Gianluca Cena

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.

Evaluation of Response Times in Industrial WLANs

The adoption of wireless communication technologies in industrial environments for supporting (soft) real-time applications heavily depends on the ability to grant bounded response times for messages, at least from a probabilistic point of view. This aspect is particularly important in factory automation systems, where response times are considered much more significant than other performance indices, such as throughput, that are usually considered in different application areas. The ever-increasing availability on the market of products and solutions based on the IEEE 802.11 standard and the introduction of the 802.11e amendment for enhancing the quality of service (QoS) and prioritizing traffic make this kind of communication technology interesting also for adoption in (loosely coupled) distributed control systems. This paper reports on some experimental measures and the related analysis that have been carried out on real 802.11g/e networks for better understanding the statistical distribution of response times and can be of help in characterizing these solutions when used to support noncritical real-time traffic.

Hybrid wired/wireless networks for real-time communications

In this article, some considerations are presented about the way several well-known industrial networks (based on both fieldbus and industrial Ethernet solutions) can be practically extended with wireless subnetworks that rely on popular technologies, such as IEEE 802.11 and 802.15.4. This results in hybrid networks, which are able to combine the advantages of both wired and wireless solutions. In particular, advantages and drawbacks of several interconnection techniques are highlighted and, depending on the wired networks specifically taken into account, some hybrid configurations that are able to cope in a satisfactory way with the tight timing requirements often imposed by industrial control systems are suggested.

An improved CAN fieldbus for industrial applications

The controller area network (CAN) was originally developed to support cheap and rather simple automotive applications. However, because of its performance and low cost, it is also being considered in automated manufacturing and process control environments to interconnect intelligent devices, such as modern sensors and actuators. Unfortunately, CAN, in its current form, is not able to either share out the system bandwidth among the different devices fairly or to grant an upper bound on the transmission times experienced by the nodes connected to the communication medium as it happens, for instance, in the token-based networks. In this paper, two slight modifications of the basic CAN protocol are presented that satisfy the above-mentioned requirements at the expense of a very small degradation of the systems performance. Both these solutions exhibit a high degree of compatibility with those devices which have already been designed for the conventional CAN fieldbus. Besides introducing the new mechanisms, this paper also presents some performance figures obtained using a specially developed software simulator, while the behavior of the new mechanisms is compared to the traditional CAN systems, in order to see how effective they are.

Evaluation of EtherCAT Distributed Clock Performance

EtherCAT is a real-time Ethernet protocol conceived explicitly for industrial applications. It is characterized by high communication efficiency, which permits control loops to be closed with short cycle times, and is provided with a suitable mechanism, known as distributed clock (DC), that enables synchronized operations to take place across the controlled system. These features can be profitably adopted, for instance, to support motion control applications. In this paper, the performance of the DC mechanism is evaluated by means of a thorough campaign of experimental measurements carried out on a real network setup. A number of factors have been taken into account that can affect accuracy and precision, and their effects studied in depth.

Advances in automotive digital communications

In the past years, control systems of cars have moved from the analog to the digital domain, and these changes affected the communication subsystems too. The next step in this direction is achieving a higher degree of performance, dependability and integration, so as to meet the needs of tomorrows more and more demanding applications. In particular, x-by-wire systems are appearing as the challenge that will drive research efforts of the whole automotive industry for the next decade. This means that new technologies have to be developed for in-car networks, in order to provide the high performance required by the next generation of automotive systems. TTCAN, TTP/C, Byteflight, FlexRay and Bluetooth are some of the most promising emerging solutions that have already been defined and can be embedded right in new projects. In the automotive context, it is of paramount importance being able to evaluate the key features of each solution, as well as to verify whether or not they are able to meet the requirements of specific application fields. In this paper these technologies are discussed and their characteristics are compared, pointing out the main advantages and drawbacks.

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.

FastCAN: a high-performance enhanced CAN-like network

The controller area network (CAN) is a real-time fieldbus network which today is very popular in industrial environments because of its very appealing characteristics and low implementation costs. It has, however, serious limitations regarding the network extension and the transmission speed which may rule out this protocol in the next few years, at least for the use in high-performance automation systems. This paper presents a modification to the conventional CAN protocol based on a dual channel topology, which overcomes those limitations and exhibits a behavior that is very similar to the original standard protocol. FastCAN features very high communication performance and ensures a certain degree of compatibility with the existing devices and applications developed for the conventional CAN networks.

On the performances of two popular fieldbuses

CAN and PROFIBUS-DP are two very popular fieldbuses, which can also be profitably also as control networks for distributed systems where a controller is connected to a number of simple devices. The use of CAN and PROFIBUS-DP as a communication support for event driven systems is considered and their performances are evaluated from this point of view. Only those aspects of these two networks which are relevant in the scenario of event driven automation systems are taken into account and formulas are derived to evaluate the responsiveness and throughput which can be achieved in a number of situations. The results obtained with our analysis show that even though the performances of the two kinds of fieldbuses are very similar, it is convenient to use CAN in small size systems while PROFIBUS-DP is more suited to poll based applications.

On the properties of the flexible time division multiple access technique

Flexible time division multiple access (FTDMA) is a medium access mechanism that some modern popular networks for use in passenger vehicles, such as Byteflight and FlexRay, rely on. Its development was driven explicitly by the new needs of automotive x-by-wire systems, which could not be satisfied by the existing solutions. The FTDMA technique was conceived primarily to overcome the limitations of controller area network and, in particular, for supporting high performance real-time communications. Even though this access scheme is mainly suitable for asynchronous transmissions, it can be adopted profitably for synchronous exchanges as well. In this paper, the main features of FTDMA are analyzed, and its performance is evaluated for different kinds of communications that have to be supported in todays vehicles.