, namely, personal information (user identity), spatial information (user position), and temporal information (identity/position + time). Secondly, we give an overview of basic principles and existing approaches to protect these privacy goals. In a third step, we classify possible attacks. Finally, we analyzed existing approaches with respect to their protection goals and their ability to resists the introduced attacks.

Marius Wernke, Pavel Skvortsov, Frank Dürr, Kurt Rothermel: A Classification of Location Privacy Attacks and Approaches. Personal and Ubiquitous Computing, 18(1), pp. 163–175, Springer-Verlag, January 2014, DOI: 10.1007/s00779-012-0633-z. [show abstract]

In recent years, location-based services have become very popular, mainly driven by the availability of modern mobile devices with integrated position sensors. Prominent examples are points of interest finders or geo-social networks like Facebook Places, Qype, Loopt. Because these services access private position information, location privacy concepts become mandatory in order to ensure that users accept these services. Many different concepts and approaches for the protection of location privacy have been described in the literature. These approaches differ with respect to the protected information and their effectiveness against different attacks. The goal of this paper is to assess the applicability and effectiveness of location privacy approaches systematically. We first identify different protection goals, namely, personal information (user identity), spatial information (user position), and temporal information (identity/position + time). Secondly, we give an overview of basic principles and existing approaches to protect these privacy goals. In a third step, we classify possible attacks. Finally, we analyzed existing approaches with respect to their protection goals and their ability to resists the introduced attacks.

Marius Wernke, Frank Dürr, Kurt Rothermel: PShare: Ensuring location privacy in non-trusted systems through multi-secret sharing. Pervasive and Mobile Computing, 9(3), Elsevier, February 2013, DOI: 10.1016/j.pmcj.2013.01.001. [show abstract]

Location-based applications such as Facebook Places, Foursquare, or Loopt typically use location services to manage mobile object positions. However, exposing precise user positions raises user privacy concerns, especially if location service providers are not fully trusted. To enable the secure management of private user positions in non-trusted systems, we present two novel position sharing approaches based on the concept of multi-secret sharing. We improve existing geometric position sharing approaches by considering continuous position updates and by increasing the robustness against various attacks. Furthermore, we present the first position sharing approach for symbolic location models.

Kurt Rothermel, Stephan Schnitzer, Ralph Lange, Frank Dürr, Tobias Farrell: Context-aware and quality-aware algorithms for efficient mobile object management. Pervasive and Mobile Computing, 8(1), pp. 131–146, February 2012, DOI: 10.1016/j.pmcj.2011.04.009. [show abstract]

The management of positions of mobile objects is an essential prerequisite for many context-aware systems such as advanced traffic management systems or personal assistance systems. In this paper, we present two approaches for the scalable tracking of mobile object trajectories and the efficient processing of continuous spatial range queries, respectively. We show in detail how both approaches utilize the basic concepts of accuracy relaxation and utilization of context information, such as movement predictions, to minimize the number of position updates, the size of trajectory data, and the number of energy-consuming position sensing operations.

Ralph Lange, Frank Dürr, Kurt Rothermel: Efficient real-time trajectory tracking. The VLDB Journal, 20(5), pp. 671–694, 2011, DOI: 10.1007/s00778-011-0237-7. [show abstract]

Moving objects databases (MOD) manage trajectory information of vehicles, animals, and other mobile objects. A crucial problem is how to efficiently track an object’s trajectory in real-time, in particular if the trajectory data is sensed at the mobile object and thus has to be communicated over a wireless network. We propose a family of tracking protocols that allow trading the communication cost and the amount of trajectory data stored at a MOD off against the spatial accuracy. With each of these protocols, the MOD manages a simplified trajectory that does not deviate by more than a certain accuracy bound from the actual movement. Moreover, the different protocols enable several trade-offs between computational costs, communication cost, and the reduction in the trajectory data: Connection-Preserving Dead Reckoning minimizes the communication cost using dead reckoning, a technique originally designed for tracking an object’s current position. Generic Remote Trajectory Simplification (GRTS) further separates between tracking of the current position and simplification of the past trajectory and can be realized with different line simplification algorithms. For both protocols, we discuss how to bound the space consumption and computing time at the moving object and thereby present an effective compression technique to optimize the reduction performance of real-time line simplification in general. Our evaluations with hundreds of real GPS traces show that a realization of GRTS with a simple simplification heuristic reaches 85-90% of the best possible reduction rate, given by retrospective offline simplification. A realization with the optimal line simplification algorithm by Imai and Iri even reaches more than 97% of the best possible reduction rate.

