Highly reliable, low-latency communication in low-power wireless networks

Low-power wireless networks consist of spatially distributed, resource-constrained devices – also referred to as nodes – that are typically equipped with integrated or external sensors and actuators. Nodes communicate with each other using wireless transceivers, and thus, relay data – e. g., collected sensor values or commands for actuators – cooperatively through the network. This way, low-power wireless networks can support a plethora of different applications, including, e. g., monitoring the air quality in urban areas or controlling the heating, ventilation and cooling of large buildings. The use of wireless communication in such monitoring and actuating applications allows for a higher flexibility and ease of deployment – and thus, overall lower costs – compared to wired solutions. However, wireless communication is notoriously error-prone. Message losses happen often and unpredictably, making it challenging to support applications requiring both high reliability and low latency. Highly reliable, low-latency communication – along with high energy-efficiency – are, however, key requirements to support several important application scenarios and most notably the open-/closed-loop control functions found in e. g., industry and factory automation applications. Communication protocols that rely on synchronous transmissions have been shown to be able to overcome this limitation. These protocols depart from traditional single-link transmissions and do not attempt to avoid concurrent transmissions from different nodes to prevent collisions. On the contrary, they make nodes send the same message at the same time over several paths. Phenomena like constructive interference and capture then ensure that messages are received correctly with high probability. While many approaches relying on synchronous transmissions have been presented in the literature, two important aspects received only little consideration: (i) reliable operation in harsh environments and (ii) support for event-based data traffic. This thesis addresses these two open challenges and proposes novel communication protocols to overcome them

Whisper: Fast Flooding for Low-Power Wireless Networks

This paper presents Whisper, a fast and reliable protocol to flood small amounts of data into a multi-hop network. Whisper relies on three main cornerstones. First, it embeds the message to be flooded into a signaling packet that is composed of multiple packlets. A packlet is a portion of the message payload that mimics the structure of an actual packet. A node must intercept only one of the packlets to know that there is an ongoing transmission. Second, Whisper exploits the structure of the signaling packet to reduce idle listening and, thus, to reduce the radio-on time of the nodes. Third, it relies on synchronous transmissions to quickly flood the signaling packet through the network. Our evaluation on the Flocklab testbed shows that Whisper achieves comparable reliability but significantly lower radio-on time than Glossy -- a state-of-the-art flooding algorithm. Specifically, Whisper can disseminate data in FlockLab twice as fast as Glossy with no loss in reliability. Further, Whisper spends 30% less time in channel sampling compared to Glossy when no data traffic must be disseminated

IEEE 802.15.4 TSCH in Sub-GHz: Design Considerations and Multi-band Support

This paper has been presented at : The 44th IEEE Conference on Local Computer Networks (LCN) October 14-17, 2019.In Press / En PrensaIn this paper, we address the support of Time-Slotted Channel Hopping (TSCH) on multiple frequency bands within a single TSCH network. This allows to simultaneously run applications with different requirements on link characteristics and to increase resilience against interference. To this end, we first enable sub-GHz communication in TSCH, which has been primarily defined for the 2.4 GHz band. Thereafter, we propose two designs to support multiple physical layers in TSCH on the same nodes. Our experimental evaluation shows that TSCH is applicable in a wide range of data rates between 1.2 kbps and 1000 kbps. We find that data rates of 50 kbps and below have a long communication range and a nearly perfect link symmetry, but also have a 20x higher channel utilization compared to higher data rates, increasing the risk of collisions. Using these findings, we show the advantages of the multi-band support on the example of synchronization accuracy when exchanging TSCH beacons with a low data rate and application data at a high data rate.This work was financed by the H2020 collaborative Europe/Taiwan research project 5G-CORAL (grant num. 761586), the ERCIM Alain Bensoussan postdoc fellowship program, and the distributed environment E-care@home, funded by the Swedish Knowledge Foundation

Highly reliable, low-latency communication in low-power wireless networks

Low-power wireless networks consist of spatially distributed, resource-constrained devices – also referred to as nodes – that are typically equipped with integrated or external sensors and actuators. Nodes communicate with each other using wireless transceivers, and thus, relay data – e. g., collected sensor values or commands for actuators – cooperatively through the network. This way, low-power wireless networks can support a plethora of different applications, including, e. g., monitoring the air quality in urban areas or controlling the heating, ventilation and cooling of large buildings. The use of wireless communication in such monitoring and actuating applications allows for a higher flexibility and ease of deployment – and thus, overall lower costs – compared to wired solutions. However, wireless communication is notoriously error-prone. Message losses happen often and unpredictably, making it challenging to support applications requiring both high reliability and low latency. Highly reliable, low-latency communication – along with high energy-efficiency – are, however, key requirements to support several important application scenarios and most notably the open-/closed-loop control functions found in e. g., industry and factory automation applications. Communication protocols that rely on synchronous transmissions have been shown to be able to overcome this limitation. These protocols depart from traditional single-link transmissions and do not attempt to avoid concurrent transmissions from different nodes to prevent collisions. On the contrary, they make nodes send the same message at the same time over several paths. Phenomena like constructive interference and capture then ensure that messages are received correctly with high probability. While many approaches relying on synchronous transmissions have been presented in the literature, two important aspects received only little consideration: (i) reliable operation in harsh environments and (ii) support for event-based data traffic. This thesis addresses these two open challenges and proposes novel communication protocols to overcome them

