WLAN Mesh Networks (WMNs) are a promising network architecture for IoT applications due to their self-healing and robust network formation capabilities. The IEEE 802.11s amendment standard integrates mesh functionality into the WLAN MAC layer, ensuring seamless peering and routing. Initially designed for testing, the Linux implementation of IEEE 802.11s includes basic peer link blocking mechanisms. Based on this, we present a new MAC address filtering approach for reliable topology control in WLAN mesh networks. Our modifications to the Linux kernel enable persistent link blocking and bidirectional link teardown, preventing the sporadic establishment of unwanted connections. Real-world experiments demonstrate the effectiveness of our approach.

Real-time capability plays an important role in modern factory communication systems. However, protocols such as MQTT, CoAP, and MQTT-SN, which are increasingly finding their way into industrial environments alongside industrial-grade protocols such as OPC UA and DDS, cannot fulfill the timing requirements without further optimization. In this paper, we present extensions of MQTT-SN to improve the real-time behavior in combination with PREEMPT_RT, which provides different real-time mechanisms for the Linux kernel. We address general, operating system-based, and hardware-specific optimizations. Their benefits are demonstrated by measurements in a practical testbed. The proposed extensions reduce delay and jitter, but do not aim to guarantee an upper bound on end-to-end delay.

Adaptive Voltage Scaling (AVS) is a well-known technique in ASICs. However, the requirements for its application in FPGAs are quite different. AVS uses run-time knowledge to determine a favorable voltage/frequency operating point. There are many suggestions in the literature for the sensors to be used to gain this run-time knowledge. However, the specific placement and routing of each sensor instance is either ignored or manually defined by hard macros or directed routing. In this paper, we propose a new approach to sensor placement. We are able to omit strict constraints like directed routing and give more freedom to the EDA tool without compromising sensor accuracy by using a proper sensor calibration. As a result, we are able to avoid placement and routing conflicts between sensors and application design. This allows the use of larger sensors with significant routing delay. An extensive experimental evaluation proves the validity of our approach.

The growing capabilities of wireless communication technologies such as WLAN-based systems enable their successful adoption in an ever-increasing range of applications. Particularly in the domain of the internet of things, the trend towards wireless interconnection is driven by its improved scalability and flexible, low-cost deployment compared to wired systems such as Ethernet. To handle the growing complexity of their design, developers need a process for early performance evaluation of these communication-intensive networked embedded systems. For this, established network simulation frameworks like ns-3 and ${OMNeT}++$ are often utilized. These frameworks require developers to model every part of their system, including the protocol stack and the target application they are developing. From the developers’ perspective, a workflow that would enable them to use real target software as much as possible is desirable. The available solutions to couple real target software with a network simulation, however, often exhibit several limitations. These range from a lacking detail level of the channel simulation to the requirement of access to the source code of every part of the system under design, and often prevent adoption of these coupling solutions. In this paper, we propose a data interface for the established simulator ns-3 that allows coupling to the Linux kernel at a very low level of the protocol stack, namely the lower WLAN MAC layer. This interface allows the performance evaluation of WLAN-based systems, in which real target code of almost the entire software stack can be used. With such an interface, developers can write real target software and test it under different wireless network scenarios and channel conditions, powered by a simulation using ns-3. We show the practicality of our interface by comparing it to a simulation solely performed in ns-3, as well as to another framework for coupling real target software to a channel simulation.

Approximate or inexact arithmetic is a promising approach towards lower power consumption for applications that can tolerate a certain amount of imprecision. As human perception is limited in its precision, this applies to image and audio processing. Beyond, other applications like neuronal networks or AI processing can benefit from such arithmetic as well, as they are inherently tolerant to a certain amount of inaccuracy. One of the most critical components of arithmetic circuits regarding power, delay, and area are multipliers. Various sophisticated approaches towards approximate multipliers are already published for ASICs. However, such ASIC approaches are under-performing in conjunction with the specific Lookup-Table (LUT)-based design of FPGAs. As FPGAs gain in importance for applications like signal processing, there is a substantial lack of approximate design methodology for FPGAs. We propose an approach towards approximate signal processing that is specifically tailored towards the LUT-based hardware of FPGAs. It allows for significant performance improvements while lowering the energy demands. While introducing an insignificant average relative error of just 0.14%, we achieve a 45.9% area reduction in terms of LUTs while decreasing the delay by 30.6% compared to the Xilinx Vivado multiplier IP core. Our proposed design is open source and available at https://github.com/niemann-c/approx-mult-for-fpga.

