Uncrewed Aerial Vehicles (UAVs) play a vital role in public safety, especially in monitoring wildfires, where early detection reduces environmental impact. In UAV-Assisted Wildfire Monitoring (UAWM) systems, jointly optimizing the data collection schedule and UAV velocity is essential to minimize the average Age of Information (AoI) for sensory data. Deep Reinforcement Learning (DRL) has been used for this optimization, but its limitations – including low sampling efficiency, discrepancies between simulation and real-world conditions, and complex training – make it unsuitable for time-critical applications such as wildfire monitoring. Recent advances in Large Language Models (LLMs) provide a promising alternative. With strong reasoning and generalization capabilities, LLMs can adapt to new tasks through In-Context Learning (ICL), which enables task adaptation using natural language prompts and example-based guidance without retraining. This paper proposes a novel online Flight Resource Allocation scheme based on LLM-Enabled In-Context Learning (FRSICL) to jointly optimize the data collection schedule and UAV velocity along the trajectory in real time, thereby asymptotically minimizing the average AoI across all ground sensors. Unlike DRL, FRSICL generates data collection schedules and velocities using natural language task descriptions and feedback from the environment, enabling dynamic decision-making without extensive retraining. Simulation results confirm the effectiveness of FRSICL compared to state-of-the-art baselines, namely Proximal Policy Optimization, Block Coordinate Descent, and Nearest Neighbor.

IoT networks require efficient intrusion detection systems that balance detection accuracy with computational constraints for edge deployment. This work introduces a novel packet-level intrusion detection method using 14 statistical features extracted from sliding windows through basic counting operations. The approach operates directly on fixed-size packet windows without requiring flow reconstruction or deep packet inspection to compute statistics. We compare this packet-level method against traditional flow-based feature extraction using tstat on the Edge-IIoTset dataset across 10 attack types. Both approaches train One-Class Support Vector Machine (OCSVM) models for fair evaluation. Results demonstrate that our packet-level method achieves superior detection rates with lower false alarms while significantly reducing feature extraction time compared to traditional flow-based methods.

Accurately recognizing and assessing pain in sheep is key to discern animal health and mitigating harmful situations. However, such accuracy is limited by the ability to manage automatic monitoring of pain in those animals. Facial expression scoring is a widely used and useful method to evaluate pain in both humans and other living beings. Researchers also analyzed the facial expressions of sheep to assess their health state and concluded that facial landmark detection and pain level prediction are essential. For this purpose, we propose a novel weighted graph neural network (WGNN) model to link sheep’s detected facial landmarks and define pain levels. Furthermore, we propose a new sheep facial landmarks dataset that adheres to the parameters of the Sheep Facial Expression Scale (SPFES). Currently, there is no comprehensive performance benchmark that specifically evaluates the use of graph neural networks (GNNs) on sheep facial landmark data to detect and measure pain levels. The YOLOv8n detector architecture achieves a mean average precision ($\mathbf{m A P}$) of $\mathbf{5 9. 3 0 \%}$ with the sheep facial landmarks dataset, among seven other detection models. The WGNN framework has an accuracy of $92.71 \%$ for tracking multiple facial parts expressions with the YOLOv8n lightweight on-board device deployment-capable model.

Unmanned Aerial Vehicles enable a wide range of applications, including search and rescue, environmental monitoring, and disaster response. These aerial platforms can form dynamic flying ad hoc networks to support both sensing and data communication. In particular, UAVs can establish a resilient communication backbone that adapts to varying propagation conditions and fluctuating traffic demands. However, maintaining reliable performance in such highly dynamic and unpredictable environments remains a critical challenge. This work investigates online link quality estimation by autonomous agents, with a focus on real-time detection of link model changes (e.g., due to mobility or interference) through model drift. Building on this, we propose and evaluate adaptive formation control strategies that adjust UAV placement to optimize the network’s Packet Delivery Ratio. Simulation results demonstrate that the proposed optimal placement strategy significantly outperforms baseline approaches for line-based UAV networks.

In Autonomous Driving (AD), accurate positioning is essential to ensure safe, efficient, and cost-effective transportation. However, malicious cyber threats, such as False Data Injection Attacks (FDIA) through Vehicle-To-Everything (V2X) communications, pose significant risks to these objectives. FDIA can manifest as deceptive obfuscation, either translational (T-OBF) – a vehicle falsely claims to be on a parallel road –, or random (R-OBF) – the vehicle reports a random position that differs from its actual position by a few meters. Our approach tackles incorrect position data by leveraging cooperation between trusted participants. These participants perform ML-based FDIA detection by exchanging Angle-Of-Arrival (AoA) measurements, and also for positioning in conjunction with an Extended Kalman Filter to reduce FDIA effects. FDIA detection achieves 89% accuracy for T-OBF and 72% for R-OBF, reducing localization errors below 10m in 77.74% and 81.84% of cases, respectively.

Vehicular platooning aims to improve fuel efficiency and traffic fluidity, and wireless protocols can support such goals. Platoons benefit all vehicle classes (e.g., passenger, truck, trailer), and can be made arbitrarily long, to the extent control and networking mechanisms allow. Vehicle-to-Vehicle (V2V) wireless links inside the platoon are susceptible to path loss, shadowing & diffraction by vehicular obstacles, and fading. In this paper we characterize V2V intra-platoon links at the physical and data-link levels, for platoons composed of same-class vehicles. Our simulation results, using a 30 vehicle-long platoon, show that vehicle dimensions affect propagation and link-level performance, notably that Packet Delivery Ratio (PDR) for edge nodes in an all-passenger vehicle scenario can be 3 times higher than for an all-trailer scenario.

Accurate channel modelling is essential for enhancing overwater communications in dynamic propagation environments, but it is hindered by variable and noisy measurement data that cause uncertainty in large-scale propagation models. This work examines data uncertainty in overwater shore-to-shore WiFi communications and explores strategies to reduce it. We show that local principal curves allow representing empirical data with lower error than conventional methods. Uncertainty and sensitivity analysis provided insights into regions (water vs. mud) and periods (rising tide) with high variability in model predictions, and the most influential parameters (antenna heights). Parameter calibration effectively mitigates the impact of input parameter uncertainty.

Unmanned Aerial Vehicles (UAVs) are increasingly being utilized in various private and commercial applications, e.g., traffic control, parcel delivery, and Search and Rescue (SAR) missions. Machine Learning (ML) methods used in UAV-Assisted Sensor Networks (UASNETs) and, especially, in Deep Reinforcement Learning (DRL) face challenges such as complex and lengthy model training, gaps between simulation and reality, and low sampling efficiency, which conflict with the urgency of emergencies, such as SAR missions. In this paper, an In-Context Learning (ICL)-Data Collection Scheduling (ICLDC) system is proposed as an alternative to DRL in emergencies. The UAV collects sensory data and transmits it to a Large Language Model (LLM), which creates a task description in natural language. From this description, the UAV receives a data collection schedule that must be executed. A verifier ensures safe UAV operations by evaluating the schedules generated by the LLM and overriding unsafe schedules based on predefined rules. The system continuously adapts by incorporating feedback into the task descriptions and using this for future decisions. This method is tested against jailbreaking attacks, where the task description is manipulated to undermine network performance, highlighting the vulnerability of LLMs to such attacks. The proposed ICLDC significantly reduces cumulative packet loss compared to both the DQN and Maximum Channel Gain baselines. ICLDC presents a promising direction for intelligent scheduling and control in UASNETs.

This issue inaugurates the 61 st volume of the Real-Time Systems journal.This is the only journal dedicated exclusively to real-time computing, its principles and applications, and it has been so since its inception in 1989.It is 36 years of uninterrupted and consistent publishing activity, hosting high-quality reference work on all aspects of research and practice in this vibrant field.Among the articles published in the journal we can find several seminal works, some of which were distinguished with awards by the real-time systems community.The Real-Time Systems journal continues providing a single source coverage of the state-of-the-art in real-time computing.In its existence the journal catered for all trends that crossed this field, from priority and deadline-driven scheduling, for uniprocessors and for multiprocessors, to tasks synchronization, real-time operating systems, distributed real-time systems, feedback scheduling, fault-tolerant real-time systems, formal verification, real-time databases, real-time middleware, real-time communications, wireless sensor networks, servers for aperiodic tasks, hierarchical scheduling, compositional design methods, hardware-software codesign, scheduling-control codesign, limited-preemption scheduling, mixed-criticality systems and more recently AI-based design and AI-supported operation of real-time systems.In any of these trends you can find most of the influential articles in this journal.The Real-Time Systems journal has also embraced extended versions of the best papers presented in real-time systems conferences, from RTSS to RTAS, RTNS, RTCSA, ECRTS, ISORC and others, nourishing a tight and prolific relationship with the real-time systems community from whom it inherited the commitment to scientific value and rigor.This commitment has always been instrumental to guarantee the quality of submitted articles, but also, and primarily, of the review pipeline, ensuring that all published articles are of the highest standard for this community.This is so independently of the quantitative metrics that today rule all journals across all

In the editorial published in March of this year we reported the significant changes the Real-Time Systems journal is undergoing this year, the last of which is the transition to continuous article publishing (CAP).Starting from January 2026, papers accepted for publication in this journal will be immediately published as they are accepted, without waiting to be compiled in separate issues made of consistent sets of papers.In this sense, this third issue of volume 61 is the last one of the current pre-CAP era.Moreover, I'm completing 6 years as Editor-in-Chief (EiC) and it is time to hand over to a new EiC who will steer the journal starting from 2026.Thus, this is also the last issue/volume published under my coordination.It is, therefore, the right moment for a brief analysis of my term as EiC, the journal situation and the challenges that lie ahead.When I started my term back in 2020, the journal was starting to show a declining number of in-topic spontaneous submissions and had a few cases of long review turn-around time.Consequently, I identified three main goals for my term: (i) reconnect the journal with the real-time systems community, (ii) speed up the review turn-around and (iii) improve the journal metrics to make it more attractive.Concerning (i) we approached the main real-time systems conferences to set up special issues with extended versions of the papers nominated for the respective best paper awards.The idea was to increase submissions in the short term with invitations and use this increase to bootstrap spontaneous submissions.Despite the successful organization of several conference-related special issues, spontaneous submissions continued low.Moreover, several concerns were raised related to the potential negative impact of organizing special issues by invitation, only, on the journal quality metrics.In the sequel to this discussion, we started publishing topical special issues in topics that were popular among the highest-ranked papers in each conference.Extended versions of such papers were still invited to submit, but these special issues

Wireless communications increasingly enable ubiquitous connectivity for a large number of nodes, applications and scenarios. One of the less explored scenarios is aquatic communications, specially when considering near-shore and short-range communications. Overwater communications are impaired by a number of distinguishing dynamic factors, such as tides, waves or node mobility, that lead to a widely fluctuating and unpredictable channel. In this work, we empirically characterize near-shore, overwater channels at 2.4 GHz under realistic conditions, including tidal variations, and relatively short TX-RX separations. To this end, we conducted experiments in a coastal estuarine region and on a harbor to characterize Shore-to-Shore (S2S) and Shore-to-Vessel (S2V) communication channels, respectively, and to identify major factors impairing communication in such scenarios. The empirical results show that constructive/destructive interference patterns, varying reflecting surface, and node mobility (i.e. travel direction and particular maneuvers) have a relevant and noticeable impact on the received signal strength. Thus, a set of parameters should be simultaneously considered for improving the performance of communication systems supporting S2S and S2V links, namely tidal variations, reflection surface changes, antenna height, TX-RX alignment and TX-RX separation. The results useful provide insights into realistic S2S and S2V link design and operation.

