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Seamless Link-Level Redundancy to Improve Reliability of Industrial Wi-Fi Networks
Abstract
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.
... All techniques aimed at improving communication reliability on wireless networks through the adoption of medium redundancy rely on the basic assumption that the sources of interference and disturbance on different channels are, to a reasonable extent, independent. Papers like [1][2] [3][4] [5] showed that, under this hypothesis, reliability of Wi-Fi can be boosted noticeably, to the point that, in some cases, it could be considered as a replacement for cables. On the other hand, designers of industrial plants are often reluctant to include wireless technologies in their equipment, unless some evidence is provided about their reliability when deployed in factory scenarios. ...
... Besides saving network bandwidth, they increase communication reliability further, while preserving real-time behavior and fairness. Basic Wi-Red requires that each redundant station (RSTA) is provided with two radio blocks (PHY) and two medium access control entities (MAC) [5]. It is worth remarking that commercial-off-the-shelf simultaneous dual-band equipment is already provided with such hardware resources. ...
... In [3][4] [5], disturbance and interference on the two channels of a redundant Wi-Fi link were modeled as independent stochastic processes. In order to complement the simulation campaigns carried out in those papers, we performed a thorough experimental evaluation aimed at assessing to which extent that assumption was acceptable. ...
p>Seamless redundancy can be profitably exploited to improve predictability of wireless networks in general and, in particular, IEEE 802.11. According to this approach, packets are transmitted by senders on two (or more) channels at the same time and duplicate copies are discarded by receivers. As long as the behavior of physical channels is uncorrelated, communication quality improves noticeably, in terms of both transmission latencies and percentage of dropped frames.
In this paper, communication over redundant links has been analyzed by means of a thorough experimental campaign, based on measurements carried out on real devices. Results confirm that, under typical operating conditions, the assumption of independence among channels in properly designed systems is verified reasonably well. Indeed, in our experiments, measured link quality indices did not differ more than 10% from what we expected from theory. This grants for redundant solutions tangible advantages over conventional Wi-Fi networks.</p
... While relying on the same principles as PoW, the Wi-Fi Redundancy (Wi-Red) approach [17] is a link-level solution where PRP and the IEEE 802.11 Medium Access Control (MAC) mechanism are intertwined in order to improve performance further. ...
... Each sub-STA includes both a MAC and a radio block, and behaves mostly the same as a conventional STA. More details can be found in [17]. ...
... Since channels can be assumed to be reasonably independent in a well-configured RBSS [19], overall spectrum consumption in PoW coincides with the sum of the traffics that would be induced if non-redundant, conventional Wi-Fi were used to send packets separately on each physical channel. The main improvement Wi-Red [17] brings over PoW [14] are duplication avoidance mechanisms. Basically, they are aimed at reducing the amount of frames sent on air. ...
p>Seamless redundancy layered atop Wi-Fi has been shown able to tangibly increase communication quality, hence offering industry-grade reliability. However, it also implies much higher network traffic, which is often unbearable as the wireless spectrum is a shared and scarce resource. To deal with this drawback the Wi-Red proposal includes suitable duplication avoidance mechanisms, which reduce spectrum consumption by preventing transmission on air of inessential frame duplicates.
In this paper, the ability of such mechanisms to save wireless bandwidth is experimentally evaluated. To this purpose, specific post-analysis techniques have been defined, which permit to carry out such an assessment on a simple testbed that relies on plain redundancy and do not require any changes to the adapters' firmware. As results show, spectrum consumption decreases noticeably without communication quality is impaired. Further saving can be obtained if a slight worsening is tolerated for latencies.</p
... While relying on the same principles as PoW, the Wi-Fi Redundancy (Wi-Red) approach [17] is a link-level solution where PRP and the IEEE 802.11 Medium Access Control (MAC) mechanism are intertwined in order to improve performance further. ...
... Each sub-STA includes both a MAC and a radio block, and behaves mostly the same as a conventional STA. More details can be found in [17]. ...
... Since channels can be assumed to be reasonably independent in a well-configured RBSS [19], overall spectrum consumption in PoW coincides with the sum of the traffics that would be induced if non-redundant, conventional Wi-Fi were used to send packets separately on each physical channel. The main improvement Wi-Red [17] brings over PoW [14] are duplication avoidance mechanisms. Basically, they are aimed at reducing the amount of frames sent on air. ...
p>Seamless redundancy layered atop Wi-Fi has been shown able to tangibly increase communication quality, hence offering industry-grade reliability. However, it also implies much higher network traffic, which is often unbearable as the wireless spectrum is a shared and scarce resource. To deal with this drawback the Wi-Red proposal includes suitable duplication avoidance mechanisms, which reduce spectrum consumption by preventing transmission on air of inessential frame duplicates.
In this paper, the ability of such mechanisms to save wireless bandwidth is experimentally evaluated. To this purpose, specific post-analysis techniques have been defined, which permit to carry out such an assessment on a simple testbed that relies on plain redundancy and do not require any changes to the adapters' firmware. As results show, spectrum consumption decreases noticeably without communication quality is impaired. Further saving can be obtained if a slight worsening is tolerated for latencies.</p
... Traditionally, industrial communications have been centred on the use of specific local network architectures for the interconnection of specialized components and applications, like Programmable Logic Controllers (PLCs), Supervisory Control and Data Acquisition (SCADA) systems and Distributed Control Systems (DCSc); these network architectures have gradually evolved from field buses to industrial Ethernet solutions and, more recently, to Wireless LAN (WLAN) technologies [6]. The latter may need some enhancements to meet the strict requirements of the industrial environment in terms of Quality of Service (QoS) and reliability; see, e.g., [7]- [10]. ...
... Specifically, the detection of possible collision courses in real time and the automated machinery control for run-time human collision avoidance would imply the satisfaction of strict latency constraints [13], [14] and extremely high reliability levels. Enhancing reliability in this kind of industrial manufacturing environment has been exactly the goal of the already cited works [7]- [10], by replicating messages over multiple independent channels in the WiFi context, along the line suggested by the Parallel Redundancy Protocol (PRP) specified in the IEC 62439-3 standard (see [15] for the latest edition). ...
... In this respect, the goal of our work is to ensure a desired reliability level adaptively by introducing seamless redundancy [15], [7], and at the same time by distinguishing between critical and non-critical operating conditions, owing to the fact that enhanced reliability requires more bandwidth and implies less scalability. Thus, whenever a critical situation is perceived by one or more UEs, a decision needs to be taken for the choice of the minimum number of radio bearers to be instantiated to ensure the desired reliability level, as well as of the gNBs toward which to activate them. ...
Factory automation in the context of Industry 4.0/5.0 requires safety levels to satisfy more stringent and tight limits than those available so far. This goal is further challenged by the extension to the wireless environment of industrial shop floor communications that were traditionally based on cabled networks. Starting with wireless LANs, the trend towards the use of industrial wireless is fostered by the advent of fifth Generation (5G) private connectivity and is bound to increase its pace in the evolution towards 6G. In particular, the interaction of human operators with industrial robots and autonomous vehicles on the shop floor is posing stringent safety requirements that in turn push forward the dependability and reliability limits of wireless connectivity. To help achieve these limits, this paper proposes a dynamic redundancy mechanism based on the real-time activation/deactivation of radio bearers instantiated between mobile devices carried by humans and machines and multiple base stations, to achieve guaranteed upper bounds on packet loss probability in the communication of data related to operational safety control loops. An optimization problem is posed, and suitable heuristics are evaluated by simulation in a 5G and beyond wireless environment, aiming to dynamically maintain the required reliability levels with small computational effort.
... While relying on the same principles as PoW, the Wi-Fi Redundancy (Wi-Red) approach [17] is a link-level solution where PRP and the IEEE 802.11 Medium Access Control (MAC) mechanism are intertwined in order to improve performance further. ...
... Each sub-STA includes both a MAC and a radio block, and behaves mostly the same as a conventional STA. More details can be found in [17]. ...
