Tandem queueing networks with neighbor blocking and back-offs

We introduce a novel class of tandem queueing networks which arise in modeling the congestion behavior of wireless multi-hop networks with distributed medium access control. These models provide valuable insight in how the network performance in terms of throughput depends on the back-off mechanism that governs the competition among neighboring nodes for access to the medium. The models fall at the interface between classical queueing networks and interacting particle systems, and give rise to high-dimensional stochastic processes that challenge existing methodologies. We present various open problems and conjectures, which are supported by partial results for special cases and limit regimes as well as simulation experiments

Aziala-net: Deploying a Scalable Multi-hop Wireless Testbed Platform for Research Purposes

Aziala-net is a flexible and scalable experimental testbed for wireless multi-hop networks based on simple off-the-shelf hardware that is able to adapt to various research purposes. It is composed of more than 50 Asus wireless routers that have been adapted to either work as fixed base station or as mobile nodes. After describing the technical details of Aziala-net, we illustrate the potential of the testbed by showing two samples of works that are currently under study in the testbed. The first example focus on the use of the IEEE 802.11 MAC layer protocol for multi-hop networks and the stability problem that it faces in the case of wireless mesh networks. The second example focus on epidemic forwarding protocols and their performance in a real testbed deployment

Demo Abstract of Net-Controller: a Network Visualization and Management Tool

Net-Controller is a user-friendly network visualization and management tool developed at EPFL in order to easily retrieve and display in real time network statistics, such as link throughput and queue occupancy from a large testbed composed of wireless routers. Additionally, Net-Controller allows to control and modify the parameters of a complete network from a central point, and to easily generate traffic between different nodes. We intend to illustrate some of the features of Net-Controller through two examples that show how easily this tool detects and helps elucidate the throughput degradation that occurs in a wireless multi-hop network. The first example shows how and why fair queuing [6] improves performance compared to the standard FIFO policy used in off-the-shelf routers. The second example shows how and why a hop-by-hop congestion control mechanism, such as EZ-Flow [4] is needed to tackle the instability problem of a multi-hop scenario

FloorNet: Deployment and Evaluation of a Multihop Wireless 802.11 Testbed

A lot of attention has been given to multihop wireless networks lately, but further research—in particular, through experimentation—is needed. This attention has motivated an increase in the number of 802.11-based deployments, both indoor and outdoor. These testbeds, which require a significant amount of resources during both deployment and maintenance, are used to run measurements in order to analyze and understand the limitation and differences between analytical or simulation-based figures and the results from real-life experimentation. This paper makes two major contributions: (i) first, we describe a novel wireless multihop testbed, which we name FloorNet, that is deployed and operated under the false floor of a lab in our Computer Science building. This false floor provides a strong physical protection that prevents disconnections or misplacements, as well as radio shielding (to some extent) thanks to the false floor panels—this later feature is assessed through experimentation; (ii) second, by running exhaustive and controlled experiments we are able to analyze the performance limits of commercial off-theshelf hardware, as well as to derive practical design criteria for the deployment and configuration of mesh networks. These results both provide valuable insights of wireless multihop performance and prove that FloorNet constitutes a valuable asset to research on wireless mesh networks.European Community's Seventh Framework ProgramPublicad

Bringing Stability to Wireless Mesh Networks

Wireless mesh networks were designed as a mean to rapidly deliver large-scale communication capabilities without the support of any prior infrastructure. Among the different properties of mesh networks, the self-organizing feature is particularly interesting for developing countries or for emergency situations. However, these benefits also bring new challenges. For example, the scheduling decision needs to be performed in a distributed manner at each node of the network. Toward this goal, most of the current mesh deployments are based on the IEEE 802.11 protocol, even if it was not designed for multi-hop communications. The main goals of this thesis are (i) to understand and model the behavior of IEEE 802.11-based mesh networks and more specifically the root causes that lead to congestion and network instability; (ii) to develop an experimental infrastructure in order to validate with measurements both the problems and the solutions discussed in this thesis; (iii) to build efficient hop-by-hop scheduling schemes that provide congestion control and inter-flow fairness in a practical way and that are backward-compatible with the current protocol; and (iv) to explain the non-monotonic relation between the end-to-end throughput and the source rate and to introduce a model to derive the rationale behind this artifact. First, we propose a Markovian model and we introduce the notion of stealing effect to explain the root causes behind the 3-hop stability boundary, where linear networks up to 3 hops are stable, and larger topologies are intrinsically unstable. We validate our analytical results both through simulations and through measurements on a small testbed deployment. Second, to support the experimental research presented in this thesis, we design and deploy a large-scale mesh network testbed on the EPFL campus. We plan our architecture to be as flexible as possible in order to support a wide range of other research areas such as IEEE 802.11 indoor localization and opportunistic routing. Third, we introduce EZ-flow, a novel hop-by-hop congestion-control mechanism that operates at the Medium Access Control layer. EZ-flow is fully backward-compatible with the existing IEEE 802.11 deployments and it works without any form of message passing. To perform its task EZ-flow takes advantage of the broadcast nature of the wireless medium in order to passively derive the queue size at the next-hop node. This information is then used by each node to adapt accordingly its channel access probability, through the contention window parameter of IEEE 802.11. After detailing the different components of EZ-flow, we analyze its performance analytically, through simulations and real measurements. Fourth, we show that hop-by-hop congestion-control can be efficiently performed at the network layer in order to not abuse the contention mechanism of IEEE 802.11. Additionally, we introduce a complete framework that jointly achieves congestion-control and fairness without requiring a prior knowledge of the network capacity region. To achieve the fairness part, we propose the Explore & Enhance algorithm that finds a fair and achievable rate allocation vector that maximizes a desired function of utility. We show experimentally that this algorithm reaches its objective by alternating between exploration phases (to discover the capacity region) and enhancement phases (to improve the utility through a gradient ascent). Finally, we note that, as opposed to wired networks, the multi-hop wireless capacity is usually unknown and time-varying. Therefore, we study how the end-to-end throughput evolves as a function of the source rate when operating both below and above the network capacity. We note that this evolution follows a non-monotonic curve and we explain, through an analytical model and simulations, the rationale behind the different transition points of this curve. Following our analysis, we show that no end-to-end congestion control can be throughput-optimal if it operates directly over IEEE 802.11. Hence, this supports the methodology of performing congestion control in a hop-by-hop manner. After validating experimentally the non-monotonicity, we compare through simulations different state-of-the-art scheduling schemes and we highlight the important tradeoff that exists in congestion-control schemes between efficiency (i.e., throughput-optimality) and robustness (i.e., no throughput collapse when the sources attempt to operate at a rate above the network capacity)

