An Investigation of Link Quality Assessment for Mobile Multi-hop and Multi-rate Wireless Networks

Wireless ad hoc networks will be an important component in future communication systems. The performance of wireless ad hoc networks can be improved by link quality-aware applications. Wireless link quality is dynamic in nature, especially in mobile scenarios. Therefore, accurate and fast packet delivery ratio estimation is a prerequisite to good performance in mobile, multi-hop and multi-rate wireless ad hoc networks. In this paper, we propose a novel packet delivery ratio estimation method that improves the accuracy and responsiveness of the packet delivery ratio estimation. The proposed link quality estimation components are implemented in a IEEE 802.11b/g test-bed. The experiment results show that the accuracy of the packet delivery ratio estimation can improve up to 50% in mobile scenarios without introducing overhead. We also show the end-to-end performance impact of this improved estimation on route selection using different routing metrics and configurations. The measurement results show that our packet delivery ratio method leads to better route selection in the form of increased end-to-end throughput compared to traditional methods, which respond slowly to the link dynamics.     

Improving throughput in multihop wireless networks

One of the main characteristics of wireless ad hoc networks is their node-centric broadcast nature of communication, leading to interferences and spatial contention between adjacent wireless links. Due to such interferences, pessimistic concerns have been recently raised with respect to the decreasing network capacity in wireless ad hoc networks when the number of nodes scales to several orders of magnitude higher. Such studies assume uniformly distributed nodes in the network and randomized traffic patterns. In this paper, we argue that in all cases of end-to-end data communications-including one-to-k unicast and multicast data dissemination as well as k-to-one data aggregation-the maximum achievable end-to-end data throughput (measured on the sources) heavily depends on the strategy of arranging the topology of transmission between sources and destinations, as well as possible per-node operations such as coding. An optimal strategy achieves better end-to-end throughput than an arbitrary one. We present theoretical studies and critical insights with respect to how these strategies may be designed so that end-to-end throughput may be increased. After all, under all circumstances-in either a lightly loaded or a congested network-increasing end-to-end throughput from its baseline is always beneficial to applications using ad hoc networks to communicate.     

Performance benchmarking of wireless Web servers

The advent of mobile computers and wireless networks enables the deployment of wireless Web servers and clients in short-lived ad hoc network environments, such as classroom area networks. The purpose of this paper is to benchmark the performance capabilities of wireless Web servers in such an environment. Network traffic measurements are conducted on an in-building IEEE 802.11b wireless ad hoc network, using a wireless-enabled Apache Web server, several wireless clients, and a wireless network traffic analyzer. The experiments focus on the HTTP transaction rate and end-to-end throughput achievable in such an ad hoc network environment, and the impacts of factors such as Web object size, number of clients, and persistent HTTP connections. The results show that the wireless network bottleneck manifests itself in several ways: inefficient HTTP performance, client-side packet losses, server-side packet losses, network thrashing, and unfairness among Web clients. Persistent HTTP connections offer up to 350% improvement in HTTP transaction rate and user-level throughput, while also improving fairness for mobile clients accessing content from a wireless Web server.     

一种低时延的短波自组网优化链路状态路由协议

优化链路状态路由(Optimized Link State Routing,OLSR)协议是一种先验式路由协议,网络中的所有节点通过周期性地发送控制消息来计算全网路由信息。在短波自组织网络中,节点周期性地发送控制消息会占据大量的信道资源,大幅增加网络的控制开销,浪费短波有限的带宽资源,导致网络通信性能急剧下降。其次,受到地形地貌、天线方向和接收性能的个体差异等影响,造成无线链路不稳定,导致网络中存在非对称链路,增加了通信端到端时延。为此,提出了一种低时延的短波自组网OLSR协议。该协议在执行MPR(Multipoint Relay)选择算法时综合考虑了节点的连接度和链路可靠性,在优化MPR节点个数的同时选择链路可靠性较大的节点作为MPR节点,在进行路由选择时能够利用网络中的非对称链路。仿真结果表明,该协议能优化数据包投递成功率、吞吐量、端到端时延和网络控制开销等性能指标。     

