On broadcast and agreement in mobile ad hoc networks

This thesis studies communication and agreement protocols in wireless ad hoc networks from both theoretical and practical perspectives. The last decade has given rise to the rapid growth of wireless telecommunications such as cellular mobile phone networks and wireless LANs. The diversity of network-aware appliances and devices is ever-increasing. Ultimately, these wireless technologies will seemingly inter-operate, offering a new range of services. This ubiquity of wireless devices will introduce a paradigm shift in the way we conceive distributed applications. Wireless ad hoc networks are self-organizing radio networks that do not rely on a preexisting infrastructure to communicate. Nodes of such networks have limited transmission range, and data packets may need to traverse multiple other nodes before reaching their destination. Research in wireless multi-hop networks was initiated 1970s by the Defense Advanced Research Projects Agency (DARPA) and has regained popularity nowadays due to the widespread availability of wireless devices. Examples of wireless ad hoc networks are i) sensor networks, which are distributed sensors that monitor environmental conditions for civil or military purposes, ii) vehicular ad hoc networks, which offer inter/intra-vehicle and infrastructure-to-vehicle communication to improve the safety, security and efficiency of transportation systems and iii) mesh networks, which provide an instantaneous, reliable and inexpensive network infrastructure to local communities. The multi-hop nature of wireless ad hoc networks, along with possible node mobility and limited energy supplies lead to highly dynamic network topology and significantly different network characteristics and behavior than their wired counterparts. Algorithms that have proved successful for traditional networks such as LANs and WANs are generally not directly transposable to such dynamic environments. This thesis is divided into two parts. In the first part we focus on basic communication primitives such as broadcast and multicast. We first study the impact of mobility on the time complexity of deterministic broadcast algorithms. We then explore the phase transition phenomenon observed in percolation theory and random graphs as a basis for reducing the overhead of broadcast through probabilistic flooding, while maintaining satisfactory message delivery reliability. Finally, we propose a novel location service solution for handling geographically scattered and mobile multicast group members, by dynamically adjusting the number and density of nodes responsible for managing multicast group membership information. The second part of the thesis studies agreement problems. Agreement-related solutions can contribute to enhance the structure and reliability to the highly disorganized and dynamic environment of self-organized networks. We first investigate the consensus problem in the context of self-organized networks. Consensus is a cornerstone of many agreement problems. We define a variant of the traditional consensus problem, by relaxing the requirement for the set of participating processes to be known by all at the beginning of the computation. We then identify the necessary and sufficient conditions under which consensus can be solved. We finally address the problem of resource allocation in mobile ad hoc networks, by designing a novel broadcast mechanism that achieves reliable and total-order delivery of messages with high probability. These properties provide the basis of a mutual exclusion algorithm.

Towards reliable communication and agreement in mobile ad-hoc networks

David Cavin

During the last decade, the number of wireless devices like digital organizers, mobile phones and laptop computers has drastically increased. At the same time, several technologies, like Bluetooth, GPRS, or WiFi have become mature and offer a high-performance wireless infrastructure to mobile users. However, there exist still many non-equipped areas where no infrastructure is available. For example, during emergency operations, rescuers may want to deploy temporarily an autonomous (or ad-hoc) network. Under these settings, mobile devices (or nodes) operate in a self-organized manner and can only communicate with their immediate wireless neighbors. To send a packet to a remote destination, a node relies on the routing capabilities of the other nodes. In a mobile ad-hoc network (MANET), problems are inherently more complicated to solve than in a regular wired network because of the dynamic topology and the absence of an underlying communication infrastructure. Many algorithms and protocols have been proposed for mobile ad-hoc networks, but very little has been done to evaluate these solutions in real environments. In most cases, algorithms are evaluated by simulation. Such a solution leads however to incorrect conclusions, as we show in the thesis. In order to avoid the shortcomings of simulations, an adequate software framework must be available, in order to ease the development and the deployment of applications or algorithms. We have developed FRANC, a lightweight Java framework dedicated to the implementation of real ad-hoc applications. FRANC is composed of an extensible library of building blocks that can be composed to build more complex applications. The thesis is not limited to practical contributions. We have also addressed two important problems that are at the root of many other distributed computing algorithms. Although extensively studied in wired networks, this research domain is however still in its early stages in MANETs. We first propose a simple broadcast protocol that disseminates efficiently and reliably a message in a one dimensional graph like a car highway. Then, we study the consensus problem and identify the necessary and sufficient conditions for solving it in self-organized networks when the participants are unknown (in the absence of crashes). Finally, we discuss a pragmatic solution to consensus with crashes based on weak ordering oracles. More specifically, we propose an implementation of these oracles and present results collected in a real small-scale wireless testbed.