... Since channels can be assumed to be reasonably independent in a well-configured RBSS [19], overall spectrum consumption in PoW coincides with the sum of the traffics that would be induced if non-redundant, conventional Wi-Fi were used to send packets separately on each physical channel. The main improvement Wi-Red [17] brings over PoW [14] are duplication avoidance mechanisms. Basically, they are aimed at reducing the amount of frames sent on air. ...
Seamless redundancy layered atop Wi-Fi has been shown able to tangibly increase communication quality, hence offering industry-grade reliability. However, it also implies much higher network traffic, which is often unbearable as the wireless spectrum is a shared and scarce resource. To deal with this drawback the Wi-Red proposal includes suitable duplication avoidance mechanisms, which reduce spectrum consumption by preventing transmission on air of inessential frame duplicates. In this paper, the ability of such mechanisms to save wireless bandwidth is experimentally evaluated. To this purpose, specific post-analysis techniques have been defined, which permit to carry out such an assessment on a simple testbed that relies on plain redundancy and do not require any changes to the adapters' firmware. As results show, spectrum consumption decreases noticeably without communication quality is impaired. Further saving can be obtained if a slight worsening is tolerated for latencies.
... All techniques aimed at improving communication reliability on wireless networks through the adoption of medium redundancy rely on the basic assumption that the sources of interference and disturbance on different channels are, to a reasonable extent, independent. Papers like [1][2] [3][4] [5] showed that, under this hypothesis, reliability of Wi-Fi can be boosted noticeably, to the point that, in some cases, it could be considered as a replacement for cables. On the other hand, designers of industrial plants are often reluctant to include wireless technologies in their equipment, unless some evidence is provided about their reliability when deployed in factory scenarios. ...
... Besides saving network bandwidth, they increase communication reliability further, while preserving real-time behavior and fairness. Basic Wi-Red requires that each redundant station (RSTA) is provided with two radio blocks (PHY) and two medium access control entities (MAC) [5]. It is worth remarking that commercial-off-the-shelf simultaneous dual-band equipment is already provided with such hardware resources. ...
... In [3][4] [5], disturbance and interference on the two channels of a redundant Wi-Fi link were modeled as independent stochastic processes. In order to complement the simulation campaigns carried out in those papers, we performed a thorough experimental evaluation aimed at assessing to which extent that assumption was acceptable. ...
p>Seamless redundancy can be profitably exploited to improve predictability of wireless networks in general and, in particular, IEEE 802.11. According to this approach, packets are transmitted by senders on two (or more) channels at the same time and duplicate copies are discarded by receivers. As long as the behavior of physical channels is uncorrelated, communication quality improves noticeably, in terms of both transmission latencies and percentage of dropped frames.
In this paper, communication over redundant links has been analyzed by means of a thorough experimental campaign, based on measurements carried out on real devices. Results confirm that, under typical operating conditions, the assumption of independence among channels in properly designed systems is verified reasonably well. Indeed, in our experiments, measured link quality indices did not differ more than 10% from what we expected from theory. This grants for redundant solutions tangible advantages over conventional Wi-Fi networks.</p
... All techniques aimed at improving communication reliability on wireless networks through the adoption of medium redundancy rely on the basic assumption that the sources of interference and disturbance on different channels are, to a reasonable extent, independent. Papers like [1][2] [3][4] [5] showed that, under this hypothesis, reliability of Wi-Fi can be boosted noticeably, to the point that, in some cases, it could be considered as a replacement for cables. On the other hand, designers of industrial plants are often reluctant to include wireless technologies in their equipment, unless some evidence is provided about their reliability when deployed in factory scenarios. ...
... Besides saving network bandwidth, they increase communication reliability further, while preserving real-time behavior and fairness. Basic Wi-Red requires that each redundant station (RSTA) is provided with two radio blocks (PHY) and two medium access control entities (MAC) [5]. It is worth remarking that commercial-off-the-shelf simultaneous dual-band equipment is already provided with such hardware resources. ...
... In [3][4] [5], disturbance and interference on the two channels of a redundant Wi-Fi link were modeled as independent stochastic processes. In order to complement the simulation campaigns carried out in those papers, we performed a thorough experimental evaluation aimed at assessing to which extent that assumption was acceptable. ...
p>Seamless redundancy can be profitably exploited to improve predictability of wireless networks in general and, in particular, IEEE 802.11. According to this approach, packets are transmitted by senders on two (or more) channels at the same time and duplicate copies are discarded by receivers. As long as the behavior of physical channels is uncorrelated, communication quality improves noticeably, in terms of both transmission latencies and percentage of dropped frames.
In this paper, communication over redundant links has been analyzed by means of a thorough experimental campaign, based on measurements carried out on real devices. Results confirm that, under typical operating conditions, the assumption of independence among channels in properly designed systems is verified reasonably well. Indeed, in our experiments, measured link quality indices did not differ more than 10% from what we expected from theory. This grants for redundant solutions tangible advantages over conventional Wi-Fi networks.</p
... Recent studies have shown that redundant wireless links can improve the reliability and latency of industrial wireless communications [10]. The Parallel Redundancy Protocol (PRP) [11] for real-time Ethernet (RTE) proposes transmitting two copies of the same frame over two different networks or network paths. ...
... This approach reduces the transmission latency and the likelihood that a packet is not delivered to the destination. This was confirmed in [10] where authors analyzed the performance of PRP over WiFi by means of simulations. Experimental studies were also presented in [8] and [9]. ...
... performance. Fig. 9 also shows that redundancy reduces the median of the experienced latency to 128.4 ms, i.e. it reduces the latency by 59.0% and 10.1% when compared to the use of AP1 and AP2 respectively 10 . ...
Factories are evolving into fully digitalized and networked structures for more adaptive and agile production ecosystems in the context of the Industry 4.0. Wireless communications will be a technical pillar of this evolution as it improves the reconfigurability of factories and the integration of mobile robots and objects. The integration of industrial wireless networks into the Industry 4.0 requires solutions capable to support highly reliable and deterministic low latency communications. This is particularly challenging for mobile industrial applications with constantly changing link quality conditions. This study experimentally evaluates for the first time the capacity of diversity and redundancy to improve the reliability and latency of wireless networks for mobile industrial applications. To this aim, a prototype is built in a collaborative robotics experimental facility. The prototype wirelessly connects a dual-arm robot and a mobile robot that collects and supplies components to the dual-arm robot. The prototype implements redundancy and diversity (using multipath TCP) for the wireless connections between both robots. The conducted trials show that both techniques improve the reliability of mobile industrial wireless communications even under the presence of interference. However, redundancy achieves lower latency levels and represents then the most attractive solution to support mobile industrial applications.
... Subsequent works postulated that seamless redundancy could be embedded directly in IEEE 802.11 specifications, as proposed for Wi-Red [7]. By doing so, duplication avoidance mechanisms can be used to lower bandwidth consumption, which is quite useful in the 2.4 GHz band. ...
... A Wi-Fi node that supports seamless redundancy will be denoted for short HR STA or HR AP, depending on its role. Their behavior resembles RSTA and RAP in [7], respectively. To comply with Wi-Fi 7, they are implemented as multi-link devices (MLD), and basically consist of a pair of affiliated STAs. ...
p>By removing wire harness, Wi-Fi is becoming increasingly pervasive in every aspect of our lives, in both the consumer and industrial worlds. Besides flexibility, the recent high efficiency and extremely high throughput versions managed to close the performance gap with Ethernet. However, it still lags behind Ethernet for what concerns dependability. To this aim, the ultra high reliability study group has been recently formed.
This paper reports on some preliminary ideas and proposals about the ways seamless redundancy can be exploited to make Wi-Fi more reliable, yet retaining a good degree of backward compatibility with existing network infrastructures.
</p
... Subsequent works postulated that seamless redundancy could be embedded directly in IEEE 802.11 specifications, as proposed for Wi-Red [7]. By doing so, duplication avoidance mechanisms can be used to lower bandwidth consumption, which is quite useful in the 2.4 GHz band. ...