A hotspot mitigation protocol for ad hoc networks

Hotspots represent transient but highly congested regions in wireless ad hoc networks that result in increased packet loss, end-to-end delay, and out-of-order packets delivery. We present a simple, effective, and scalable hotspot mitigation protocol (HMP) where mobile nodes independently monitor local buffer occupancy, packet loss, and MAC contention and delay conditions, and take local actions in response to the emergence of hotspots, such as, suppressing new route requests and rate controlling TCP flows. We use analysis, simulation, and experimental results from a wireless testbed to demonstrate the effectiveness of HMP in mobile ad hoc networks. HMP balances resource consumption among neighboring nodes, and improves end-to-end throughput, delay, and packet loss. Our results indicate that HMP can also improve the network connectivity preventing premature network partitions. We present analysis of hotspots, and the detailed design of HMP. We evaluate the protocol’s ability to effectively mitigate hotspots in mobile ad hoc networks that are based on on-demand and proactive routing protocols.     

Energy-Efficient Routing Algorithms Using Directional Antennas for Mobile Ad Hoc Networks

Power consumption is an important issue in the wireless ad hoc networking environment. In this paper, we present several energy-efficient routing algorithms using directional antennas for wireless ad hoc networks. These algorithms are simple to implement and are distributed and can be applied to mobile environments. We evaluate how directional antennas improve system throughput. We study the influence of the battery recovery effect and mobility on the network throughput during a network lifetime. We also present an algorithm that exploits the broadcast nature of the wireless communication environment to improve end-to-end bit error performance for a Rayleigh fading channel.     

Cross-Layer Interaction of TCP and Ad Hoc Routing Protocols in Multihop IEEE 802.11 Networks

In this research, we first investigate the cross-layer interaction between TCP and routing protocols in the IEEE 802.11 ad hoc network. On-demand ad hoc routing protocols respond to network events such as channel noise, mobility, and congestion in the same manner, which, in association with TCP, deteriorates the quality of an existing end-to-end connection. The poor end-to-end connectivity deteriorates TCP's performance in turn. Based on the well-known TCP-friendly equation, we conduct a quantitative study on the TCP operation range using static routing and long-lived TCP flows and show that the additive-increase, multiplicative-decrease (AIMD) behavior of the TCP window mechanism is aggressive for a typical multihop IEEE 802.11 network with a low-bandwidth-delay product. Then, to address these problems, we propose two complementary mechanisms, that is, the TCP fractional window increment (FeW) scheme and the Route-failure notification using BUIk-losS Trigger (ROBUST) policy. The TCP FeW scheme is a preventive solution used to reduce the congestion-driven wireless link loss. The ROBUST policy is a corrective solution that enables on-demand routing protocols to suppress overreactions induced by the aggressive TCP behavior. It is shown by computer simulation that these two mechanisms result in a significant improvement of TCP throughput without modifying the basic TCP window or the wireless MAC mechanisms.     

Extending Global IP Connectivity for Ad Hoc Networks

Ad hoc networks have thus far been regarded as stand-alone networks without assumed connectivity to wired IP networks and the Internet. With wireless broadband communications and portable devices with appropriate CPU, memory and battery performance, ad hoc connectivity will become more feasible and demand for global connectivity through ad hoc networking is likely to rapidly grow. In this paper we propose an algorithm and describe a developed prototype for connectivity between an ad hoc network running the ad hoc on-demand distance-vector protocol and a wired IP network where mobile IP is used for mobility management. Implementation issues and performance metrics are also discussed.     