Concurrent Transmissions for Communication Protocols in the Internet of Things

Standard Internet communication protocols are key enablers for the Internet of Things (IoT). Recent technological advances have made it possible to run such protocols on resource-constrained devices. Yet these devices often use energy-efficient, low-level communication technologies, like IEEE 802.15.4, which suffer from low-reliability and high latency. These drawbacks can be significantly reduced if communication occurs using concurrent transmissions - a novel communication paradigm for resource-constrained devices. In this paper, we show that Internet protocols like TCP/UDP and CoAP can run efficiently on top of a routing substrate based on concurrent transmissions. We call this substrate LaneFlood and demonstrate its effectiveness through extensive experiments on Flocklab, a publicly available testbed. Our results show that LaneFlood improves upon CXFS - a representative competitor - in terms of both duty cycle and reliability. Furthermore, LaneFlood can transport IoT traffic with an end-to-end latency of less than 300 ms over several hops

Keep the Beat: On-The-Fly Clock Offset Compensation for Synchronous Transmissions in Low-Power Networks

Emerging protocols for low-power wireless networks increasingly exploit constructive interference and the capture effect. The basic idea is that the synchronous transmission of identical packets by neighboring nodes leads to constructive interference - or at least do not cause destructive interference. This requires that the temporal displacement of packets at receiving nodes is lower than 0.5 s when employing IEEE 802.15.4 radios. However, commonly used sensor nodes are equipped with cheap and imprecise clocks that show high frequency deviations across nodes, making constructive interference difficult to achieve. Such deviations further increase when individual nodes are exposed to different temperatures. In this paper we introduce Flock, a novel approach to compensate for differences in clock frequency across synchronously transmitting nodes. We implemented Flock in Contiki on the example of Glossy, a flooding protocol based on synchronous transmissions. Our results confirm that Flock can achieve constructive interference on real sensor nodes in over 98% of the cases. Overall, Flock makes protocols that exploit synchronous transmissions more robust to operate even in challenging environments

Shadow-based Hand Gesture Recognition in one Packet

The ubiquity of wirelessly connected sensing devices in IoT applications provides the opportunity to enable various types of interaction with our digitally connected environment. Currently, low processing capabilities and high energy costs for communication limit the use of energy-constrained devices for this purpose. In this paper, we address this challenge by exploring the new possibilities highly capable deep neural network classifiers present. To reduce the energy consumption for transferring continuously sampled data, we propose to compress the sensed data and perform classification at the edge. We evaluate several compression methods in the context of a shadow-based hand gesture detection application, where the classification is performed using a convolutional neural network. We show that simple data reduction methods allow us to compress the sensed data into a single IEEE 802.15.4 packet while maintaining a classification accuracy of 93%. We further show the generality of our compression methods in an audio-based interaction scenario

Whisper: Fast Flooding for Low-Power Wireless Networks

This article presents Whisper, a fast and reliable protocol to flood small amounts of data into a multi-hop network. Whisper makes use of synchronous transmissions, a technique first introduced by the Glossy flooding protocol. In contrast to Glossy, Whisper does not let the radio switch from receive to transmit mode between messages. Instead, it makes nodes continuously transmit identical copies of the message and eliminates the gaps between subsequent transmissions. To this end, Whisper embeds the message to be flooded into a signaling packet that is composed of multiple packlets-where a packlet is a portion of the message payload that mimics the structure of an actual packet. A node must intercept only one of the packlets to detect that there is an ongoing transmission and that it should start forwarding the message. This allows Whisper to speed up the propagation of the flood and, thus, to reduce the overall radio-on time of the nodes. Our evaluation on the FlockLab testbed shows that Whisper achieves comparable reliability but 2x lower radio-on time than Glossy. We further show that by embedding Whisper in an existing data collection application, we can more than double the lifetime of the network

Topology Control in Wireless Sensor Networks: What Blocks the Breakthrough?

Graph-based topology control adapts wireless topologies to achieve certain target graph structures. Wireless sensor networks seem well-suited for the expectations (in particular those on provided energy savings) raised by topology control. Nevertheless, topology control has never made the breakthrough in real-world deployments. This work explores the reasons for this, identifying five practical obstacles of today’s topology control: (i) unrealistic assumptions, (ii) unsuitable graph structures, (iii) application agnosticism, (iv) unclear role in the stack, and (v) insufficient framework support. To address the latter obstacle, we provide a re-usable framework for the implementation and evaluation of topology control. Based on this framework, we conduct a testbed-based evaluation for two application scenarios and three topology control algorithms including a novel application-specific algorithm. Indeed, the identified obstacles hinder topology control from boosting the application. However, the achieved graph structures show the practical feasibility of topology control in principle