This article presents a novel centrality-driven gateway designation framework for the improved real-time performance of low-power wireless sensor networks (WSNs) at system design time. We target time-synchronized channel hopping (TSCH) WSNs with centralized network management and multiple gateways with the objective of enhancing traffic schedulability by design . To this aim, we propose a novel network centrality metric termed minimal-overlap centrality that characterizes the overall number of path overlaps between all the active flows in the network when a given node is selected as gateway. The metric is used as a gateway designation criterion to elect as a gateway the node leading to the minimal number of overlaps. The method is then extended to multiple gateways with the aid of the unsupervised learning method of spectral clustering . Concretely, after a given number of clusters are identified, we use the new metric at each cluster to designate as cluster gateway the node with the least overall number of overlaps. Extensive simulations with random topologies under centralized earliest-deadline-first (EDF) scheduling and shortest-path routing suggest our approach is dominant over traditional centrality metrics from social network analysis, namely, eigenvector , closeness , betweenness , and degree . Notably, our approach reduces by up to 40% the worst-case end-to-end deadline misses achieved by classical centrality-driven gateway designation methods.

MQTT is a widely used application-layer protocol for Internet-of-Things (IoT) and Industrial IoT networks. However, its centralized architecture, which relies on a broker that mediates all data transmissions, can lead to traffic congestion and reduced network capability, particularly in networks with a large number of devices, such as edge networks. This paper proposes an optimized SDN-based real-time MQTT architecture for edge networks, called Multicast Real-Time MQTT (MRT-MQTT) that leverages multicast routing mechanisms to efficiently distribute data while supporting timeliness requirements and minimizing network usage. Extensive emulation experiments show the effectiveness of the proposed approach and its superiority over Direct Multicast-MQTT (DM-MQTT) and Standard MQTT (STD-MQTT). We achieved a reduction in transmission delay of 29% and 23% for QoS=0 and QoS=1, respectively, when compared to DM-MQTT and 55% and 43% compared to STD-MQTT. In this last case, MRT-MQTT also reduced network usage by 58.0% and 45.0% for QoS=0 and QoS=1, respectively.

IEEE 802.11 suite of protocols is the de facto standard for wireless local area networks, extending to a wide breadth of application domains. One such domain is robotics and teams of cooperating mobile agents. This is a challenging domain due to ad-hoc links and highly dynamic topology. Improving the use of the Radio-Frequency channel requires some level of coordination of transmissions in time and space. However, achieving this level of coordination requires that the network interface cards (NICs) behave in a predictable manner, at least following the Medium Access Control (MAC) protocol. In this preliminary work we report early intriguing observations with existing NICs that are lenient to interpretations of the standard MAC resulting in different ways to handle interference in the medium. Some NICs do not seem to respect an interference zone as commonly defined and silently impose very different buffering/delay trade-offs, with consequences for overlay protocols, particularly TDMA.

Embedded systems (ES) are wide-spread in our world and responsible for many critical systems.More recently, machine learning (ML) tools have become a well-established solution for data-intensive tasks, but their application in embedded systems is still gaining traction and their real-time performance is often unclear.We provide a (non-extensive) review of the ML tools that may be suited for deployment in ES, from which we selected two representative tools -the wellestablished Python-based Scikit-Learn, and the interoperabilityoriented ONNX Runtime -to compare their response time.Using archetypal datasets and four pre-trained ML models, we measure the prediction time for each sample, for each model, in Scikit-Learn and ONNX Runtime in a standard desktop (to compare performance of the tools in the same platform), and for ONNX Runtime in a representative ES, a Raspberry Pi v4 (to compare performance of the same tool across platforms).We report that ONNX considerably improves over Scikit-Learn, and experiences a negligible performance degradation when ported to the RPi.

To access all the potential value present in IoT, the IoT devices need to be interoperable. Some works in the literature target this issue, but it is not yet entirely solved, mainly because the proposed solutions are not standard-based at the semantic level. This paper presents the detailed implementation of our standard-based software framework targeting IoT interoperability, named NextGenGW. With NextGenGW, we propose the first integration of IETF SDF with the MQTT protocol. We define an evaluation baseline for validating IoT gateway performance while focusing on interoperability. Our evaluation results show the NextGenGW suitability for deployment in devices with reduced resources and for use cases that require high scalability both in terms of connected IoT end nodes and the number of requests per time interval.

Implementing, managing and updating large-scale complex IoT applications is still a non-trivial challenge. Service Oriented Architectures (SOA) are increasingly used for this purpose, given their well-defined communication interfaces and configurable service-to-service links that ease the implementation of complex data flows. However, these architectures usually stop at the edge, not supporting far-edge resource-constrained devices. This paper proposes embServe, a new service-oriented framework that enables SOA on networks of constrained devices. It allows users to deploy services compiled independently of the base application and configure their connections using standard IoT protocols and data models. We compared a reference implementation against traditional approaches. Results show that embServe reduces resource overhead with a subtle impact on computing performance while improving application response time and uptime.

Nowadays, custom components are increasingly being replaced by commercially available off-the-shelf hardware and standard protocols. Additionally, emerging industrial paradigms like Industry 4.0 and IoT place new demands on requirements like scalability, transparency, adaptability and efficiency. Accordingly, application layer protocols like the Message Queuing Telemetry Transport Protocol (MQTT) are becoming more and more popular in these fields, thanks to their simplicity, scalability, low resource-usage and decoupling between end nodes. However, these protocols are not deterministic, thus being unsuitable for real-time applications. Recently the authors proposed a set of extensions to the MQTT protocol, allowing applications to explicitly specify real-time requirements that are then used by a resource manager, implemented in Software Defined Networking (SDN), to create real-time channels. This paper extends the work, providing worst-case analysis using the Holistic and Trajectory approaches. The paper also includes a set of experimental results aiming to verify the correctness of both analysis and evaluate its performance in several scenarios.

To access all the potential value present in IoT, devices need to be interoperable. IoT devices are heterogeneous in nature and modelling them, both syntactically and semantically, is a problem difficult to tackle. In this work, we propose the Next Generation Gateway, a software architecture designed with scalability in mind that addresses IoT interoperability from both the syntactic and semantic perspectives, and harmonizes the various IoT end nodes communication protocols in a single one.

Private vehicles are expected to continue representing a large share of the urban traffic requiring intelligent management to provide safe and efficient urban mobility. In this context, it is imperative to mitigate traffic congestion and associated travel delays to improve the quality of life of urban dwellers. This paper explores the global performance of grid networks of independent intersections using different intersection management protocols. We particularly aim to compare the performance achieved when using the intelligent intersection management architecture (IIMA) that relies on the synchronous intersection management protocol (SIMP), against two conventional (Round-robin - RR and trivial traffic light control - TTLC) and two adaptive (Max-pressure control algorithm - MCA and Websters traffic light control - WTLC) intersection management approaches. We consider four-way two-lane intersections with two crossing configurations, namely dedicated and shared left lane, on a 2×2 grid network of intersections. Simulation results with SUMO show that composing intersections with synchronous management considerably improves the network throughput and reduces travel delays.

The IEEE Time-Sensitive Networking (TSN) Task Group specifies a set of standards that enables real-time communication with predictable and bounded delays over the Ethernet. Specifically, TSN introduces a new set of so-called shapers, which regulate traffic arrival and transmission in the networks. Prominent among those are the IEEE 802.1 Qbv Time Aware Shaper (TAS) and IEEE 802.1Qav Credit-Based Shaper (CBS). Another traffic control mechanism is the IEEE 802.1Qbu Frame Preemption. Most works in the literature have focused on the quantitative performance comparison between these mechanisms. However, the discussion on how they compare in terms of implementation cost has received less attention. In this paper, we provide a comprehensive comparison of the implementation cost of the aforementioned TSN traffic control mechanisms. This comparison can help system designers in choosing which of the mechanism(s) to deploy for their applications.

MQTT is an application-layer protocol that eventually became popular in the Internet of Things (IoT) and Industrial IoT (IIoT) thanks to its simplicity and effective publisher-subscriber messaging model that enables its use in embedded resource-constrained devices. However, MQTT features a limited set of Quality-of-Service classes addressing exclusively message delivery, impairing its use in IIoT applications subject to timeliness requirements. This limitation of MQTT has been addressed in the literature, but with focus on the broker real-time operation, only. This paper adds to the state-of-the-art, by proposing a set of extensions to the MQTT protocol grounded on Software-Defined Networking (SDN) that enable, at the network level, attaining real-time communication services. Simulation results validate the benefits of the proposed extensions.

Intersections are critical elements of urban traffic management and are identified as bottlenecks prone to traffic congestion and accidents. Intelligent intersection management plays a significant role in improving traffic efficiency and safety determining, among other metrics, the waiting time that vehicles incur when crossing an intersection. This work presents a preliminary analysis of the worst-case response time of intersection management protocols that handle mixed traffic with autonomous and human-driven vehicles. We deduce theoretical bounds for such time considered as the interval between the injection of a vehicle in the road system and its departure from the intersection, considering different intersection management protocols for mixed traffic, namely the Synchronous Intersection Management Protocol (SIMP) and several configurations of the conventional Round-Robin (RR) policy. Simulation results validate the analytical bounds partially. Ongoing work addresses the queue dynamics and its reliable detection by traffic simulators.

This paper proposes network centrality as a criterion to designate a gateway or sink in real-time wireless sensor-actuator networks (WSAN). The objective is to improve network schedulability by design, particularly, by means of a centrality-driven gateway designation. To this purpose, four classical centrality metrics taken from social network analysis are explored, namely, (i) degree centrality, (ii) closeness centrality, (iii) betweenness centrality, and (iv) eigenvector centrality. We assume time-synchronized channel-hopping (TSCH) WSANs under centralized shortest-path routing and earliest-deadline-first (EDF) scheduling. Simulation results under varying configurations show that assigning the gateway role based on network centrality is, in general, an effective and promising approach to improve real-time performance in this type of networks. To the best of our knowledge, this work pioneers the use of a centrality-driven gateway designation as a mean to improve schedulability in WSANs.

