Lucia Seno

On the Performance of IEEE 802.11e Wireless Infrastructures for Soft-Real-Time Industrial Applications

Nowadays, wireless communication technologies are being employed in an ever increasing number of different application areas, including industrial environments. Benefits deriving from such a choice are manifold and include, among the others, reduced deployment costs, enhanced flexibility and support for mobility. Unfortunately, because of a number of reasons that have been largely debated in the literature, wireless systems cannot be thought of as a means able to fully replace wired networks in production plants, in particular, when real-time behavior is a key issue. In this paper, an analysis of the real-time performance that can be achieved in quality-of-service (QoS)-enabled 802.11 networks has been carried out. In particular, a detailed analysis of latencies and packet loss ratios for a typical enhanced distributed channel access (EDCA) infrastructure wireless local area network (WLAN) is presented, obtained through numerical simulations. A number of aspects that may affect suitability for the use in control systems have been taken into account, including the Transmission Opportunity (TXOP) mechanism, the internal architecture of the AP, the use of a time-division multiple access (TDMA)-based communication scheme as well as the adoption of broadcast communications.

Performance analysis of Ethernet Powerlink networks for distributed control and automation systems

Industrial communication networks are a key element for developing advanced distributed control and automation systems. On the one hand, high performance and low costs are generally required to cope with more and more demanding application requirements, while, on the other hand, real-time capabilities are often needed in an increasing number of automation scenarios. Communication solutions based on Ethernet technologies are becoming popular in many industrial and factory environments and a number of soft/hard real-time competing products have been proposed in the last few years. This paper focuses on the Ethernet Powerlink standard, in general, and on some of its real-time characteristics, in particular. In fact, we present some basic results concerning both cyclic and acyclic real-time traffic in Ethernet Powerlink networks, obtained by extensive simulation, which can be used as a basic step for a better understanding of the real-time behavior of this protocol.

Real-time Ethernet networks for motion control

Communication networks have been traditionally employed in motion control applications, especially within factory automation systems. While in the past they were merely used to exchange non time critical data (e.g. parameters and configuration data) nowadays they allow for much more powerful performance. In particular, the recently introduced Real-time Ethernet (RTE) networks, have been explicitly designed in order to cope with very tight timing constraints in terms of both determinism and real-time. In this paper we focus on two popular RTE networks, namely Ethernet POWERLINK and EtherCAT, and provide an example of their employment for a coordinated motion control application. In particular, we consider the tracking of a circular trajectory by the coordinated motion of two independent axes where the velocity set-points are transmitted to the electrical drives implementing the axis control by means of the RTE networks. After providing some essential features of the two networks, we describe the configurations adopted for the coordinated motion control application. Then we check the effectiveness of the solution proposed by means of numerical simulations which take into consideration possible error scenarios deriving from the adoption of the communication networks such as transmission errors, communication delays and cable breaks.

On the Rate Adaptation Techniques of IEEE 802.11 Networks for Industrial Applications

The performance of the IEEE 802.11 WLAN are influenced by the wireless channel characteristics that reflect on the signal-to-noise ratio (SNR), particularly in industrial communication systems, that often operate in harsh environments. In order to cope with SNR reductions, the IEEE 802.11 WLAN specification suggests to adapt (reduce) the transmission rate, since the modulation techniques employed at the lower rates are more robust. However, the standard does not define any rate adaptation (RA) technique, leaving the actual implementation to the device manufacturers choice. In this paper we focus on RA techniques for industrial communication systems that are typically subjected to tight reliability and timing requirements. In detail, we compare the performance figures of a general purpose widespread technique, namely the automatic rate fallback (ARF), with those of the RA techniques actually implemented on two commercially available IEEE 802.11 devices via a set of practical experiments. The obtained results show that these techniques are characterized by a relevant number of packet retransmissions that may introduce a considerable randomness on the service time, possibly leading to performance degradation. Consequently, we propose two new techniques and evaluate their behavior by means of numerical simulations carried out for typical industrial traffic profiles. The outcomes are encouraging since the proposed RA techniques show in most cases better performance than ARF.

