Cognitive Wireless Mesh Networks with Dynamic Spectrum Access

Wireless Mesh Networks (WMNs) are envisaged to extend Internet access and other networking services in personal, local, campus, and metropolitan areas. Mesh routers (MR) form the connectivity backbone while performing the dual tasks of packet forwarding as well as providing network access to the mesh clients. However, the performance of such networks is limited by traffic congestion, as only limited bandwidth is available for supporting the large number of nodes in close proximity. This problem can be alleviated by the cognitive radio paradigm that aims at devising spectrum sensing and management techniques, thereby allowing radios to intelligently locate and use frequencies other than those in the 2.4 GHz ISM band. These promising technologies are integrated in our proposed Cognitive Mesh NETwork (COMNET) algorithmic framework, thus realizing an intelligent frequency-shifting self-managed mesh network. The contribution of this paper is threefold: (1) A new approach for spectrum sensing is devised without any change to the working of existing de facto mesh protocols. (2) An analytical model is proposed that allows MRs to estimate the power in a given channel and location due to neighboring wireless LAN traffic, thus creating a virtual map in space and frequency domains. (3) These models are used to formulate the task of channel assignment within the mesh network as an optimization problem, which is solved in a decentralized manner. Our analytical models are validated through simulation study, and results reveal the benefits of load sharing by adopting unused frequencies for WMN traffic.     

分布式队列服务算法在无线网状网包调度中的应用

根据无线网状网的包调度特点,结合已有的差分队列服务算法和分布式贝尔曼-福特算 法,将有线网络中的差分队列服务算法改进为分布式队列服务算法（DQS）,使之实用于无 线网状网中多任务条件下实现系统的吞吐量最大化。仿真实验证明了DQS算法能有效地避免 传统多径传输中的按“类”或 “流”来进行调度的缺陷,有效地减少了数据包的端到端 延时和缓冲区需求,尤其是DQS算法的实际平均吞吐量性能有了很大的提高。     

Asymptotic Capacity of Infrastructure Wireless Mesh Networks

An infrastructure wireless mesh network (WMN) is a hierarchical network consisting of mesh clients, mesh routers and gateways. Mesh routers constitute a wireless mesh backbone, to which mesh clients are connected as a star topology, and gateways are chosen among mesh routers providing Internet access. In this paper, the throughput capacity of infrastructure WMNs is studied. For such a network with Nc randomly distributed mesh clients, Nr regularly placed mesh routers and Ng gateways, assuming that each mesh router can transmit at W bits/s, the per-client throughput capacity has been derived as a function of Nc , Nr , Ng and W . The result illustrates that, in order to achieve high capacity performance, the number of mesh routers and the number of gateways must be properly chosen. It also reveals that an infrastructure WMN can achieve the same asymptotic throughput capacity as that of a hybrid ad hoc network by choosing only a small number of mesh routers as gateways. This property makes WMNs a very promising solution for future wireless networking.     

The impact of multihop wireless channel on TCP performance

This paper studies TCP performance in a stationary multihop wireless network using IEEE 802.11 for channel access control. We first show that, given a specific network topology and flow patterns, there exists an optimal window size W* at which TCP achieves the highest throughput via maximum spatial reuse of the shared wireless channel. However, TCP grows its window size much larger than W* leading to throughput reduction. We then explain the TCP throughput decrease using our observations and analysis of the packet loss in an overloaded multihop wireless network. We find out that the network overload is typically first signified by packet drops due to wireless link-layer contention, rather than buffer overflow-induced losses observed in the wired Internet. As the offered load increases, the probability of packet drops due to link contention also increases, and eventually saturates. Unfortunately the link-layer drop probability is insufficient to keep the TCP window size around W'*. We model and analyze the link contention behavior, based on which we propose link RED that fine-tunes the link-layer packet dropping probability to stabilize the TCP window size around W*. We further devise adaptive pacing to better coordinate channel access along the packet forwarding path. Our simulations demonstrate 5 to 30 percent improvement of TCP throughput using the proposed two techniques.     