Platooning is a promising concept used within the Intelligent Transportation System to increase efficiency and safety of road transportation. It is based on periodically sharing the kinematic status of the platoon members to allow reducing inter-vehicle distances in a safe way. This coordination is automatic and depends heavily on the wireless channel. A common technique to improve the channel properties is to use Time-Division Multiple Access (TDMA) that organizes the access to the wireless medium in slots assigned exclusively to each vehicle. However, while platoons are physical and dynamic, the corresponding dynamic reconfiguration of a logical TDMA frame is non-trivial. In this paper we address this Cyber-Physical problem resorting to the RA-TDMAp protocol to track the dynamics of a platoon, specifically joining, merging and leaving. In our solution, we include an adequate admission control block, to verify whether joining or merging can be accepted, and we present the state-machine that handles the reconfiguration process. We validate our TDMA reconfiguration mechanism with simulations using the Plexe/Vein/OMNeT++ framework. We show the effectiveness of the proposed mechanisms which ensures a synchronized start of the platoon control with the TDMA frame reconfiguration.

Measurement-Based Probabilistic Timing Analysis (MBPTA) produces Probabilistic Worst-Case Execution Times (pWCETs), i.e., WCET estimates associated with known low exceedance probabilities. Despite applicability and goodness-of-fit tests being used within MBPTA, any method based on the sampling of a population is subject to a degree of uncertainty. The acceptance of MBPTA in industrial engineering processes depends on obtaining enough evidence that the produced pWCETs are indeed reliable. In this paper we propose a statistical hypothesis test to check the reliability of pWCET estimates, done at a specified significance level. We assume as null hypothesis that the pWCET estimate is reliable, and as alternative hypothesis that it is optimistic. Both Type I and Type II errors are considered. The reliability test is based on a binomial experiment and it is complementary to applicability and goodness-of-fit tests. We evaluated the test using multiple synthetic and real-hardware execution time samples, and applied it on 20 pWCET estimates generated for each of them. The combined use of the proposed reliability test with applicability and goodness-of-fit tests could detect most of the knowingly unreliable estimates on synthetic samples. Similar behaviour was observed for real-hardware samples, evidencing the test's usefulness for selecting pWCET estimates with increased confidence.

In the design of shore-to-shore and shore-to-vessel wireless links, the impact of the ray reflected on the surface is often neglected. It adds that, in some coastal areas, the geometry of the reflection changes over time due to tides. When choosing an antenna height for an inshore node, often the largest possible height is used, but this approach can lead to signal degradation. The two-ray model is the most fundamental path loss model to account for the contribution of the reflected ray. We carried out experimental measurements at the shores of a freshwater body to verify that the two-ray model can predict the major trends of the path loss experienced by a 2.4 GHz over-water wireless link. We focus on short-to-medium distance links, with antennas installed a few meters above surface. We observed considerable consistency between measurements and model estimates, leading us to conclude that the two-ray model may bring benefits when applied to the network design of over-water links affected by tidal variations, which is our end-goal.

The rising adoption of wireless technologies in the In- dustrial Internet of Things has stressed the need for traffic schedulability validation at system design-time to support safety and time critical streams (e.g., process control and emergency response). In this context, the demand-based schedulability tests have recently been proposed in the literature. This work revisits two well-established techniques borrowed from the multi-processor scheduling theory, namely the demand-bound-function (DBF) and the forced-forward-demand-bound-function (FFDBF), and evaluates their performances when adapted to the field of wireless sensor-actuator networks. Simulation experiments when varying network configurations confirm the equal or better accuracy of FFDBF over DBF to estimate both network demand and schedulability. In future work, we aim at building upon these promising results in order to design novel admission control and adaptation strategies that improve network schedulability under varying workload conditions.

Context-awareness is a prominently desired feature in computing systems. Smartphones, smart cards or tags, wearables, sensor nodes, and many other devices enable a system to compute context for different users and environment. With ever increasing advances in hardware for such devices, the interactions with users are increasing every day. This enables the collection of a large amount of data about users, systems, and physical environment. With such data available to be leveraged, context-awareness will soon become a necessity. Such type of data collection happens most frequently in sensing applications enabled by wireless sensor network (WSN) devices. This paper discusses the concept of context for sensing applications, specifically related to Cyber Physical Systems (CPS). The paper highlights key aspects of context and its definition. This paper proposes, to the best of the author's knowledge, the first programming approach to build context-aware applications for WSN-based CPS. This paper provides a proof of concept for a framework to detect, manage and deploy context-aware applications.

Industry 4.0, Industrial Internet of Things, Cyber-Physical Production Systems and Smart Factories are closely related emerging concepts expected to drive significant improvements in industrial production systems, with gains in efficiency, cost and customer satisfaction. These concepts are intimately associated with highly distributed and cooperative architectures that rely, naturally, on the network infrastructure. However, traditional industrial communication technologies hardly provide the required level of integration, flexibility and performance. Seeking a solution to this mismatch, we assess two technologies that appeared recently in the industrial realm, namely IEEE 802.1 Time-Sensitive Networking (TSN) and Software-Defined Networking (SDN). TSN and SDN are fundamentally different, thus having distinct strengths and weaknesses. This paper reviews their fundamental operation principles, evaluating them qualitatively against the specific requirements posed by Industry 4.0.

Urban traffic management (UTM) is responsible for planning and controlling traffic on road infrastructures, including lane closures, full freeway closures, and pedestrian access. An essential element in UTM is the Intersection Management (IM) that deals with traffic control and is vulnerable to traffic congestion and accidents. In this paper, we propose an intelligent intersection management architecture along with the synchronous intersection management protocol (SIMP) instantiated in two versions. Simulation results show the advantages of SIMP-M (one of the versions) over the well known TraCI IM protocol, in terms of both worst-case and average vehicle speed passing through one intersection.

No abstract available.

The Internet of Things (IoT) is experiencing rapid growth and being adopted across multiple domains. For example, in industry it supports the connectivity needed to integrate smart machines, components and products in the ongoing Industry 4.0 trend. However, there is a myriad of communication technologies that complicate the needed integration, requiring gateways to connect to the Internet. Conversely, using IEEE 802.11 (WiFi) devices can connect to existing WiFi infrastructures directly and access the Internet with shorter communication delays and lower system cost. However, WiFi is energy consuming, impacting autonomy of the end devices. In this work we characterize a recent WiFi-enabled device, namely the ESP8266 module, that is low cost and branded as ultra-low-power, but whose performance for IoT applications is still undocumented. We explore the built-in sleep modes and we measure the impact of infrastructure parameters beacon interval and DTIM period on energy consumption, as well as packet delivery ratio and received signal strength as a function of distance and module antenna orientation to assert area coverage. The ESP8266 module showed suitability for battery powered IoT applications that allow 2-4 days recharge cycles on a 1000mAh battery with seconds-scale transmission intervals.

Minimum Cycle Time is a common performance indicator adopted to compare Real-Time Ethernet protocols. Though serving its purpose, Minimum Cycle Time excludes the delays inside the sending and receiving nodes, so it is insufficient to estimate the end-to-end latency. In this work, we describe some implementation possibilities of an Ethernet node in a System-on-Chip and present measurements of the delay to send/receive packets from/to the application layer. We chose different points in the software to make the measurement, so the results cover more use-cases. We found the Ethernet Lite Media Access Controller (MAC) to be faster than the hard MAC (GEM) and the Lightweight IP stack to add less than 2.2 μs. Finally, we show how a hardware accelerator can reduce the delay of high-priority packets by 1.4 μs.

Bicycle-to-bicycle (Bi2Bi) communication can be implemented by well-established technologies in the 2.4GHz ISM band: IEEE 802.11, Bluetooth or IEEE 802.15.4. These technologies have distinct performance due to different physical and data link layers. In this paper, we characterize the mentioned 2.4 GHz-operating technologies over opportunistic links established between bicycles using commodity hardware. We find that, in Bi2Bi links, Blue-tooth, IEEE 802.11 at 24 Mbit/s, and IEEE 802.11 with automatic rate adaptation can communicate only in the immediate surroundings (under 15m of range), to maxima of 1.5 Mbit/s, 17 Mbit/s and 25 Mbit/s, respectively. IEEE 802.15.4 and IEEE 802.11 at 1 Mbit/s sustain connectivity up to 30 and 40 meters and peak transfer rates of 50 kbit/s and 800 kbit/s respectively. In addition, we observed that, in all measurement scenarios, link performance depended strongly on whether bicycles were approaching or moving away, rather than on whether one was at the front or back of the other.

Cooperating mobile robots are real-time systems that often require mutual synchronisation, either to carry out cooperative sensing and actuation, or to improve the quality of wireless communications. Concerning this last aspect, a common technique to improve the communication channel is to eliminate access collisions by allocating predefined disjoint time slots to robots, in a circular list, which is known as Time Division Multiple Access (TDMA). This technique typically requires a global clock to identify each slot. However, this method is not robust with respect to asynchronous transmissions generated by external or joining nodes. Consequently, this work proposes a global TDMA protocol that allows for real-time and guaranteed delivery of messages within deadlines, given its predictable schedule, and that: i) applies to dynamic mesh networks of cooperating mobile robots; ii) synchronises slots in a relative fashion using locally perceived delays of message exchanges that are globalised throughout the network, thus not relying on a global clock; and iii) tolerates external traffic and asynchronous joining robots using underneath standard ad-hoc wireless RF technologies that provide CSMA-type arbitration. We describe our protocol and prove that under common operating conditions all robots eventually reach synchronisation. We also propose a heuristic for the few cases that were not covered by the previous proof, which always led to consensus under extensive simulation testing. To the best of our knowledge, this is the first guaranteed clockless synchronisation approach for ad-hoc networks of mobile robots that works over commodity wireless protocols.

This paper presents a time-triggered protocol designed for decentralized control applications that use Ethernet line or ring networks. The protocol was developed based on the requirements of Modular Multilevel Converters currently used in electric power systems. It reduces the nodes forwarding delay and cycle time to a minimum by exploring the combination of a global set-point dissemination with distributed local adjustments. We have validated the overall concept with an OMNeT++ model and tested the forwarding implementation using a Field Programmable Gate Array. A comparison with EtherCAT, Profinet IRT and VABs shows that the proposed protocol has shorter cycle times.

Ethernet is a promising candidate for networking real-time embedded systems such as automobiles, and it is already used in several embedded scopes, such as airplanes and trains. Moreover, its higher bandwidth enables applications made of dispersed embedded systems that exchange large amounts of data through the so-called Internet-of-Things, such as wide-area video sensing. In this realm, network reservations are an important design element that favor composability in the time domain thereby supporting the design of complex systems. However, reservations may impact negatively on the network bandwidth efficiency due to over-specification, particularly when they are designed to meet worst-case delay constraints. In this work, we use a specific Ethernet protocol that provides real-time reservations, namely FTT-SE, and we assess through extensive simulations the efficiency of a worst-case network delay analysis for sporadic reservations associated to asynchronous messages. Essentially, we compare the analytical worst-case delay with the one observed in the simulations using two data sets comprising up to 20000 and 100000 message sets, respectively, and we change the system configuration, namely properties of the message sets and network and protocol configuration parameters. We find that the analysis is accurate, i.e. matches observations, for a significant percentage of messages in the message sets (up to 60% on average). Conversely, we found that a small percentage of message sets (below 6%) generated rather pessimistic analytic estimates exceeding the observations by 6 times, thus with a negative impact on efficiency. The results we present in the paper, particularly their distributions, inform system designers of how to tune their designs to improve network bandwidth efficiency under strict timing constraints.

