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.

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.

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.

The adoption of wireless communications and, in particular, Wi-Fi, at the lowest level of the factory automation hierarchy has not increased as fast as expected so far, mainly because of serious issues concerning determinism. Actually, besides the random access scheme, disturbance and interference prevent reliable communication over the air and, as a matter of fact, make wireless networks unable to support distributed real-time control applications properly. Several papers recently appearing in literature suggest that diversity could be leveraged to overcome this limitation effectively. In this paper, a reference architecture is introduced, which describes how seamless link-level redundancy can be applied to Wi-Fi. The framework is general enough to serve as a basis for future protocol enhancements, and also includes two optimizations aimed at improving the quality of wireless communication by avoiding unnecessary replicated transmissions. Some relevant solutions have been analyzed by means of a thorough simulation campaign, in order to highlight their benefits when compared with conventional Wi-Fi. Results show that both packet losses and network latencies improve noticeably.

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.

This paper describes reference broadcast infrastructure synchronization (RBIS), a clock synchronization protocol for IEEE 802.11 infrastructure wireless networks. The protocol is especially tailored for industrial and home automation networks, and in many application contexts, it offers several advantages compared with other solutions targeted at similar purposes. RBIS has been conceived to rely on conventional Wi-Fi equipment and, in particular, on unmodified access points. It is based on the master/slave approach and follows the receiver/receiver paradigm. An implementation of RBIS-carried out completely in software and based on timestamps taken at the interrupt handler level-has been developed, which achieves a synchronization error below 3 μs. Then, a simple distributed hard real-time control application has been setup, which consists in two PCs running real-time application interface for Linux (RTAI) and connected through Wi-Fi. The actuation error, measured on the generation of synchronous pulses, is strictly below 13 μs.