... A Wi-Fi node that supports seamless redundancy will be denoted for short HR STA or HR AP, depending on its role. Their behavior resembles RSTA and RAP in [7], respectively. To comply with Wi-Fi 7, they are implemented as multi-link devices (MLD), and basically consist of a pair of affiliated STAs. ...
p>By removing wire harness, Wi-Fi is becoming increasingly pervasive in every aspect of our lives, in both the consumer and industrial worlds. Besides flexibility, the recent high efficiency and extremely high throughput versions managed to close the performance gap with Ethernet. However, it still lags behind Ethernet for what concerns dependability. To this aim, the ultra high reliability study group has been recently formed.
This paper reports on some preliminary ideas and proposals about the ways seamless redundancy can be exploited to make Wi-Fi more reliable, yet retaining a good degree of backward compatibility with existing network infrastructures.
</p
... Subsequent works postulated that seamless redundancy could be embedded directly in IEEE 802.11 specifications, as proposed for Wi-Red [7]. By doing so, duplication avoidance mechanisms can be used to lower bandwidth consumption, which is quite useful in the 2.4 GHz band. ...
... A Wi-Fi node that supports seamless redundancy will be denoted for short HR STA or HR AP, depending on its role. Their behavior resembles RSTA and RAP in [7], respectively. To comply with Wi-Fi 7, they are implemented as multi-link devices (MLD), and basically consist of a pair of affiliated STAs. ...
... The concept of PRP can be applied to a range of wireless technologies. Recently, Cena et al. [7] proposed Wi-Red which is essentially seamless redundancy, as defined by PRP, applied to Wi-Fi networks. Similarly, Papadopoulos et al. [8] developed Leapfrog Collaboration which applies the PRP principle in the form of parallel transmissions over two paths in case of 6TiSCH wireless networks. ...
Enabling ultra-reliable low latency communications (uRLLC) over 5G wireless networks creates challenging design requirements, particularly on the air-interface. The stringent latency and reliability targets require enhancements at different layers of the protocol stack. On the other hand, the parallel redundancy protocol (PRP), wherein each data packet is duplicated and transmitted concurrently over two independent networks, provides a simple solution for improving reliability and reducing latency in wireless networks. PRP can be realized in cellular networks through the dual connectivity (DC) solution. Recently, 3GPP has introduced packet duplication functionality in 5G wireless networks. To this end, this paper provides an overview of the packet duplication functionality in 5G, in light of recent developments within 3GPP, and also highlights the related technical challenges.
... Statistical methods hinge on historical data about link quality, constructing a stochastic model for predicting the future state of wireless channels. On the other hand, ML leverages algorithms to discern the connections among factors influencing wireless channel quality [8], [9], [10], [11], [12], [13], thus enabling more accurate predictions of its future state. In recent times, ML approaches have emerged as promising contenders, showcasing their prowess in capturing intricate patterns and relationships within data. ...
Predicting the behavior of a wireless link in terms of, e.g., the frame delivery ratio, is a critical task for optimizing the performance of wireless industrial communication systems. This is because industrial applications are typically characterized by stringent dependability and end-to-end latency requirements, which are adversely affected by channel quality degradation. In this work, we studied two neural network models for Wi-Fi link quality prediction in dense indoor environments. Experimental results show that their accuracy outperforms conventional methods based on exponential moving averages, due to their ability to capture complex patterns about communications, including the effects of shadowing and multipath propagation, which are particularly pronounced in industrial scenarios. This highlights the potential of neural networks for predicting spectrum behavior in challenging operating conditions, and suggests that they can be exploited to improve determinism and dependability of wireless communications, fostering their adoption in the industry.
... For example, Seno et al. [32] attempted to enhance determinism in WiFi communications by introducing a coordinator node to enable soft real time applications in industries. The work in [12] proposes Wi-Red, a parallel redundancy protocol which aims to improve timeliness and dependability in links by transmitting duplicate packets on redundant links. These works are orthogonal to our approach of utilizing OFDMA for deadline constrained settings. ...
A key strategy for making production in factories more efficient is to collect data about the functioning of machines, and dynamically adapt their working. Such smart factories have data packets with a mix of stringent and non-stringent deadlines with varying levels of importance that need to be delivered via a wireless network. However, the scheduling of packets in the wireless network is crucial to satisfy the deadlines. In this work, we propose a technique of utilizing IEEE 802.11ax, popularly known as WiFi 6, for such applications. IEEE 802.11ax has a few unique characteristics, such as specific configurations of dividing the channels into resource units (RU) for packet transmission and synchronized parallel transmissions. We model the problem of scheduling packets by assigning profit to each packet and then maximizing the sum of profits. We first show that this problem is strongly NP-Hard, and then propose an approximation algorithm with a 12-approximate algorithm. Our approximation algorithm uses a variant of local search to associate the right RU configuration to each packet and identify the duration of each parallel transmission. Finally, we extensively simulate different scenarios to show that our algorithm works better than other benchmarks.
... In this respect, wireless communication technologies are one of the main enablers, by supporting mobility and automatic (re)configurability of industrial systems [4]. The ever increasing need for wire-free solutions in industries, homes, and cities, implies that specific research activities, aimed at making the related technologies capable to fulfill the very strict requirements, in terms of dependability and end-to-end latency, of industrial networks [5], [6], [7], [8], [9], [10], are more and more demanded. Besides communication, services like localization [11], timesynchronization [12], and roaming [13] have to be also provided for enabling fully wireless industrial systems (required, e.g., when devices are characterized by mobility), by adapting the existing approaches to the specific wireless technology. ...
... In the past years, a fair amount of research activities were spent for improving the quality of communication on wireless networks by exploiting techniques like seamless redundancy [16], transmission scheduling [17], software-defined networking [18], automatic network configuration [19], and so on. Some solutions rely on enhanced medium access con-trol (MAC) mechanisms that enable deterministic behavior. ...
One of the aspects that mainly characterize wireless networks is their apparent unpredictability. Although several attempts were made in the past years to define for them deterministic medium access techniques, for instance by having data exchanges scheduled by an access point, as a matter of fact they remain a partial solution and are unable to ensure the same behavior as wired infrastructures, since interference may also come from devices outside the network, which obey different rules. A possible way to cope with disturbance on air, both internal and external to the network, is to obtain some knowledge about it by analyzing what happened in the recent past. This information, usually expressed in terms of suitable metrics, is then employed to optimize network operation, for example by prioritizing time-sensitive traffic when needed. In the simplest approaches such metrics coincide with statistical indices evaluated on transmission outcomes, like the failure rate. In this paper we analyze a more sophisticated solution that relies on machine learning, and in particular on artificial neural networks, to predict the behavior of a Wi-Fi link in terms of its frame delivery ratio. Results confirm that more accurate predictions than simpler methods (e.g., moving average) are possible, even when training is partially independent from the specific conditions experienced on the different channels.
... To comply to the new production paradigms and satisfy the previously listed goals, industrial networks are becoming increasingly heterogeneous [6], which implies that the requirements they are expected to meet in terms of reliability, determinism, and timeliness are becoming more and more strict. Research works aimed at improving the different wireless communication technologies to meet these constraints [7], [8], [9], [10], [11] represent concrete contributions toward their widespread adoption in the industry. In addition, research on (ultra-)low power solutions and energy-saving mechanisms This work was partially supported by the European Union under the Italian National Recovery and Resilience Plan (NRRP) of NextGenerationEU, partnership on "Telecommunications of the Future" (PE00000001 -program "RESTART"). ...
p>The ability to reliably predict the future quality of a wireless channel, as seen by the media access control layer, is a key enabler to improve performance of future industrial networks that do not rely on wires. Knowing in advance how much channel behavior may change can speed up procedures for adaptively selecting the best channel, making the network more deterministic, reliable, and less energy-hungry, possibly improving device roaming capabilities at the same time.