无线MESH网络拓扑控制研究

作为下一代无线网络中的一种新型网络结构,无线MESH网受到了广泛关注。MESH网络节点间的自组织组网方式,使得网络具有良好的灵活性、鲁棒性,同时也使得MESH网络的性能称为研究的重点。从功率控制和信道分配等方面研究了MESH网络的虚拟骨干结构的拓扑优化,功率控制通过调节节点发射功率来改变其对邻居节点的影响;信道分配利用正交信道的相互不连通性质改变节点对于邻居节点的影响,以降低链路间的干扰、维护网络连通性、提高系统容量。     

Traffic load metrics for multihomed mobile IP and global connectivity

Support for host mobility an essential and necessary feature for roaming users who connect to wireless networks via access points. Access points may have different capabilities, be connected to different networks and be installed by different providers. A mobile host will discover multiple access points in this environment. In such an environment, a mobile host should be able to use the best available connection to communicate with a correspondent host and perhaps use multiple connections for different hosts. In areas with wireless local area network access, pockets with limited or no coverage could exist. Such restricted connectivity could be compensated by neighbor hosts who form an ad hoc network and relay packets until they reach an access point. This paper describes and discusses a proposed solution towards enabling and supporting connectivity in wireless networks. In the proposed solution the network layer software will evaluate and decide which wireless network connections to use. A Running Variance Metric (RVM) and a Relative Network Load(RNL) are used to measure the traffic load of access points in wireless access networks. RVM and RNL can be efficiently used for both infrastructure networks and ad hoc networks. Multihomed Mobile IP (M-MIP) is an extension of Mobile IP that enables mobile hosts to use multiple care-of addresses simultaneously. The extension enhances network connectivity by enabling the mobile host, the home agent and correspondent hosts to evaluate and select the best connection. A proposed gateway architecture using M-MIP that integrates wired IP networks with ad hoc networks is described. The M-MIP and gateway architecture using the RVM and RNL metrics have been validated with simulation studies and results are presented.     

Link scheduling with power control for throughput enhancement in multihop wireless networks

Joint scheduling and power control schemes have previously been proposed to reduce power dissipation in wireless ad hoc networks. However, instead of power consumption, throughput is a more important performance concern for some emerging multihop wireless networks, such as wireless mesh networks. This paper examines joint link scheduling and power control with the objective of throughput improvement. The MAximum THroughput link Scheduling with Power Control (MATH-SPC) problem is first formulated and then a mixed integer linear programming (MILP) formulation is presented to provide optimal solutions. However, simply maximizing the throughput may lead to a severe bias on bandwidth allocation among links. To achieve a good tradeoff between throughput and fairness, a new parameter called the demand satisfaction factor (DSF) to characterize the fairness of bandwidth allocation and formulate the MAximum Throughput fAir link Scheduling with Power Control (MATA-SPC) problem is defined. An MILP formulation and an effective polynomial-time heuristic algorithm, namely, the serial linear programming rounding (SLPR) heuristic, to solve the MATA-SPC problem are also presented. Numerical results show that bandwidth can be fairly allocated among all links/flows by solving the MILP formulation or by using the heuristic algorithm at the cost of a minor reduction of network throughput. In addition, extensions to end-to-end throughput and fairness and multiradio wireless multihop networks are discussed.     

ATCP: TCP for mobile ad hoc networks

Transport connections set up in wireless ad hoc networks are plagued by problems such as high bit error rates, frequent route changes, and partitions. If we run the transmission control protocol (TCP) over such connections, the throughput of the connection is observed to be extremely poor because TCP treats lost or delayed acknowledgments as congestion. We present an approach where we implement a thin layer between Internet protocol and standard TCP that corrects these problems and maintains high end-to-end TCP throughput. We have implemented our protocol in FreeBSD, and we present results from extensive experimentation done in an ad hoc network. We show that our solution improves the TCP's throughput by a factor of 2-3     

Power-aware routing protocols in ad hoc wireless networks

An ad hoc wireless network has no fixed networking infrastructure. It consists of multiple, possibly mobile, nodes that maintain network connectivity through wireless communications. Such a network has practical applications in areas where it may not be economically practical or physically possible to provide a conventional networking infrastructure. The nodes in an ad hoc wireless network are typically powered by batteries with a limited energy supply. One of the most important and challenging issues in ad hoc wireless networks is how to conserve energy, maximizing the lifetime of its nodes and thus of the network itself. Since routing is an essential function in these networks, developing power-aware routing protocols for ad hoc wireless networks has been an intensive research area in recent years. As a result, many power-aware routing protocols have been proposed from a variety of perspectives. This article surveys the current state of power-aware routing protocols in ad hoc wireless networks.     

