Christian Rohner

A long-term study of correlations between meteorological conditions and 802.15.4 link performance

Outdoor wireless sensor networks are all exposed to a constantly changing environment that influences the performance of the network. In this paper, we study how variations in meteorological conditions influence IEEE 802.15.4 links. We show that the performance varies over both long and short periods of time, and correlate these variations to changes in meteorological conditions. The case study is based on six months of data from a sensor network deployed next to a meteorological research station running a continuous experiment, collecting both high-quality link and meteorological measurements. We present observations from the deployment, highlighting variations in packet reception ratio and signal strength. Furthermore, we show how the variations correlate with four selected meteorological factors, temperature, absolute humidity, precipitation and sunlight. Our results show that packet reception ratio and signal strength correlate the most with temperature and the correlation with other factors are less pronounced. We also identify a diurnal cycle as well as a seasonal variation in the packet reception ratio aggregated over all links. We discuss the implication of the findings and how they can be used when designing wireless sensor networks.

Congestion avoidance in a data-centric opportunistic network

In order to achieve data delivery in an opportunistic network, data is replicated when it is transmitted to nodes within communication reach and that are likely to be able to forward it closer to the destination. This replication and the unpredictable contact times due to mobility necessitate buffer management strategies to avoid buffer overflow on nodes. In this paper, we investigate buffer management strategies based on local forwarding statistics and relevance of the data for other nodes. The results obtained on our emulation platform for opportunistic networks show that strategies with a high data refresh rate achieve the most efficient delivery and generate the smallest overhead on our community and mobility scenarios.

Haggle: Opportunistic mobile content sharing using search

We present Haggle, a content-sharing system for mobile devices, allowing users to opportunistically share content without the support of infrastructure. Mobile devices share content and interests over direct WiFi or Bluetooth links, and may store-carry-forward content on behalf of others based on interests, bridging otherwise disconnected devices. Unlike traditional Internet-based content sharing systems, Haggle faces disconnections, unpredictable mobility, and time-limited contacts, which pose unique challenges to the systems design and implementation. While similar content-sharing systems typically value every content item the same, Haggle uses a ranked search to judiciously decide which content to exchange, and in which order. The search matches a devices locally stored content against the interests of other users that the device has collected, prioritizing relevant content when contacts are time limited and resources scarce. Thus, search enables dissemination of content in order of how strongly users desire it, offering delay and resource savings by exchanging the content that matters. An optional content delegation mechanism allows Haggle to altruistically disseminate a limited amount of items based on the interests of third-party nodes, increasing the benefit of the network as a whole, and protecting against networks that are partitioned along interests. Ranked searches, combined with delegation, allow Haggle to balance the short-term benefit of exchanging a content item between two nodes against the long-term benefit to the network as a whole. We evaluate Haggle through a real-world experiment with mobile phones, running a picture sharing application, complemented by trace-based emulations. Our results show that a content item whose interest group (the nodes that desire it) has strong interests is delivered with lower delay, and to a higher fraction of members, than an item with a similar sized group with weak interests. Compared to a relevance-agnostic system, Haggle can deliver the most relevant items in one third of the time and at a lower cost.

Haggle: a data-centric network architecture for mobile devices

Delay-tolerant and opportunistic networks relax the traditional assumption of end-to-end connectivity. Such networks are therefore suitable for content dissemination in sparsely connected regions of the world, and for complementing existing infrastructure by operating cost-free, high bandwidth, albeit high latency, content delivery services. In this work we argue, however, that content dissemination in the above contexts requires us to revisit delay-tolerant communication at the architectural level, looking at multiple issues such as naming and addressing, congestion control and application interfaces. We propose a new architecture, called Haggle, that leverages the principles of search, as known from desktop operating systems and the Web, in order to achieve truly data-centric communication. Searching is naturally data-centric and embeds principles, such as ranking, that can be used to bind data to interested receivers and to prioritize the data to send during node encounters. We herein give an overview of the Haggle architecture and its basic design.

Evaluating Battery Models in Wireless Sensor Networks

Recent measurements highlight the importance of battery-aware evaluation of energy efficiency in wireless sensor networks. However, existing battery models been not investigated in the context of the low duty cycle, short duration loads that are typical of sensor networks. We evaluate three battery models with regard to their applicability in the WSN context. Our evaluation focuses on how the models reflect two key battery discharge behaviors, the rate capacity effect and charge recovery. We find that the models handle the former better than the latter and are more sensitive to a load’s peak current than to its timing.