Due to the increasing complexity of distributed embedded systems, the development process demands early substantiation of design decisions for the targeted system. Virtual prototypes enable an early validation of hardware/software systems and they have proven to be very beneficial for target software development. However, these prototypes often do not include models for inter-device communication aspects of distributed systems. Especially the use of wireless communication has a significant impact on the overall system behavior and performance. This paper introduces a SystemC model of an essential part of IEEE 802.11 Wireless LAN communication, the Enhanced Distributed Channel Access (EDCA) protocol. In comparison to widely used network simulations, the modeling with SystemC promises an easier integration into existing virtual prototypes. EDCA is the main part of the lower medium access layer (MAC) of IEEE 802.11 and interfaces to the physical-layer communication as well as to non-time-critical higher MAC-layer functions. We conducted experiments with an additional simple physical communication channel model to compare our solution in terms of precision and performance to the well-established network simulator ns-3. We show that our model differs by $\approx 0.3 \%$ of retransmitted frames and an accompanied difference of the overall simulated transmission time of $\approx 0.45 \%$. It meets the overall expectations for the stochastic EDCA protocol. Moreover, the presented model has a significantly better simulation performance with $3 \cdot 10^{-5}$ s wall-clock time per frame, which is at least two orders of magnitude better than with the compared ns-3 simulation.

In Smart Buildings there is a large number of connected devices. Each of them is possibly vulnerable, so that an attacker could make use of a single smart device to run attacks in the Smart Building network. We propose a concept to partition the network into trust zones depending on the application layer, so that devices that are logically linked on application layer are able to communicate on MAC layer. A trusted device is used to bootstrap new devices and reconfigure existing trust zones. We restrict the communication as far as possible, so that the potential damage caused by a compromised device is limited. In principle, malicious behavior of devices could lead to an exclusion on MAC layer. The general concept is described using a typical IoT protocol stack containing IEEE 802.11s, IP, UDP/DTLS, and CoAP.

WLAN mesh networks are one of the key technologies for upcoming smart city applications and characterized by a flexible and low-cost deployment. The amendment IEEE 802.11s introduces low-level mesh interoperability at the WLAN MAC layer. On the physical layer, IEEE 802.11n introduced major improvements such as HT data rates, MIMO techniques, and frame aggregation. However, building large-scale 802.11n/s testbeds and reproducible setups is challenging and costly. On the other hand, existing attempts for down-scaling real-world setups are limited to works without support for 802.11n and 802.11s. We therefore present Mini-Mesh, a miniaturized indoor 802.11n/s testbed. Following a transmission range scaling approach, we deploy a 6×6-node mesh grid on an area of only 1 m2. We validate the applicability of our method via comparative measurements, exhibiting a deviation of less than 6 % between a scaled indoor and unscaled outdoor setup. Based on these results, we parameterize a path loss model helping us to estimate outdoor dimensions for arbitrary indoor mesh topologies.

Vendor-independent interoperability is one of the key-enablers for medical devices in future operating rooms, intensive care units, and medical care in general. Using the paradigm of a Service-Oriented Architecture (SOA) is a promising approach realized by the new IEEE 11073 SDC family of standards. Standard compliant communication stacks will be used to build up systems of networked medical devices. The performance of the stack implementation is crucial for the usability in real-world medical environments. Therefore, we investigate the latency of currently available middleware stacks: SoftICE, openSDC, and OSCLib. The aim is to evaluate the suitability of the underlying concepts, understanding the communication behavior using different hard- and software platforms, and finding problems to support future development. For the latency measurements we build up a use-case independent testbed and instrument the libraries to get more information. On the one hand, our investigations substantiate the suitability of the underlying concept and the available middleware stack implementations. On the other hand, unexpected results occurred, like a strong dependency of communication latency on the combination of hardware platform, Java Virtual Machine (JVM), and JVM configuration and even a strong dependency on the intensity of exchanged data when using Java middleware implementations.