Christian Becker, Frank Dürr, Mirko Knoll, Daniela Nicklas, Torben Weis: Entwicklung ortsbezogener Anwendungen. Praxis der Informationsverarbeitung und Kommunikation (PIK), 29(1), pp. 30–36, K. G. Saur, January 2006, DOI: 10.1515/PIKO.2006.30. [show abstract]

Ortsbezogene Anwendungen versorgen Benutzer mit Diensten und Informationen in Abhängigkeit ihrer aktuellen Position. Beispiele sind im Bereich der Telematikdienste, Touristenführer und Informationsdienste zu finden. Anwendungen können sich an den Ort oder ganz allgemein an den Kontext des Benutzers anpassen, indem sie ortsspezifische Informationen anzeigen oder Daten der jeweiligen Situation entsprechend präsentieren. Es existieren verschieden Ansätze und Architekturen, um ortsbezogene Anwendungen zu realisieren. In diesem Beitrag stellen wir unterschiedliche Ansätze vor, diskutieren deren Vor- und Nachteile und leiten daraus ein abstraktes Anwendungsmodell für ortsbezogene Anwendungen ab. Des Weiteren stellen wir Entwicklungswerkzeuge vor, die wir auf Basis dieses Anwendungsmodells konzipiert haben. Die Kombination aus Anwendungsmodell und spezialisierten Werkzeugen wird die Entwicklung ortsbezogener Anwendungen systematisieren und wesentlich vereinfachen.

Christian Becker, Frank Dürr: On Location Models for Ubiquitous Computing. Personal and Ubiquitous Computing, 9(1), pp. 20–31, Springer-Verlag, January 2005, DOI: 10.1007/s00779-004-0270-2. [show abstract]

Common queries regarding information processing in ubiquitous computing are based on the location of physical objects. No matter if the next printer, next restaurant, or friend is searched for, a notion of distances between objects is required. A search for all objects in a certain geographic area requires the possibility to define spatial ranges and spatial inclusion of locations. In this paper we discuss general properties of symbolic and geometric coordinates. Based on that, we present an overview of existing location models allowing for position, range, and nearest neighbor queries. The location models are classified according to their suitability with respect to the query processing and the involved modeling effort along with other requirements. Besides an overview of existing location models and approaches the classification of location models with respect to application requirements can assist developers in their design decisions.

Books and Edited Proceedings

Kurt Rothermel (editor), Dieter Fritsch (editor), Wolfgang Blochinger (editor), Frank Dürr (editor): Quality of Context – Proceedings of the First International Workshop on Quality of Context (QuaCon 2009). Lecture Notes in Computer Science 5786, Springer-Verlag, June 2009, DOI: 10.1007/978-3-642-04559-2. [show abstract]

