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19
Neighborhoodcentric congestion control for multihop wireless mesh networks
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Closedform Throughput Expressions for CSMA Networks with Collisions and Hidden Terminals
"... Abstract—We present a novel modeling approach to derive closedform throughput expressions for CSMA networks with hidden terminals. The key modeling principle is to break the interdependenceof events in a wireless network usingconditional expressions that capture the effect of a specific factor each ..."
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Abstract—We present a novel modeling approach to derive closedform throughput expressions for CSMA networks with hidden terminals. The key modeling principle is to break the interdependenceof events in a wireless network usingconditional expressions that capture the effect of a specific factor each, yet preserve the required dependences when combined together. Different from existing models that use numerical aggregation techniques, our approach is the first to jointly characterize the three main critical factors affecting flow throughput (referred to as hidden terminals, information asymmetry and flowinthemiddle) withina single analytical expression. We have developed a symbolic implementation of the model, that we use for validation against realistic simulations and experiments with real wireless hardware, observing high model accuracy in the evaluated scenarios. The derived closedform expressions enable new analytical studies of capacity and protocol performance that would not be possible with prior models. We illustrate this through an application of network utility maximization in complex networks with collisions, hidden terminals, asymmetric interference and flowinthemiddle instances. Despite that such problematic scenarios make utility maximization a challenging problem, the modelbased optimization yields vast fairness gains and an average perflow throughput gain higher than 500 % with respect to 802.11 in the evaluated networks. I.
Models of 802.11 MultiHop Networks: Theoretical Insights and Experimental Validation
, 2011
"... ... to analyze. On the one hand, their dynamics are complex and rather subtle effects may severely affect their performance. Yet, understanding these effects is critical to operate upper layer protocols, such as TCP/IP. On the other hand, their models tend to be very complex in order to reproduce al ..."
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... to analyze. On the one hand, their dynamics are complex and rather subtle effects may severely affect their performance. Yet, understanding these effects is critical to operate upper layer protocols, such as TCP/IP. On the other hand, their models tend to be very complex in order to reproduce all the features of the protocol. As a result, they do not convey much insight into the essential features. We review two models of 802.11 protocols, which are simple enough to first explain why a tradeoff needs to be found between fairness and spatial reuse (throughput) in saturated wireless networks (where all nodes have packets to transmit to their neighbors); and then to explain why nonsaturated networks (where only some nodes, the sources, have packets to transmit to their destinations in a multihop fashion) that are more than 3 hops longs suffer from instability. We confront both models either to realistic simulations in ns2 or to experiments with a testbed deployed at EPFL. We find that the predictions of both models help us understand the performance of the 802.11 protocol, and provide hints about the changes that need to be brought to the protocol.
Symptotics: A Framework for Analyzing the Scalability of RealWorld Wireless Networks
"... We present a framework for nonasymptotic analysis of realworld wireless networks that captures protocol overhead, congestion bottlenecks, traffic heterogeneity and other realworld concerns. The framework introduces the definition of symptotic 1 scalability, and a metric called change impact value ..."
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We present a framework for nonasymptotic analysis of realworld wireless networks that captures protocol overhead, congestion bottlenecks, traffic heterogeneity and other realworld concerns. The framework introduces the definition of symptotic 1 scalability, and a metric called change impact value (CIV) for comparing the impact of underlying system parameters on network scalability. A key idea is to divide analysis into generic and specific parts connected via a signature – a set of governing parameters of a network scenario – such that analyzing a new network scenario reduces mainly to identifying its signature. Using this framework, we present approximate scalability expressions for line, mesh and clique topologies using TDMA and 802.11, for unicast and broadcast traffic. We compare the analysis with discrete event simulations and show that the model provides sufficiently accurate estimates of scalability. Based on the symptotic expressions, we study the change impact value of underlying parameters on network scalability. We show how impact analysis can be used to tune network features to meet a scaling requirement, and determine the regimes in which reducing routing overhead impacts performance.