To this aim, popular approaches based on moving averages and regression were compared, using multiple key performance indicators, on data captured from a real Wi-Fi setup. Moreover, a simple technique based on a linear combination of outcomes from different techniques was presented and analyzed, to further reduce the prediction error, and some considerations about lower bounds on achievable errors have been reported. We found that the best model is the exponential moving verage, which managed to predict the frame delivery ratio with a 2.10% average error and, at the same time, has lower computational complexity and memory consumption than the other models we analyzed.</p
... To comply to the new production paradigms and satisfy the previously listed goals, industrial networks are becoming increasingly heterogeneous [6], which implies that the requirements they are expected to meet in terms of reliability, determinism, and timeliness are becoming more and more strict. Research works aimed at improving the different wireless communication technologies to meet these constraints [7], [8], [9], [10], [11] represent concrete contributions toward their widespread adoption in the industry. In addition, research on (ultra-)low power solutions and energy-saving mechanisms This work was partially supported by the European Union under the Italian National Recovery and Resilience Plan (NRRP) of NextGenerationEU, partnership on "Telecommunications of the Future" (PE00000001 -program "RESTART"). ...
p>The ability to reliably predict the future quality of a wireless channel, as seen by the media access control layer, is a key enabler to improve performance of future industrial networks that do not rely on wires. Knowing in advance how much channel behavior may change can speed up procedures for adaptively selecting the best channel, making the network more deterministic, reliable, and less energy-hungry, possibly improving device roaming capabilities at the same time.
To this aim, popular approaches based on moving averages and regression were compared, using multiple key performance indicators, on data captured from a real Wi-Fi setup. Moreover, a simple technique based on a linear combination of outcomes from different techniques was presented and analyzed, to further reduce the prediction error, and some considerations about lower bounds on achievable errors have been reported. We found that the best model is the exponential moving verage, which managed to predict the frame delivery ratio with a 2.10% average error and, at the same time, has lower computational complexity and memory consumption than the other models we analyzed.</p
... Not every frame contains information useful in the control room. Removing useless frames before sending them to the network improves the latency [31]. Blurring video frames is another tactic to reduce the amount of information from each frame [32]. ...
The imaging devices sense light reflected from objects and reconstruct images using the 2D-sensor matrix. It is a 2D Cartesian coordinate system where the depth dimension is absent. The absence of a depth axis on 2D images imposes challenges in locating and tracking objects in a 3D environment. Real-time object tracking faces another challenge imposed by network latency. This paper presents the development and analysis of a real-time, real-world object tracker called Trackez, which is capable of tracking within the top hemisphere. It uses Machine Vision at the IoT Edge (Mez) technology to mitigate latency sensitivity. A novel algorithm, Follow-Satisfy-Loop (FSL), has been developed and implemented in this paper that optimally tracks the target. It does not require the depth-axis. The simple and innovative design and incorporation of Mez technology have made the proposed object tracker a latency-insensitive, Z-axis-independent, and effective system. The Trackez reduces the average latency by 85.08% and improves the average accuracy by 81.71%. The object tracker accurately tracks objects moving in regular and irregular patterns at up to 5.4
ft/s
speed. This accurate, latency tolerant, and Z-axis independent tracking system contributes to developing a better robotics system that requires object tracking.
... To comply to the new production paradigms and satisfy the previously listed goals, industrial networks are becoming increasingly heterogeneous [6], which implies that the requirements they are expected to meet in terms of reliability, determinism, and timeliness are becoming more and more strict. Research works aimed at improving the different wireless communication technologies to meet these constraints [7], [8], [9], [10], [11] represent concrete contributions toward their widespread adoption in the industry. In addition, research on (ultra-)low power solutions and energy-saving mechanisms This work was partially supported by the European Union under the Italian National Recovery and Resilience Plan (NRRP) of NextGenerationEU, partnership on "Telecommunications of the Future" (PE00000001 -program "RESTART"). ...
p>The ability to reliably predict the future quality of a wireless channel, as seen by the media access control layer, is a key enabler to improve performance of future industrial networks that do not rely on wires. Knowing in advance how much channel behavior may change can speed up procedures for adaptively selecting the best channel, making the network more deterministic, reliable, and less energy-hungry, possibly improving device roaming capabilities at the same time.
To this aim, popular approaches based on moving averages and regression were compared, using multiple key performance indicators, on data captured from a real Wi-Fi setup. Moreover, a simple technique based on a linear combination of outcomes from different techniques was presented and analyzed, to further reduce the prediction error, and some considerations about lower bounds on achievable errors have been reported. We found that the best model is the exponential moving verage, which managed to predict the frame delivery ratio with a 2.10% average error and, at the same time, has lower computational complexity and memory consumption than the other models we analyzed.</p
... To comply to the new production paradigms and satisfy the previously listed goals, industrial networks are becoming increasingly heterogeneous [6], which implies that the requirements they are expected to meet in terms of reliability, determinism, and timeliness are becoming more and more strict. Research works aimed at improving the different wireless communication technologies to meet these constraints [7], [8], [9], [10], [11] represent concrete contributions toward their widespread adoption in the industry. In addition, research on (ultra-)low power solutions and energy-saving mechanisms This work was partially supported by the European Union under the Italian National Recovery and Resilience Plan (NRRP) of NextGenerationEU, partnership on "Telecommunications of the Future" (PE00000001 -program "RESTART"). ...
p>The ability to reliably predict the future quality of a wireless channel, as seen by the media access control layer, is a key enabler to improve performance of future industrial networks that do not rely on wires. Knowing in advance how much channel behavior may change can speed up procedures for adaptively selecting the best channel, making the network more deterministic, reliable, and less energy-hungry, possibly improving device roaming capabilities at the same time.
To this aim, popular approaches based on moving averages and regression were compared, using multiple key performance indicators, on data captured from a real Wi-Fi setup. Moreover, a simple technique based on a linear combination of outcomes from different techniques was presented and analyzed, to further reduce the prediction error, and some considerations about lower bounds on achievable errors have been reported. We found that the best model is the exponential moving verage, which managed to predict the frame delivery ratio with a 2.10% average error and, at the same time, has lower computational complexity and memory consumption than the other models we analyzed.</p
... It is worth noting that, in PoW, packet duplication and deduplication at both ends of a redundant wireless link can also be carried out in software, with significantly lower implementation costs. The Wi-Fi Redundancy (Wi-Red) proposal [23] is quite similar to PoW, but it defines specific duplicate avoidance mechanisms, preliminarily introduced in [24] and [25], which are aimed at reducing network traffic [26] and enhancing the overall system behavior. ...
p>Reliability and determinism of Wi-Fi can be tangibly improved by means of seamless redundancy, to the point of making this technology suitable for industrial environments. As pointed out in recent papers, the most benefits can be achieved when no phenomena can simultaneously affect transmissions on all channels of a redundant link. In this paper, several aspects are analyzed which, if not properly counteracted, may worsen seamless redundancy effectiveness. Effects they cause on communication have been experimentally evaluated in real testbeds, which rely on commercial Wi-Fi devices. Then, practical guidelines are provided, which aim at preventing joint interference through a careful system design. Results show that measured communication quality can be made as good as expected in theory.</p
... It is worth noting that, in PoW, packet duplication and deduplication at both ends of a redundant wireless link can also be carried out in software, with significantly lower implementation costs. The Wi-Fi Redundancy (Wi-Red) proposal [23] is quite similar to PoW, but it defines specific duplicate avoidance mechanisms, preliminarily introduced in [24] and [25], which are aimed at reducing network traffic [26] and enhancing the overall system behavior. ...
p>Reliability and determinism of Wi-Fi can be tangibly improved by means of seamless redundancy, to the point of making this technology suitable for industrial environments. As pointed out in recent papers, the most benefits can be achieved when no phenomena can simultaneously affect transmissions on all channels of a redundant link. In this paper, several aspects are analyzed which, if not properly counteracted, may worsen seamless redundancy effectiveness. Effects they cause on communication have been experimentally evaluated in real testbeds, which rely on commercial Wi-Fi devices. Then, practical guidelines are provided, which aim at preventing joint interference through a careful system design. Results show that measured communication quality can be made as good as expected in theory.</p
... It is worth noting that, in PoW, packet duplication and deduplication at both ends of a redundant wireless link can also be carried out in software, with significantly lower implementation costs. The Wi-Fi Redundancy (Wi-Red) proposal [23] is quite similar to PoW, but it defines specific duplicate avoidance mechanisms, preliminarily introduced in [24] and [25], which are aimed at reducing network traffic [26] and enhancing the overall system behavior. ...