EPFL2006

Distributed routing algorithms for sensor networks

Guillermo Barrenetxea Kobas

Recent advances in wireless communications and computing technology are enabling the emergence of low-cost devices that incorporate sensing, processing, and communication functionalities. A large number of these devices are deployed in the field to create a sensor network for both monitoring and control purposes. Sensor networks are currently an active research area mainly due to the potential of their applications. However, the deployment of a working large scale sensor network still requires solutions to a number of technical challenges that stem primarily from the constraints imposed by simple sensor devices: limited power, limited communication bandwidth, and small storage capacity. In view of all these particular constraints, we require a new paradigm for communication, which consists of new algorithms specifically conceived for sensor networks. This thesis concentrates on the routing problem, that is, moving data among different network locations, and on the interactions between routing and coding, that is, how sensors code the observations. We start by designing efficient and computationally simple decentralized algorithms to transmit data from one single source to one single destination. We formalize the corresponding routing problem as a problem of constructing suitably constrained random walks on random graphs and derive distributed algorithms to compute the local parameters of the random walk that induces a uniform load distribution in the network. The main feature of this routing formulation is that it is possible to route messages along all possible routes between the source and the destination node, without performing explicit route discovery/repair computations and without maintaining explicit state information about available routes at the nodes. A natural extension to the single-source/single-destination scenario is to consider multiple sources and/or multiple destinations. Depending on the structure and goal of the network, nodes exhibit different communication patterns. We analyze the problem of routing under three different communication models, namely uniform communication, central data gathering, and border data gathering. For each of these models, we derive capacity limits and propose constructive routing strategies that achieve this capacity. An important constraint of sensor networks is the limited storage capacity available at the nodes. We analyze the problem of routing in networks with small buffers. We develop new approximation models to compute the distribution on the queue size at the nodes which provide a more accurate distribution than the usual Jackson's Theorem. Using these models, we design routing algorithms that minimize buffer overflow losses. Routing in large and unreliable networks, such as sensor networks, becomes prohibitively complex in terms of both computation and communication: due to temporary node failures, the set of available routes between any two nodes changes randomly. We demonstrate that achieving robust communications and maximizing the achievable rate per node are incompatible goals: while robust communications require the use of as many paths as possible between the source and the destination, maximizing the rate per node requires using only a few of the available paths. We propose a family of routing algorithms that explores this trade-off, depending on the degree of reliability of the network. The performance of routing algorithms in sensor networks can be significantly improved by considering the interaction of the source coding mechanism with the transport mechanism. We jointly optimize both the source coding and the routing algorithm in a common scenario encountered in sensor network, namely, real-time data transmission. We demonstrate that the combination of specially designed coding techniques, such as multiple description coding, and multipath routing algorithms, performs significantly better that the usual routing and coding schemes. In summary, this thesis revisits the classic routing problem in the light of distributed schemes for networks with resource-limited nodes.