Optimal resource allocation in wireless ad hoc networks: a price-based approach

The shared-medium multihop nature of wireless ad hoc networks poses fundamental challenges to the design of effective resource allocation algorithms that are optimal with respect to resource utilization and fair across different network flows. None of the existing resource allocation algorithms in wireless ad hoc networks have realistically considered end-to-end flows spanning multiple hops. Moreover, strategies proposed in wireline networks are not applicable in the context of wireless ad hoc networks, due to their unique characteristics of location-dependent contention. In this paper, we propose a new price-based resource allocation framework in wireless ad hoc networks to achieve optimal resource utilization and fairness among competing end-to-end flows. We build our pricing framework on the notion of maximal cliques in wireless ad hoc networks, as compared to individual links in traditional wide-area wireline networks. Based on such a price-based theoretical framework, we present a two-tier iterative algorithm. Distributed across wireless nodes, the algorithm converges to a global network optimum with respect to resource allocations. We further improve the algorithm toward asynchronous network settings and prove its convergence. Extensive simulations under a variety of network environments have been conducted to validate our theoretical claims.     

Comparative analysis of link quality metrics and routing protocols for optimal route construction in wireless mesh networks

We provide a comparative analysis of various routing strategies that affect the end-to-end performance in wireless mesh networks. We first improve well-known link quality metrics and routing algorithms to enhance performance in wireless mesh environments. We then investigate the route optimality, i.e., whether the best end-to-end route with respect to a given link quality metric is established, and its impact on the network performance. Network topologies, number of concurrent flows, and interference types are varied in our evaluation and we find that a non-optimal route is often established because of the routing protocol’s misbehavior, inaccurate link metric design, interflow interference, and their interplay. Through extensive simulation analysis, we present insights on how to design wireless link metrics and routing algorithms to enhance the network capacity and provide reliable connectivity.     

Modeling throughput gain of network coding in multi-channel multi-radio wireless ad hoc networks

In this paper, we model the network throughput gains of two types of wireless network coding (NC) schemes, including the conventional NC and the analog NC schemes, over the traditional non-NC transmission scheduling schemes in multihop, multi-channel, and multi-radio wireless ad hoc networks. In particular, we first show that the network throughput gains of the conventional NC and analog NC are (2n)/(2n-1) and n/(n-1), respectively, for the n-way relay networks where n ges 2. Second, we propose an analytical framework for deriving the network throughput gain of the wireless NC schemes over general wireless network topologies. By solving the problem of maximizing the network throughput subject to the fairness requirements under our proposed framework, we quantitatively analyze the network throughput gains of these two types of wireless NC schemes for a variety of wireless ad hoc network topologies with different routing strategies. Finally, we develop a heuristic joint link scheduling, channel assignment, and routing algorithm that aims at approaching the optimal solution to the optimization problem under our proposed framework.     

Performance analysis of multicast flow control algorithms overcombined wired/wireless networks

A multicast flow control framework for data traffic traversing both a wired and wireless network is proposed. Markov-modulated fluid (MMF) models are used for the receivers to capture the dynamics of the wireless links. Our study shows that the phase differences of the instantaneous throughput capabilities of the receivers are a distinctive feature of multicast connections. The objectives of the multicast flow control algorithms are to cope with the receiver phase differences (RPDs) cost effectively in addition to the general goals such as maximizing throughput and minimizing delay. Three ad hoc algorithms have been studied: listen to slowest request (LSQ), source estimation (SE), and open-loop control. A fluid-flow analysis technique is applied to study the effect of receiver phase differences assuming zero propagation delay. The effect of propagation delay in multicast connections is then discussed. Simulation results are presented to verify the analysis for the zero-delay case and to compare the performance of the algorithms under nonnegligible delays. It turns out that the zero-delay case reveals the characteristics of the multicast algorithms and provides good performance bounds for the cases with nonnegligible propagation delays     

Guest Editorial: Geometry and Random Graphs for the Analysis and Design of Wireless Networks