Reliability and determinism of Wi-Fi can be tangibly improved by means of seamless redundancy, to the point of making this technology suitable for industrial environments. As pointed out in recent papers, the most benefits can be achieved when no phenomena can simultaneously affect transmissions on all channels of a redundant link. In this paper, several aspects are analyzed which, if not properly counteracted, may worsen seamless redundancy effectiveness. Effects they cause on communication have been experimentally evaluated in real testbeds, which rely on commercial Wi-Fi devices. Then, practical guidelines are provided, which aim at preventing joint interference through a careful system design. Results show that measured communication quality can be made as good as expected in theory.
... Due to the advantages of compatibility, cost, high rate, etc., many research works focused on the modification of IEEE 802.11 protocols based on the COTS network interfaces towards WTSN. In order to improve the reliability of industrial Wi-Fi networks, the authors in [16] proposed Wi-Fi Redundancy (Wi-Red) solution to offer seamless link-level redundancy. However, each independent Wi-Fi network in Wi-Red still uses legacy CSMA/CA, which cannot guarantee deterministic latency. ...
Wireless Time-Sensitive Networking (WTSN) has emerged as a promising technology for Industrial Internet of Things (IIoT) applications. To meet the latency requirements of WTSN, wireless local area network (WLAN) such as IEEE 802.11 protocol with the time division multiple access (TDMA) mechanism is shown to be a practical solution. In this paper, we propose the RT-WiFiQA protocol with two novel schemes to improve the latency and reliability performance: real-time quality of service (RT-QoS) and fine-grained aggregation (FGA) for TDMA-based 802.11 systems. The RT-QoS is designed to guarantee the quality-of-service requirements of different traffic and to support the FGA mechanism. The FGA mechanism aggregates frames for different stations to reduce the physical layer transmission overhead. The trade-off between the reliability and FGA packet size is analyzed with numerical results. Specifically, we derive a critical threshold such that the FGA can achieve higher reliability when the aggregated packet size is smaller than the critical threshold. Otherwise, the non-aggregation scheme outperforms the FGA scheme. Extensive experiments are conducted on the commercial off-the-shelf 802.11 interface. The experiment results show that compared with the existing TDMA-based 802.11 system, the developed RT-WiFiQA protocol can achieve deterministic bounded real-time latency and greatly improves the reliability performance.
... The network is divided into groups, where there will be fixed number of users whose transmissions will be scheduled. To improve determinism, in realtime industrial communication, in [9] authors propose a framework that provide seamless link-level redundancy and redundant WLAN equipment assuming that all WLAN devices are dual band and AP supports simultaneous dual band operations as well. [10] proposes a hybrid channel access mechanism along with temporal redundancy techniques to provide deterministic and reliable connectivity for the industrial real-time communication. ...
Nowadays multiple wireless communication systems operate in industrial environments side by side. In such an environment performance of one wireless network can be degraded by the collocated hostile wireless network having higher transmission power or higher carrier sensing threshold. Unlike the previous research works which considered IEEE 802.15.4 for the Industrial Wireless communication systems (iWCS) this paper examines the coexistence of IEEE 802.11 based iWCS used for delay-stringent communication in process automation and gWLAN (general-purpose WLAN) used for non-real time communication. In this paper, we present a Markov chain-based performance model that described the transmission failure of iWCS due to geographical collision with gWLAN. The presented analytic model accurately determines throughput, packet transaction delay, and packet loss probability of iWCS when it is collocated with gWLAN. The results of the Markov model match more than 90% with our simulation results. Furthermore, we proposed an adaptive transmission power control technique for iWCS to overcome the potential interferences caused by the gWLAN transmissions. The simulation results show that the proposed technique significantly improves iWCS performance in terms of throughput, packet transaction, and cycle period reduction. Moreover, it enables the industrial network for the use of delay critical applications in the presence of gWLAN without affecting its performance.
... Redundancy over Wi-Fi has been recently investigated under the name of Wi-Red (Wi-Fi Redundancy) [41]. Wi-Red aims at providing seamless link-level redundancy in IEEE 802.11 networks for industrial reliable wireless communications. ...
New use cases and applications in factory automation scenarios impose demanding requirements for traditional industrial communications. In particular, latency and reliability are considered as some of the most representative Key Performance Indicators (KPI) that limit the technological choices addressing wireless communications. Indeed, there is a considerable research effort ongoing in the area of wireless systems, not only from academia, but also from companies, towards novel solutions that fit Industry 4.0 KPIs. A major limitation for traditional wireless architectures is related to the harsh nature of the industrial propagation channel. Accordingly, this paper addresses these challenges by studying the reliability and latency performance of the joint use of different retransmission schemes in combination with Non-Orthogonal Multiple Access (NOMA) techniques. Two general retransmission schemes have been tested: time-based and spatial diversity-based retransmissions. An adaptive injection level NOMA solution has been combined with the retransmission schemes to improve the reliability of critical information. In all cases, a particular set of simulations has been carried out varying the main parameters, such as modulation, code rate and the injection level. Moreover, the impact of the number of transmitters in relation to the communication reliability has been analyzed. Results show that spatial diversity-based retransmissions overcome considerably the reliability obtained with time-domain retransmissions while maintaining assumable latency rates.
... Improving the reliability and determinism of Wi-Fi networks has been the focus of some recent studies. Cena et al. [13] proposed Wi-Red which extends the concept of PRP to Wi-Fi networks. Wi-Red requires nodes to be equipped with multiple radios with each radio connected to an independent Wi-Fi networks. ...
Communication for control-centric industrial applications is characterized by the requirements of very high reliability, very low and deterministic latency and high scalability. Typically, IEEE 802.11-based wireless local area networks (WLANs), also known as Wi-Fi networks, are deemed ineligible for industrial control applications owing to insufficient reliability and non-deterministic latency. This paper proposes a novel solution for providing reliable and deterministic communication, through Wi-Fi, in industrial environments. The proposed solution, termed as
{\textsf{HAR}^{{\text{2}}}\textsf{D-Fi}
, adopts hybrid channel access mechanisms for achieving deterministic communication. It also provides temporal redundancy for enhanced reliability.
{\textsf{HAR}^{{\text{2}}}\textsf{D-Fi}
implements different medium access control (MAC) designs that build on the standard physical (PHY) layer. Such designs can be classified into two categories: (a) MAC designs with pre-defined (physical) time-slotted schedule, and (b) MAC designs with virtual time-slotted schedule. Performance evaluation, based on analysis and system-level simulations, demonstrates the viability of
{\textsf{HAR}^{{\text{2}}}\textsf{D-Fi}
for control-centric industrial applications.
... Improving the reliability and determinism of Wi-Fi networks has been the focus of some recent studies. Cena et al. [13] proposed Wi-Red which extends the concept of PRP to Wi-Fi networks. Wi-Red requires nodes to be equipped with multiple radios with each radio connected to an independent Wi-Fi networks. ...
Communication for control-centric industrial applications is characterized by the requirements of very high reliability, very low and deterministic latency and high scalability. Typically, IEEE 802.11-based wireless local area networks (WLANs), also known as Wi-Fi networks, are deemed ineligible for industrial control applications owing to insufficient reliability and non-deterministic latency. This paper proposes a novel solution for providing reliable and deterministic communication, through Wi-Fi, in industrial environments. The proposed solution, termed as \textsf{HARD-Fi} (\underline{H}ybrid channel \underline{A}ccess with \underline{R}edundancy for \underline{R}eliable and \underline{D}eterministic Wi-\underline{Fi}), adopts hybrid channel access mechanisms for achieving deterministic communication. It also provides temporal redundancy for enhanced reliability. \textsf{HARD-Fi} implements different medium access control (MAC) designs that build on the standard physical (PHY) layer. Such designs can be classified into two categories: (a) MAC designs with pre-defined (physical) time-slotted schedule, and (b) MAC designs with virtual time-slotted schedule. Performance evaluation, based on analysis and system-level simulations, demonstrates the viability of \textsf{HARD-Fi} for control-centric industrial applications.
