Nils Müllner

Modeling options for demand side participation of thermostatically controlled loads

Residential thermostatically controlled loads (TCLs) have potential for participation in electricity markets. This is because we can control a large group of these loads to achieve aggregate system behavior such as providing frequency reserves while ensuring the control actions are non-disruptive to the end users. A main challenge in controlling aggregations of TCLs is developing dynamical system models that are simple enough for optimization and control, but rich enough to capture the behavior of the loads. In this work, we propose three classes of models that approximate aggregate TCL dynamics. We analyze these models in terms of their accuracy and computational tractability. The models demonstrate a progression from models that help us analyze and predict TCL population behavior to those that help us develop large-scale automatic control strategies. Specifically, we demonstrate how formal methods from computer science and optimal control can be used to derive bounds on model error, guarantees for trajectory tracking, and algorithms for price arbitrage. We find that the accuracy of the analytic results decreases as TCL parameter heterogeneity is introduced. Thus, we motivate further development of analytical tools and modeling approaches to investigate realistic TCL behavior in power systems.

Derivation of Fault Tolerance Measures of Self-Stabilizing Algorithms by Simulation

Fault tolerance measures can be used to distinguish between different self-stabilizing solutions to the same problem. However, derivation of these measures via analysis suffers from limitations with respect to scalability of and applicability to a wide class of self-stabilizing distributed algorithms. We describe a simulation framework to derive fault tolerance measures for self-stabilizing algorithms which can deal with the complete class of self-stabilizing algorithms. We show the advantages of the simulation framework in contrast to the analytical approach not only by means of accuracy of results, range of applicable scenarios and performance, but also for investigation of the influence of schedulers on a meta level and the possibility to simulate large scale systems featuring dynamic fault probabilities.

The Degree of Masking Fault Tolerance vs. Temporal Redundancy

Self-stabilizing systems, intended to run for a long time, commonly have to cope with transient faults during their mission. We model the behavior of a distributed self-stabilizing system under such a fault model as a Markov chain. Adding fault detection to a self-correcting non-masking fault tolerant system is required to progress from non-masking systems towards their masking fault tolerant functional equivalents. We introduce a novel measure, called limiting window availability (LWA) and apply it on self-stabilizing systems in order to quantify the probability of (masked) stabilization against the time that is needed for stabilization. We show how to calculate LWA based on Markov chains: first, by a straightforward Markov chain modeling and second, by using a suitable abstraction resulting in a space-reduced Markov chain. The proposed abstraction can in particular be applied to spot fault tolerance bottlenecks in the system design.

Cooperation and network coding based MAC protocol for VANETs

In the last decade, cooperative communication and network coding techniques gained rising attention. Based on these techniques, this paper introduces a novel protocol called cooperation and network coding based MAC (CNCMAC) for vehicular ad-hoc networks (VANETs). It employs cooperative automatic repeat request (ARQ) and network coding techniques to enhance throughput. The CNC-MAC protocol works in two phases. In the first phase - cooperation - data is sent to relay nodes. The second phase - network coding - minimizes received packets. The performance of the CNC-MAC protocol is compared with cooperative ARQ-MAC, another common protocol in this domain. The simulation results indicate that CNC-MAC outperforms ARQ-MAC.

Combining decomposition and reduction for state space analysis of a self-stabilizing system☆

Fault tolerance measures of distributed systems can be calculated precisely by state space analysis of the Markov chain corresponding to the product of the system components. The power of such an approach is inherently confined by the state space explosion, i.e. the exponential growth of the Markov chain in the size of the underlying system. We propose a decompositional method to alleviate this limitation. Lumping is a well-known reduction technique facilitating computation of the relevant measures on the quotient of the Markov chain under lumping equivalence. In order to avoid computation of lumping equivalences on intractably large Markov chains, we propose a system decomposition supporting local lumping on the considerably smaller Markov chains of the subsystems. Recomposing the lumped Markov chains of the subsystems constructs a lumped transition model of the whole system, thus avoiding the construction of the full product chain. An example demonstrates how the limiting window availability (i.e. a particular fault tolerance measure) can be computed for a self-stabilizing system exploiting lumping and decomposition.

Combining Decomposition and Reduction for State Space Analysis of a Self-Stabilizing System

Verifying fault tolerance properties of a distributed system can be achieved by state space analysis via Markov chains. Yet, the power of such exact analytic methods is confined by exponential growth of the chains state space in the size of the system modeled. We propose a method that alleviates this limit. Lumping is a well known reduction technique that can be applied to a Markov chain to prune redundant information. We propose a system decomposition to employ lumping piecewise on the considerably smaller Markov chains of the subsystems which are much more likely to be tractable. Recomposing the lumped Markov chains of the subsystems results in a state space that is likely to be considerably smaller. An example demonstrates how the limiting window availability (i.e. a fault tolerance property) can be computed for a system while exploiting the combination of lumping and decomposition.