Multidimensional Scaling (MDS) can be used to localize a set of nodes (tags) by evaluating their distances from another set of nodes having known location (anchors). Node localization with MDS generally requires that the proximity graph be fully connected. This implies that matrices generated from tag-anchor ranging for which tag-to-tag distances are missing can not be used directly with the MDS algorithm without the use of estimates for the missing data. These estimates, however, unavoidably introduce some approximations in the localization process, which can become relatively large depending on the number of missing measurements and the amount of noise in the pair-wise distance measurements. This paper proposes a specialized form of the anchored MDS algorithm that undermines missing tag-to-tag distances in the connectivity matrix. We show that decoupling tag-to-tag interactions in the Scaling by MAjorizing a COmplicated Function (SMACOF) algorithm can undermine the effects of missing tag-to-tag distances and produce tag configurations that are inferred directly from only anchor-tag pairwise distances.

Emerging concepts such as Smart Production, Industrial Internet of Things and Industry 4.0 bring a radically new set of requirements to the way industrial systems are engineered. In what concerns the communication infrastructure, support to dynamic environments, interoperability and heterogeneity, combined with a significant increase in the number of devices, are just a few of the challenges that must be faced. Software-defined networking is a disruptive networking paradigm that emerged on campus networks but was soon considered for use at industrial level. This paper presents a set of extensions to the Software Defined Networking (SDN) OpenFlow protocol that complement its functionality, namely supporting real-time reservations, which is one of its more notorious limitations when considering industrial scenarios. We explain how the extensions are implemented in the OpenFlow side and enforced using a Flexible Time-Triggered Ethernet network. The extensions are validated experimentally, showing that the platform supports dynamically reconfigurable heterogeneous traffic classes.

An emerging trend in industry 4.0 is to use wireless communication infrastructure and mesh networks in applications requiring high reliability and safety. Although not a typical industrial production process, railway vehicular networks are also an industrial application which come with stringent reliability and safety requirements. Current research is focusing on using vehicular networks as an enabling technology to actively control the separation between two consecutive vehicles, enforcing a safe distance which is nevertheless much shorter than currently used to maintain vehicle separation. In this respect, we analyze a hopping strategy for Time-Slotted Channel-Hopping (TSCH), which was introduced in the IEEE 802.15.4e amendment with a view of improving the reliability of IEEE 802.15.4 networks. We define a probability framework to estimate the chance of successful hopping assuming two previously merged vehicles, and we design a zero-sum game and propose a payoff function to always place communicating nodes in a Nash equilibrium by choosing whether to hop or not, and therefore maximizing the communication throughput by mitigating jamming signals.

Recent advances in Unmanned Aerial Vehicles (UAVs) have enabled countless new applications in the domain of aerial sensing. In scenarios such as intrusion detection, target tracking and facility monitoring it is important to reach a given area of interest (AOI), and create an online data streaming connection to a monitoring ground station (GS) for immediate delivery of content to the operator. In previous work, we showed that a multi-hop line network can increase the range of the mission by finding the optimal number of relay UAVs, and their optimal placement. In this demo, we show that CSMA (typical 802.11's medium access protocol) behaves poorly in this type of networks due to mutual interference, and that TDMA is a better alternative. We will also discuss how changing slot width online can overcome typical and less known TDMA in-efficiencies, and therefore reach maximum end-to-end throughput and low delay.

Unmanned Aerial Vehicles (UAVs) are rapidly becoming an important tool for applications like surveillance, target tracking and facility monitoring. In many of these contexts, one or more UAVs need to reach an area of interest (AOI) while streaming live video to a ground station (GS) where one or more operators inspect the AOI and carry out fine control of UAVs position. In remote areas, intermediate UAVs can act as relays and form a line network to extend range. Interactive control requires a live video stream where both throughput and delay are important. In this paper, we show that routing packets over CSMA/CA (native medium access protocol of WiFi, the most common wireless technology among UAVs) behaves poorly in this context due to link asymmetries. We propose a novel distributed, adaptive and self-synchronized TDMA protocol (DVSP) that both enhances delay and packet delivery while operating on commodity hardware and leveraging a standard UDP/IP protocol stack. We prove that DVSP converges to a global solution that minimizes delay using local information, only, thus in a fully distributed manner. Real world experiments with multiple UAVs show gains in delay up to 75%, and packet delivery up to 50%, without sacrificing goodput.

Modern computing systems are often formed by multiple components that interact with each other through the use of shared resources (e.g., CPU, network bandwidth, storage). In this paper, we consider a representative scenario of one such system in the context of an Internet of Things application. The system consists of a network of self-adaptive cameras that share a communication channel, transmitting streams of frames to a central node. The cameras can modify a quality parameter to adapt the amount of information encoded and to affect their bandwidth requirements and usage. A critical design choice for such a system is scheduling channel access, i.e., how to determine the amount of channel capacity that should be used by each of the cameras at any point in time. Two main issues have to be considered for the choice of a bandwidth allocation scheme: (i) camera adaptation and network access scheduling may interfere with one another, (ii) bandwidth distribution should be triggered only when necessary, to limit additional overhead. This paper proposes the first formally verified event-triggered adaptation scheme for bandwidth allocation, designed to minimize additional overhead in the network. Desired properties of the system are verified using model checking. The paper also describes experimental results obtained with an implementation of the scheme.

Contemporary distributed embedded systems in many domains have become highly complex due to ever-increasing demand on advanced computer controlled functionality. The resource reservation techniques can be effective in lowering the software complexity, ensuring predictability and allowing flexibility during the development and execution of these systems. This paper proposes a novel end-to-end resource reservation model for distributed embedded systems. In order to support the development of predictable systems using the proposed model, the paper provides a method to design resource reservations and an end-to-end timing analysis. The reservation design can be subjected to different optimization criteria with respect to runtime footprint, overhead or performance. The paper also presents and evaluates a case study to show the usability of the proposed model, reservation design method and end-to-end timing analysis.

Unmanned aerial vehicles (UAVs) recently enabled a myriad of new applications spanning domains from personal entertainment to surveillance and monitoring. In this paper, we focus on using several small UAVs collaboratively to provide extended reach to an online video monitoring system for inspection of industrial installations. We make use of 802.11 radios on low-cost commercial-off-the-shelf UAVs, set up a time-division multiple access overlay protocol to avoid mutual interference, and enable high channel utilization in multihop networks. In particular, we provide a model for the quality of the UAV-to-UAV link, in terms of packet delivery ratio as a function of distance, packet size, and orientation, based on an extensive measurement campaign. We show that this platform is not omnidirectional in the horizontal plane and that UAV-to-UAV communication ceases around 75 m. Concerning the operation in a multihop mode to allow extending the network, the paper derives the optimal number of hops that maximize the end-to-end throughput, as well as the corresponding hop lengths. We validate our mathematical model with extensive experimental measurements transmitting payloads up to 200 m (over 802.11 g at 54 MBps).

Wireless communication will play an increasingly important role in future factory automation and process control, where the presence of mobile autonomous devices is expected to grow. However, wireless links are prone to errors due to shadowing and multi-path fading, which is even more severe in dynamic environments. These problems can be attenuated by using a mesh backbone to which mobile node connect to, using a hand-off algorithm. This solution is particularly important under real-time requirements typically found in factory automation. In this paper, we devise the Double-Threshold Hand-off (DoTHa) algorithm, a novel hand-off mechanism that triggers a hand-off in various environmental conditions. As a case study, we carry out the tele-operation of a mobile robot through a wireless mesh network in an indoor setting, using a wireless chain network protocol (WICKPro-SRT) that supports soft real-time traffic. We empirically compared DoTHa with two existing hand-off algorithms based on single and double hysteresis margin. The results revealed that DoTHa achieves Data Delivery Ratio (DDR) close to 100% whereas the single hysteresis-based hand-off suffers from frequent disconnections, dropping DDR to 88%. The double hysteresis-based hand-off shows higher ping-pong effect than DoTHa, doubling the number of hand-offs in some scenarios.

There is a growing interest in developing embedded systems capable of being deployed in dynamic environments that may change in unpredictable manners. When such systems are Distributed Embedded Systems (DESs) they must exhibit flexibility at all levels of their architecture, including the network. On the other hand, there is a clear trend in industry towards using Ethernet-based protocols at the network level of DESs. Nevertheless, Ethernet lacks appropriate support for real-time (RT) communications, mixing different RT traffic and on-line management of the Quality of Service (QoS). Several implementations of the Flexible Time-Tiggered (FTT) protocol over Ethernet were proposed to cope with these drawbacks. FTT is a master/multi-slave protocol that is able to simultaneously convey real and non-real-time traffic and provides mechanisms for dynamically changing the QoS of the network, including Admission Control (AC). The AC is a fundamental component for on-line network management, since it guarantees that each participant gets the required QoS. This paper presents the implementation in OMNeT++ of a simulation model of the AC in the FTT HaRTES switch as well as a preliminary performance study using that model.

The resource reservation techniques provide effective means to lower the software complexity, ensure predictability and allow flexibility during the development and execution of complex distributed embedded systems. In this paper we propose a new end-to-end resource reservation model for distributed embedded systems. The model is comprehensive in such a way that it supports end-to-end resource reservations on distributed transactions with various activation patterns that are commonly used in industrial control systems. The model allows resource reservations on processors and real-time network protocols. We also present timing analysis for the distributed embedded systems that are developed using the proposed model. The timing analysis computes the end-to-end response times as well as delays such as data age and reaction delays. The presented analysis also supports real-time networks that can autonomously initiate transmissions. Such networks are not supported by the existing analyses. We also include a case study to show the usability of the model and end-to-end timing analysis with resource reservations.

Industrial applications have been shifting towards wireless multi-hop networks in recent years due to their lower cost of deployment and reconfiguration compared with their wired counterparts. These wireless networks usually must support real-time communication to meet the application requirements. For this reason, Wireless Mesh Networks (WMNs) are potential candidates for industrial applications as they support a fixed infrastructure of static nodes for relaying packets. To meet the application demands, we modify the wireless chain network protocol (WICKPro) to support soft real-time traffic in WMNs with chain topologies over IEEE 802.11. We employ tele-operation of mobile robots as our case study, and perform extensive simulation and laboratory experiments. We show that the data delivery ratio is increased up to 42% in a scenario with 7 nodes, when the maximum end-to-end delay tolerated by the application is doubled. This is particularly suited to soft real-time applications that can trade longer delays by higher reliability. Moreover, when compared with a distributed priority-based token-passing protocol (RT-WMP), the lower overhead of WICKPro allows, in an error-free scenario, obtaining a throughput improvement of 33.42% on average.

Ethernet is gaining importance in fields such as automation, avionics and automotive. In these fields novel multimedia-based applications must coexist with traditional control systems, which leads to high diversity in size, intensity and timing requirements of the traffic traversing the channel. Multimedia traffic is characterised by having large size, low intensity and soft real-time requirements, while control traffic usually conveys small amounts of information with a high intensity and hard real-time requirements. Moreover, many modern applications must support on-line connection and disconnection of participants. Since Ethernet was designed as a general purpose data network protocol it lacks appropriate support for real-time communications and dynamic quality of service management. Several protocols were proposed to cope with these drawbacks, including Flexible Time-Triggered Switched Ethernet and, more recently, Audio Video Bridging. In this paper we discuss the importance of the admission control and make a comparison of the implementations carried out in the aforementioned protocols.