Industrial wireless sensor networks for factory automation are globally considered as an integral part of smart manufacturing. Wireless networks for Industrial Automation-Factory Automation (WIA-FA) is the first and only international standard specifying industrial wireless sensor networks for time-sensitive automation applications. In this paper, a Channel Hopping based Retransmission (CHR) scheme for WIA-FA networks is proposed. To take advantage of strictly limited communication resources, we design CHR as a hybrid scheduling-based retransmission scheme that is composed of a cyclic reservation diversity retransmission scheme and an on-demand retransmission scheme. For device fairness and channel diversity, we propose to combine time-slotted channel hopping with retransmission schemes. Furthermore, we perform the reliability analysis to CHR, guiding the dynamic selection of retransmission schemes according to the actual traffic and available communication resources. Simulation results demonstrate that CHR outperforms existing retransmission schemes in transmission reliability and worst-case latency.
Factory automation strongly relies on industrial wireless networks that are able to guarantee reliable and real-time transmissions. To this end, a recently approved International Electrotechnical Commission standard Wireless networks for Industrial Automation-Factory Automation (WIA-FA) is proposed to specify wireless networks for time-critical industrial applications. This paper proposes a Flexible Retransmission Scheme for reliable and real-time transmissions in WIA-FA networks, termed as FRS, which combines the advantages of a cyclic reservation diversity retransmission scheme and an on-demand retransmission scheme to make full use of the strictly limited communication resources. Further, we perform the reliability analysis by analytical modeling to FRS, whose major contribution lies in the dynamic selection of retransmission schemes according to the reliability analysis. Simulation results finally show that this work outperforms other retransmission schemes in terms of transmission reliability for different scenarios.
The arrival of Wi-Fi 6 in late 2019, which is based on IEEE 802.11 ax, marked a giant leap forward in improving the capacity, efficiency, and coverage in most WLAN environments. While Wi-Fi 6 introduced various features to enhance the network performance and user experience such as in high-dense deployment scenarios, emerging applications like 4K/8K video, augmented reality, virtual reality, industrial Internet of Things, and real-time collaborations demand more deterministic connectivity. Meeting the most stringent requirements in throughput and latency in a deterministic fashion is beyond the capabilities of Wi-Fi 6 and therefore motivates the development of a new Wi-Fi 7 generation. As a result, the IEEE 802.11 Task Group be has been formed to define extreme high throughput (EHT) PHY and MAC layers capable of supporting a maximum throughput of at least 30 Gb/s, as well as reducing worst case latency and jitter to improve support for latency-sensitive applications. In this article, our primary goal is to identify and describe the main PHY and MAC elements that will shape Wi-Fi 7, which will operate in the 2.4 GHz, 5 GHz, and 6 GHz bands. For each of the main features, including key PHY enhancements, multi-link operation, and enhanced quality of service management, we discuss how Wi-Fi 7 is designing the corresponding enabling mechanism and present performance results as appropriate.
Industrial automation requires communication with low latencies and high reliability between controllers, sensors and actuators. State of the art is usually a wired connection via Ethernet in combination with specialized industrial protocols. Wired connections are difficult to realize in certain applications. New wireless technologies such as WiFi 6 and 5G promise to achieve reliability and low latencies as currently only possible via wired connections. It is necessary to consider during the engineering phase whether the requirements of an industrial application are met by a selected communication technology. This paper presents an approach to evaluate communication technologies for industrial control systems during the system engineering phase, considering future application conditions. The focus of the approach is on the evaluation of communication latency. It is necessary to examine different technological variants or parameters in order to determine their respective influence on communication and the overall system. The approach makes it possible to identify relevant parameters affecting performance of the communication. Thus, predictions about the expected performance of the application with regard to communication can be made during engineering. The approach was implemented and tested with a private 5G standalone network and a WiFi 6 network, using Ethernet as reference.
This paper addresses the H∞ finite-time asynchronous state estimation issue for Markov jump systems with partial transition probabilities. A hidden-Markov-chain-based redundant channel model (HMCb-RCM) is established to reflect a more practical situation. Based on the output of the HMCb-RCM, firstly an asynchronous full-order state estimator is devised for the jump system with partial transition probabilities. Then, new sufficient criteria are derived such that the state estimation error is H∞ stochastically finite-time bounded. The relationship between the partial transition probabilities and asynchronous modes is revealed as few attempts. The conditional transition probability matrix (CTPM) for the HMCb-RCM is not fixed but designable accordingly; a co-design strategy is newly developed to synthesize the CTPM and the state estimator simultaneously, which produces less conservatism than that with fixed CTPM. Finally, the theoretical results are applied to a one-link robotic manipulator to validate the proposed results.
The fourth industrial revolution is paving the way for Industrial Internet of Things applications where large number of wireless nodes, equipped with sensors and actuators, monitor the production cycle of industrial goods. This paper proposes and analyses LoRaIN, a network architecture and MAC-layer protocol thought for on-demand monitoring of industrial machines. Our proprietary system is an energy-efficient, reliable and scalable solution, where the protocol is built on top of LoRa at 2.4 GHz. Indeed, the low-power characteristics of LoRa allow to reduce energy consumption, while Wireless Power Transfer is used to recharge batteries, avoiding periodic battery replacement. High reliability is obtained through the joint use of Frequency and Time Division Multiple Access. A dynamic LoRaIN scheduler manages the communication and recharging phases depending on the tasks assigned to the nodes, as well as the number of monitoring devices. Performance is measured in terms of network throughput, energy consumption and latency. Results demonstrate that the proposed solution is suitable for monitoring applications of industry machines.
The use of wireless communications in Industrial Internet of Things (IIoT) enables unparalleled levels of flexibility and instantaneous reconfiguration for autonomous industrial processes. In this paper, the focus is on optimizing and evaluating Wi-Fi 6 and 5G New Radio (NR) licensed and unlicensed wireless networks for meeting the packet latency and reliability requirements of critical IIoT applications. The study is based on extensive system simulations using a 3GPP-defined IIoT indoor factory framework and application traffic models. Each radio technology is individually optimized leveraging the pros and cons of that technology to maximize the carried load in the network while fulfilling the delay requirements at a specified reliability level of 99.999 %. In addition to a performance comparison, the paper also provides deployment guidance for applying each radio technology in the considered IIoT setting. With proposed latency aware scheduling and when operated in interference free spectrum, Wi-Fi 6 can support <1 ms applications at a very low load, whereas the performance gap with respect to 5G NR reduces as delay requirements are relaxed to 10-100 ms. Conditioned on the fulfilment of the application latency and reliability requirements, unlicensed 5G NR shows nearly 2x the spectral efficiency of Wi-Fi 6 in all available configurations. Licensed 5G NR shows generally the best performance, especially for delay requirement <1 ms, supporting 2-4x the spectral efficiency achievable by unlicensed technologies.
This article investigates the ultra-reliable communication for industrial wireless sensor networks (IWSNs), where frequency diversity, path diversity and cooperative diversity are jointly investigated. We propose a novel IWSN equipped with the full-duplex (FD) relays and carrier aggregation (CA) technique, to improve the reliability of all sensors. We derive a closed-form expression for the reliability characterization based on signal-to-interference-plus-noise (SINR) model for sensors connecting to sink node either via FD relays or direct communications. We formulate a joint resource allocation problem for reliability maximization, considering sub-carrier assignment, relay selection and power control. We propose to apply the distributed decision making (DDM) framework to decouple the problem into two sub-problems and find the joint optimal solution in an iterative manner. We further propose an improved artificial bee colony algorithm to obtain the optimal results for each sub-problem. Our simulation results showcase the reliability increases with increasing the number of FD-relays and sub-carriers. We also show that the FD and CA techniques along with our proposed algorithm can be an effective way for improving the reliability of IWSNs.
WIA-FA and IEEE 802.11 are two most widely adopted industrial wireless standards in discrete manufacturing. However, comprehensive performance comparisons between WIA-FA and IEEE 802.11 are still missing and industrial applications urgently need experimental methods to guide the selection of appropriate wireless technologies. To this end, this paper performs extensive experiments between WIA-FA and IEEE 802.11 in two practical industrial scenarios, with one ordered scenario defining the transmission order of devices and the other order-free scenario imposing no order constraints to the transmission order of devices. Network performance indices of the WIA-FA and IEEE 802.11 networks, including reliability, delay, delay jitter, and disorder rate are compared for different network sizes and data generation periods. Experimental results show that the WIA-FA protocol provides stable network performance, while the network performance of the IEEE 802.11 protocol is random. Additionally, we perform preliminary comparisons of WIA-FA with IEEE 802.11ax and 5G New Radio.
