http://www.maths.qmul.ac.uk/%7Elatora/report_06.pdf

Complex networks: Structure and dynamics

Recent advances in miniaturization and low-cost, low-power design have led to active research in large-scale networks of small, wireless, low-power sensors and actuators. Time synchronization is critical in sensor networks for diverse purposes including sensor data fusion, coordinated actuation, and power-efficient duty cycling. Though the clock accuracy and precision requirements are often stricter than in traditional distributed systems, strict energy constraints limit the resources available to meet these goals.We present Reference-Broadcast Synchronization, a scheme in which nodes send reference beacons to their neighbors using physical-layer broadcasts. A reference broadcast does not contain an explicit timestamp; instead, receivers use its arrival time as a point of reference for comparing their clocks. In this paper, we use measurements from two wireless implementations to show that removing the sender's nondeterminism from the critical path in this way produces high-precision clock agreement (1.85 ± 1.28μsec, using off-the-shelf 802.11 wireless Ethernet), while using minimal energy. We also describe a novel algorithm that uses this same broadcast property to federate clocks across broadcast domains with a slow decay in precision (3.68 ± 2.57μsec after 4 hops). RBS can be used without external references, forming a precise relative timescale, or can maintain microsecond-level synchronization to an external timescale such as UTC. We show a significant improvement over the Network Time Protocol (NTP) under similar conditions.

/pdf/fine-grained-network-time-synchronization-using-reference-3go0i1u94g.pdf

Fine-grained network time synchronization using reference broadcasts

Wireless ad-hoc sensor networks have emerged as an interesting and important research area in the last few years. The applications envisioned for such networks require collaborative execution of a distributed task amongst a large set of sensor nodes. This is realized by exchanging messages that are time-stamped using the local clocks on the nodes. Therefore, time synchronization becomes an indispensable piece of infrastructure in such systems. For years, protocols such as NTP have kept the clocks of networked systems in perfect synchrony. However, this new class of networks has a large density of nodes and very limited energy resource at every node; this leads to scalability requirements while limiting the resources that can be used to achieve them. A new approach to time synchronization is needed for sensor networks.In this paper, we present Timing-sync Protocol for Sensor Networks (TPSN) that aims at providing network-wide time synchronization in a sensor network. The algorithm works in two steps. In the first step, a hierarchical structure is established in the network and then a pair wise synchronization is performed along the edges of this structure to establish a global timescale throughout the network. Eventually all nodes in the network synchronize their clocks to a reference node. We implement our algorithm on Berkeley motes and show that it can synchronize a pair of neighboring motes to an average accuracy of less than 20ms. We argue that TPSN roughly gives a 2x better performance as compared to Reference Broadcast Synchronization (RBS) and verify this by implementing RBS on motes. We also show the performance of TPSN over small multihop networks of motes and use simulations to verify its accuracy over large-scale networks. We show that the synchronization accuracy does not degrade significantly with the increase in number of nodes being deployed, making TPSN completely scalable.

/pdf/timing-sync-protocol-for-sensor-networks-53aov77qpj.pdf

Timing-sync protocol for sensor networks

The Internet of Things (IoT) makes smart objects the ultimate building blocks in the development of cyber-physical smart pervasive frameworks. The IoT has a variety of application domains, including health care. The IoT revolution is redesigning modern health care with promising technological, economic, and social prospects. This paper surveys advances in IoT-based health care technologies and reviews the state-of-the-art network architectures/platforms, applications, and industrial trends in IoT-based health care solutions. In addition, this paper analyzes distinct IoT security and privacy features, including security requirements, threat models, and attack taxonomies from the health care perspective. Further, this paper proposes an intelligent collaborative security model to minimize security risk; discusses how different innovations such as big data, ambient intelligence, and wearables can be leveraged in a health care context; addresses various IoT and eHealth policies and regulations across the world to determine how they can facilitate economies and societies in terms of sustainable development; and provides some avenues for future research on IoT-based health care based on a set of open issues and challenges.