The ability of reconfiguring a system during runtime is essential for dynamic real-time applications in which resource usage is traded online for quality of service. The HaRTES switch, which is a modified Ethernet switch, holds this ability for the network resource, and at the same time it provides hard real-time support for both periodic and sporadic traffic. Although the HaRTES switch technologically caters this ability, a protocol to actually perform the dynamic reconfiguration is missing in multi-hop HaRTES networks. In this paper we introduce such a protocol that is compatible with the traffic scheduling method used in the architecture. We prove the correctness of the protocol using a model checking technique. Moreover, we conduct a set of simulation experiments to show the performance of the protocol and we also show that the reconfiguration process is terminated within a bounded time.

Recent advances in Unmanned Aerial Vehicles (UAVs) have enabled a myriad of new applications in many different domains from personal entertainment to process and infrastructure online monitoring in large industrial sites, among other. Our work focuses on how one can use several small UAVs collaboratively to provide extended reach to an online video monitoring system. We demonstrate how a TDMA overlay using 802.11 radios on low-cost commercial-off-the-shelf (COTS) UAVs can be used to enable high channel utilization in multi-hop networks, by avoiding mutual interference. This paper presents an extensive network characterisation and modelling of the quality of the UAV-to-UAV link, in terms of packet delivery ratio as a function of distance, packet size and orientation. We show that this platform is non-omnidirectional in the flight plane and that UAV-to-UAV communication ceases around 75m. Then, we solve the mathematical problem of finding the optimal link length and number of hops that maximize the end-to-end throughput, as we extend the network. We validate our mathematical model with extensive experimental campaigns transmitting payloads up to 200m (over 802.11g @ 54MBps).

The Vehicular Time-Triggered Protocol (V-FTT) is an infra-structured communication protocol for vehicular communications, based on IEEE802.11p/ETSI ITS-G5 standards, that brings improved timeliness and determinism by taking advantage of cooperative roadside units interconnected by a backhauling network. In a previous work, the use of a Spatial TDMA framework (STDMA) to improve both coverage and communication reliability on V-FTT networks was proposed. A set of experiments, carried out in a real scenario, proved that STDMA brings coverage and reliability improvements but with limitations. Particularly, the obtained results show that in order to obtain low message error rates, a prohibitively high number of roadside units may be required. Selective message retransmission is a well-known time-domain redundancy technique, which has been also explored for vehicular communications. However, the simultaneous combination of both space- and time-domain techniques has yet to be explored. This paper presents a preliminary study of the potential benefits of such approach. Data extracted from an experimental system deployment in a Portuguese highway was used to derive a realistic error-rate model. Based on this model, simulations were carried out to assess and validate the potential benefits of such hybrid approach.

Using low duty-cycle is the most common technique to extend the system lifetime in WSNs. However, it also implies limited throughput and long delay and the penalty is even higher under variable traffic patterns. In this paper, we present iQueue-MAC, a hybrid CSMA/TDMA MAC that adapts to variable/bursty traffic. With light load, iQueue-MAC uses a contention-based CSMA mechanism that provides low delay with scattered transmissions. When traffic increases, detected by a forming backlog in the sender, iQueue-MAC changes to a contention-free TDMA mechanism allocating transmission slots. Thus, iQueue-MAC mitigates packet buffering and reduces packet delay, combining the best of TDMA and CSMA. In this paper we also show how iQueue-MAC can operate in both single and multi channel modes. We implemented it on SIM32W108 chips together with other reference WSN protocols for comparison. iQueue-MAC exhibits similar figures during light traffic. However, with bursty traffic its throughput can be five times that of CoSenS and Ri-MAC-MC and its delay 20 times lower. Finally, iQueue-MAC is able to effectively use multiple channels, duplicating its throughput when compared to single channel operation.

Mobile devices in industrial settings became commonplace with different handheld equipment and mobile robots using COTS WiFi interfaces for communication. This growth in the use of the wireless medium increases the potential for overload and consequent long delays and high losses. Traffic segregation with different Quality of Service (QoS) classes can attenuate the problem but does not solve it within each class. In this paper we propose an extension to the Reconfigurable and Adaptive TDMA protocol to cope with generic traffic and improve the performance of highly loaded WiFi networks. We benefit from the self-synchronization feature of the protocol to create disjoint slots without clock synchronization which are then made available to the protocol stack through a TUN/TAP virtual interface. Then we propose a cooperative bandwidth management to adjust the traffic generated by each node through the virtual interface to match the available bandwidth, granting high bandwidth utilization while avoiding overload. We validate our approach with several practical experiments that confirm the capacity to measure and track the actual bandwidth available for communication through the protocol, removing the bandwidth consumed by interference.

The number of mobile devices accessing the Internet is continuously increasing, creating new problems. For example, when many wireless clients try to access the same video stream in the same hot spot, multicast transmission must be used. However, multicast packets are unacknowledged and may present a high loss rate under adverse conditions which may jeopardize the quality of the streaming. In a severe scenario, packet losses may affect all video frames, preventing the clients from receiving the video stream at all. Therefore, we propose a novel reliable multicast protocol, tailored to streaming applications, that also scales to many clients under the same access point. It uses a feedback repeat request mechanism per client and such requests are adequately scheduled by the streaming server. This mechanism, which is the focus of this paper, also allows differentiating the quality of service provided to the clients, polling their feedback independently. We show experimental and simulation results that confirm the desired scalability and reliable streaming, with efficient bandwidth usage. The results also show a clear superiority in mitigating lost packets with respect to other approaches to the same problem, with benefits reaching and even going beyond one order of magnitude in packets recovery.

Small teams of cooperating robots have been shown to benefit from the increased reliability of synchronised message exchanges provided by TDMA-based schemes. However, such schemes may also impose a long propagation delay to communications between non neighbour robots. Such negative impact is further increased by long TDMA rounds, which favour reduced medium utilisation and energy consumption, and by decentralised mesh topologies, which increase the team coverage and layout flexibility. For small size teams, say up to 10 units, it is feasible to set-up a global TDMA framework as well as tracking the instantaneous network topology, making it available to each robot. In this work we use this knowledge to forward packets along their path within each TDMA slot. To that end, we present a novel communication protocol that combines global TDMA and multi-hop routing. It maintains the reliability benefits of the TDMA schemes while strongly reducing the end-to-end propagation delay of interactions between non neighbour robots. We validate our protocol with simulation results using OMNET++. In a worst-case topology scenario we achieved an end-to-end delay that can be as low as 35% that of a traditional TDMA implementation that forwards the packets to the immediate one-hop neighbours, only. In a concrete audio streaming application scenario reported in the literature for an alternative real-time token-passing protocol, our proposal achieves similar delays with significantly less management bandwidth.

On-the-fly adaptability and reconfigurability are recently becoming an interest in real-time communications. To assure a continued real-time behavior, the admission control with a quality-of-service mechanism is required, that screen all adaptation and reconfiguration requests. In the context of switched Ethernet networks, the FTT-SE protocol provides adaptive real-time communication. Recently, we proposed two methods to perform the online reconfiguration in multi-hop FTT-SE architectures. However, the methods lack the experimental evaluation. In this paper, we evaluate both methods in terms of the reconfiguration time.

The manual partitioning of virtual machines with real-time requirements onto a multi-core platform is expensive, does not guarantee to find an optimal solution, and does not scale with regard to the upcoming higher number of both virtual machines and processor cores. This work proposes an algorithmic solution. As a prerequisite, the partitioning problem is defined in a formal manner by the abstraction of computation time demand of virtual machines and computation time supply of a shared processor core. In particular, we propose a branch-and-bound partitioning algorithm that systematically generates and evaluates candidate solutions. Combined with a computation time server based scheduling of the virtual machines that are mapped to the same core, it is guaranteed that the computation time demand of all virtual machines is satisfied. The utilization is optimized by transforming to harmonic server periods. The partitioning either minimizes the required number of cores or maximizes the distribution of critical virtual machines. The different outcomes of the algorithm according to these two goals are illustrated exemplarily and evaluated with random workloads.

Distributed systems rely in communication networks, typically a bus, in order to exchange messages and fulfill their goals. However, message transmission is subject to interferences that ultimately can lead to message corruption. In systems where a high-reliability is sought, error recovery mechanisms can be deployed in order to give the required reliability level, and this can be done in the spatial or temporal domain. In the scope of the FTT paradigm, and applied to the FTT-CAN protocol, the authors have previously presented a time domain recovery method using message retransmissions controlled by a server. In this article we assess the impact of different scheduling policies for the server, presenting a qualitative evaluation of the alternatives, complemented by a simulation study, in order to verify their advantages and weak points.

In safety-critical systems a global high-reliability is sought, including in the communication network when a distributed control system is used. The FlexRay protocol was developed within the automotive industry with the necessary characteristics to respond to the demands of safety-critical applications, e.g. X-by-wire. Nevertheless, the FlexRay protocol does not define a mechanism to guarantee message delivery, nor defines global error signaling, leaving the resolution of these problems to the application. This paper presents a preliminary proposal for a mechanism that uses temporal redundancy to recover transient errors in time-triggered messages. An extra node is used to trigger message retransmissions whenever errors or omissions do effectively occur. This mechanism uses the FlexRay dynamic segment to implement message retransmissions, leading to a minimum recovery time, typically one cycle, together with a very small bandwidth usage.

With the growth of complexity in the embedded domain, the use of distributed systems to support multiple realtime applications has become commonplace. These applications may share processor and network resources, and real-time scheduling policies can guarantee that these applications do not interfere with each other's ability to meet their temporal constraints. We believe that these applications should also be able to transparently share services and chains of services, without the coupling that such sharing typically implies. To solve this problem, we propose D-RES, a resource management system that guarantees temporal isolation between service-sharing applications in a distributed system. D-RES transparently tracks which application uses which service, billing the correct application even in case of nested service calls. We implemented D-RES, and demonstrate its ability to isolate service-sharing applications even in case of overload.

Hypervisor-based virtualization provides a natural way to integrate formerly distinct systems into a single mixed-criticality multicore system by consolidating in separated virtual machines. We propose an adaptive computation bandwidth management for such architectures, which is compatible with a potential certification based on the guarantee of specified bandwidth minimums and the isolation of overruns of virtual machines. This management uses periodic servers and an elastic task model to combine analyzability at design time with adaptability at runtime. Mode changes or early termination of VMs trigger a resource redistribution that reassigns spare capacity. In this paper we focus on the integration of an adaptive reservation policy into a virtualization software stack and the co-design of hypervisor and paravirtualized guest operating system. In a concrete implementation on a PowerPC 405, the bandwidth distribution policy incurred in a memory footprint below 2.7KB and a worst-case execution time for the redistribution function below 4 microseconds for realistic low numbers of VMs. Simulations over synthetically generated sets of VMs with random mode changes showed a gain of 13% of computation bandwidth when compared to an approach with fixed partitions and provided a relative error of allocated bandwidth to desired bandwidth 4 times lower.