The real and effective ground of all new concepts dedicated to the current advanced factories, as well as to the future digital ones, is close cooperativity of scattered applications in highly heterogeneous systems. Communication is the key enabling component, and all new approaches are inspired in practice to the demanding characteristics of industrial networks. These kinds of computer networks, together with new technologies derived from distant application fields, are the main technological means to accelerate the fast evolution of modern factory systems. Due to various communication requirements coming from the plurality of structures, components and application contexts, communication subsystems must be increasingly heterogeneous. Let us say clearly: this evolution cannot be stopped at this stage, no special universal solution is possible, and thinking about monogamous networking is a kind of dreamland. This paper is an analysis of the state of the art in the matter of heterogeneous networking in industry. It deeply investigates both wired and wireless technologies from the point of view of technological aspects and relevant key performance indicators, such as those related to dependability, and it contains a prospective estimation of future trends.
In distributed control systems where devices are connected through Wi-Fi, direct access to low-level medium access control (MAC) operations may help applications to meet their timing constraints. In particular, the ability to timely control single transmission attempts on air, by means of software programs running at the user space level, eases the implementation of mechanisms aimed at improving communication timeliness and reliability. Relevant examples are deterministic traffic scheduling, seamless channel redundancy, rate adaptation algorithms, and so on. In this paper, a novel architecture is defined, we call software-defined MAC (SDMAC), which in its current embodiment relies on conventional Linux PCs equipped with commercial Wi-Fi adapters. Preliminary SDMAC implementation on a real testbed and its experimental evaluation showed that integrating this paradigm in the existing protocol stacks constitutes a viable option, whose performance suits a wide range of applications characterized by soft real-time requirements.
Cyber physical systems are based on a number of nodes connected through a communication network, which can interact with the environment. In this chapter, a completely open-source architecture of a cyber physical system based on off-the-shelf components will be presented. Its main characteristics are high real-time capabilities and the use of both wired and IEEE 802.11 wireless technologies for communication. The Linux operating system installed on common personal computers and communication technologies derived from the IT world make the proposed architecture highly customizable, inexpensive, and performing. Moreover, the presence of a time synchronization service allows the sharing of time between nodes. Specific software and techniques, some based on synchronized nodes, are used to increase determinism and reliability in both wired and wireless extensions.
div>Communication for control-centric industrial applications is characterized by the requirements of very high reliability, very low and deterministic latency and high scalability. This paper proposes a novel solution for providing reliable and deterministic communication, through Wi-Fi, in industrial environments. The proposed solution, termed as HAR2D-Fi, adopts hybrid channel access mechanisms for achieving deterministic communication. It also provides temporal redundancy for enhanced reliability. HAR2D-Fi implements different medium access control (MAC) designs that build on the standard physical (PHY) layer. Such designs can be classified into two categories: (a) MAC designs with pre-defined (physical) time-slotted schedule, and (b) MAC designs with virtual time-slotted schedule. Performance evaluation, based on analysis and system-level simulations, demonstrates the viability of HAR2D-Fi for control-centric industrial applications.</div
The performance of OFDM systems over a multipath channel can strongly degrade due to the propagation delay spread. The distortion of the received signal over the fast Fourier transform window is referred to as multipath noise. This work aims to analytically determine the performance loss due to multipath noise as a function of OFDM and channel parameters for narrowband OFDM systems. First, it is investigated whether it is possible to describe the multipath noise, varying over different OFDM packets due to the temporal variation of the channel, by an effective noise factor , from which the loss factor is directly determined. Second, the theory of room electromagnetics is applied to develop a closed-form expression for as a function of the OFDM and reverberation parameters. This analytical method is validated with excellent agreement. Finally, the loss factor is determined for IEEE 802.11 based on channel measurements in two large conference rooms, providing values up to 19 dB for an 800 ns cyclic prefix length.
Cable-less avionics implementation will clearly improve the efficiency of aircraft while reducing weight and maintenance costs. Therefore, with the technological progress of wireless technologies, an alternative avionic communication architecture based on Ultra WideBand (UWB) technology is proposed to fulfill these new needs. To adapt this wireless technology to safety-critical avionics, first, the tuning process of the MAC layer and the integration of accurate reliability mechanisms to achieve timely and reliable communications are presented. Then, an efficient timing and reliability analysis of such a communication network based on Network Calculus is detailed. This analysis integrates the impact of non-preemptive message transmission, various service policies in end-systems, e.g., First In First Out (FIFO) and Fixed Priority (FP), and transmission errors. Third, this general analysis is illustrated in the case of a realistic avionic application to replace the AFDX backup network with our proposed UWB-based network to support timely and reliable communications.
WLAN according to standard IEEE 802.11 is widely regarded unsuitable as communication channel for real-time and safety applications. Non-determinism and interference liability leads to packet loss, exceeded and variable latency times due to retransmissions. This work proposes a method that compensates such consequences of stochastic channel fading by the parallel operation of diverse wireless channels, applying frequency and space diversity techniques. A fault-tolerant wireless “black channel” is achieved that is able to fulfill soft real-time availability plus providing redundancy. This is realized with standard WLAN components and the “Parallel Redundancy Protocol” (PRP) according to IEC 62439-3. Reliability and performance characteristics are derived from measurements on an experimental setup with SafetyNET p nodes.
Wireless technologies are increasingly deployed in factory automation systems, because they are able to provide several advantages, e.g., a higher level of mobility, more flexibility, and at the same time causing lower costs. However, the requirements of industrial applications in terms of real-time communication can not be satisfied by existing wireless technologies, such as IEEE 802.11 Wireless Local Area Networks (WLANs). This is due to their unpredictable behaviour and the introduction of delays beyond acceptable bounds, mainly caused by a non-deterministic medium access. Hence, the proposed wireless communication system provides real-time communication services based on the IEEE 802.11 protocol family. In this paper we present a new isochronous medium access control mechanism, called IsoMAC. The paper addresses IsoMAC and shows how, in combination with a suitable wireless clock synchronization, IsoMAC is able to satisfy the typical constraints of soft real-time flows found on the factory floor. A simulation case study is provided that shows the achievable update times of the proposed combination.
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.
Building networked control systems over wireless networks is an extremely challenging task, as the wireless communication characteristics such as random packet losses and delay, significantly affect the stability and the performance of the control systems. We present a novel approach to the design of wireless networked control system. This approach decomposes the design concerns into two factors and addresses them separately in two design spaces -- stability of the system is ensured using a passivity-based architecture at the control layer, while the performance of the system is optimized at the communication layer by adjusting the network operation parameters. This paper focuses on the design of IEEE 802.11-based wireless network. In particular, we present a MAC controller that dynamically adjusts the retransmission limit to track the optimal trade-off between packet losses and transmission delays and thus optimizes the overall control system performance. Simulation results show that our approach significantly improves the performance of the networked control systems.
Currently, there is a trend towards the implementation of industrial communication systems using wireless networks. However, keeping up with the timing constraints of real-time traffic in wireless environments is a hard task. The main reason is that real-time devices must share the same communication medium with timing unconstrained devices. The VTP-CSMA architecture has been proposed to deal with this problem. It considers an unified wireless system in one frequency band, where the communication bandwidth is shared by real-time and non-real-time communicating devices. The proposed architecture is based on a virtual token passing (VTP) procedure that circulates a virtual token among real-time devices. This virtual token is complemented by an underlying traffic separation mechanism that prioritizes the real-time traffic over the non-real-time traffic. This is one of the most innovative aspects of the proposed architecture, as most part of real-time communication approaches are not able to handle timing unconstrained traffic sharing the same communication medium. A ring management procedure for the VTP-CSMA architecture is also proposed, allowing real-time stations to adequately join/leave the virtual ring.