Understanding and Tackling the Root Causes of Instability in Wireless Mesh Networks
 IEEE/ACM TRANSACTIONS ON NETWORKING
"... We investigate, both theoretically and experimentally, the stability of CSMAbased wireless mesh networks, where a network is said to be stable if and only if the queue of each relay node remains (almost surely) finite. We identify two key factors that impact stability: the network size and the soc ..."
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We investigate, both theoretically and experimentally, the stability of CSMAbased wireless mesh networks, where a network is said to be stable if and only if the queue of each relay node remains (almost surely) finite. We identify two key factors that impact stability: the network size and the socalled “stealing effect”, a consequence of the hidden node problem and nonzero transmission delays. We consider the case of a greedy source and prove, by using Foster’s theorem, that 3hop networks are stable, but only if the stealing effect is accounted for. We also prove that 4hop networks are, on the contrary, always unstable (even with the stealing effect) and show by simulations that instability extends to more complex linear and nonlinear topologies. To tackle this instability problem, we propose and evaluate a novel, distributed flowcontrol mechanism, called EZflow. EZflow is fully compatible with the IEEE 802.11 standard (i.e., it does not modify headers in packets), can be implemented using offtheshelf hardware, and does not entail any communication overhead. EZflow operates by adapting the minimum congestion window parameter at each relay node, based on an estimation of the buffer occupancy at its successor node in the mesh. We show how such an estimation can be conducted passively by taking advantage of the broadcast nature of the wireless channel. Real experiments, run on a 9node testbed deployed over 4 different buildings, show that EZflow effectively smoothes traffic and improves delay, throughput, and fairness performance.
CapEst: A Measurementbased Approach to Estimating Link Capacity in Wireless Networks
, 2012
"... Estimating link capacity in a wireless network is a complex task because the available capacity at a link is a function of not only the current arrival rate at that link, but also of the arrival rate at links which interfere with that link as well as of the nature of interference between these link ..."
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Estimating link capacity in a wireless network is a complex task because the available capacity at a link is a function of not only the current arrival rate at that link, but also of the arrival rate at links which interfere with that link as well as of the nature of interference between these links. Models which accurately characterize this dependence are either too computationally complex to be useful or lack accuracy. Further, they have a high implementation overhead and make restrictive assumptions, which makes them inapplicable to real networks. In this paper, we propose CapEst, a general, simple yet accurate, measurementbased approach to estimating link capacity in a wireless network. To be computationally light, CapEst allows inaccuracy in estimation; however, using measurements, it can correct this inaccuracy in an iterative fashion and converge to the correct estimate. Our evaluation shows that CapEst always converged to within 5 percent of the correct value in less than 18 iterations. CapEst is modelindependent; hence, it is applicable to any MAC/PHY layer and works with autorate adaptation. Moreover, it has a low implementation overhead, can be used with any application which requires an estimate of residual capacity on a wireless link and can be implemented completely at the network layer without any support from the underlying chipset.
Making the Case for Random Access Scheduling in Wireless Multihop Networks
"... Abstract—This paper formally establishes that random access scheduling schemes, and, more specifically CSMACA, yields exceptionally good performance in the context of wireless multihop networks. While it is believed that CSMACA performs significantly worse than optimal, this belief is usually base ..."
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Abstract—This paper formally establishes that random access scheduling schemes, and, more specifically CSMACA, yields exceptionally good performance in the context of wireless multihop networks. While it is believed that CSMACA performs significantly worse than optimal, this belief is usually based on experiments that use rate allocation mechanisms which grossly underutilize the available capacity that random access provides. To establish our thesis we compare the maxmin rate allocation achieved by CSMACA and optimal in multihop topologies and find that: (i) CSMACA is never worse than 16 % of the optimal when ignoring physical layer constraints, (ii) in any realistic topology with geometric constraints due to the physical layer, CSMACA is never worse than 30 % of the optimal. Considering that maximal scheduling achieves much lower bounds than the above, and greedy maximal scheduling, which is one of the best known distributed approximation of an optimal scheduler, achieves similar worst case bounds, CSMACA is surprisingly efficient. I.