In the context of switched Ethernet networks, multi-hop communication is essential as the networks in industrial applications comprise a high amount of nodes, that is far beyond the capability of a single switch. In this paper, we focus on multi-hop communication using HaRTES switches. The HaRTES switch is a modified Ethernet switch that provides real-time traffic scheduling, dynamic Quality-of-Service and temporal isolation between real-time and non-real-time traffic. Herein, we propose a method, called Reduced Buffering Scheme, to conduct the traffic through multiple HaRTES switches in a multi-hop HaRTES architecture. In order to enable the new scheduling method we propose to modify the HaRTES switch structure. Moreover, we develop a response time analysis for the new method. We also compare the proposed method with a method previously proposed, called Distributed Global Scheduling, based on their traffic response times. We show that, the new method forwards all types of traffic including the highest, the medium and the lowest priority, faster than the previous method in most of the cases. Furthermore, we show that the new method performs even better for larger networks compared with the previous one.

Distributed embedded systems have traditionally been designed using static approaches, i.e., assuming a static environment. Such approaches, however, cannot guarantee continuous operation under dynamic environments that impose new requirements upon a system as time passes. As a solution, flexible approaches have been proposed. One such approach that allows a system to adapt to changing real-time requirements is the Flexible Time-Triggered (FTT) communication paradigm. Nevertheless, if continuous operation under dynamic environments is desired, then flexibility is not enough. Indeed, it is also crucial for the system to be sufficiently reliable. In this paper we therefore explore some design ideas to make FTT highly reliable through fault tolerance by using replication. As a starting point we will use the switch of the Hard Real-Time Ethernet Switching (HaRTES) implementation of FTT.

Distributed Embedded Systems are subject to transient communication faults that need being detected and mitigated in safety-critical scopes. This paper addresses error recovery in time-triggered systems based on the Controller Area Network (CAN). It extends a recent work that proposed using online traffic scheduling, combined with servers, to implement dynamic message retransmissions. In particular, we provide a schedulability analysis that considers the interference of the error-recovery server in the time-triggered traffic, as well as a methodology to compute the worst-case response time of messages affected by errors. We also present a comparison with related error-recovery methods that confirms the superiority of the proposed method.

Duty-cycling technique has been widely adopted in MAC protocols for wireless sensor networks to conserve energy. However, low duty-cycle also leads to limited throughput in most of existing solutions. In this paper, we propose iQueue-MAC to provide immediate yet energy-efficient throughput enhancement for dealing with burst or heavy traffic. Combined with CSMA/CA, iQueue-MAC makes use of queue length of each sensor node and allocates suitable TDMA slots to them for packets transmission. During light traffic period, no extra slots will be allocated; iQueue-MAC acts like other low duty-cycle MACs to conserve power. While in burst or heavy traffic period, iQueue-MAC senses the build up of packet queues and dynamically schedules adequate number of slots for packet transmission. We have implemented iQueue-MAC on STM32W108 chips that offer IEEE 802.15.4 standard communication. We set up several real-world experimental scenarios, including a 46 nodes multi-hop test-bed for simulating a general application, and conducted numerous experiments to evaluate iQueue-MAC, in comparison with other traffic adaptive duty-cycle protocols, such as multi-channel version RI-MAC and CoSenS. Results clearly show that iQueue-MAC outperforms multi-channel version of RI-MAC and CoSenS in terms of packet delay and throughput.

Video applications, which include industrial uses like machine vision, object tracking, surveillance, driving aids, etc. are becoming increasingly common. These sensors produce large amounts of data, being normally compressed at the source nodes to save network bandwidth. As a side effect, video streams exhibit a large variability in their bandwidth utilization. On the other hand, many video applications are highly dynamic. For instance, a video surveillance application can meet its goals with a low frame-rate video, when the environment being monitored is static, but require an high frame-rate when the environment changes. Another example is applications that have several video sources that are activated on demand. For instance, a rear-view video camera, nowadays commonly found in cars and trucks, is only necessary during certain maneuvers. Applications like the ones presented above can be efficiently supported by enhanced Ethernet switching that provides hierarchical server-based traffic scheduling, in particular by the FTT-enabled switch [1] [2] that resulted from instantiating the FTT (Flexible Time-Triggered) communication paradigm [3] onto a customized Ethernet switch. Servers are, in fact, adequate abstractions to handle real-time video transmission since:1) servers discipline the use of the network, thus coping with the large variability of compressed video sources, 2) the adaptability, obtained by changing the servers budget, permits varying dynamically the bandwidth allocated to each video stream, 3) the reconfigurability permits enabling and disabling video-streams. We demonstrate these features with a simplified video surveillance demo, using the setup shown in Figure 1.

Growing interest in facilitating the construction of real-time applications from reusable components and their run-time reconfiguration is motivating the use of Service-Oriented approaches in the real-time domain. The iLAND ARTEMIS project researches such direction, aiming at providing an SO middleware that supports dynamic reconfiguration, namely addition and removal of services and QoS-based hot swap of service implementations. In this demonstration we exhibit a few simple video sensing applications that illustrate the desired prompt reconfigurations without significant glitches and within bounded time as required for real-time applications.

The CAN bus is one of the most used networks in distributed real-time systems. The CAN bus can be divided by means of bridges in order to distribute the network load among different segments and contribute positively to the schedulability of the system. On the other hand, CAN supports the fragmentation of a message in multiple packets when it does not fit into one single CAN frame. The schedulability analysis of CAN has been extensively studied in the literature. Although the proposed methods analyze non-segmented CAN buses, they can be applied to segmented networks. However, the techniques from the related literature obtain quite pessimistic results in the analysis of segmented CAN networks with multi-packet messages. This paper proposes a less pessimistic CAN schedulability analysis method for the mentioned kind of networks. Moreover, this optimized technique can be integrated with methods oriented to the holistic schedulability analysis of a complete distributed real-time system.

Nodes, among the components of distributed embedded systems, exhibit the greatest permanent failure rate. Thus, providing tolerance to nodes faults is mandatory whenever high-reliability is required, being node replication the most common technique for that purpose. This paper proposes a novel technique suitable for CAN-based systems that simplifies existing approaches taking advantage of a star topology and the FTT protocol.

Despite the significant advantages of the Controller Area Network (CAN) there is an extended belief that CAN is not suitable for critical applications, mainly because of several dependability limitations. During the CANbids project each one of these limitations has been addressed and a complete architecture for CAN-based fault-tolerant systems has been devised. This architecture allows building highly-reliable systems. This paper describes the design of such an architecture and the prototyping of its fundamental parts.

The complexity of Networked Embedded Systems (NES) has been growing steeply, due to increases both in size and functionality, and is becoming a major development concern. This situation is pushing for paradigm changes in NES design methodologies towards higher composability and flexibility. Component-oriented design technologies, in particular supported by server-based scheduling, seem to be good candidates to provide the needed properties.

The assignment of fixed priorities to tasks and messages in distributed real-time systems is known to be an NP-hard problem, and thus there is no optimal method to accomplish it in polynomial time. This fact makes it a suitable problem to be approached with generic search and optimization algorithms. In this paper we propose a genetic algorithm with a permutational solution encoding for the assignment of fixed priorities to tasks and messages in distributed real-time systems using a holistic approach. This paper shows that the genetic algorithm can find more and better schedulable priority assignments than HO PA, which is, as far as we know, one of the best methods for the fixed priority assignment in distributed real-time systems.

Dynamic resource management is generating growing interest as a way to simplify systems deployment, react to changing operational conditions and improve efficiency in using system resources. Buttazzo et al proposed the elastic task model (ETM) that dynamically moves the tasks instantiation period around a nominal value, increasing or decreasing the bandwidth that each task uses from the resource. However such computation requires a number of iteration steps. This paper identifies the QoS-based resource distribution problem and proposes a generic framework that encompasses the ETM. An algorithm is presented for the resource distribution that computes the results in minimum time, outperforming the original ETM scheme in terms of computational complexity.

Supporting real-time traffic in a wireless network is a challenging task due to the high probability of errors and to the interference that can be provoked by other networks in the same area. There may be situations in which alien traffic is present by default and communication systems have to deal with difficulty by adapting their characteristics to provide the highest possible level of timing guarantees and performance. In this paper we propose a technique to make real-time protocols tolerant to alien traffic and able to provide soft real-time behavior even in the presence of high traffic loads, using a variable timeout window that allows belated acknowledgments to be accepted also.

Although the use of star topologies to improve dependability in field-buses is gaining in importance, as in TTP/C and FlexRay, a mature technology such as the Controller Area Network (CAN) remained essentially a bus-only network. Thus, we proposed a CAN-compliant replicated star topology called ReCANcentrate, which has advanced error-containment and fault-tolerance mechanisms. Its two hubs are coupled with each other and create a single logical broadcast domain that allowed us to propose, in a previous work, a strategy for each node to easily manage the replicated star by means of a software driver that abstracts away the details of the replication. This paper describes the main functionalities of this driver, as well as the first tests we have conducted, on a real ReCANcentrate prototype, to verify the correctness and the performance of the driver in the absence and in the presence of faults.

Modern distributed real-time systems (DRTS) are typically formed by several heterogeneous networked devices. Sometimes these devices must adapt to dynamic environments in which devices may join or leave the network and must respond in run-time to changing requirements ensuring certain levels of QoS. These characteristics yield to systems that are intrinsically complex to develop, test and maintain. In this scenario middleware platforms, and particularly Service-Oriented Architectures (SOA), are becoming increasingly popular because they ease the creation of complex distributed applications and reduce the time-to-market. In this paper we show how the recently developed Flexible Time-Triggered CORBA (FTT-CORBA) can be used to support real-time Service-Oriented Architectures that are deterministically and dynamically reconfigurable.

Ethernet switches are typically found in many large-scale distributed real-time systems providing low-end transactions as well as bulk backbone routing to real-time applications. The FTT-SE protocol (Flexible Time-Triggered communication over Switched Ethernet) is a recent proposal to bypass the limitations of conventional switches in terms of real-time behavior while catering for growing requirements on dynamic reconfigurability and adaptability. For this end, this paper develops linear time-complexity and memory-efficient on-line admission control tests based on utilization bounds for Rate-Monotonic and EDF scheduling on Ethernet switches using FTT-SE, which are suited for dynamic Quality of Service (QoS) management. Our analysis also has broader applicability in general periodic task sets with bounded release delays. For FTT-SE with 100 Mbps links and 1500 bytes of maximum packet size, our sufficient schedulability condition achieves an utilization bound of 61% for RMS and 88% for EDF. Simulation results on randomly generated task sets demonstrate that such bounds are within 18% and 5% utilization of the ideal tests for RMS and EDF, respectively.

