Publikationen

Abstract: Using wireless sensor and actuator networks in industrial applications promises timely and fine-grained feedback and control of plants. However, these applications call for very high reliability that cannot be fulfilled with contention-based medium access. Therefore, the IEEE 802.15.4 standard was extended with multiple time-slotted as well as frequency-agile medium access techniques. The Deterministic and Synchronous Multi-Channel Extension (DSME) is of particular interest due to its extensive set of standardized methods for distributed slot management. This chapter presents openDSME, a comprehensive implementation of DSME to be used in the OMNeT++ simulator as well as on real-life wireless sensor nodes. The main features of DSME are presented, together with implementation details of openDSME. The chapter concludes with a step-by-step tutorial to get started with openDSME.

Abstract: Using event-based simulations is an excellent method for demonstrating and learning the functionality of computer networks. OMNeT++ provides many features for building and analyzing networks and is widely used in research and teaching. It is, however, difficult to influence a running simulation and the interfaces are more optimized for in-depth analyses so it is easy to get distracted from the main point of interest. This motivation led to the development of a remote interface for the OMNeT++ simulator that facilitates live modifications of parameters as well as monitoring of events. It is based on web technologies and allows for convenient creation of customized interactive interfaces for conferences, fairs, or teaching environments.

Abstract: Wireless mesh networks are a promising technology for connecting sensors and actuators with high flexibility and low investment costs. In industrial applications, however, reliability is essential. Therefore, two time-slotted medium access methods, DSME and TSCH, were added to the IEEE 802.15.4 standard. They allow collision-free communication in multi-hop networks and provide channel hopping for mitigating external interferences. The slot schedule used in these networks is of high importance for the network performance. This paper supports the development of efficient schedules by providing an analytical model for the assessment of such schedules, focused on TSCH. A Markov chain model for the finite queue on every node is introduced that takes the slot distribution into account. The models of all nodes are interconnected to calculate network metrics such as packet delivery ratio, end-to-end delay, and throughput. An evaluation compares the model with a simulation of the Orchestra schedule. The model is applied to Orchestra as well as to two simple distributed scheduling algorithms to demonstrate the importance of traffic-awareness for achieving high throughput.

Abstract: Wireless communications techniques are finding their way into industrial applications, but commercial wireless technologies lack the dependability required for critical applications in very large networks. In many wireless-controlled industrial facilities, safety regulations mandate real-time communications for tasks such as emergency shutdowns. This requires a wireless communications architecture that supports the coexistence of non-real-time and real-time tasks. To address this requirement, this paper proposes an architecture that augments a wireless mesh network with a unidirectional long-range communications system. Various configuration parameters of the system, including its security features, are optimized. Additionally, a physical realization of the architecture is evaluated using a series of experiments, including some performed on a real solar tower power plant. The experimental results demonstrate that the extension of a wireless mesh network with a unidirectional long-range communications system provides safety and scalability for industrial applications while promoting cost effectiveness and energy efficiency.

Abstract: Wireless mesh networks using IEEE 802.15.4 are getting increasingly popular for industrial applications because of low energy consumption and low maintenance costs. The IEEE 802.15.4 standard introduces DSME (Deterministic and Synchronous Multi-channel Extension). DSME uses time-slotted channel access to guarantee timely data delivery, multi-channel communication, and frequency hopping to mitigate the effects of external interferences. A distinguishing feature of DSME is its flexibility and adaptability to time-varying network traffic and to changes in the network topology. In this paper we evaluate the ability of DSME to adapt to time-varying network traffic. We examine the limits for slot allocation rates for different topologies. The evaluation is performed with openDSME, an open-source implementation of DSME.