Transmission scheduling for wireless mesh network systems equipped with multiple directional antennas

All existing broadcast scheduling algorithms (BSAs) were designed for omnidirectional antenna (OA) packet radio (PR) nodes of a wireless mesh network (WMN). However, when WMN nodes are equipped with multiple directional antennas (DAs), a signal sent from a neighboring node may be received by more than one DA of the receiving node, and some nodes may receive signals from multiple DAs sent from the same node. When existing OA BSAs are used for scheduling transmission time-slots in time division multiple access (TDMA), due to the multiple DA signal detection phenomenon a weak performance in average time delay and channel utilization is obtained. Therefore, a novel transmission scheduling algorithm (TSA) for WMN nodes equipped with multiple DAs is proposed. Simulation results demonstrate that a significant performance gain can be obtained from using the proposed DA-TSA scheme.     

无线Mesh网络的新型建模方法研究

对无线Mesh网络准确建模是开展Mesh网络各项研究的基础,基于传统802.11协议模型来分析Mesh网络性能会导致较大的偏差。提出了一种适用于无线Mesh网络的新型模型用以计算网络饱和状态下性能参数的理论值,如丢包率、节点发送概率等,性能分析和仿真结果表明,该方法更能精确地评估无线Mesh网性能。同时在此基础上给出了路由判据ETX理论值的计算方法,该方法可用来预测ETX值而无需实测,从而有效的减少了测量误差和带宽消耗。     

无线Mesh网络路由协议研究

无线Mesh网络WMN（wireless mesh networks）是一种新型的无线网络,它融合了无线局域网（WLAN）和Ad Hoc网络的优势,成为宽带接入的一种有效手段。首先介绍了无线Mesh网的网络结构和特点．并在此基础上讨论了无线Mesh网络对路由协议的要求。无线Mesh网络的路由算法是Mesh领域的研究难点,通过分析比较4种针对WMN的路由协议,总结了现有的路由协议的优缺点,并对今后的研究方向做出了展望。     

TCP Throughput Enhancement over Wireless Mesh Networks

TCP is the predominant technology used on the Internet to support upper layer applications with reliable data transfer and congestion control services. Furthermore, it is expected that traditional TCP applications (e.g., Internet access) will continue to constitute the major traffic component during the initial deployment of wireless mesh networks. However, TCP is known for its poor throughput performance in wireless multihop transmission environments. For this article, we conducted simulations to examine the impact of two channel interference problems, the hidden terminal and exposed terminal, on TCP transmissions over wireless mesh networks. We also propose a multichannel assignment algorithm for constructing a wireless mesh network that satisfies the spatial channel reuse property and eliminates the hidden terminal problem. The simulation results demonstrate the effectiveness of the proposed approach in improving the performance of TCP in wireless multihop networks.     

Network/hardware cross-layer evaluation for ROHC and packet aggregation on wireless mesh networks

This paper proposes a profile-based network and hardware co-simulation method to investigate the overall performance and real-timing characteristics of a wireless mesh network (WMN) affected by hardware capabilities, speed and complexity. For the sophisticated algorithms to be assisted by a hardware realization, we adopt the RObust Header Compression (ROHC) and packet aggregation that provide high and reliable data transmission over unstable wireless links, which is proven in the preliminary works. To verify the hardware support needs to get the benefit of the two algorithms, we measure the ROHC processing time from Intel Pentium 4 and RouterBOARD, and identify the deterioration of sensor throughput and successful voice calls under various NS-2 simulation scenarios. The co-simulation method integrates the network level simulator, NS-2 and hardware level simulator, SystemC. In this approach, we first insert the modules of ROHC and packet aggregation algorithms into the network simulator hierarchy, and measure the packet arrival times. Then, the corresponding hardware architecture is designed by SystemC for profiling the hardware delay appeared in encoding and decoding packets. The hardware is suitably designed to reduce the complexity and make a sufficient speedup in the packet processing. Finally, the traced hardware delays are applied into the network level simulator to extract real-timing WMN behaviors changed by the hardware operations in each mesh router.     

Performance Optimization of Interference-Limited Multihop Networks

The performance of a multihop wireless network is typically affected by the interference caused by transmissions in the same network. In a statistical fading environment, the interference effects become harder to predict. Information sources in a multihop wireless network can improve throughput and delay performance of data streams by implementing interference-aware packet injection mechanisms. Forcing packets to wait at the head of queues and coordinating packet injections among different sources enable effective control of copacket interference. In this paper, throughput and delay performance in interference-limited multihop networks is analyzed. Using nonlinear probabilistic hopping models, waiting times which jointly optimize throughput and delay performances are derived. Optimal coordinated injection strategies are also investigated as functions of the number of information sources and their separations. The resulting analysis demonstrates the interaction of performance constraints and achievable capacity in a wireless multihop network.      