The middleware is an important software component when designing an application, facilitating the development and deployment of the applications. In the case of Distributed Embedded Systems (DES), the middleware should provide basic functionalities to abstract the complexity that results from network distribution, namely data consistency, events synchronization and resource management. Often DES applications exhibit real-time requirements and have to deal with dynamic environments that present evolving requirements. While some middleware architectures have been proposed to address resource provisioning and QoS management, none of those middle-wares supports dynamic resource reconfiguration while providing real-time guarantees. This paper proposes a middleware layer, based on the services provided by a flexible real-time communication protocol, addressing distribution abstraction, dynamic reconfiguration and dynamic QoS management under real-time constraints.

Ethernet is becoming a common network technology for industrial and factory automation systems and, in recent years, a big effort has been made in enabling real-time communications using Ethernet technology. Many of these systems are complex, extend over relatively large places and/or integrate a significant number of nodes, thus requiring the use of multiple switches (hop). In this paper we look into the usage of Flexible Time-Triggered (FTT) enabled Ethernet switches in this class of systems, more specifically using the recently proposed server-based scheduling mechanism supported by this protocol. The paper proposes and validates a resource reservation protocol, presents a method for computing the end-to-end deadlines and discusses possible strategies for the deadline partitioning.

ZigBee/IEEE 802.15.4 beacon-enabled mode allows low duty-cycle operation with high energy saving in the wireless sensor network. However, low duty cycles introduce higher end-to-end delay in event propagation and generally lower reactivity of the network with respect to the actual sensing. In target tracking systems, both qualities should be satisfied, namely, low energy consumption while no target is being tracked and low end-to-end delay in target tracking mode. Hence, we need to define a trade-off between the energy consumption and the end-to-end delay. In this paper, we propose a dynamic dual-rate beacon scheduling method with two rate switching schemes modifying ZigBee/IEEE 802.15.4 to enhance its real-time property but still maintain the advantage of low energy consumption. We evaluate our scheme through a simulation using the OPNET simulator. The results show that our scheme exhibits low energy consumption while presenting reduced end-to-end delay, outperforming usual single rate working modes.

Current trends in real-time systems identify Java as a new alternative to develop both centralized and distributed real-time systems. Many efforts have been devoted to develop the Real-Time Specification for Java (RTSJ), and there is substantial ongoing activity to produce a straightforward and valuable Distributed Real-Time Specification for Java (DRTSJ). The current paper provides a contribution to this latter activity defining, from different angles, a synchronous scheduling service aligned with principles of some popular real-time architectures. This service orchestrates the system in such a way that it provides end-to-end guarantees in the distributed transactions, guaranteeing their timely execution across the network and nodes. The service is described from two points of view: the system one, characterizing a portable model; and the programmer one, defining a distributed object-oriented implementation of a model based on Real-Time Remote Method Invocation (RTRMI). Finally, it also presents results of an implementation carried out to judge the efficiency of the service, offering a preliminary predictability and performance assessment of a distributed real-time Java technology.

Star topologies are rising the interest of newer field-bus communication technologies like TTP/C and FlexRay, given the dependability advantages stars can offer. However, it is also possible to take advantage of a mature technology such as Controller Area Network (CAN), while benefiting from stars. For that, we developed a CAN-compliant replicated star called ReCANcentrate. It includes two hubs that are coupled with each other, thereby forcing a single broadcast domain that allowed us to define, in a previous work, a strategy for each node to easily manage the replicated star. To demonstrate the feasibility of this management, this paper presents its on-going implementation as a driver to be executed at each node.

There is a growing interest in using star topologies instead of buses as the communication infrastructure for highly-reliable distributed control systems, given the better dependability stars are supposed to provide. For the controller area network (CAN), we developed a simplex and a replicated star called CANcentrate and ReCANcentrate respectively. In a previous work we modelled the dependability of the CAN bus and CANcentrate using stochastic activity networks (SANs). There we presented the first quantitative analysis of the error-containment benefits of a simplex star when considering permanent hardware faults. This paper quantitatively analyzes, for the first time, how a replicated star such as ReCANcentrate can improve both error-containment and reliability, also considering permanent hardware faults. We explain our modelling strategy using SANs and show some first and novel results.

The Power Line technology has received an increasing attention in the last decades due to its inherent benefits, mainly related to the reduction of cabling and associated costs. Power Line Communication (PLC) was first employed in power utilities, and since the 80s in home automation, too. However, its use in the automotive field received relatively little attention. This paper extends previous works towards assessing PLC technology for communications within the automotive domain. In particular, it focuses on the real-time behavior of such technology, namely the DCB500 adaptors supplied by the Yamar company, showing experimental results of transmission delays, communication overheads and effectiveness of the medium access control policies.

This paper presents a model for quality-of-service (QoS)-aware service composition in distributed systems with real-time and fault-tolerance requirements. This model can be applied in application domains like, for example, remote monitoring, control and surveillance. Classic approaches to real-time systems do not provide the flexibility and fault-tolerance required in new emerging environments that need to combine a high degree of dynamism with temporal predictability. Our approach addresses these new challenges by combining concepts from the service oriented paradigm and distributed real-time systems. We propose a concrete system model based on a holistic time-triggered-based approach for design and configuration. Based on this model, we propose two algorithms for the composition of QoS-aware service-based applications with temporal requirements: an exhaustive algorithm that computes the optimal service combination in terms of a figure of merit, suitable for offline composition; and an improved algorithm based on heuristics and partial figures of merit, suitable for online composition. Experimental results show that the latter reduces dramatically the number of combinations explored with a minimal degradation in the quality of the solution, making it feasible for online execution in dynamic environments.

To overcome some dependability limitations of CAN that arise from its non-redundant bus topology, we have proposed a CAN-compliant replicated star topology, ReCANcentrate, whose hubs incorporate the necessary fault-treatment and fault-tolerance mechanisms. This paper presents ongoing work regarding the design and formal verification of the strategy each node of ReCANcentrate uses to manage the transmissions and the receptions on the replicated star, as well as to tolerate faults.

Industrial devices are increasingly adopting RTOSs or improved versions of general purpose OSs. This fact introduces new possibilities like wrapping the devices with open middleware technologies like CORBA. Thus, by using object oriented interfaces that should be provided by the vendors of the devices, heterogeneous industrial devices could be integrated in distributed applications. However, CORBA-like architectures may result relatively complex for automation engineers. The use of tools that substitute coding tasks for configuration tasks could be beneficial for them. This paper presents an interface for a generic anthropomorphic robotic arm which is being implemented for a particular case and how this interface could be used by an external synchronization service that has been presented in previous works.

Interest on using mobile autonomous agents has been growing, recently, due to their capacity to cooperate for diverse purposes, from rescue to demining and security. However, such cooperation requires the exchange of state data that is time sensitive while achieving timeliness with RF communication is intrinsically difficult due to the openess of the medium. This paper describes a communication layer that improves the timeliness of periodic data exchanges among the team reducing the chances of lost packets caused by collisions between team members. In particular, the paper extends a previous proposal for an adaptive TDMA protocol with new self-configuration capabilities according to the current number of active team members. This feature further reduces the likelyhood of collisions within the team. Several experimental results with an actual system implementation show the effectiveness of the proposed solution.

Recently, the application domains where the development of systems with temporal guarantees is needed have increased. The majority of such domains are dynamic environments where the classical design approach is no longer applicable since it is too pessimistic. Thus, new challenges arise to adapt the current methodologies used in different architectural levels to these new environments. New directions and paradigms are needed to allow combining functional flexibility and dynamism with temporal predictability. The current work addresses these new challenges through the exploration of the applicability of concepts from the service oriented paradigm to distributed real-time systems. We propose a concrete system model based on a holistic time-triggered-based approach for design and configuration. Based on this model, we analyse the architectural entities required for two different architectural design approaches: static, when no reconfigurations at run-time are admitted, and dynamic, when these reconfigurations are possible. An exhaustive composition algorithm is also proposed.

In recent years, approaches to control performance and resource optimization for embedded control systems have been receiving increased attention. Most of them focus on theory, whereas practical aspects are omitted. Theoretical advances demand flexible real-time kernel support for multitasking and preemption, thus requiring more sophisticated and expensive software/hardware solutions. On the other hand, embedded control systems often have cost constraints related with mass production and strong industrial competition, thus demanding low-cost solutions. In this paper, it is shown that these conflicting demands can be softened and that a compromise solution can be reached. We advocate that recent research results on optimal resource management for control tasks can be implemented on simple multitasking preemptive real-time kernels targeting low-cost microprocessors, which can be easily built in-house and tailored to actual application needs. The experimental evaluation shows that significant control performance improvement can be achieved without increasing hardware costs.

The design of safety-critical systems has typically adopted static techniques to simplify error detection and fault tolerance. However, economic pressure to reduce costs is exposing the limitations of those techniques in terms of efficiency in the use of system resources. In some industrial domains, such as the automotive, this pressure is too high, and other approaches to safety must be found, e.g., capable of providing some kind of fault tolerance but with graceful degradation to lower costs, or also capable of adapting to instantaneous requirements to better use the computational/communication resources.

Robots are becoming commonplace in unstructured and dynamic environments, ranging from homes to offices, public sites, catastrophe sites, military scenarios. Achieving adequate performance in such circumstances requires complex control architectures, mixing adequately deliberative and reactive capabilities. This mixing needs to be properly addressed from both the software and hardware architectures point of view and, particularly, the mapping of the former onto the latter, in order to reduce mutual interference between concurrent behaviors and support the desired coordination with adequate level of reactivity. This paper discusses the benefits of using hierarchical distributed hardware architectures and presents the case study of the CAMBADA soccer robots developed at the University of Aveiro, Portugal. These robots use a distributed hardware architecture with a central computer to carry out vision sensing, global coordination and deliberative functions and a low-level distributed sensing and actuation system based on a set of simple microcontroller nodes interconnected with a Controller Area Network (CAN).

In this paper we overview the integration of a framework that generically manages the system resources in the form of contracts, namely the FRESCOR framework, with a flexible network resource. We describe how a network resource, namely FTT-SE, supports the FRESCOR framework services and, likewise, how the network services are made available to the application through the contracting framework. In a designer perspective, we also describe how a typical distributed application can be easily deployed using such a framework.

The use of multimedia within industrial applications has become commonplace, targetting improved process monitoring and machine vision. In both cases, the multimedia information is commonly distributed and subject to time constraints that must be met across networks while avoiding untolerable interference over typical control flows. This can be achieved with Constant Bit-Rate (CBR) channels, typically supported by real-time protocols. However, the compressors used to reduce the amount of information to transfer generate Variable Bit-Rate (VBR) patterns. Adapting a VBR source to a CBR channel requires specific care in order to avoid wasting channel bandwidth or dropping video frames. This paper focuses on MJPEG transmission over Ethernet and proposes a bidimensional adaptation using the JPEG quantification factor q on the source side and frame acquisition/transmission period T on the source/network side respectively, using the FTT-SE protocol and its support for dynamic QoS management. The paper also shows several experiments with pre-recorded video streams that illustrate the advantages of the proposed approach.