Experimenting with commodity 802.11 hardware: overview and future directions

The huge adoption of 802.11 technologies has triggered a vast amount of experimentally-driven research works. These works range from performance analysis to protocol enhancements, including the proposal of novel applications and services. Due to the affordability of the technology, this experimental research is typically based on commercial off-the-shelf (COTS) devices, and, given the rate at which 802.11 releases new standards (which are adopted into new, affordable devices), the field is likely to continue to produce results. In this paper, we review and categorise the most prevalent works carried out with 802.11 COTS devices over the past 15 years, to present a timely snapshot of the areas that have attracted the most attention so far, through a taxonomy that distinguishes between performance studies, enhancements, services, and methodology. In this way, we provide a quick overview of the results achieved by the research community that enables prospective authors to identify potential areas of new research, some of which are discussed after the presentation of the survey.This work has been partly supported by the European Community through the CROWD project (FP7-ICT-318115) and by the Madrid Regional Government through the TIGRE5-CM program (S2013/ICE-2919).Publicad

Mobile Ad Hoc Networks

Guiding readers through the basics of these rapidly emerging networks to more advanced concepts and future expectations, Mobile Ad hoc Networks: Current Status and Future Trends identifies and examines the most pressing research issues in Mobile Ad hoc Networks (MANETs). Containing the contributions of leading researchers, industry professionals, and academics, this forward-looking reference provides an authoritative perspective of the state of the art in MANETs. The book includes surveys of recent publications that investigate key areas of interest such as limited resources and the mobility of mobile nodes. It considers routing, multicast, energy, security, channel assignment, and ensuring quality of service. Also suitable as a text for graduate students, the book is organized into three sections: Fundamentals of MANET Modeling and Simulation—Describes how MANETs operate and perform through simulations and models Communication Protocols of MANETs—Presents cutting-edge research on key issues, including MAC layer issues and routing in high mobility Future Networks Inspired By MANETs—Tackles open research issues and emerging trends Illustrating the role MANETs are likely to play in future networks, this book supplies the foundation and insight you will need to make your own contributions to the field. It includes coverage of routing protocols, modeling and simulations tools, intelligent optimization techniques to multicriteria routing, security issues in FHAMIPv6, connecting moving smart objects to the Internet, underwater sensor networks, wireless mesh network architecture and protocols, adaptive routing provision using Bayesian inference, and adaptive flow control in transport layer using genetic algorithms