Wireless communications suffer from disturbance and interference, which prevent reliable data exchanges and preclude their adoption in the real-time applications found at the lower levels of factory automation systems. This is particularly true for Wi-Fi, for which mechanisms like channel hopping and blacklisting are not available. Several solutions were defined in the past few years, which range from the adoption of general-purpose redundancy protocols over conventional communication equipment to proposals like Wi-Red, which offers link-level redundancy. In this paper, an enhancement is presented for the latter solution, which achieves better performance by deferring transmission of duplicate frames. The net effect is a sort of load balancing between wireless channels, which reduces network traffic and increases reliability further.
This "Special Section on Real-Time Fault Diagnosis and Fault-Tolerant Control" of the IEEE Transactions on Industrial Electronics is motivated to provide a forum for academic and industrial communities to report recent theoretic/application results in real-time monitoring, diagnosis, and fault-tolerant design, and exchange the ideas about the emerging research direction in this field. Twenty-three papers were eventually selected through a strict peer-reviewed procedure, which represent the most recent progress on real-time fault diagnosis, fault-tolerant control design, and their applications. Twelve selected papers pay attention on fault diagnosis methods and applications, and the other eleven papers are concentrated on realtime fault-tolerant control and applications. We are going to overview the selected papers following fault diagnosis techniques and fault-tolerant control techniques, sequentially.
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 . Then, a simple distributed hard real-time control application has been set up, 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 .
Wireless communications are becoming increasingly appealing for many application areas in industrial environments, including control systems at the shop-floor. IEEE 802.11 resembles Ethernet closely and, in the newest versions, it manages to provide similar throughput. Unfortunately, it fails in ensuring a comparable degree of reliability, mostly because its physical layer is quite sensitive to interference and disturbance. To overcome these limitations, very interesting solutions have appeared recently that layer a redundancy protocol above conventional Wi-Fi equipment. In this paper, a new approach is introduced and evaluated, which enhances reliability further by making redundant wireless adapters share information about the outcome of acknowledged transmissions. Doing so decreases both transmission latencies and the amount of lost packets.
Strict reliability and delay requirements of factory monitoring and control applications pose challenges for wireless communications in dynamic and cluttered industrial environments. To reduce outage in such fading-rich areas, cooperative relays can be used to overhear source-destination transmissions and forward data packets that a source fails to deliver. This article presents the results of an experimental study of selective cooperative relaying protocols that are implemented in off-the-shelf IEEE 802.15.4-compatible devices and evaluated in an industrial production plant. Three practical relay update schemes, which define when a new relay selection is triggered, are investigated: periodic, adaptive, and reactive relay selections. The results show that all relaying protocols outperform conventional time diversity retransmissions in delivery ratio and number of retransmissions for packet delivery. Reactive selection provides the best overall delivery ratio of nearly 99% over the tested network. There is a tradeoff, however, between achievable delivery ratio and required selection overhead. This tradeoff depends on protocol and network parameters, and is studied via protocol emulation using empirical channel values.
Recently, the International Society of Automation (ISA) released ISA100.11a as an open standard for reliable wireless networks for industrial automation. ISA100.11a operates in the 2.4-GHz industrial, scientific, and medical (ISM) unlicensed band, and may suffer from interference from other radio technologies operating in the same band. This coexistence issue can lead to significant degradation of ISA100.11a performance. In this work, the performance of the ISA100.11a industrial wireless network under interference from an IEEE 802.11b wireless local area network (WLAN) is evaluated. An analytical model for the coexistence between ISA100.11a and IEEE 802.11b is suggested. The packet error rate (PER) and average end-to-end delay are evaluated, where the PER is obtained from the bit error probability and collision time, whereas the average end-to-end delay is investigated from the waiting time in buffer and the transmission time. Simulation results from the OPNET modeler are presented to validate the numerical analysis. It is demonstrated that ISA100.11a achieves acceptable PER and satisfies the delay requirement for industrial process control and monitoring, even under significant WLAN interference.
Wide deployment of wireless sensor and actuator networks in cyber-physical systems requires systematic design tools to enable dynamic tradeoff of network resources and control performance. In this paper, we consider three recently proposed aperiodic control algorithms which have the potential to address this problem. By showing how these controllers can be implemented over the IEEE 802.15.4 standard, a practical wireless control system architecture with guaranteed closed-loop performance is detailed. Event-based predictive and hybrid sensor and actuator communication schemes are compared with respect to their capabilities and implementation complexity. A two double-tank laboratory experimental setup, mimicking some typical industrial process control loops, is used to demonstrate the applicability of the proposed approach. Experimental results show how the sensor communication adapts to the changing demands of the control loops and the network resources, allowing for lower energy consumption and efficient bandwidth utilization.
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.
The error structure on communication channels used for data transmission may be so complex as to preclude the feasibility of accurately predicting the performance of given codes when employed on these channels. Use of an approximate error rate as an estimate of performance allows the complex statistics of errors to be reduced to a manageable table of parameters and used in an economical evaluation of large collections of error detecting codes. Exemplary evaluations of error detecting codes on the switched telephone network are included in this paper.
On channels which may be represented by Gilbert's model of a burst-noise channel, the probabilities of error or of retransmission may be calculated without approximations for both error correcting and error detecting codes.
The first decade of the new millennium has been a stage for the rapid development of wireless communication technologies for low-cost, low-power wireless solutions capable of robust and reliable communication [1]. IEEE Standard 802.15.4 for low-rate wireless personal area networks (WPANs) [2] has been the enabling technology for numerous applications within the field of wireless sensor networks (WSNs) [3], and more recently, wireless instrumentation. Although WSNs quickly found their way into a wide variety of applications, the adoption of wireless technology in the process automation and manufacturing industries has been slow.
A model of a burst-noise binary channel uses a Markov chain with two states G and B. In state G, transmission is error-free. In state B, the channel has only probability h of transmitting a digit correctly. For suitably small values of the probabilities, p, P of the B rA B transitions, the model simulates burst-noise channels. Probability formulas relate the parameters p, P, h to easily measured statistics and provide run distributions for comparison with experimental measurements. The capacity C of the model channel exceeds the capacity C(sym. bin.) of a memoryless symmetric binary channel with the same error probability. However, the difference is slight for some values of h, p, P; then, time-division encoding schemes may be fairly efficient.
There is currently high activity in the area of redundancy for Ethernet networks. A large number of solutions have emerged among the switch manufacturers during the last few years. This has been accompanied by standardization activities in the IEC. This paper looks briefly into industrial requirements on network redundancy and existing solutions with special focus on ring redundancy solutions. The Rapid Spanning Tree protocol (RSTP) is currently the best-performing Ethernet redundancy standard. RSTP is commonly used in meshed topology networks but performance data from use in ring topology networks is very limited. This paper therefore presents new results from experiments with this protocol in a ring network. This helps in determining where RSTP can be used in industrial networks where the ring topology is common.
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 paper we propose the use of a coordination layer to handle real-time communication in infrastructured WiFi networks. This layer combines a TDMA scheme with a traffic separation mechanism (FCR MAC), which enables the prioritization of real-time (RT) traffic over uncontrolled (external) traffic sources. The target of this paper is to assess the behavior of this coordination layer when supporting RT communication and to compare these results with those obtained with IEEE 802.11e EDCA. The simulation assessment considers an open communication environment, where a set of RT and non-RT stations share the same coverage area and frequency band. A realistic error-prone channel was used to measure the impact of interferences against an error-free channel. We show that the proposed solution offers a significant improvement when compared with EDCA, in what concerns average deadline losses and delay.
The design and simulation of coding schemes, medium access control and link layer protocols for future industrial wireless local area networks can be supported by some understanding of the statistic properties of the bit error patterns delivered by a wireless link (which is an ensemble of transmitter, channel, receiver, modems). We present results of bit error measurements taken with an IEEE 802.11-compliant physical layer hardware in an industrial environment. In addition to reporting the most important results, we draw some conclusions for the design of protocols and for the stochastic simulation of wireless channels. Besides discussing some well-known bit error models, we introduce the class of bipartite models.
Guest editorial special section on industrial wireless sensor networks