Advances in sensor technology, wireless communication, and mobile devices lead to the proliferation of sensors in our physical environment. At the same time detailed digital models of buildings, towns, or even the globe become widely available. Integrating the huge amount of sensor data into spatial models results in highly dynamic models of the real world, often called context models. A wide range of applications can substantially benefit from context models. However, context data are inherently associated with uncertainty. In general, quality of context information has to be taken into account by both context management and applications. For example, the accuracy, completeness, and trustworthiness of spatial context information such as street or building data are very important for navigation and guidance systems. QuaCon 2009 was the first international scientific meeting that specifically focused on the different aspects of quality of context data. Research in context management and, in particular, context quality, requires an interdisciplinary approach. Therefore, the QuaCon workshop aimed to bring together researchers from various fields to discuss approaches to context quality and to make a consolidated contribution toward an integrated way of treating this topic. We received 19 high-quality paper submissions by researchers from Europe, USA, and Asia. The International Program Committee selected 11 papers for presentation at the workshop. Additionally, five invited contributions by internationally renowned experts in the field were included in the workshop program. The presentations at the workshop showed many facets of quality of context from different research fields including context data management, spatial models, context reasoning, privacy, and system frameworks. The lively discussions underlined the great interest in this topic and in particular led to a deeper understanding of the relations between the various aspects of quality of context. The success of QuaCon 2009 was the result of a team effort. We are grateful to the members of the Program Committee and the external reviewers for their thorough and timely reviews as well as to the authors for their high-quality submissions and interesting talks. We would like to extend special thanks to our invited speakers for their excellent and inspiring keynotes. Finally, we wish to thank all persons involved in the organization of the QuaCon 2009 workshop who did really a great job.

Workshop Papers

Christoph Stach, Frank Dürr, Kai Mindermann, Saravana Murthy Palanisamy, Muhammad Adnan Tariq, Bernhard Mitschang, Stefan Wagner: PATRON – Datenschutz in Datenstromverarbeitungssystemen. In Informatik 2017: Digitale Kulturen, Tagungsband der 47. Jahrestagung der Gesellschaft für Informatik e.V. (GI), September 2017. [show abstract]

Boris Koldehofe, Frank Dürr, Muhammad Adnan Tariq, Kurt Rothermel: The Power of Software-defined Networking: Line-rate Content-based Routing Using OpenFlow. In Proceedings of the 7th Middleware for Next Generation Internet Computing Workshop of the 13th International Middleware Conference 2012 (MW4NG 2012), December 2012, DOI: 10.1145/2405178.2405181. [show abstract]

A lot of research effort has been invested to support efficient content-based routing. Nevertheless, practitioners often fall back to far less expressive communication paradigms like multicast groups. The benefits of content-based routing in minimizing bandwidth consumption are often rendered useless by simpler communication paradigms that rely on line-rate processing of data packets at the switches of the network providers. Contrary content-based routing protocols face the inherent overhead in matching the content of events against subscriptions leading to far lower throughput rates and higher end-to-end delays. However, recent trends in networking such as software defined networking in combination with network virtualization have tremendous potential to change the picture. In our opinion this will significantly increase acceptance of sophisticated middleware like content-based routing in the future. To support our claims we outline in this paper a reference architecture that may be used to build middleware for Future Internet applications. Furthermore, we provide a solution for realizing content-based routing at line-rate relying on this reference architecture and illustrate research problems that need to be addressed.

Patrick Baier, Harald Weinschrott, Frank Dürr, Kurt Rothermel: Effiziente automatisierte Erstellung von Straßenkarten. In 7. GI/ITG KuVS Fachgespräch Ortsbezogene Anwendungen und Dienste., pp. 85–92, Logos Verlag Berlin GmbH, September 2011. [show abstract]

Das relativ junge Paradigma des Urban Sensing ermöglicht die kostengünstige Bereitstellung von Sensordaten, welche in diesem Umfang bisher nicht zugänglich waren. Ein potentieller Verwendungszweck dieser Daten liegt im Bereich der Kartografie, indem von Mobilger”\aten erfasste GPS-Daten genutzt werden, um Straßenkarten automatisch zu erstellen. Dadurch kann eine Ersparnis hinsichtlich Aufwand und Kosten, im Vergleich zu konventionellen Methoden der Kartenerstellung, erzielt werden. Diese Arbeit stellt einen solchen Ansatz zur effizienten, automatisierten Erstellung von Straßenkarten mithilfe von GPS-Sensordaten vor. Diese Daten werden dabei automatisch von Personen gesammelt, die ihre Mobilgeräte wie gewohnt mit sich führen, zusätzlich aber auf ihren alltäglichen Wegen GPS-Positionsinformationen erfassen, welche sie einem zentralen System zur Verfügung stellen. Dies geschieht automatisch, ohne dass eine Interaktion dieser Personen nötig ist. Im Gegenzug soll der Ressourcenverbrauch der teilnehmenden Mobilgeräte möglichst minimiert werden. Daher koordiniert der in dieser Arbeit vorgestellte Ansatz die Erfassung der GPS-Daten so, dass die Mobilgeräte diese möglichst nur dann erfassen, wenn sie sich in einem Gebiet befinden, welches bis zu diesem Zeitpunkt noch nicht ausreichend kartografisch erfasst wurde. Um diese gezielte Koordination der Mobilgeräte zu ermöglichen, werden im Rahmen dieser Arbeit Qualitätsmetriken f\ur Geodaten vorgestellt, welche den erfassten Straßen quantitative Größen zuordnen, um so einen Vergleich dieser Daten zu ermöglichen.