Abstract: Testbeds are a key element in the evaluation of wireless multi-hop networks. In order to relieve researchers from the hassle of deploying their own testbeds, remotely controllable testbeds, such as the FIT/IoT-LAB, are built. However, while the IoT-LAB has a high number of nodes deployed in constraint areas. This, together with the complex nature of radio propagation, makes an ad-hoc construction of multi-hop topologies with higher number of hops difficult. This work presents a strategic approach to solve this problem and proposes algorithms to generate topologies with desired properties. The implementation is provided as open-source software. The algorithms are evaluated for the IoT-LAB testbeds. The results show that preset topologies of various types can be built even in dense wireless testbeds.

Abstract: Discrete event simulators, such as OMNeT++, provide fast and convenient methods for the assessment of algorithms and protocols, especially in the context of wired and wireless networks. Usually, simulation parameters such as topology and traffic patterns are predefined to observe the behaviour reproducibly. However, for learning about the dynamic behaviour of a system, a live interaction that allows changing parameters on the fly is very helpful. This is especially interesting for providing interactive demonstrations at conferences and fairs. In this paper, we present a remote interface to OMNeT++ simulations that can be used to control the simulations while visualising real-time data merged from multiple OMNeT++ instances. We explain the software architecture behind our framework and how it can be used to build demonstrations on the foundation of OMNeT++.

Abstract: In cyber-physical systems, such as modern industrial plants, complex software is an essential part that enables cost-effective and flexible operation. However, this complexity increases the probability of problems that only reveal themselves after the deployment. This is even more important if security aspects are involved. Therefore, providing the possibility for software updates is an important building block in the design of industrial plants. This paper presents a holistic concept for software updates in an industrial plant with thousands of wirelessly connected embedded devices. Using wireless technology imposes additional difficulties in terms of data rate, packet size and reliability that have to be addressed in particular. The contribution also includes an analytical model to estimate the time until a new firmware is distributed. Evaluations carried out on hardware as well as in the OMNeT++ simulator demonstrate the applicability and scalability of the proposed approach.

Abstract: The Deterministic and Synchronous Multi-Channel Extension (DSME) of the IEEE 802.15.4 standard provides a data link layer for time division multiple access in wireless mesh networks. The authors present openDSME, a portable implementation for hardware and simulators which promises reliable message transfer suitable for applications in demanding industrial environments. A demonstration has been developed to illustrate the performance of openDSME in a simulated network and to show its benefits over CSMA/CA.

Abstract: Providing dependability is still a major issue for wireless mesh networks, which restrains their application in industrial contexts. The widespread CSMA/CA medium access can provide high throughput and low latency, but can not prevent packet loss due to collisions, especially in very large and dense networks. Time slotted medium access techniques together with a distributed slot management, as proposed by the Distributed Synchronous Multi-channel Extension (DSME) of the IEEE 802.15.4 standard, are promising to provide low packet loss, high scalability and bounded end-to-end delays. However, our implementation, openDSME, exposed some weaknesses. While the allocated slots allow for reliable data transmission, the slot management itself is conducted via CSMA/CA and is thus vulnerable to packet loss, eventually leading to an inconsistent slot allocation. This paper uses the UPPAAL framework for formal analysis and verification of the slot management process. The analysis identifies weaknesses of the slot allocation process under communication and node failures. However, it is shown that inconsistencies are eventually resolved and improvements to the procedure are proposed that reduce the negative impact of failed slot allocation procedures significantly.

Abstract: Reliable wireless solutions for large-scale automation are a major challenge today. The IEEE 802.15.4 standard forms the basis for many open and proprietary implementations. To reflect current state-of-the-art techniques, the IEEE has amended standard 802.15.4 with new MAC-layers such as TSCH, which resembles WirelessHART, and the Deterministic and Synchronous Multi-Channel Extension (DSME). This paper introduces openDSME, our implementation of IEEE 802.15.4 DSME. DSME aims at preventing packet collisions through slot reservation in networks where conventional CSMA/CA is not reliable enough. In this document, we will outline core features of DSME and openDSME, and present details of our implementation. Additionally, current research efforts on connected topics will be highlighted.