Contention techniques for opportunistic communication in wireless mesh networks

Auf dem Gebiet der drahtlosen Kommunikation und insbesondere auf den tieferen Netzwerkschichten sind gewaltige Fortschritte zu verzeichnen. Innovative Konzepte und Technologien auf der physikalischen Schicht (PHY) gehen dabei zeitnah in zelluläre Netze ein. Drahtlose Maschennetzwerke (WMNs) können mit diesem Innovationstempo nicht mithalten. Die Mehrnutzer-Kommunikation ist ein Grundpfeiler vieler angewandter PHY Technologien, die sich in WMNs nur ungenügend auf die etablierte Schichtenarchitektur abbilden lässt. Insbesondere ist das Problem des Scheduling in WMNs inhärent komplex. Erstaunlicherweise ist der Mehrfachzugriff mit Trägerprüfung (CSMA) in WMNs asymptotisch optimal obwohl das Verfahren eine geringe Durchführungskomplexität aufweist. Daher stellt sich die Frage, in welcher Weise das dem CSMA zugrunde liegende Konzept des konkurrierenden Wettbewerbs (engl. Contention) für die Integration innovativer PHY Technologien verwendet werden kann. Opportunistische Kommunikation ist eine Technik, die die inhärenten Besonderheiten des drahtlosen Kanals ausnutzt. In der vorliegenden Dissertation werden CSMA-basierte Protokolle für die opportunistische Kommunikation in WMNs entwickelt und evaluiert. Es werden dabei opportunistisches Routing (OR) im zustandslosen Kanal und opportunistisches Scheduling (OS) im zustandsbehafteten Kanal betrachtet. Ziel ist es, den Durchsatz von elastischen Paketflüssen gerecht zu maximieren. Es werden Modelle für Überlastkontrolle, Routing und konkurrenzbasierte opportunistische Kommunikation vorgestellt. Am Beispiel von IEEE 802.11 wird illustriert, wie der schichtübergreifende Entwurf in einem Netzwerksimulator prototypisch implementiert werden kann. Auf Grundlage der Evaluationsresultate kann der Schluss gezogen werden, dass die opportunistische Kommunikation konkurrenzbasiert realisierbar ist. Darüber hinaus steigern die vorgestellten Protokolle den Durchsatz im Vergleich zu etablierten Lösungen wie etwa DCF, DSR, ExOR, RBAR und ETT.In the field of wireless communication, a tremendous progress can be observed especially at the lower layers. Innovative physical layer (PHY) concepts and technologies can be rapidly assimilated in cellular networks. Wireless mesh networks (WMNs), on the other hand, cannot keep up with the speed of innovation at the PHY due to their flat and decentralized architecture. Many innovative PHY technologies rely on multi-user communication, so that the established abstraction of the network stack does not work well for WMNs. The scheduling problem in WMNs is inherent complex. Surprisingly, carrier sense multiple access (CSMA) in WMNs is asymptotically utility-optimal even though it has a low computational complexity and does not involve message exchange. Hence, the question arises whether CSMA and the underlying concept of contention allows for the assimilation of advanced PHY technologies into WMNs. In this thesis, we design and evaluate contention protocols based on CSMA for opportunistic communication in WMNs. Opportunistic communication is a technique that relies on multi-user diversity in order to exploit the inherent characteristics of the wireless channel. In particular, we consider opportunistic routing (OR) and opportunistic scheduling (OS) in memoryless and slow fading channels, respectively. We present models for congestion control, routing and contention-based opportunistic communication in WMNs in order to maximize both throughput and fairness of elastic unicast traffic flows. At the instance of IEEE 802.11, we illustrate how the cross-layer algorithms can be implemented within a network simulator prototype. Our evaluation results lead to the conclusion that contention-based opportunistic communication is feasible. Furthermore, the proposed protocols increase both throughput and fairness in comparison to state-of-the-art approaches like DCF, DSR, ExOR, RBAR and ETT

The Boston University Photonics Center annual report 2015-2016

This repository item contains an annual report that summarizes activities of the Boston University Photonics Center in the 2015-2016 academic year. The report provides quantitative and descriptive information regarding photonics programs in education, interdisciplinary research, business innovation, and technology development. The Boston University Photonics Center (BUPC) is an interdisciplinary hub for education, research, scholarship, innovation, and technology development associated with practical uses of light.This has been a good year for the Photonics Center. In the following pages, you will see that this year the Center’s faculty received prodigious honors and awards, generated more than 100 notable scholarly publications in the leading journals in our field, and attracted $18.9M in new research grants/contracts. Faculty and staff also expanded their efforts in education and training, and cooperated in supporting National Science Foundation sponsored Sites for Research Experiences for Undergraduates and for Research Experiences for Teachers. As a community, we emphasized the theme of “Frontiers in Plasmonics as Enabling Science in Photonics and Beyond” at our annual symposium, hosted by Bjoern Reinhard. We continued to support the National Photonics Initiative, and contributed as a cooperating site in the American Institute for Manufacturing Integrated Photonics (AIM Photonics) which began this year as a new photonics-themed node in the National Network of Manufacturing Institutes. Highlights of our research achievements for the year include an ambitious new DoD-sponsored grant for Development of Less Toxic Treatment Strategies for Metastatic and Drug Resistant Breast Cancer Using Noninvasive Optical Monitoring led by Professor Darren Roblyer, continued support of our NIH-sponsored, Center for Innovation in Point of Care Technologies for the Future of Cancer Care led by Professor Cathy Klapperich, and an exciting confluence of new grant awards in the area of Neurophotonics led by Professors Christopher Gabel, Timothy Gardner, Xue Han, Jerome Mertz, Siddharth Ramachandran, Jason Ritt, and John White. Neurophotonics is fast becoming a leading area of strength of the Photonics Center. The Industry/University Collaborative Research Center, which has become the centerpiece of our translational biophotonics program, continues to focus onadvancing the health care and medical device industries, and has entered its sixth year of operation with a strong record of achievement and with the support of an enthusiastic industrial membership base