Modelling the IEEE 802.11 protocol in wireless multi-hop networks

IEEE 802.11 is probably the most widely used, medium access control protocol in current wireless networks. In the Wireless LAN (i.e., single-hop) setting, its performance is by now quite well understood. However, in the multi-hop setting where relay nodes are used to achieve end-to-end communication, there is, to date, no widely accepted model. Consequently, when confronted with experimental results, people often find it hard to interpret them. The goals of this thesis are (i) to model protocols "à la 802.11" in the context of multi-hop ad hoc networks, (ii) to derive theoretical limits for their performance, (iii) to contrast the performance of the current IEEE 802.11 protocol with these limits and (iv) to identify all the factors that prevent IEEE 802.11 from reaching these limits. Most of this thesis is dedicated to achieving the two first goals. We begin by proposing an idealized version of IEEE 802.11. We model this idealized protocol using a continuous Markov chain. We then use the properties and the stationary distribution of this Markov chain to derive the performance of the idealized 802.11 protocol. We first look at its spatial reuse or, in other words, at its ability to schedule a large number of concurrent successful transmissions. We show that the idealized 802.11 protocol organizes the transmissions in space in such a way that it leads to an optimal spatial reuse when its access intensity is large. This is encouraging, as it shows that a protocol using only local interactions can find a global optimum in a completely decentralize way. We then consider the short and long-term fairness properties of the idealized 802.11 protocol. We observe a clear trade-off between its spatial reuse and its fairness. At low access intensities, its fairness is high but its spatial reuse is low; whereas at high access intensities, the reverse is true. As a result, the access intensity of the protocol can be used to adapt its performance to fit the requirements of the applications running on top of it. The fairness performance of 802.11 also highly depends on the underlying network topology – 802.11 only amplifies the existing topological inequalities. In regular lattice topologies these inequalities arise only at the border where the nodes have fewer neighbors than the nodes inside the network. We demonstrate that, in large line networks and for all finite access-intensities, this border effect does not propagate inside the network, as a result 802.11 is fair. In contrast, we demonstrate that in large grid topologies a phase transition occurs. Under a certain access intensity, the border effect fades away; whereas above a certain access intensity, it propagates throughout the network, and the protocol is severely unfair. Finally, after extending our model to consider different node sensing and capture capabilities, we compare the performance of the ns-2 implementation of IEEE 802.11 and of the idealized protocol. We observe a large gap between the theoretical and practical performance. We identify the three problems that are responsible for this gap. We then propose a remedy to address each of these problems, and show that a 'cured' IEEE 802.11 can achieve the level of performance of the idealized 802.11 protocol

Demo: An Interoperability Development and Performance Diagnosis Environment

Interoperability is key to widespread adoption of sensor network technology, but interoperable systems have traditionally been difficult to develop and test. We demonstrate an interoperable system development and performance diagnosis environment in which different systems, different software, and different hardware can be simulated in a single network configuration. This allows both development, verification, and performance diagnosis of interoperable systems. Estimating the performance is important since even when systems interoperate, the performance can be sub-optimal, as shown in our companion paper that has been conditionally accepted for SenSys 2011

Understanding the Gap between the IEEE 802.11 Protocol Performance and the Theoretical Limits

The ability of the IEEE 802.11 Medium Access Control (MAC) protocol to perform well in multi-hop ad hoc networks has been recently questioned. We observe levels of spatial reuse that are 30% to 50% away from the theoretical limit. The goal of this paper is to answer the following question: what prevents the IEEE 802.11 MAC protocol from operating at the limit determined by its physical layer? We identify three problems in the contention resolution mechanism of the IEEE 802.11 MAC protocol, and we show that they account for most of the gap separating the actual and optimal performances of the protocol. For each of the problems, we propose a solution that, once implemented, allows us to quantify the impact of the problem on the performance of the IEEE 802.11 MAC protocol. The resulting protocol operates 10% to 15% away from the theoretical limit. Finally, we show that reducing the overhead of the protocol to some negligible quantity brings the spatial reuse of the protocol to the theoretical limits. It also makes apparent the powerful organizing capacity of the IEEE 802.11 MAC protocol

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

Enabling Cyber Physical Systems with Wireless Sensor Networking Technologies

[[abstract]]Over the last few years, we have witnessed a growing interest in Cyber Physical Systems (CPSs) that rely on a strong synergy between computational and physical components. CPSs are expected to have a tremendous impact on many critical sectors (such as energy, manufacturing, healthcare, transportation, aerospace, etc) of the economy. CPSs have the ability to transform the way human-to-human, human-toobject, and object-to-object interactions take place in the physical and virtual worlds. The increasing pervasiveness of Wireless Sensor Networking (WSN) technologies in many applications make them an important component of emerging CPS designs. We present some of the most important design requirements of CPS architectures. We discuss key sensor network characteristics that can be leveraged in CPS designs. In addition, we also review a few well-known CPS application domains that depend on WSNs in their design architectures and implementations. Finally, we present some of the challenges that still need to be addressed to enable seamless integration of WSN with CPS designs.[[incitationindex]]SCI[[booktype]]紙