Although distributed real-time (RT) systems are intrinsically complex to develop and maintain, the use of certain object-oriented (OO) technologies such as CORBA and CORBA-RT facilitates those tasks. This is the main reason why these technologies are becoming increasingly popular. Moreover, CORBA is complemented with additional services that solve specific requirements for some applications, so developers may concentrate on the application functionality. Following this approach, this paper presents a new service that allows the synchronous scheduling of different activities in a distributed RT system in a simple and flexible way. This paper also explains how this service can be used for optimizing that access to shared resources in a distributed system and presents several experimental data that show the service performance

Recently, new trends in application development for distributed platforms, such as the composable services model, attempt to provide more flexibility in system design, deployment and execution. Such trends, and particularly the referred composable services model, can also be beneficial in real-time distributed embedded systems, also providing a means to support more adaptive behaviors, reacting to the execution environment or coping with system reconfiguration. This paper explores a relatively new direction, which is the extension of the service-based model to dynamic, i.e. at run-time, composition in real-time distributed environments, in order to support the level of flexibility and adaptibility referred above. The paper proposes an architecture to support such dynamic service composition that is based on the flexible time triggered communication paradigm (FTT).To achieve the desired goal, we also redefine the concepts of service and service-based application in the concept of the FTT paradigm. Finally, we show experimental results obtained with a prototype implementation of the proposed architecture that confirm its feasibility and good temporal behavior

This paper describes an approach towards the definition and implementation of a synchronization service on top of a remote object model offered by an unicast real-time remote object paradigm. Also an architecture model based on the RTSJ (real-time specification for Java) and the distribution middleware RMI (remote method invocation) specifications is proposed in order to give support to the model, defining a convergence layer that manages the underlying resources involved in a master-slave communication through a new API. Finally, preliminary results from an implementation prototype show the feasibility of the model and provide an initial estimation of jitters and the performance of the synchronization service

CORBA, in particular the real-time specification, has proven to be a reliable and mature technology that has been successfully used in a great number of industrial distributed computing applications. However, its current version does not provide specific mechanisms to support the deployment of synchronous activities in a flexible way. This fact may complicate the application of this technology to certain applications where synchronous behaviour is expected. Therefore, this paper presents a new service, built over CORBA-RT, that allows the synchronization of different activities of a distributed system in an easy way. The paper also explains how this service can be used to optimize, in general, resource utilization in a distributed system

This paper presents a fault-tolerant modular control architecture for an electrical vehicle (VEIL) equipped with x-by-wire sub-systems. The proposed architecture is based on COTS components and includes steer-by-wire, brake-by-wire and accelerate-by-wire safety critical functions. The communication infrastructure is based on the FTT-CAN protocol, which provides the joint scheduling of message and tasks, according to a holistic approach.

The controller area network (CAN) is a field bus that is nowadays widespread in distributed embedded systems due to its electrical robustness, low price, and deterministic access delay. However, its use in safety-critical applications has been controversial due to dependability limitations, such as those arising from its bus topology. In particular, in a CAN bus, there are multiple components such that if any of them is faulty, a general failure of the communication system may happen. In this paper, we propose a design for an active star topology called CANcentrate. Our design solves the limitations indicated above by means of an active hub, which prevents error propagation from any of its ports to the others. Due to the specific characteristics of this hub, CANcentrate is fully compatible with existing CAN controllers. This paper compares bus and star topologies, analyzes related work, describes the CANcentrate basics, paying special attention to the mechanisms used for detecting faulty ports, and finally describes the implementation and test of a CANcentrate prototype.

The traditional approaches to the design of distributed safety-critical systems, due to fault-tolerance reasons, have mostly considered static cyclic table-based traffic scheduling. However, there is a growing demand for operational flexibility and integration, mainly to improve efficiency in the use of system resources, with the network playing a central role to support such properties. This calls for dynamic online traffic scheduling techniques so that dynamic communication requirements are adequately supported. Nevertheless, using dynamic traffic management mechanisms raises additional problems, in terms of fault-tolerance, related with the weaker knowledge of the future system state caused by the higher level of operational flexibility. Such problems have been recently addressed in the scope of using flexible time-triggered CAN (FTT-CAN) in safety-critical applications in order to benefit from the high operational flexibility of this protocol. This paper gathers and reviews the main mechanisms that were developed to provide dependability to the protocol, namely, master replication and fail-silence enforcement.

The management of ad-hoc networks raises interesting problems, that are particularly challenging for networks of mobile nodes. Considering the inherent complexity of these systems, the development of distributed applications relying on wireless communication protocols would be greatly simplified by the use of specific tools for supporting testing and step-by-step debugging. In this paper we describe WISE, a flexible interactive simulation environment for the development of wireless ad-hoc networks consisting of mobile units. A graphical interface allows the user to create/delete nodes, change their positions and parameters, and select specific mobility models in order to verify the network behavior in dynamic conditions. The simulator also provides a useful support for the verification of agreement protocols, synchronization algorithms and distributed scheduling, allowing the user to display a step-by-step evolution of the algorithms in a suitable graphical representation

The design of embedded real-time systems requires skills from multiple specific disciplines, including, but not limited to, control, computer science, and electronics. This often involves experts from differing backgrounds, who do not recognize that they address similar, if not identical, issues from complementary angles. Design methodologies are lacking in rigor and discipline so that demonstrating correctness of an embedded design, if at all possible, is a very expensive proposition that may delay significantly the introduction of a critical product. While the economic importance of embedded systems is widely acknowledged, academia has not paid enough attention to the education of a community of high-quality embedded system designers, an obvious difficulty being the need of interdisciplinarity in a period where specialization has been the target of most education systems. This paper presents the reflections that took place in the European Network of Excellence Artist leading us to propose principles and structured contents for building curricula on embedded software and systems.

Ethernet was not originally developed to meet the requirements of real-time industrial automation systems and it was commonly considered unsuited for applications at the field level. Hence, several techniques were developed to make this protocol exhibit real-time behavior, some of them requiring specialized hardware, others providing soft-real-time guarantees only, or others achieving hard real-time guarantees with different levels of bandwidth efficiency. More recently, there has been an effort to support quality-of-service (QoS) negotiation and enforcement but there is not yet an Ethernet-based data link protocol capable of providing dynamic QoS management to further exploit the variable requirements of dynamic applications. This paper presents the FTT-Ethernet protocol, which efficiently supports hard-real-time operation in a flexible way, seamlessly over shared or switched Ethernet. The FTT-Ethernet protocol employs an efficient master/multislave transmission control technique and combines online scheduling with online admission control, to guarantee continued real-time operation under dynamic communication requirements, together with data structures and mechanisms that are tailored to support dynamic QoS management. The paper includes a sample application, aiming at the management of video streams, which highlights the protocol's ability to support dynamic QoS management with real-time guarantees.

As the bandwidth of CPUs and networks continues to grow, it becomes more attractive, for efficiency reasons, to share such resources among several applications with the minimum level of interference. This can be achieved using temporal partitions, with each application assigned to its own partition and executing as if it was executing alone on a resource with lower bandwidth. The partitions are associated to servers that execute the application tasks according to a given application-level scheduler. On the other hand, the set of servers is scheduled by a system-level scheduler. This paper addresses the particular case of fixed priorities-based application-level schedulers together with a periodic server model at the system level. It starts with an adequate response time analysis based on the notion of server availability for a known server. Then it addresses the inverse problem of designing a server with minimum system-level resource requirements to fulfill the application time constraints. In this context, the paper shows that response time based schedulability tests with linear time bounds do not need to consider all tasks but just a small subset, which may lead to substantial speed-ups. The proposed method goes a step further with respect to other recent works in the literature by considering a more complete task model, effectively computing the server parameters and establishing a better trade-off concerning complexity and tightness.

Summary form only given. Real-time distributed application complexity is steadily increasing. A well-known technique used to manage such complexity consists of decomposing the whole system into different quasiindependent distributed subsystems. Inter-subsystem communication, when necessary, is performed via gateway nodes that filter ingoing and outgoing traffic. For real-time systems, this architecture poses additional design challenges, since it becomes necessary to consider both intra and inter-network message exchanges with real-time constraints. The FTT communication paradigm has been provided with tools for supporting flexible real-time communication on isolated networks. We present an approach to incorporate multisegment support into the FTT protocol family. Two approaches are presented, analyzed and compared, which allow breaking end-to-end deadlines into parameters that are local to each one of the interconnected networks.

Wireless communication technology is spreading quickly in almost all the information technology areas as a consequence of a gradual enhancement in quality and security of the communication, together with a decrease in the related costs. This facilitates the development of relatively low-cost teams of autonomous (robotic) mobile units that cooperate to achieve a common goal. Providing real-time communication among the team units is highly desirable for guaranteeing a predictable behavior while operating autonomously in unstructured environments. This paper proposes a MAC protocol for wireless communication that supports dynamic resource reservation for small teams of cooperative robots. The protocol uses a slotted time-triggered medium access transmission control that is collision-free, even in the presence of hidden nodes. The transmissions are scheduled according to the earliest deadline first scheduling policy. An adequate admission control guarantees the timing constraints of the team communication requirements, including when new nodes dynamically join or leave the team. The paper describes the protocol focusing on the consensus procedure that supports coherent changes in the global system. Finally, a set of simulation results are shown that illustrate the effectiveness of the proposed protocol.

Despite having been designed to interconnect office equipment such as computers and printers, since its early daysEthernet has also been considered for use in the industrial domain. However, it was not originally developed to meet the requirements of real-time industrial automation systems and it was commonly considered unsuited for applications at the field level, i.e.to interconnect sensors, actuators and controllers. Therefore, along its 30 years of existence, several proposals have been presented to make this protocol exhibit real-timebehaviour. Nevertheless, these proposals either require specialised hardware, or are suited to soft-real-time operation only, or are bandwidth or response-time inefficient. This paper presents an overview about the work previously done towards real-time communication on Ethernet. Then, it presents a new protocol, FTT-Ethernet, which relies on common network adapters and on a new transmission control named master/multi-slave that efficiently supports hard-real-time operation in a flexible way.

The requirement for flexible operation is becoming increasingly important in many distributed computer controlled systems, in order to cope with dynamic operational changes in a resource efficient way. This requirement naturally extends to the low level of such systems where, typically, field equipment such as sensors, actuators and controllers, are interconnected by a fieldbus communication system. The paper presents the DISCO project (DIStributed embeddable systems for COntrol applications) that started in September 2000, in the University of Aveiro, aiming at exploring specification, architectural and management issues in flexible distributed computer control systems for embedded applications. The project addresses the issues of control requirements definition in a QoS perspective, flexible scheduling of both network traffic as well as nodes' computing load, and global system management techniques. The paper shows the results obtained so far as well as the status of the current work in the scope of the DISCO project.

Robotic competitions for students have been around for more than two decades. Although the benefits of these competitions can be exploited at various levels of education, recent examples have been following a trend towards higher complexity. In order to bring the pedagogical advantages of robotics competitions down to a level where they can be exploited by undergraduate and high school students, a somewhat different strategy is presented in this paper. Since 1995, the authors have been organising the Micro-Rato Contest of the University of Aveiro, a competition of small autonomous and mobile robots. This paper provides an overview of the contest, describing the strategy followed by the organisers, the rules and technical specifications, as well as a brief profile of some of the robots.