A Fair Scheduling for Wireless Mesh Networks

Wireless Mesh Networks (WMNs) represent a new and promising paradigm that uses multi-hop communications to extend WiFi networks: By deploying only one hot spot (directly connected to the Internet) and several transient access points (TAPs), an Internet Service Provider (ISP) can extend its coverage and serve a large number of clients using a single broadband connection. Unfortunately, if the medium access protocol is poorly designed or inadequate, it can lead to severe unfairness and low bandwidth utilization. In this paper, we propose a fair scheduling mechanism that optimizes the bandwidth utilization in the mesh network. Our solution assigns transmission rights to the links in the WMN and maximizes the spatial reuse (i.e., the possibility for links that do not contend to be activated at the same time). We show that our solution is fair and collision-free, and we evaluate its efficiency by means of simulations

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)

Achieving Fairness in 802.11-Based Multi-channel Wireless Mesh Networks

Multi-hop wireless networks based on 802. 11 are being used more widely as an alternative technology for last-mile broadband Internet access. Their benefits include ease of deployment and lower cost. Such networks are not without problems. Current research on such networks aims at a number of challenges, including overcoming capacity limitation and poor fairness. The focus of our research is for achieving fairness in multi-channel multi-hop wireless networks. First, we review the literature for different methods for representing link-contention areas, and the existing single-channel fairness computational model. Second, we generalize the fairness constraints applied to each link-contention area, defined in the existing single-channel fairness reference model, to multi-channel models. Third, by adopting the concepts of link-usage matrix and medium-usage matrix to represent network topology and flow status, and using Collision Domain theory and Clique Graph theory to represent link-contention area, we develop a computational model to compute optimal MAC-layer bandwidth allocated to each flow in a multi-channel multi-hop WMN. We simulate various network configurations to evaluate the performance of the fairness algorithm based on the above computational model in different scenarios. We have found that in the multi-channel environment, our extension to the Collision Domain model generally provides a more accurate estimation of network capacity. Based on this model, we have extended the source-rate-limiting mechanism, which limits the flow rate to its fair share computed by the computational model. Experimental results that validate these findings are presented in this thesis

Secure Incentives to Cooperate for Wireless Networks

The operating principle of certain wireless networks makes essential the cooperation between the mobile nodes. However, if each node is an autonomous selfish entity, cooperation is not guaranteed and therefore we need to use incentive techniques. In this thesis, we study cooperation in three different types of networks: WiFi networks, Wireless Mesh Networks (WMNs), and Hybrid Ad-hoc networks. Cooperation has a different goal for each of these networks, we thus propose incentive mechanisms adapted to each case. In the first chapter of this thesis, we consider WiFi networks whose wide-scale adoption is impeded by two major hurdles: the lack of a seamless roaming scheme and the variable QoS experienced by the users. We devise a reputation-based solution that (i) allows a mobile node to connect to a foreign Wireless ISP in a secure way while preserving his anonymity and (ii) encourages the WISPs to cooperate, i.e., to provide the mobile clients with a good QoS. Cooperation appears here twofold: First, the mobile clients have to collaborate in order to build and maintain the reputation system and second, the use of this reputation system encourages the WISPs to cooperate. We show, by means of simulations, that our reputation model indeed encourages the WISPs to behave correctly and we analyze the robustness of our solution against various attacks. In the second chapter of the thesis, we consider Wireless Mesh Networks (WMNs), a new and promising paradigm that uses multi-hop communications to extend WiFi networks. Indeed, by connecting only one hot spot to the Internet and by deploying several Transit Access Points (TAPs), a WISP can extend its coverage and serve a large number of clients at a very low cost. We analyze the characteristics of WMNs and deduce three fundamental network operations that need to be secured: (i) the routing protocol, (ii) the detection of corrupt TAPs and (iii) the enforcement of a proper fairness metric in WMNs. We focus on the fairness problem and propose FAME, an adaptive max-min fair resource allocation mechanism for WMNs. FAME provides a fair, collision-free capacity use of the WMN and automatically adjusts to the traffic demand fluctuations of the mobile clients. We develop the foundations of the mechanism and demonstrate its efficiency by means of simulations. We also experimentally assess the utility of our solution when TAPs are equipped with directional antennas and distinct sending and receiving interfaces in the Magnets testbed deployed in Berlin. In the third and last chapter of this thesis, we consider Hybrid Ad-hoc networks, i.e., infrastructured networks that are extended using multi-hop communications. We propose a secure set of protocols to encourage the most fundamental operation in these networks, namely packet forwarding. This solution is based on a charging and rewarding system. We use "MAC layering" to reduce the space overhead in the packets and a stream cipher encryption mechanism to provide "implicit authentication" of the nodes involved in the communication. We analyze the robustness of our protocols against rational and malicious attacks. We show that the use of our solution makes cooperation rational for selfish nodes. We also show that our protocols thwart rational attacks and detect malicious attacks