Frank Dürr, Marius Wernke, Pavel Skvortsov, Kurt Rothermel: Towards a Position Sharing Approach for Location-based Services. In Proceedings of W3C Workshop on Privacy for Advanced Web APIs, July 2010. [show abstract]

Frank Dürr, Harald Weinschrott, Kurt Rothermel: Geocast Routing of Symbolically Addressed Messages in Wireless Mesh Networks (invited paper). In Proceedings of the 8th IEEE International Conference on Pervasive Computing and Communications Workshops (PerCom Workshops 2010), pp. 552–557, IEEE Press, March 2010, DOI: 10.1109/PERCOMW.2010.5470499. [show abstract]

Geocast protocols can be used to send messages to all receivers in a geographic target area. In this paper we present geocast routing algorithms for Wireless Mesh Networks that are tailored to symbolic addressing using symbolic location names like floor or room numbers. Since in particular indoors no geometric information is available, our algorithms use symbolic location models to derive directional information for routing. Moreover, we show how to integrate geometric and symbolic geographic routing algorithms into a hybrid routing approach which is applicable to larger areas consisting of symbolically and geometrically defined locations.

Ralph Lange, Harald Weinschrott, Lars Geiger, Andre Blessing, Frank Dürr, Kurt Rothermel, Hinrich Schütze: On a Generic Uncertainty Model for Position Information. In Quality of Context – Proceedings of the First International Workshop on Quality of Context (QuaCon 2009), Lecture Notes in Computer Science (LNCS) 5786, pp. 76-87, Springer-Verlag, June 2009, DOI: 10.1007/978-3-642-04559-2. [show abstract]

Position information of moving as well as stationary objects is generally subject to uncertainties due to inherent measuring errors of positioning technologies, explicit tolerances of position update protocols, and approximations by interpolation algorithms. There exist a variety of approaches for specifying these uncertainties by mathematical uncertainty models such as tolerance regions or the Dilution of Precision (DOP) values of GPS. In this paper we propose a principled generic uncertainty model that integrates the different approaches and derive a comprehensive query interface for processing spatial queries on uncertain position information of different sources based on this model. Finally, we show how to implement our approach with prevalent existing uncertainty models.

Frank Dürr, Jonas Palauro, Lars Geiger, Ralph Lange, Kurt Rothermel: Ein kontextbezogener Instant-Messaging-Dienst auf Basis des XMPP-Protokolls. In 5. GI/ITG KuVS Fachgespräch Ortsbezogene Anwendungen und Dienste., Sonderdruck Schriftenreihe der Georg-Simon-Ohm-Hochschule Nürnberg 42, pp. 23–28, Georg-Simon-Ohm-Hochschule Nürnberg, September 2008. [show abstract]

Dieser Beitrag beschreibt die Verteilung kontextbezogener Informationen mittels eines erweiterten Instant-Messaging-Dienstes. Dieser Dienst ermöglicht das Senden von Nachrichten an alle Teilnehmer, die einen bestimmten Kontext besitzen und sich z.B. an einem bestimmten Ort aufhalten oder bestimmte Interessen besitzen. Als Basis dient das Extensible Messaging and Presence Protokoll (XMPP) sowie ein XMPP-basierter Instant-Messaging-Dienst. Wir beschreiben Protokoll- und Architekturerweiterungen für die Integration von Kontextinformationen in das XMPP-Protokoll und die Server-Infrastruktur. Der erweiterte Dienst unterstützt insbesondere die Server-seitige Filterung von Nachrichten aufgrund von Kontextinformationen und ermöglicht dadurch die effiziente Nachrichtenverteilung.