Analysis of Ethernet Powerlink Wireless Extensions Based on the IEEE 802.11 WLAN

The industrial communication scenario is experiencing the introduction of wireless networks at all levels of factory automation systems. The benefits deriving from such an innovation are manifold, even if wireless systems cannot be thought as a complete replacement of wired networks. Rather, they will be even more used in the near future to realize hybrid (wired/wireless) configurations. In particular, it is envisaged that wireless networks may be employed to implement extensions of (possibly already installed) wired systems. In this paper we consider wireless extensions of Ethernet Powerlink, a very popular Real-Time Ethernet network, implemented by means of the IEEE 802.11 WLAN. Specifically, we focus on a widespread network configuration and address two types of extensions based on, respectively, bridge and gateway devices. In the second part of the paper we provide an analysis of the hybrid networks aimed at evaluating the most relevant performance indexes. Since the reliability of wireless networks may represent a critical aspect, the analysis is carried out taking into account the presence of interference as well as fading in the wireless segment. The results we show, obtained from a theoretical analysis and validated by numerical simulations, allow to get some useful insights on the overall performance of the hybrid networks.

Real Time Ethernet networks evaluation using performance indicators

The employment of Real Time Ethernet networks in factory automation systems is rapidly increasing and several commercial products, with different characteristics, are already available from various manufacturers. Most of these networks have been included in both the IEC 61158 and IEC 61784 International Standards that, in addition, define a set of Performance Indicators. In this paper we focus on two popular Real Time Ethernet networks, namely Ethernet POWERLINK and EtherCAT, and we evaluate their performance for a typically deployed factory automation configuration. Specifically, we compute the most relevant performance indicators introduced by IEC 61784 standard and two (purposely defined) additional ones, namely minimum cycle time and jitter, which are suitable for the two networks considered.

Industrial Wireless Networks: The Significance of Timeliness in Communication Systems

In the last few years, wireless networks have gained significant importance in the context of industrial communication systems [1], where their deployment is bringing several noticeable benefits, ranging from replacement of cables to the connection of devices that cannot be reached by traditional wired systems. These features make the adoption of wireless networks for industrial applications very attractive, and they are envisaged to be deployed even more in the future, either as stand-alone systems or arranged in hybrid (wired/wireless) configurations. Unfortunately, wireless communication systems are often characterized by well-known problems, such as fading, multipath propagation, shadowing, and interference, that have the undesired effect of increasing the bit error rate (BER), resulting in the introduction of delays as well as randomness in packet delivery. Moreover, in the context of industrial communication, these aspects may be exacerbated by the specific nature of the environment. Indeed, the rapid movement of machineries along with the possible presence of electromagnetic interference sources, which are typical of manufacturing sites, may introduce considerable fluctuations of the BER values that contribute to further degradation in communication quality.

Experimental evaluation of the service time for industrial hybrid (wired/wireless) networks under non-ideal environmental conditions

In industrial communication systems, the unavailability of models of real components, in particular of their behavior in the presence of non-ideal channels, often leads to consistent differences between theoretical/simulative analysis and experimental results. In this paper we consider a hybrid (Ethernet/IEEE 802.11) network employing access points from different vendors and measure a specific performance index, namely the service time, for various transmission rates focusing on the effects of both internal component behaviors and channel conditions. The obtained results show how, in case of small payload packets transmitted on noisy channels, the choice of a fixed low transmission rate guarantees better service times. Indeed, decreasing the transmission rate on the wireless segment below 54 Mb/s reveals effective, since it limits the occurrence of packet errors and the consequent retransmissions reducing the service time variability to the intrinsic randomness of the employed devices.

A simulation approach to a Real-Time Ethernet protocol: EtherCAT

In the last years the use of Ethernet as the communication technology for automation systems has been sensibly increasing. In particular, real-time Ethernet networks have become widely adopted. In this paper we consider one of the most popular Real-Time Ethernet networks, namely EtherCAT. We firstly provide a short outline of the protocol, along with a description of a simulation model we have developed. Subsequently, we focus on some typical network configurations and provide some insights on their performance by means of both theoretical analysis and numerical simulations.

Performance of Industrial Communication Systems: Real Application Contexts

The analysis carried out in this article along with the proposed examples, highlighted that the practical(actually measured) performance figures of industrial communication systems are often worse than those expected, as derived from theoretical/simulative models. As could be seen, the causes of such discrepancies are mainly related to the internal behaviors of the employed components as well as to the features of the environments where the networks are deployed.