Scalable and interference aware wi-fi mesh networks using cots devices

A crescente tendencia no acesso móvel tem sido potenciada pela tecnologia IEEE 802.11. Contudo, estas redes têm alcance rádio limitado. Para a extensão da sua cobertura é possível recorrer a redes emalhadas sem fios baseadas na tecnologia IEEE 802.11, com vantagem do ponto de vista do custo e da flexibilidade de instalação, face a soluções cabladas. Redes emalhadas sem fios constituídas por nós com apenas uma interface têm escalabilidade reduzida. A principal razão dessa limitação deve-se ao uso do mecanismo de acesso ao meio partilhado Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA) em topologias multi-hop. Especificamente, o CSMA/CA não evita o problema do nó escondido levando ao aumento do número de colisões e correspondente degradação de desempenho com impacto direto no throughput e na latência. Com a redução da tecnologia rádio torna-se viável a utilização de múltiplos rádios por nó, sem com isso aumentar significativamente o custo da solução final de comunicações. A utilização de mais do que um rádio por nó de comuniações permite superar os problemas de desempenho inerentes ás redes formadas por nós com apenas um rádio. O objetivo desta tese, passa por desenvolver uma nova solução para redes emalhadas multi-cana, duar-radio, utilizando para isso novos mecanismos que complementam os mecanismos definidos no IEEE 802.11 para o estabelecimento de um Basic Service Set (BSS). A solução é baseada na solução WiFIX, um protocolo de routing para redes emalhadas de interface única e reutiliza os mecanismos já implementados nas redes IEEE 802.11 para difundir métricas que permitam à rede escalar de forma eficaz minimizando o impacto na performance. A rede multi-hop é formada por nós equipados com duas interfaces, organizados numa topologia hierárquica sobre múltiplas relações Access Point (AP) – Station (STA). Os resultados experimentais obtidos mostram a eficácia e o bom desempenho da solução proposta face à solução WiFIX original.The increasing trend on mobile access has been mainly potentied for IEEE 802.11 technology. However these networks suffer from reduced radio range. The extension of coverage can be potentiated by mesh deployments since they provide an ease, robust, flexible and cost effective solution for this problem. These networks are built upon nodes scattered in a mesh topology that form the backbone of an extended basic service set. Single radio Wireless Mesh Networks (WMN) however suffer from reduced scalability. The main reason to such limitation is the use of Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA) in the multi-hop topology. Specifically, CSMA/CA fails to prevent the hidden and exposed node occurrence, which respectively, lead to an increase on the number of collisions and flow retentions. The direct impact on throughput and latency reduces the overall network performance to values that no longer match user increasing demands. As radio technology becomes cheaper, it became possible to equip nodes with multiple interfaces and operate them in multiple channels in order the reduce interference from links operating on a common channel. Therefore the goal of this thesis is to develop a new WMN Multi-Radio Multi-Channel (MRMC) solution addressing new mechanisms not yet covered in state of art. The proposed solution, is based on WiFIX, a Single Radio (SR) WMN routing protocol and reuses the mechanisms already implemented in IEEE 802.11 networks to broadcast metrics that enable the network to auto-configure efficiently and to scale with minimum overhead. The multi-hop backbone is formed by nodes equipped with two interfaces disposed in a hierarchical topology, under multiple Access Point (AP) - Station (STA) relations. The results obtained from an experimental testbed clearly show the effectiveness of the solution compared with the original WiFIX and its capability to scale resulting from the overhead control and co-channel interference reduction

State of the art in Wireless Mesh Networks - delivrable L3.01 - RNRT project "Airnet"

This delivrable presents a state of the art on management related issues in Wireless Mesh Networks. We describe existant work focusing on the five functional domains of the management plane: fault management, configuration management, accounting, performance and security

The Efficacy of Source Rate Control in Achieving Fairness in Wireless Mesh Networks

The use of 802.11-based wireless mesh networks (WMNs) as an alternative network backbone technology is growing rapidly. The primary advantages of this approach are ease of deployment and lower cost. However, such networks typically exhibit poor fairness properties, often starving nodes if they are too many hops distant from the gateway. Researchers have shown a growing interest in this problem in recent years. Many solutions proposed amount to some level of source rate control, either by policing directly at the source, or via TCP congestion control reacting to a gateway-enforced rate limit. However, there has been limited study on the effectiveness of source rate control. In this thesis we first demonstrate that source rate control can only partially solve the fairness issue in 802.11-based WMNs, with some routers experiencing an undesirable degree of unfairness, which we call structural unfairness. We then identify the four necessary factors that cause structural unfairness. If we can eliminate or reduce any one of these conditions, we can eliminate or ameliorate the unfairness problem. We first investigate two techniques to improve 802.11 MAC scheduling: fixing the contention window and packet spacing at every router node, both means achievable with commodity 802.11 hardware. We show that the combination of these mechanisms provides a significant gain in fairness. We also perform case studies using another three techniques, channel re-assignment, routing changes, and careful router placement, to remove or reduce other necessary conditions. We demonstrate that these techniques, whenever applicable, can eliminate the unfairness problem entirely at times, or at least improve the situation

Towards versatile access networks (Chapter 3)

Compared to its previous generations, the 5th generation (5G) cellular network features an additional type of densification, i.e., a large number of active antennas per access point (AP) can be deployed. This technique is known as massive multipleinput multiple-output (mMIMO) [1]. Meanwhile, multiple-input multiple-output (MIMO) evolution, e.g., in channel state information (CSI) enhancement, and also on the study of a larger number of orthogonal demodulation reference signal (DMRS) ports for MU-MIMO, was one of the Release 18 of 3rd generation partnership project (3GPP Rel-18) work item. This release (3GPP Rel-18) package approval, in the fourth quarter of 2021, marked the start of the 5G Advanced evolution in 3GPP. The other items in 3GPP Rel-18 are to study and add functionality in the areas of network energy savings, coverage, mobility support, multicast broadcast services, and positionin