EPFL2006

Interference management in impulse-radio ultra-wide band networks

Ruben Merz

We consider networks of impulse-radio ultra-wide band (IR-UWB) devices. We are interested in the architecture, design, and performance evaluation of these networks in a low data-rate, self-organized, and multi-hop setting. IR-UWB is a potential physical layer for sensor networks and emerging pervasive wireless networks. These networks are likely to have no particular infrastructure, might have nodes embedded in everyday life objects and have a size ranging from a few dozen nodes to large-scale networks composed of hundreds of nodes. Their average data-rate is low, on the order of a few megabits per second. IR-UWB physical layers are attractive for these networks because they potentially combine low-power consumption, robustness to multipath fading and to interference, and location/ranging capability. The features of an IR-UWB physical layer greatly differ from the features of the narrow-band physical layers used in existing wireless networks. First, the bandwidth of an IR-UWB physical layer is at least 500 MHz, which is easily two orders of magnitude larger than the bandwidth used by a typical narrow-band physical layer. Second, this large bandwidth implies stringent radio spectrum regulations because UWB systems might occupy a portion of the spectrum that is already in use. Consequently, UWB systems exhibit extremely low power spectral densities. Finally IR-UWB physical layers offer multi-channel capabilities for multiple and concurrent access to the physical layer. Hence, the architecture and design of IR-UWB networks are likely to differ significantly from narrow-band wireless networks. For the network to operate efficiently, it must be designed and implemented to take into account the features of IR-UWB and to take advantage of them. In this thesis, we focus on both the medium access control (MAC) layer and the physical layer. Our main objectives are to understand and determine (1) the architecture and design principles of IR-UWB networks, and (2) how to implement them in practical schemes. In the first part of this thesis, we explore the design space of IR-UWB networks and analyze the fundamental design choices. We show that interference from concurrent transmissions should not be prevented as in protocols that use mutual exclusion (for instance, IEEE 802.11). Instead, interference must be managed with rate adaptation, and an interference mitigation scheme should be used at the physical layer. Power control is useless. Based on these findings, we develop a practical PHY-aware MAC protocol that takes into account the specific nature of IR-UWB and that is able to adapt its rate to interference. We evaluate the performance obtained with this design: It clearly outperforms traditional designs that, instead, use mutual exclusion or power control. One crucial aspect of IR-UWB networks is packet detection and timing acquisition. In this context, a network design choice is whether to use a common or private acquisition preamble for timing acquisition. Therefore, we evaluate how this network design issue affects the network throughput. Our analysis shows that a private acquisition preamble yields a tremendous increase in throughput, compared with a common acquisition preamble. In addition, simulations on multi-hop topologies with TCP flows demonstrate that a network using private acquisition preambles has a stable throughput. On the contrary, using a common acquisition preamble exhibits an effect similar to exposed terminal issues in 802.11 networks: the throughput is severely degraded and flow starvation might occur. In the second part of this thesis, we are interested in IEEE 802.15.4a, a standard for low data-rate, low complexity networks that employs an IR-UWB physical layer. Due to its low complexity, energy detection is appealing for the implementation of practical receivers. But it is less robust to multi-user interference (MUI) than a coherent receiver. Hence, we evaluate the performance of an IEEE 802.15.4a physical layer with an energy detection receiver to find out whether a satisfactory performance is still obtained. Our results show that MUI severely degrades the performance in this case. The energy detection receiver significantly diminishes one of the most appealing benefits of UWB, specifically its robustness to MUI and thus the possibility of allowing for parallel transmissions. This performance analysis leads to the development of an IR-UWB receiver architecture, based on energy detection, that is robust to MUI and adapted to the peculiarities of IEEE 802.15.4a. This architecture greatly improves the performance and entails only a moderate increase in complexity. Finally, we present the architecture of an IR-UWB physical layer implementation in ns-2, a well-known network simulator. This architecture is generic and allows for the simulation of several multiple-access physical layers. In addition, it comprises a model of packet detection and timing acquisition. Network simulators also need to have efficient algorithms to accurately compute bit or packet error rates. Hence, we present a fast algorithm to compute the bit error rate of an IR-UWB physical layer in a network setting with MUI. It is based on a novel combination of large deviation theory and importance sampling.

EPFL2008

Interference management in impulse-radio ultra-wide band networks

Ruben Merz

EPFL2008

Towards reliable communication and agreement in mobile ad-hoc networks

David Cavin

EPFL2006

Distributed routing algorithms for sensor networks

Guillermo Barrenetxea Kobas

EPFL2006