Andreas Willig

Recent and Emerging Topics in Wireless Industrial Communications: A Selection

In this paper we discuss a selection of promising and interesting research areas in the design of protocols and systems for wireless industrial communications. We have selected topics that have either emerged as hot topics in the industrial communications community in the last few years (like wireless sensor networks), or which could be worthwhile research topics in the next few years (for example cooperative diversity techniques for error control, cognitive radio/opportunistic spectrum access for mitigation of external interferences).

Wireless Technology in Industrial Networks

With the success of wireless technologies in consumer electronics, standard wireless technologies are envisioned for the deployment in industrial environments as well. Industrial applications involving mobile subsystems or just the desire to save cabling make wireless technologies attractive. Nevertheless, these applications often have stringent requirements on reliability and timing. In wired environments, timing and reliability are well catered for by fieldbus systems (which are a mature technology designed to enable communication between digital controllers and the sensors and actuators interfacing to a physical process). When wireless links are included, reliability and timing requirements are significantly more difficult to meet, due to the adverse properties of the radio channels. In this paper, we thus discuss some key issues coming up in wireless fieldbus and wireless industrial communication systems: 1) fundamental problems like achieving timely and reliable transmission despite channel errors; 2) the usage of existing wireless technologies for this specific field of applications; and 3) the creation of hybrid systems in which wireless stations are incorporated into existing wired systems.

Measurements of a wireless link in an industrial environment using an IEEE 802.11-compliant physical layer

The design and simulation of coding schemes, medium access control (MAC), and link-layer protocols for future industrial wireless local area networks can be supported by some understanding of the statistical properties of the bit error patterns delivered by a wireless link (which is an ensemble of transmitter, channel, receiver, modems). The authors present results of bit error measurements taken with an IEEE 802.11-compliant radio modem in an industrial environment. In addition to reporting the most important results, they draw some conclusions for the design of MAC and link-layer protocols. Furthermore, they show that the popular Gilbert/Elliot model and a modified version of it are a useful tool for simulating bit errors on a wireless link, despite their simplicity and failure to match certain measured statistics.

Chaotic maps as parsimonious bit error models of wireless channels

The error patterns of a wireless digital communication channel can be described by looking at consecutively correct or erroneous bits (runs and bursts) and at the distribution function of these run and burst lengths. A number of stochastic models exist that can be used to describe these distributions for wireless channels, e.g., the Gilbert-Elliot model. When attempting to apply these models to actually measured error sequences, they fail: measured data gives raise to two essentially different types of error patterns which can not be described using simple error models like Gilbert-Elliot. These two types are distinguished by their run length distribution; one type in particular is characterized by a heavy-tailed run length distribution. This paper shows how the chaotic map model can be used to describe these error types and how to parameterize this model on the basis of measurement data. We show that the chaotic map model is a superior stochastic bit error model for such channels by comparing it with both simple and complex error models. Chaotic maps achieve a modeling accuracy that is far superior to that of simple models and competitive with that of much more complex models, despite needing only six parameters. Furthermore, these parameters have a clear intuitive meaning and are amenable to direct manipulation. In addition, we show how the second type of channels can be well described by a semiMarkov model using a quantized lognormal state holding time distribution.

Polling-based MAC protocols for improving real-time performance in a wireless PROFIBUS

The idea of having a wireless PROFIBUS is appealing, since this can bring benefits like reduced cabling need and mobile stations to the factory floor. However, unfortunately, wireless transmission is error prone, which affects the timeliness and reliability behavior users expect from a fieldbus system (hard real time). In this paper, we compare two different approaches for the medium access control (MAC) and link layer of a wireless PROFIBUS system with respect to their so-called real-time performance in the presence of transmission errors. Specifically, we compare the existing PROFIBUS MAC and link layer protocol with a simple round-robin protocol. It is shown that round-robin delivers significantly better real-time performance than the PROFIBUS protocol under bursty error conditions. In a second step, we propose three add-ons to round-robin and we show that they further increase the real-time performance of round-robin. The add-ons take certain characteristics of the wireless medium into account.

A new class of packet- and bit-level models for wireless channels

Stochastic channel error models are an important part of performance evaluations of wireless protocols. In fact, for a given protocol, the performance measures of interest seen in simulations often depend on the choice of the error models, simpler models often giving poorer quality performance predictions. The paper presents a new class of Markovian-based channel models, called bipartite models. They allow the user to choose freely the desired model complexity and therefore the model accuracy. We demonstrate through simulations of an example system that this model class gives much more accurate predictions of performance parameters than other popular channel models.

Ring stability of the PROFIBUS token-passing protocol over error-prone links

The PROFIBUS is a well-known and widely used fieldbus. On the medium access control layer, it employs a token-passing protocol where all active stations form a logical ring on top of a broadcast medium. This protocol is designed to deliver real-time data transmission services in harsh industrial environments. A necessary prerequisite for timeliness and quality of service is the ring membership stability of the logical ring in the presence of transmission errors, since only ring members are allowed to transmit data. In this paper, the ring membership stability under high error rates and using different error models is analyzed. The choice of the error behavior is in turn inspired by properties of possible future transmission technologies, e.g., wireless LANs. It is shown that the protocol has serious stability problems. To attack these problems, two changes to the protocol and its parameters are proposed, which can be implemented in a purely local manner. We show that they significantly improve ring stability.

Data Transport Reliability in Wireless Sensor Networks. A Survey of Issues and Solutions

ABSTRACT Reliable data transport is an important facet of dependability and quality of service in wireless sensor networks. This paper gives an introduction to the reliable data transport problem and surveys protocols and approaches for this protocol, often developed for particular applications to reflect the application-specific dependability requirements. A joint characteristic of many of the discussed protocols is that they combine mechanisms from several layers to achieve their reliability goals while being energy-efficient. This very need to be energy-efficient precludes Internet-style approaches to reliability – handle it in the end system – and necessitates in-network solutions.

An architecture for wireless extension of PROFIBUS

Industrial communication systems and fieldbus systems can benefit in many ways from wireless LAN technology. However, the large amount of already deployed fieldbus LANs motivates to focus on wireless extensions of existing fieldbus systems, allowing to run wireless and wired stations in the same fieldbus LAN. There exist different architectural options for the coupling of wireless stations to a wired fieldbus LAN. For the PROFIBUS, a well-known and standardized fieldbus system we present and motivate a specific coupling architecture, the virtual ring extension. In this architecture the lower layer protocols of the PROFIBUS (medium access control layer and link layer) are not used on the wireless side. Instead, a set of specifically tailored protocols is used, which allows to take the error behavior of wireless channels into account, which is significantly different from that of wired channels. The key elements of this architecture are presented and some operational aspects are discussed in more detail.

Guest Editorial Special Section on Wireless Technologies in Factory and Industrial Automation—Part II

The three papers in this special section focus on wireless technologies in factory and industrial automation. The papers which appear in this second part cover everything from protocol design and evaluation to the design and assessment of system-level solutions for wireless sensor networks in industrial automation.