The Internet of Things for Health Care: A Comprehensive Survey

IEEE Transactions on Pattern Analysis and Machine Intelligence

When deployed in a real-world setting, many sensor networks fail to meet application requirements even though they have been tested in the lab prior to deployment. Hence, concepts and tools for inspection are needed to identify failure causes in situ on the deployment site. Tools for inspection should minimize the interference with the sensor network to, firstly, ensure that failures of the sensor network do not break the inspection mechanism, and, secondly, to ensure that the inspection mechanism does not change the behavior of the sensor network. In this position paper, we propose passive distributed assertions (PDA) as a novel tool for identifying failure causes. PDA allow a programmer to assert certain predicates over distributed node states. Packet sniffing is used to detect failed assertions, thus minimizing the interference with the sensor networks.

/pdf/increasing-the-visibility-of-sensor-networks-with-passive-ukw1uzqknj.pdf

Increasing the visibility of sensor networks with passive distributed assertions

Internet of Things (IoT) applications in several domains such as surveillance of civil infrastructure, smart grids, and smart healthcare are of utmost importance for our society and require dependable performance. Guaranteeing that application-specific dependability requirements are met is however still an open research challenge. The IoT indeed exposes highly resource-constrained computing devices to harsh environmental conditions (e.g., heat, mechanical shock, electromagnetic radiation) and physical attacks. Unfortunately, traditional methods to withstand these threats heavily rely on redundancy, a concept that is incompatible with the resource constraints of common IoT devices.

/pdf/dependability-for-the-internet-of-things-from-dependable-3pjakkfq7y.pdf

Dependability for the Internet of Things—from dependable networking in harsh environments to a holistic view on dependability

Many sensor network applications feature correlated traffic bursts: after a period of idle time with almost no network traffic, many nodes have to transmit large amounts of data simultaneously. One example is volcano monitoring [17], where rare eruptions trigger many nodes simultaneously to transmit seismic data traces. For such applications, a MAC protocol should exhibit a low overhead both in idle mode and during correlated traffic bursts. Existing MAC protocols only meet one of these requirements, but not both at the same time. Contention-based protocols such as SCP-MAP have very low overhead in idle situations, but exhibit high overhead during correlated traffic bursts due to contention. Scheduled protocols such as LMAC avoid contention overhead during correlated bursts, but exhibit a significant overhead in idle mode due to control traffic [10]. We propose BurstMAC, a new MAC protocol specifically designed for applications with correlated traffic bursts. BurstMAC's idle overhead is almost as low as that of SCP-MAC and at the same time provides better throughput and lower energy overhead than LMAC during correlated traffic bursts.

http://www.vs.inf.ethz.ch/publ/papers/mringwal_burstmac_inss2009.pdf

BurstMAC — An efficient MAC protocol for correlated traffic bursts

EWSN Dependability Competition: Experiences and Lessons Learned

Smart camera networks provide the opportunity to detect, classify, and trace visual events by means of a wireless network of embedded computing devices equipped with camera sensors Previous research in this area has largely focused on custom hardware solutions In contrast, we propose to use mobile phones as a portable and low-cost platform to implement smart camera networks Due to mass production, the price of mobile phones is constantly dropping while additional functionality is being added In particular, the addition of cameras and ad hoc networking enable the use of mobile phones for smart camera networks Also, software developed for mobile phones is highly portable due to standardized programming environments and APIsIn this paper, we present Facet, a self-organizing network of smart cameras and its software architecture, which addresses the specific challenges of the mobile phone platform An initial prototype of Facet has been developed and is used to obtain first performance results in an office setting

http://www.vs.inf.ethz.ch/publ/papers/bolligph-facet2007.pdf

Kay Römer

Papers

Increasing the visibility of sensor networks with passive distributed assertions

Dependability for the Internet of Things—from dependable networking in harsh environments to a holistic view on dependability

BurstMAC — An efficient MAC protocol for correlated traffic bursts

EWSN Dependability Competition: Experiences and Lessons Learned

Facet: towards a smart camera network of mobile phones