无线Mesh网络集中式信道分配算法设计

以集中式无线Mesh网络(WMN)为基础,分析和研究了传统多信道分配算法,并在此基础上提出了以节点优先级和分组为特点的多接口多信道分配算法(Channel Assignment based on Rank of Node and Link group,CAR-NL),该算法结合节点分级和链路负载预期评估机制,通过节点链路分组按级分配信道。通过仿真实验表明,该算法能有效提高无线Mesh网络多业务流并发执行时系统整体吞吐量,并实现较低的丢包率。     

A delay‐based transport layer mechanism for fair TCP throughput over 802.11 multihop wireless mesh networks

Cities worldwide have planned and implemented large‐scale wireless mesh network (WMN) deployments. These mesh deployments are expected to provide broadband mobile access to the Internet at a low cost to the user. This paper considers the fairness problem affecting nodes in multihop WMNs. Many existing approaches to coping with the fairness problem are unsuitable because they necessitate modifications to the IEEE 802.11 MAC protocol or node queueing operations. Such modifications require a change in hardware/firmware at every node in the WMN. Thus, these approaches are less favorable from the points of view of interoperability and cost. Without modifying the lower layer protocols, this study identifies TCP parameters that impact throughput fairness and proposes an adjustment to these parameters to reduce frame collisions and to improve throughput fairness. Using simple mathematical formulations and ns2 simulations, this study shows that the frame transmissions from each node can be effectively controlled by properly controlling the delayed ACK timer and by using a suitable advertised window. The proposed method, in addition to fairness, requires fewer buffer resources than other methods. Moreover, it is not sensitive to the carrier sense range. It is also simple and easy to deploy. Copyright © 2011 John Wiley & Sons, Ltd.     

Wireless Mesh Networks: Current Challenges and Future Directions of Web-In-The-Sky

Within the short span of a decade, Wi-Fi hotspots have revolutionized Internet service provisioning. With the increasing popularity and rising demand for more public Wi-Fi hotspots, network service providers are facing a daunting task. Wi-Fi hotspots typically require extensive wired infrastructure to access the backhaul network, which is often expensive and time consuming to provide in such situations. wireless mesh networks (WMNs) offer an easy and economical alternative for providing broadband wireless Internet connectivity and could be called the web-in-the-sky. In place of an underlying wired backbone, a WMN forms a wireless backhaul network, thus obviating the need for extensive cabling. They are based on multihop communication paradigms that dynamically form a connected network. However, multihop wireless communication is severely plagued by many limitations such as low throughput and limited capacity. In this article we point out key challenges that are impeding the rapid progress of this upcoming technology. We systematically examine each layer of the network and discuss the feasibility of some state-of-the-art technologies/protocols for adequately addressing these challenges. We also provide broader and deeper insight to many other issues that are of paramount importance for the successful deployment and wider acceptance of WMNs.     

Design and Performance Study for a Mobility Management Mechanism (WMM) Using Location Cache for Wireless Mesh Networks

Wireless mesh networks (WMNs) have emerged as one of the major technologies for 4G high-speed mobile networks. In a WMN, a mesh backhaul connects the WMN with the Internet, and mesh access points (MAPs) provide wireless network access service to mobile stations (MSs). The MAPs are stationary and connected through the wireless mesh links. Due to MS mobility in WMNs, mobility management (MM) is required to efficiently and correctly route the packets to MSs. We propose an MM mechanism named Wireless mesh Mobility Management (WMM). The WMM adopts the location cache approach, where mesh backhaul and MAPs (referred to as mesh nodes (MNs)) cache the MS's location information while routing the data for the MS. The MM is exercised when MNs route the packets. We implement the WMM and conduct an analytical model and simulation experiments to investigate the performance of WMM. We compare the signaling and routing cost between WMM and other existing MM protocols. Our study shows that WMM has light signaling overhead and low implementation cost.     