Abstract: Using wireless communication instead of laying cables promises high cost savings in many applications. Motivated by the intended deployment of wireless networks in solar tower power plants with up to hundreds of thousands of possible clients, this research shall explore the limits of large-scale wireless networks. Apart from the pure number of supported clients, maintaining reliability and time-bounded communication is a key requirement for such industrial control systems.

Abstract: Ensuring reliability is a key requirement for using wireless mesh networks in industrial applications. Analytical models provide a valuable support in the design of reliable networks, especially for large scale applications, where simulations do not provide the required scalability. This paper proposes an analytical model for multi-hop IEEE 802.15.4 networks that incorporates significant improvements compared to previous models. Comparisons of the model with simulations using the MiXiM framework for OMNeT++ exhibit a very good compliance. The analytical model is therefore adequate for assessment of very large wireless mesh networks. Additionally, the model yields new insights into the characteristics of IEEE 802.15.4 networks, regarding the impact of acknowledgment collisions and the influence of hidden nodes on retransmissions.

Abstract: The AutoR-project takes a holistic approach to reduce the cost of heliostat fields: Wireless control and energy supply enables to use smaller heliostats which need less steel per mirror area (but usually have high wiring cost). A low cost but high efficient drive system is chosen which reduces energy consumption to a minimum amount and leads to low cost for PV cell and energy storage. The usual boundary layer wind tunnels tests for heliostats are proven regarding energy spectra to avoid oversizing of steel structure and drives or failures because of underestimations of the loads. The concepts for wireless control and energy supply, the wind tunnel investigations and the first rim drive heliostat prototype are presented.

Abstract: Developing scalable software for large-scale applications, particularly for networks and circuits, is challenging due to the underlying unstructured and irregular geometry of the problem. We present a programming framework recently added to the PETSc library to easily express network problems, and thereby reduce the application development time. A brief overview of the framework is presented and two application examples, one from power grid and the other from radio networks, are discussed.

Abstract: Today's public Internet availability and capabilities allow manifold applications in the field of multimedia that were not possible a few years ago. One emerging application is the so-called Networked Music Performance, standing for the online, low-latency interaction of musicians. This work proposes a stand-alone device for that specific purpose and is based on a Raspberry Pi running a Linux-based operating system.

Abstract: Combining energy harvesting with energy-aware scheduling enables perpetually operating sensor networks. The practical realization of this goal yet requires reliable and precise holistic online energy assessment. While the building blocks--assessing residual energy, predicting energy intake, and tracing energy consumption--have been studied in detail, the analysis of their interaction on a real platform has been neglected. This paper answers the question, whether these techniques can be easily joined to give a precise and correct picture of a sensor node's energetic state and behavior. For this purpose, we model the energy flow of a prototype energy-harvesting sensor node and evaluate the joint performance of state-of-the-art energy assessment based on a field test. We verify the system model and show the feasibility of holistic energy assessment, which tolerates small configuration errors, achievable with a combination of generic configuration and online calibration. We also analyze the feasibility of forecasting a node's future energetic state, and find that the presented method gives sufficient results for uniformly distributed consumption profiles.

Abstract: This paper presents an algorithm to dynamically determine the maximum supported uniform demand for energy of sensor nodes powered by energy harvesters using supercapacitors as energy buffers. Knowledge about the maximum uniform consumption is required to adapt the sensor node's duty cycle or task schedule to achieve uniform, utility-maximizing, and depletion-safe operation. Our algorithm makes use of a supercapacitors' relationship between state-of-charge and voltage, is particularly designed to handle the non-linear system model, and is lightweight enough to run on low-power sensor node hardware. We define three energy policies, evaluate their performance using a real-world solar-harvesting trace, and analyze the influence of the supercapacitor's capacity and errors of the energy forecast.

Abstract: Combining energy harvesting with energy-aware scheduling enables perpetually operating sensor networks. Practical realization yet requires precise holistic online energy assessment. The building blocks are available, but the analysis of their interaction has been neglected. To close the gap, we evaluate the joint performance of energy assessment components. Our experiments substantiate that holistic energy assessment is feasible and that small configuration errors are tolerable.