Lars Geiger, Frank Dürr: Kontextbezogene Kommunikation. In 4. GI/ITG KuVS Fachgespräch Ortsbezogene Anwendungen und Dienste, pp. 22–26, Verlag Dr. Hut, September 2007. [show abstract]

Verbindet man die Position eines Benutzers mit weiteren Attributen wie beispielsweise Typ, Alter oder Status zu seinem Kontext, so ergeben sich daraus interessante neue Anwendungsmöglichkeiten. In diesem Artikel soll ein Verfahren vorgestellt werden, wie ein solcher Kontext benutzt werden kann, um Nachrichten an bestimmte Gruppen von Benutzern – bestimmt durch ihren Kontext – zu versenden. Ein solcher Kommunikationsmechanismus könnte beispielsweise eingesetzt werden, um Verkehrsnachrichten gezielt an alle Fahrzeuge innerhalb eines bestimmten Gebietes und mit einer bestimmten Fahrtrichtung zu übermitteln.

Martin Saternus, Torben Weis, Mirko Knoll, Frank Dürr: A Middleware for Context-Aware Applications and Services Based on Messenger Protocols. In Proceedings of the Fifth Annual IEEE International Conference on Pervasive Computing and Communications Workshops (PerComW 2007), pp. 467–471, IEEE Press, March 2007. [show abstract]

Nowadays most context-aware applications are independent proprietary pieces of software. A general framework or middleware deployed in the field does not exist; therefore the implementation of context-aware applications and services assumes the development of the whole application stack for each application. Along these development issues, deployment and privacy problems are to be solved. This imposes the following challenges concerning a middleware for context-aware applications: (1) We need an architecture that allows to implement and deploy services easily on the network. (2) We need a user interface that is widespread, well known to users and allows to mange one’s privacy settings for every single service transparently. In this paper we describe our middleware’s architecture for context-aware applications, based on messenger protocols.

Frank Dürr, Nicola Hönle, Daniela Nicklas, Christian Becker, Kurt Rothermel: Nexus—A Platform for Context-Aware Applications. In 1. GI/ITG Fachgespräch Ortsbezogene Anwendungen und Dienste, Informatik-Berichte der FernUniversität in Hagen 317, pp. 15–18, FernUniversität in Hagen, June 2004. [show abstract]

In this paper we present the Nexus Platform for context-aware applications. This platform allows to share the effort of setting up a global and detailed context model between different providers by federating their partial models. Applications can query data directly, use push-based communication through an event service, or use value-added services like a navigation or map service for special tasks. Additional context-aware services like hoarding or geocast can be implemented on basis of the platform. For the latter we present different approaches for addressing and efficient message forwarding based on the Nexus Platform.

Demos

Ralph Lange, Frank Dürr, Kurt Rothermel: Efficient Tracking of Moving Objects using Generic Remote Trajectory Simplification. In Proceedings of the 8th IEEE International Conference on Pervasive Computing and Communications Workshops (PerCom Workshops 2010), pp. 829–831, IEEE Press, March 2010, DOI: 10.1109/PERCOMW.2010.5470553. [show abstract]

Position information of moving objects plays a vital role in many pervasive applications. Therefore, moving objects databases (MODs), which can manage trajectory data of a number objects, are used in many pervasive systems. A crucial problem with MODs is how to efficiently track a remote object’s trajectory in real-time, i.e. how to continuously report the sensed trajectory data to the MOD with minimal effort. For this purpose, we present a prototypical implementation of the Generic Remote Trajectory Simplification (GRTS) protocol, which optimizes storage consumption, processing, and communication costs. Our prototypical system includes a fully functional MOD as well as map-based mobile applications for subnotebooks and smartphones to illustrate the functioning of GRTS.