Confronted with the difficulties of analyzing large wireless networks such as cellular, ad hoc, and sensor networks, researchers have realized that the mathematical techniques used need to incorporate and explicitly model the network geometry, which is crucial to their connectivity, capacity, and reliability. As a consequence, stochastic geometry and the theory of random geometric graphs have emerged as essential tools in the analysis and design of large wireless systems. In the last decade, these techniques have led to important results and insights into the fundamental limits of wireless networks and the coverage of sensor networks. Specifically, point process theory and percolation theory were instrumental in recent breakthroughs. The deployment of wireless networks will continually become more dense and ubiquitous as time progresses, and hence become increasingly interference-limited. Therefore, the importance of the geometry of the transmitters and receivers in the network will increase further and understanding its effects will be crucial to the design of future wireless communication systems.     

Performance of energy-aware and link-adaptive routing metrics for ultra wideband sensor networks

Wireless sensor networks are energy constrained since sensors operate with limited battery capacity. Thus, energy consumption is one of the most critical issues in the design of routing protocols. In addition, the link quality needs to be taken into account in the route decision for a wireless multihop network in order to efficiently exploit the inherent spatial diversity. In this paper we consider energy-aware and link-adaptive routing strategies for UWB (Ultra Wide Band) sensor networks. We utilize the ranging capabilities offered by UWB and employ adaptive modulation to take advantage of favorable link conditions. Different routing metrics are proposed based on the availability of sensor node’s location, link quality and next hop battery capacity information. These routing metrics integrate the measure of next hop remaining battery capacity with the throughput performance measures, Maximum Forward Progress (MFP) or Maximum Information Progress (MIP). The effectiveness of these metrics is evaluated in different simulation scenarios in terms of network throughput and lifetime for both random and grid sensor network topologies. Jinghao Xuis a D.Sc. candidate of the Department of Electrical and Computer Engineering at the George Washington University. He received his B.Sc. and M.S. degrees in telecommunication and electrical engineering from the Tianjin University, China, in 1993 and 1996, respectively. From 1996 to 2001, he was with Research Institute of Telecommunication Transmission of Ministry of Information Industry, Beijing, China, and China Wireless Telecommunication Standards group, where he was involved in the standardization of IMT-2000 communication systems for China. His research interests include performance evaluation and modeling of wireless ad hoc networks, Ultra Wide Band systems and multiuser detection techniques. Bojan M. Peric received the B.S. degree in electrical engineering from the University of Belgrade, Belgrade, Serbia and Montenegro in 2001. He is currently working toward the D.Sc. degree in electrical engineering at the George Washington University, Washington, DC. His research interests include wireless communications, with an emphasis on ad hoc networks. Branimir R. Vojcic is a professor in, and a past Chairman of, the Department of Electrical and Computer Engineering at the George Washington University. He has received his Dipl. Ing., M.Sc. and D.Sc. degrees from the University of Belgrade in Serbia and Montenegro in 1980, 1986 and 1989, respectively. His current research interests are in the areas of communication theory, performance evaluation and modeling mobile and wireless networks, mobile internet, code division multiple access, multiuser detection, adaptive antenna arrays, space-time coding and ad-hoc networks. He has also been an industry consultant and has published and lectured extensively in these areas. He co-authored the book: The cdma2000 System for Mobile Communications. Dr Vojcic received NSF CAREER Award in 1995. He is a Senior Member of IEEE, was an Associate Editor for IEEE Communications Letters and is presently an Associate Editor for Journal on Communications and Networks.     

A distributed Newton algorithm for network utility maximization in wireless ad hoc networks

The existing distributed Newton algorithm for network utility maximization cannot be directly applied into the wireless ad hoc networks, as it does not consider the wireless link capacity variation and transmission power consumption. Regarding these, a new joint network utility maximization problem is formulated to optimize the wireless ad hoc network resource utility. The distributed Newton algorithm is implemented to solve for the dual variables and Newton directions by using the local information at each session source and link, respectively. Furthermore, a new iterative method is proposed to improve the convergence rate of the existing matrix‐splitting method. Simulation results validate the efficiency and efficacy of the proposed distributed Newton algorithm for wireless ad hoc network utility maximization.