The new standard IEEE 802.11s enables vendor-independent wireless mesh networks based on the 802.11 WLAN technology. Transmission Control Protocol (TCP) is the most widespread transport protocol for reliable data delivery and still the basis for many network applications. TCP supports different mechanisms for flow and congestion control. However, designed for wired networks, it does not consider the dynamics of wireless networks and especially multi-hop wireless mesh networks. In addition, 802.11s provides own mechanisms such as Automatic Repeat Request (ARQ) for frame retransmissions to hide wireless loss from the upper layers. Being transparent to each other, retransmission schemes on both layers may interfere and operate redundantly, if not properly adjusted. We study the effects of ARQ retry limit variation on TCP throughput in a real-world multi-hop 802.11s test bed. As a result, we suggest ARQ adaptation based on the 802.11s standard's Airtime Link Metric (ALM) for path selection, serving as indicator for overall frame travel time. Our proposed approach solely relies on standard features and imposes no modifications to 802.11s or TCP.

The new standard amendment IEEE 802.11s enables low-level interoperability for future WLAN mesh networks. Support of the Hybrid Wireless Mesh Protocol (HWMP) and the Airtime Link Metric (ALM) for MAC-layer routing is mandatory. Its default distance vector routing mode facilitates scalability but also results in a limited network view per mesh node. Moreover, mesh mechanisms operate transparently to higher layers which makes the management and optimization of 802.11s networks a challenging task. Available on every standard-compliant node, ALM offers the potential to derive mesh topology information. We present AKadeMesh (Adaptive Kad-enhanced Mesh), a cross-layer approach specifically designed for 802.11s networks. It is based on the P2P protocol Kad and dynamically adapts its logical overlay to the physical mesh underlay by directly considering ALM. The resulting topology-aware P2P overlay is used to realize logical clustering for the distributed management of 802.11s networks, thereby maintaining unrestrained interoperability to the mesh standard. Our solution was implemented and evaluated in a real-world test bed. Results demonstrate its practical feasibility and verify the expected clustering benefit.

Real-time systems play a major role in the realm of industrial automation. It is predicted for the number of smart interconnected devices that participate in such systems to grow significantly in the future. This development is also referred to as Industrial Internet of Things (IIoT) or Industry 4.0. The high number of devices results in highly distributed applications. Therefore, it is no longer sufficient for each device to be real-time capable. In fact, the influence of the communication on the overall timing behavior of the applications grows. Taking this into account, a variety of real-time capable Ethernet approaches called Industrial Ethernet (IE) emerged. However, the established IE solutions rely on proprietary hardware and/or non standard conform protocol adaptations. This leads to very expensive hardware and incompatibilities with other IE solutions or common Ethernet, and thus degrades the interoperability. HaRTKad describes a purely software-based P2P approach that allows deterministic communication over common Ethernet. Although it solely relies on well-known standards and allows real-time communication, it suffers from a multitude of problems. In this paper, the low network utilization, the handling of hash collisions and the traffic prioritization are revealed as the most significant limitations of HaRTKad and possible solutions to these problems are presented.

In the recent past, the development of applications and protocols for the Internet of Things (IoT) made a big leap forward. New approaches have emerged to adopt IoT technologies in the realm of industrial automation. This development is also referred to as Industrial Internet of Things (IIoT) or Industry 4.0. It is predicted for the number of smart interconnected devices participating in automation systems to grow significantly in the future. However, the industrial domain introduces new requirements for IoT technologies regarding the timeliness of interactions. Current IoT protocols, like the Constrained Application Protocol (CoAP), do not yet provide real-time behavior for the inter-device communication. In our previous work, we have already proposed a real-time extension for CoAP that enables deterministic network behavior through a TDMA-based approach. We have shown that the proposed mechanisms for time synchronization, time slot management, and access control can be realized purely software-based. However, a central instance is needed as a time server. This introduces a Single Point of Failure (SPoF) to the system, limiting the robustness and scalability of the approach. In this paper, we introduce a concept for a distributed time server for CoAP. The proposed concept includes a refined time synchronization mechanism as well as strategies to select multiple time servers and share information between them. Furthermore, the described amendments to the real-time extension are integrated into the lightweight platform-independent jCoAP communication stack and evaluated in a multi-device real-world test bed.