Poster: A hybrid MAC scheme for emergency systems in urban VANETs environment

Driven by road safety requirements and intelligent traffic control, Vehicular Ad-hoc Networks (VANETs) have been attracting significant interest in both academia and industry. One key goal of VANETs is to provide sufficient Quality-of-Service (QoS) for real-time safety applications. The MAC protocol for VANETs is crucial to provide for an efficient and reliable medium access. The challenge is to design it to be able to cope with rapid topology changes, high node mobility [1] and different QoS requirements. Safety applications require a high reliability and a bounded delay while non-critical applications are rather throughput sensitive [2]. These applications facilitate wireless broadcast warning messages between neighboring vehicles to inform drivers on time about dangerous situations like accidents. The IEEE 1609.4 standard defines a MAC layer implementation for multichannel operations in VANETs for such broadcasts. In this poster paper, we present a distributed hybrid MAC scheme for emergency systems. The proposed scheme improves the Control Channel (CCH) utilization and uniformly distributes the load on Service Channels (SCHs). In the proposed scheme, the network switches its mode from general to emergency in order to effectively utilize common service channels thus increase the probability for timely reception of messages. In addition, the proposed scheme reduces the rate of transmission collisions and latency, thus resulting in high probability of message delivery. The proposed MAC scheme is simulated and compared with the existing competitors. The obtained results depict that the proposed hybrid scheme outperforms its competitors and thus validates its performance.

Composing Thermostatically Controlled Loads to Determine the Reliability against Blackouts

Power grids are parallel systems in which consumers demand a shared resource independent of each other. A blackout occurs when the total demand increases or decreases too rapidly. This paper combines methods and concepts from three domains. The first stems from estimating the power consumption based on thermostatically controlled loads via Markov chains. The second domain provides the composition of parallel systems enriched by intermediate lumping to construct a minimal aggregate transition model, in this case of a community of housings. The third domain provides reasoning about fault tolerance properties by introducing limiting window reliability as measure, suitable to account for the continuous risk of blackouts. Combined, the three methods and concepts allow to determine the risk of blackout of a community over time.

A Traffic Aware Segment-based Routing protocol for VANETs in urban scenarios

Abstract Vehicular ad-hoc networks (VANETs) offer a diverse set of applications and therefore gain more and more attention from both academic and industrial communities. However, the deployment of VANETs is not very straight-forward. One challenge is highlighted by an uphill task of establishing and subsequently sustaining a robust communication. The need to obviate extra relay infrastructure in dynamically fluctuating topologies plus concurring shielding obstacles only magnifies this arduous task. In this context, information about traffic-density and about its estimated progress are valuable assets to tackle this issue. This paper proposes a novel routing protocol called Traffic Aware Segment-based Routing (TASR) protocol. The proposed protocol comprises two major parts: 1) Real-time vehicular traffic information for route selection allows for calculating the Expected Connectivity Degree (ECD) on different segments, and 2) a new forwarding method based on geographical information transfers packets from source to destination node. The new metric ECD takes vehicle densities into account, estimating the connectivity on each segment and thus the connectivity of nodes and data delivery ratio for transmitting packets. Furthermore, extensive simulations help analyzing the efficiency of TASR, indicating that it outperforms competing routing protocols.

Simulation-Based Safety Testing Brake-by-Wire

Mechanical systems in cars are replaced by electronic equivalents. To be authorized for the road, validation that the replacements are at least as good as the old systems is required. For electronic braking systems (brake-by-wire), this goodness translates to safety in terms of maintaining timing constraints. Yet, in the future, the safety of braking maneuvres will depend, not only, on electronic brakes, but also on cooperative driving maneuvres orchestrated among many cars. Connecting both brake-by-wire on the microscopic level with cooperative braking on the macroscopic level allows for determining safety on a broader scale, as both systems feed from the same resource: Time. This paper discusses work-in-progress, introducing and combining two threads: electronic brakes and cooperative braking. Discussing safety on two levels simultaneously motivates connecting a Simulink model of an electronic brake-by-wire system with the traffic simulator SUMO for conducting the required combined validation. How safe is a car in relation to a given maximal braking distance? What is the optimal distribution of reaction time between electronic brakes and cooperative braking? The validation focuses on non-functional safety limited by temporal constraints (translated to braking distance). It can be exploited in an early validation approach to help reduce costs of more expensive real world experimentation. It can also determine the boundaries at which sufficient safety can be guaranteed.