DEAPspace: transient ad-hoc networking of pervasive devices

The DEAPspace project is building an interaction framework for connecting pervasive devices over the wireless medium, supporting the development of new proximity-based collective distributed applications. The main components of this framework are the discovery algorithm and the service description model. DEAPspace provides devices with useful information about the other devices in their surroundings. This information can be queried in a consistent way that will tolerate evolutions, and allow legacy devices to continue to function in the fast-developing world of pervasive gadgets. The discovery is done in a power-efficient, and network-friendly way, and will adapt to a wide range of error conditions. This framework has been implemented, and allows the development of distributed applications that use ad-hoc transient networking as part of their function. The primary implementation was in Java. A subset of the code was also written in C, to allow the use of machines which do not have a JVM. In addition to simulation, the code has been tested over TCP/IP and the Ethernet interface of an 802.11 link.

Evaluating wireless multi-hop networks using a combination of simulation, emulation, and real world experiments

Mobile ad hoc networks suffer from complex interactions between protocols on different levels. To identify and explain anomalies found in real world experiments we propose to combine real world testing with simulation and emulation. By inspecting the differences in the results obtained using the three evaluation methods we can efficiently identify when anomalies occur and get a hint about where to further investigate. Our approach has proved to be valuable in an extensive comparison of the routing protocols AODV, DSR, and OLSR. The study resulted in two gigabytes of log data that required an efficient and structured method to isolate important time regions for further manual analysis. We discuss our approach and present some key findings.

How do the dynamics of battery discharge affect sensor lifetime

Evaluation of energy consumption and device lifetime in battery-powered wireless sensor networks (WSN) is almost exclusively based on estimates of the total charge (i.e. mA-h) consumed by the device. In reality, batteries are complex electro-chemical systems and their discharge behavior depends heavily on the timing and intensity of the applied load. However, there is very little empirical data or reliable models available for the kinds of batteries and loads that are typically used in WSN. The effect of battery dynamics on sensor lifetime is therefore not well understood. We characterize CR2032 Li coin cells using carefully controlled synthetic loads and a wide range of WSN-typical load parameters. Our results are the first to quantify in-depth the discharge behavior of primary batteries in the WSN context. We report that in some common cases, observed lifetimes can differ from predicted ones by almost a factor of three. Furthermore, loads with similar average currents - which would be expected to have similar lifetimes - can vary significantly in the amount of capacity they can utilize, with short duration loads generally faring better. The results show that energy evaluation based on a mA-h consumed model has significant limitations. This has important implications for the design and evaluation of WSN applications, as well as for practical problems in network dimensioning and lifetime prediction.

Trace-based performance analysis of opportunistic forwarding under imperfect node cooperation

The paper proposes an innovative method for the performance analysis of opportunistic forwarding protocols over files logging mobile node encounters (contact traces). The method is modular and evolves in three steps. It first carries out contact filtering to isolate contacts that constitute message forwarding opportunities for givenmessage coordinates and forwarding rules. It then draws on graph expansion techniques to capture these forwarding contacts into sparse space-time graph constructs. Finally, it runs standard shortest path algorithms over these constructs and derives typical performance metrics such as message delivery delay and path hopcount. The method is flexible in that it can easily assess the protocol operation under various expressions of imperfect node cooperation. We describe it in detail, analyze its complexity, and evaluate it against discrete event simulations for three representative randomized forwarding schemes. The match with the simulation results is excellent and obtained with run times up to three orders of size smaller than the duration of the simulations, thus rendering our method a valuable tool for the performance analysis of opportunistic forwarding schemes.

LoRea: A Backscatter Architecture that Achieves a Long Communication Range

There is the long-standing assumption that radio communication in the range of hundreds of meters needs to consume mWs of power at the transmitting device. In this paper, we demonstrate that this is not necessarily the case for some devices equipped with backscatter radios. We present LOREA an architecture consisting of a tag, a reader and multiple carrier generators that overcomes the power, cost and range limitations of existing systems such as Computational Radio Frequency Identification (CRFID). LOREA achieves this by: First, generating narrow-band backscatter transmissions that improve receiver sensitivity. Second, mitigating self-interference without the complex designs employed on RFID readers by keeping carrier signal and backscattered signal apart in frequency. Finally, decoupling carrier generation from the reader and using devices such as WiFi routers and sensor nodes as a source of the carrier signal. An off-the-shelf implementation of LOREA costs 70 USD, a drastic reduction in price considering commercial RFID readers cost 2000 USD. LOREAs range scales with the carrier strength, and proximity to the carrier source and achieves a maximum range of 3.4 km when the tag is located at 1 m distance from a 28 dBm carrier source while consuming 70 μW at the tag. When the tag is equidistant from the carrier source and the receiver, we can communicate upto 75 m, a significant improvement over existing RFID readers.