Current IoT protocols, like the Constrained Application Protocol (CoAP), do not yet provide real-time behavior for the inter-device communication. In this paper, we propose a real-time extension for the CoAP standard that defines interfaces for the time synchronization among nodes and the time slot management. These interfaces enable a controlled exclusive network access based on a Time Division Multiple Access (TDMA) approach. With this extension, it is possible to realize access control on the application layer without the modification of lower layer protocols. The described interfaces are prototypically implemented within the jCoAP communication stack and evaluated in a multi-device real-world testbed. In our prototype, we used established algorithms for the time synchronization. The results, reveal the weaknesses of the chosen synchronization algorithm. However, the interface definition allows the usage of more accurate algorithms.

WLAN mesh networks are characterized by their flexible and low-cost deployment, scalability, and self-healing capabilities. The new WLAN standard IEEE 802.11s introduces low-level mesh interoperability. However, building large-scale real-world test beds and reproducible setups is challenging and costly. In the majority of research works, network simulation is preferred over practical measurements. Here, the main disadvantage exists in simplified device and protocol models restricting the comparability to practical implementations. In contrast, using device emulation still requires the simulation of wireless channel and environment models. Consequently, a combination of both emulation and simulation is needed to enable virtual prototyping of real applications and protocols in WLAN mesh networks. Nevertheless, the computation of complex wireless channel effects requires a decoupling of wall clock and simulation time. Therefore, we present ViPMesh, a virtual prototyping framework for IEEE 802.11s and its Linux reference implementation. ViPMesh relies on WLAN device emulation and nested virtualization using QEMU and Linux containers to support the analysis of real applications on top of an unmodified protocol stack. Adopting an alternative time source approach for QEMU, ViPMesh acts as discrete-event simulator. It further integrates channel and environment models with support for IEEE 802.11n MIMO techniques, high throughput modes, multi-channel operation, and node mobility. To the best of our knowledge, this is the first approach that combines the IEEE 802.11s reference implementation with the described simulation features. The functionality of ViPMesh is demonstrated in different example scenarios.

The broad availability of WLAN-capable off-the-shelf hardware lets WLAN mesh networks appear as promising technology for future distributed wireless applications. Featuring automatic device discovery, interconnection and routing, they provide a higher scalability, flexibility, and robustness compared to common centralized WLAN infrastructures. Besides these advantages, characteristics such as variable network topologies and link qualities imply new technical challenges for administration and real-world operation. Adopted in late 2011, IEEE 802.11s appears as new WLAN standard amendment, enabling vendor-independent mesh networks based on the widespread WLAN technology. However, network monitoring and management fall out of the standardization scope and are therefore not specified. In this paper, we present a novel 802.11s management solution based on the SNMP protocol. It covers dynamic mesh bootstrapping, error recovery, status monitoring and remote configuration. The presented solution was implemented and evaluated in a real-world testbed comprising more than 10 mesh nodes.

The term Internet of Things (IoT) describes a scenario where embedded systems are integrated into everyday objects, turning them into smart devices to assist the user in his everyday life. Each of these smart objects only offers a very limited amount of computational power since it is only specialized in a limited set of tasks. In order to achieve complex goals, the devices have to interact with each other. Therefore, they do not only need to be interconnected either by wire or through wireless technology but also need a set of common protocols to enable vendor-independent communication. In the past years, various protocols pursuing this objective have emerged. One of the most promising approaches is the Constrained Application Protocol (CoAP) because it offers high interoperability and very low communication overhead at the same time. Typical IoT applications include the observation and manipulation of their environment through sensors and actuators. Since the physical world is continuous in time and does not wait for calculations to finish, it is essential that the execution times of the applications stay within certain boundaries. These timing constraints are referred to as real-time requirements. However, current protocol implementations do not consider real-time requirements for IoT applications. In this paper, we introduce the jCoAP communication stack as a lightweight Java implementation of CoAP. We give a brief introduction to real-time communication and CoAP and provide insight in the design concept of jCoAP and the offered functionalities. Furthermore, a performance evaluation is done in order to point out the suitability of the jCoAP framework for real-time IoT applications.