On the Capacity of Wireless CSMA/CA Multihop Networks
"... Abstract—Due to a poor understanding of the interactions among transmitters, wireless multihop networks have commonly been stigmatized as unpredictable in nature. Even elementary questions regarding the throughput limitations of these networks cannot be answered in general. In this paper we investig ..."
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Abstract—Due to a poor understanding of the interactions among transmitters, wireless multihop networks have commonly been stigmatized as unpredictable in nature. Even elementary questions regarding the throughput limitations of these networks cannot be answered in general. In this paper we investigate the behavior of wireless multihop networks using carrier sense multiple access with collision avoidance (CSMA/CA). Our goal is to understand how the transmissions of a particular node affect the medium access, and ultimately the throughput, of other nodes in the network. We introduce a theory which accurately models the behavior of these networks and show that, contrary to popular belief, their performance is easily predictable and can be described by a system of equations. Using the proposed theory, we provide the analytical expressions necessary to fully characterize the capacity region of any wireless CSMA/CA multihop network. We show that this region is nonconvex in general and entirely agnostic to the probability distributions of all network parameters, depending only on their expected values. I.
On the Efficiency of CSMACA Scheduling in Wireless Multihop Networks
 IEEE/ACM TRANSACTIONS ON NETWORKING
, 2012
"... This paper establishes that random access scheduling schemes, and more specifically CSMACA, yield exceptionally good performance in the context of wireless multihop networks. While it is believed that CSMACA performs significantly worse than optimal, this belief is usually based on experiments tha ..."
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This paper establishes that random access scheduling schemes, and more specifically CSMACA, yield exceptionally good performance in the context of wireless multihop networks. While it is believed that CSMACA performs significantly worse than optimal, this belief is usually based on experiments that use rate allocation mechanisms that grossly underutilize the available capacity that random access provides. To establish our thesis, we first compare the achievable rate region of CSMACA and optimal in a number of carefully constructed multihop topologies and find that CSMACA is always within 48 % of the optimal. Motivated by this result, we next characterize the worstcase performance of CSMACA in neighborhood topologies representing the congested regions of larger multihop topologies by deriving the neighborhood topology that yields the worstcase throughput ratio for CSMACA and find that in neighborhood topologies with less than 20 edges: 1) CSMACA is never worse than 16 % of the optimal when ignoring physicallayer constraints; and 2) in any realistic topology with geometric constraints due to the physical layer, CSMACA is never worse than 30 % of the optimal. Considering that maximal scheduling achieves much lower bounds than the above, and greedy maximal scheduling, which is one of the best known distributed approximation of an optimal scheduler, achieves similar worstcase bounds, CSMACA is surprisingly efficient.
Procedia Computer Science Mitigating Multipath Fading in a Mobile Mesh Network
"... By using robots as routers, a team of networked robots can provide a communication substrate to establish a wireless mesh network. The mobile mesh network can autonomously optimize its configuration, increasing performance. One of the main sources of radio signal fading in such a network is multi ..."
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By using robots as routers, a team of networked robots can provide a communication substrate to establish a wireless mesh network. The mobile mesh network can autonomously optimize its configuration, increasing performance. One of the main sources of radio signal fading in such a network is multipath propagation, which can be mitigated by moving the senders or the receivers on the distance of the order of a wavelength. In this paper, we measure the performance gain when robots are allowed to make such small movements and find that it may be as much as 270%. Our main contribution is the design of a system that allows robots to cooperate and improve the realworld network throughput via a practical solution. We model the problem of which robots to move as a distributed constraint optimization problem (DCOP). Our study includes four local metrics to estimate global throughput.