André Herms

Heuristic constructive algorithms for open shop scheduling to minimize mean flow time

In this paper, we consider the problem of scheduling n jobs on m machines in an open shop environment so that the sum of completion times or mean flow time becomes minimal. For this strongly NP-hard problem, we develop and discuss different constructive heuristic algorithms. Extensive computational results are presented for problems with up to 50 jobs and 50 machines, respectively. The quality of the solutions is evaluated by a lower bound for the corresponding preemptive open shop problem and by an alternative estimate of mean flow time. We observe that the recommendation of an appropriate constructive algorithm strongly depends on the ratio n/m.

Precise Admission Control for Bandwidth Reservation in Wireless Mesh Networks

Quality of service in wireless mesh networks is an often requested feature for various kinds of applications. A common approach is the establishment of routes based on hop-by-hop reservation of bandwidth. In this paper we address the problems of admission control in wireless mesh networks. We show that the existing solutions suffer from temporal inconsistencies during the distributed admission process, which lead to high admission failure rates of approximately 2% to 10% in typical topologies, even with the best approach. A solution for this problem is presented, based on the two phase commit protocol. It prevents inconsistencies and the corresponding admission failures.

Exploiting publish/subscribe communication in wireless mesh networks for industrial scenarios

This paper addresses questions of using wireless mesh networks (WMNs) in heterogeneous industrial infrastructures. This implies several problems like global addressing, handling of QoS requirements, interconnection with embedded networks. Our proposed solution is a publish/subscribe middleware. We discuss how it solves the mentioned problems. Real-world measurements and simulation results give an idea of QoS properties. A case study of controlling a mobile robot is presented. The results show that the middleware is well suited for non-critical control and monitoring tasks.

Measurement-Based Detection of Interfering Neighbors for QoS in Wireless Mesh Networks

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

Real-Time Mesh Networks for Industrial Applications

Wireless LANs (IEEE 802.11) are increasingly used in industrial applications. They reduce cabling costs, increase flexibility and enable mobile applications for maintenance or logistics tasks. Mesh networks provide a self-configuring and -healing wireless backbone for large scale deployments (e.g. in process automation). This paper presents a routing algorithm which provides QoS in wireless mesh networks, thus leveraging their use in industrial applications. It allows reserving bandwidth for real-time flows based on measurements of the physically available bandwidth. Thus it fully utilizes the bandwidth while still preventing congestion. Simulation results demonstrate the reliability of the algorithm and its advantage over previous works.

Preventing admission failures of bandwidth reservation in wireless mesh networks

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

Interval based off-line clock synchronization for wireless mesh networks

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

Dependable wireless mesh networks: An integrated approach

Wireless Mesh Networks (WMNs) are replacing wireless Infrastructure networks in many areas because of their lower cost and higher flexibility. However, applications in the Process Industry and Telerobotics field require not only flexible but also dependable service, which is not provided by existing solutions. To make a WMN dependable, many problems have to be solved on different layers. With this paper, we describe our ongoing work to provide an integrated solution to increase the dependability of WMNs. Our approach combines network coverage planning on the physical layer, bandwidth management on the link layer and live network monitoring to improve the reliability, availability and maintainability of a WMN. We provide fine-grained means to improve the predictability of the network components, thus making the WMN more dependable. In this paper, we present first results of our ongoing work, and describe how they are interleaved.

An integrated approach for reliability and dependability of Wireless Mesh Networks

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

A COMPARISON OF HEURISTICS FOR MEAN FLOW TIME OPEN SHOP SCHEDULING

Abstract In this paper, the problem of scheduling n jobs on m machines in an open shop environment is considered so that mean flow time becomes minimal. Since this problem is strongly NP-hard, different constructive and iterative heuristic algorithms are developed and discussed. Computational results are presented for problems with up to 50 jobs and 50 machines, respectively. The quality of the solutions is estimated by a lower bound for the corresponding preemptive open shop problem and by an alternative estimation of mean flow time.