基于WMN变节节点多径检测的Petri网模型分析

针对WMN中出现的安全问题,提出了利用WMN多径特性的密钥协商模型,可以在防止中间人攻击的同时发现网络中的变节节点。采用Petri网理论对模型结构特性及安全性进行形式化分析,对发现的漏洞给出解决方案。通过引入新的变节节点的检测安全模型和算法,使WMN的安全性得到增强。     

无线网格网中一种基于树的先验式路由协议

Wireless Mesh Network (WMN) is seen as an effective Internet access solution for dynamic wireless applications . For the low mobility of mesh routers in WMN, the backbone topography can be effectively maintained by proactive routing protocol. Preproposals like Tree Based Routing (TBR) protocol and Root Driven Routing (RDR) protocol are so centralized that they make the gateway become a bottleneck which severely restricts the network performance. We proposed an Optimized Tree-based Routing (OTR) protocol that logically separated the proactive tree into pieces. Route is partly computed by the branches instead of root. We also discussed the operation of multiple Internet gateways which is a main issue in WMN. The new proposal lightens the load in root, reduces the overhead and improves the throughput. Numerical analysis and simulation results confirm that the performance of WMN is improved and OTR is more suitable for large scale WMN.     

Mesh IP Video Surveillance Systems Model Design and Performance Evaluation

This paper proposes a model for Internet Protocol (IP) mesh video surveillance systems and presents the performance evaluation of Ah-hoc On-demand Distance Vector (AODV) and Open Shortest Path First (OSPF) routing protocols in wireless mesh video surveillance system. A wireless mesh video surveillance network consists of IP cameras linked to mesh routers which are then linked to the mesh gateways. Local monitoring can be done by inserting a switch or router between the gateways and the Internet while remote monitoring can be done through the Internet. Routing provides selection, constructing and management of routes in order to maximize throughput and minimize video packet losses, end-to-end delays and Jitter. Results show that the OSPF routing protocol outperformed the AODV in throughput, packet loss, end-to- end delay and Jitter terms with a throughput advantage of 35%.     

Improving Delay and Jitter Performance in Wireless Mesh Networks for Mobile IPTV Services

Wireless mesh networking has recently emerged as a promising technology for the next-generation wireless networks. In wireless mesh networks (WMNs), it is practically attractive to support the low-cost quality-of-service (QoS) guaranteed mobile TV service. To meet this need, our study addresses how to improve the delay and jitter performance of mobile IPTV services over IEEE 802.11 based WMN. Particularly, we first discuss the adaptation of IEEE 802.11 MAC layer to construct a WMN with emphasis on mobile IPTV service; we then develop an enhanced version of Guaranteed-Rate (GR) packet scheduling algorithm, namely virtual reserved rate GR (VRR-GR), to further reduce the delay and suppress the jitter in multiservice network environment. Simulation results show that our proposed approach can satisfyingly prioritize mobile IPTV services in WMN, while providing non-IPTV services with what they need as well.      

无线mesh网络安全研究

无线mesh网络（wireless mesh networks,WMN）是下一代网络中的新型技术,和传统网络不同,它不依赖任何固定的设施,主机可以相互依赖保持网络连接,WISP可以利用它提供快速、简单、低廉的网络部署,然而存在一个主要的问题是容易遭受攻击。介绍了无线mesh网络的体系机构以及特点,分析并研究其存在的安全性威胁以及现有的关键安全解决机制。     

DCMA: A Label Switching MAC for Efficient Packet Forwarding in Multihop Wireless Networks

This paper addresses the problem of efficient packet forwarding in a multihop, wireless "mesh" network. We present an efficient interface contained forwarding (ICF) architecture for a "wireless router," i.e., a forwarding node with a single wireless network interface card (NIC) in a multihop wireless network that allows a packet to be forwarded entirely within the NIC of the forwarding node without requiring per-packet intervention by the node's CPU. To effectively forward packets in a pipelined fashion without incurring the 802.11-related overheads of multiple independent channel accesses, we specify a slightly modified version of the 802.11 MAC, called data driven cut-through multiple access (DCMA) that uses multiprotocol label switching (MPLS)-like labels in the control packets, in conjunction with a combined ACK/RTS packet, to reduce 802.11 channel access latencies. Our proposed technique can be used in combination with "frame bursting" as specified by the IEEE 802.11e standard to provide an end-to-end cut-through channel access. Using extensive simulations, we compare the performance of DCMA with 802.11 DCF MAC with respect to throughput and latency and suggest a suitable operating region to get maximum benefits using our mechanism as compared to 802.11