Abstract: Wireless technology promises flexible and cost-efficient machine-to-machine communication. However, high packet loss can emerge from simultaneous transmissions of many devices, undermining the reliability required for industrial applications. This thesis analyzes and develops techniques for time-slotted multi-hop communication with focus on IEEE 802.15.4 DSME. In a holistic approach, simulations, formal and analytical analyses and testbed experiments are combined, concluding the utility of the proposed methods. For future usage in research and real-world deployments, openDSME is developed as open-source implementation of DSME and application-specific insights are provided.

Abstract: Wireless communication is a key element in the realization of the Industrial Internet of Things for flexible and cost-efficient monitoring and control of industrial processes. Wireless mesh networks using IEEE 802.15.4 have a high potential for executing monitoring and control tasks with low energy consumption and low costs for deployment and maintenance. However, conventional medium access techniques based on carrier sensing cannot provide the required reliability for industrial applications. Therefore, the standard was extended with techniques for time-slotted medium access on multiple channels. In this paper, we present openDSME, a comprehensive implementation of the Deterministic and Synchronous Multi-channel Extension (DSME) and propose a method for traffic-aware and decentralized slot scheduling to enable scalable wireless industrial networks. The performance of DSME and our implementation is demonstrated in the OMNeT++ simulator and on a physically deployed wireless network in the FIT/IoT-LAB. It is shown that in the given scenarios, twice as much traffic can be delivered reliably by using DSME instead of CSMA/CA and that the energy consumption can be reduced significantly. The paper is completed by presenting important trade-offs for parameter selection and by uncovering open issues of the current specification that call for further effort in research and standardization.

Abstract: Wireless mesh networks are a promising technology for connecting sensors and actuators with high flexibility and low investment costs. In industrial applications, however, reliability is essential. Therefore, two time-slotted medium access methods, DSME and TSCH, were added to the IEEE 802.15.4 standard. They allow collision-free communication in multi-hop networks and provide channel hopping for mitigating external interferences. The slot schedule used in these networks is of high importance for the network performance. This paper supports the development of efficient schedules by providing an analytical model for the assessment of such schedules, focused on TSCH. A Markov chain model for the finite queue on every node is introduced that takes the slot distribution into account. The models of all nodes are interconnected to calculate network metrics such as packet delivery ratio, end-to-end delay and throughput. An evaluation compares the model with a simulation of the Orchestra schedule. The model is applied to Orchestra as well as to two simple distributed scheduling algorithms to demonstrate the importance of traffic-awareness for achieving high throughput.

Abstract: Testbeds are a key element in the evaluation of wireless multi-hop networks. In order to relieve researchers from the hassle of deploying their own testbeds, remotely controllable testbeds, such as the FIT/IoT-LAB, are built. However, while the IoT-LAB has a high num- ber of nodes, they are deployed in constraint areas. This, together with the complex nature of radio propagation, makes an ad-hoc construction of multi-hop topologies with a high number of hops difficult. This work presents a strategic approach to solve this problem and proposes algo- rithms to generate topologies with desired properties. The implementa- tion is evaluated for the IoT-LAB testbeds and is provided as open-source software. The results show that preset topologies of various types can be built even in dense wireless testbeds.

Abstract: The IEEE 802.15.4 standard allows for the deployment of cost-effective and energy-efficient multi-hop networks. This document features an in-depth presentation of an analytical model for assessing the performance of such networks. It considers a generic, static topology with Poisson distributed data-collection as well as data-dissemination traffic. The unslotted CSMA/CA MAC layer of IEEE 802.15.4 is closely modeled as well as an enhanced model of the neighborhood allows for consideration of collisions of packets including interferences with acknowledgements. The hidden node problem is taken into account as well as a formerly disregarded effect of repeated collisions of retransmissions. The model has been shown to be suitable to estimate the capacity of large-scale multi-hop networks.