Mathilde Durvy

Making sensor networks IPv6 ready

With emerging IPv6-based standards such as 6LowPAN and ISA100a, full IPv6 sensor networks are the next major step. With millions of deployed embedded IPv6 devices, interoperability is of major importance, both within the sensor networks and between the sensors and the Internet hosts. We present uIPv6, the first IPv6 stack for memory-constrained devices that passes all Phase-1 IPv6 Ready certification tests. This is an important step for end-to-end interoperability between IPv6 sensors and any IPv6 capable device. To allow widespread community adoption, we release uIPv6 under a permissive open source license that allows both commercial and non-commercial use.

A Packing Approach to Compare Slotted and Non-Slotted Medium Access Control

In multi-hop ad hoc networks, the efficiency of a medium access control protocol under heavy traffic load depends mainly on its ability to schedule a large number of simultaneous non-interfering transmissions. However, as each node has only a local view of the network, it is difficult to globally synchronize transmission times over the whole network. How does the lack of global coordination affect spatial reuse in multi-hop wireless networks? We show that in a de-centralized network the spatial reuse does not benefit from global clock synchronization. On the contrary, we demonstrate that non-slotted protocols using collision avoidance mechanisms can achieve a higher spatial reuse than the corresponding slotted protocols. By means of a simple backoff mechanism, one can thus favor the spontaneous emergence of spatially dense transmission schedules.

Self-Organization Properties of CSMA/CA Systems and Their Consequences on Fairness

Decentralized medium access control schemes for wireless networks based on CSMA/CA, such as the IEEE 802.11 protocol, are known to be unfair. In multihop networks, they can even favor some links to such an extent that the others suffer from virtually complete starvation. This observation has been reported in quite a few works, but the factors causing it are still not well understood. We find that the capture effect and the relative values of the receive and carrier sensing ranges play a crucial role in the performance of these protocols. Using a simple Markovian model, we show that an idealized CSMA/CA protocol suffers from starvation when the receiving and sensing ranges are equal, but quite surprisingly that this unfairness is reduced or even disappears when these two ranges are sufficiently different. We also show that starvation has a positive counterpart, namely organization. When its access intensity is large the protocol organizes the transmissions in space in such a way that it maximizes the number of concurrent successful transmissions. We obtain exact formula for the so-called spatial reuse of the protocol on large line networks.

Modeling the 802.11 Protocol Under Different Capture and Sensing Capabilities

Decentralized medium access control schemes for wireless networks based on CSMA/CA, such as the 802.11 protocol, are known to be unfair. In multi-hop networks, they can even favor some connections to such an extent that the others suffer from virtually complete starvation. This observation has been reported in quite a few works, but the factors causing it are still not well understood. We find that the capture effect and the relative values of the receiving and carrier sensing ranges play a crucial role in the unfairness of these protocols. We show that an idealized 802.11 protocol does suffer from starvation when the receiving and sensing ranges are equal, but quite surprisingly this unfairness is reduced or even disappears when these two ranges are sufficiently different. Using a Markovian model, we explain why apparently benign variations in these ranges have such a dramatic impact on the 802.11 protocol performance.

Industry: beyond interoperability: pushing the performance of sensor network IP stacks

Interoperability is essential for the commercial adoption of wireless sensor networks. However, existing sensor network architectures have been developed in isolation and thus interoperability has not been a concern. Recently, IP has been proposed as a solution to the interoperability problem of low-power and lossy networks (LLNs), considering its open and standards-based architecture at the network, transport, and application layers. We present two complete and interoperable implementations of the IPv6 protocol stack for LLNs, one for Contiki and one for TinyOS, and show that the cost of interoperability is low: their performance and overhead is on par with state-of-the-art protocol stacks custom built for the two platforms. At the same time, extensive testbed results show that the ensemble performance of a mixed network with nodes running the two interoperable stacks depends heavily on implementation decisions and parameters set at multiple protocol layers. In turn, these results argue that the current industry practice of interoperability testing does not cover the crucial topic of the performance and motivate the need for generic techniques that quantify the performance of such networks and configure their run-time behavior.

On the fairness of large CSMA networks

We characterize the fairness of decentralized medium access control protocols based on CSMA/CA, in large multi-hop wireless networks. In particular, we show that the widely observed unfairness of these protocols in small network topologies does not always persist in large topologies. In regular networks, this unfairness is essentially due to the unfair advantage of nodes at the border of the network, which have a restricted neighborhood and thus a higher probability to access the communication channel. In large 1D lattice networks these border effects do not propagate inside the network, and nodes sufficiently far away from the border have equal access to the channel; as a result the protocol is long-term fair. In 2D lattice networks, we observe a phase transition. If the access intensity of the protocol is small, the border effects remain local and the protocol behaves similarly as in one-dimensional networks. However, if the access intensity of the protocol is large enough, the border effects persist independently of the size of the network and the protocol is strongly unfair. In irregular networks, the topology is inherently unfair. This unfairness increases with the access intensity of the protocol, but in a much smoother way than in regular two-dimensional networks. Finally, in situations where the protocol is long-term fair, we provide a characterization of its short-term fairness.

Border Effects, Fairness, and Phase Transition in Large Wireless Networks

We characterize the fairness of decentralized medium access control protocols based on CSMA/CA, such as IEEE 802.11, in large multi-hop wireless networks. In particular, we show that the widely observed unfairness of the protocol in small network topologies does not always persist in large topologies. This unfairness is essentially due to the unfair advantage of nodes at the border of the network, which have a restricted neighborhood and thus a higher probability to access the communication channel. In large one-dimensional networks these border effects do not propagate inside the network, and nodes sufficiently far away from the border have equal access to the channel; as a result the protocol is long-term fair. In two-dimensional networks, we observe a phase transition. If the access intensity of the protocol is small, the border effects remain local and the protocol behaves similarly as in one- dimensional networks. However, if the access intensity of the protocol is large enough, the border effects persist independently of the size of the network and the protocol is strongly unfair. Finally, in situations where the protocol is long-term fair, we provide a characterization of its short-term fairness.

Towards Reliable Broadcasting using ACKs

We propose a mechanism for reliable broadcasting in wireless networks, that consists of two components: a method for bandwidth efficient acknowledgment collection, and a coding scheme that uses acknowledgments. Our approach combines ideas from network coding and distributed space time coding.

Reaction-diffusion based transmission patterns for ad hoc networks

We present a new scheme that mimics pattern formation in biological systems to create transmission patterns in multi-hop ad hoc networks. Our scheme is decentralized and relies exclusively on local interactions between the network nodes to create global transmission patterns. A transmission inhibits other transmissions in its immediate surrounding and encourages nodes located further away to transmit. The transmission patterns created by our medium access control scheme combine the efficiency of allocation-based schemes at high traffic loads and the flexibility of random access schemes. Moreover, we show that with appropriately chosen parameters our scheme converges to collision free transmission patterns that guarantee some degree of spatial reuse.

Low-power IPv6 for the Internet of Things

The Internet of Things allows physical objects, sensors, and actuators to be connected to each other as well as cloud services. Over the past few years, advances in wireless protocols, software systems, and standardization for IPv6 points to a future where every physical object that can benefit from an Internet connection will be connected to the Internet. This paper presents the IPv6 networking support in the Contiki operating system, the system that first introduced the concept of IP networking for low-power wireless systems. We discuss the design and implementation of IPv6 in Contiki as well as the methods used to develop and evaluate low-power mechanisms.