Analyzing the effective throughput in multi-hop IEEE 802.11n networks

In this paper we characterize the effective throughput for multi-hop paths in IEEE 802.11n based wireless mesh networks. We derive an analytical model capturing the effects of frame aggregation and block acknowledgements, features found in the new IEEE 802.11n standard. We describe the throughput at MAC layer as a function of physical data rate, error rate, aggregation level and path length. While being mathematically tractable, the proposed model is flexible enough to account for complex and realistic error characteristics of the wireless channel, such as long-term fluctuations and burstiness. We further show how to integrate the well-known Gilbert-Elliot channel model into our model and evaluate both models in our indoor wireless testbed.     

IEEE 802.11无线局域网标准研究*

介绍了IEEE 802.11全系列标准,研究了IEEE 802.11系列各标准的发展轨迹和相互关系,建立了该系列标准的层次模型。研究并分析了IEEE 802.11、IEEE 802.11a、IEEE 802.11b、IEEE 802.11g和IEEE 802.11n这几种常见标准,并对相应物理层和媒质访问控制层的关键技术作了重点分析。     

Enhancing multi-hop communication over multi-radio multi-channel wireless mesh networks: A cross-layer approach

The multi-channel multi-radio technology represents a straightforward approach to expand the capacity of wireless mesh networks (WMNs) in broadband wireless access scenarios. However, the effective leveraging of this technology in WMNs requires (i) enhanced MAC protocols, to coordinate the access to multiple channels with a limited number of radio interfaces, and (ii) efficient channel allocation schemes, to mitigate the impact of co-channel interference. The design of channel assignment schemes and MAC protocols is strictly interrelated, so that joint design should be considered to optimize the mesh network performance. In this paper, a channel assignment and fast MAC architecture (CAFMA) is proposed, which exploits the benefits provided by the multi-channel multi-radio technology to (i) enhance the performance of multi-hop communications, (ii) maximize the resource utilization, and (iii) support differentiation of traffic classes with different quality of service (QoS) requirements. CAFMA is designed with a cross-layer approach and includes (1) a novel MAC scheme, which provides multi-channel coordination and fast data relaying over multi-hop topologies, and (2) a distributed channel allocation scheme, which works in cooperation with the routing protocol. Simulation results confirm the effectiveness of CAFMA when compared with other single-layer and cross-layer solutions for multi-radio multi-channel WMNs.     

支持多速率自适应的IEEE 802.11网络性能分析

针对基于自动速率回退(ARF)算法的多速率IEEE 802.11网络,给出了ARF算法分析的一种离散时间Markov链模型.模型的建立,考虑了节点在发送过程中,通过Basic与RTS/CTS模式的选择,对是否因为信道传输误码导致的失败进行的区分.利用算法模型,分析了节点在以随机概率选择不同发送模式的方式下,其采用各个传输速率的稳态概率分布,并在此基础上结合协议退避机制的研究,给出了多速率网络系统吞吐量性能的计算方法.通过大量的仿真实验,验证了模型分析方法具有很好的准确性,并讨论了算法以及MAC协议参数设置对网络性能的影响.     

On the end-to-end flow allocation and channel assignment in multi-channel multi-radio wireless mesh networks with partially overlapped channels

The performance of wireless mesh networks (WMNs) can be improved significantly with the increase in number of channels and radios. Despite the availability of multiple channels in several of the current wireless standards, only a small fraction of them are non-overlapping and many channels are partially overlapped. In this paper, we formulate the joint channel assignment and flow allocation problem for multi-channel multi-radio WMNs as a Mixed Integer Linear Program (MILP). Unlike most of the previous studies, we consider the case when both non-overlapped and partially overlapped channels are being used. We consider an objective of maximizing aggregate end-to-end throughput and minimizing queueing delay in the network, instead of the sum of link capacities, since the traffic characteristics of a multi-hop WMN are quite different from a single hop wireless network. Our static channel assignment algorithm incorporates network traffic information, i.e., it is load aware. Our formulation takes into consideration several important network parameters such as the transmission power of each node, path loss information, the signal to interference plus noise ratio at a node, and the frequency response of the filters used in the transmitter and receiver. We show by simulations that our MILP formulation makes efficient use of the spectrum, by providing superior channel assignments and flow allocations with the addition of partially overlapped channels, without the use of any additional spectrum. We also justify the need to consider alternative objective functions such as, minimizing average queueing in the network. We also propose a polynomially bounded heuristic algorithm to scale the proposed algorithm to bigger network topologies.     

Route Maintenance in IEEE 802.11 wireless mesh networks

In this paper, we propose a novel Route Maintenance scheme for IEEE 802.11 wireless mesh networks. Despite lack of mobility and energy constraints, reactive routing protocols such as AODV and DSR suffer from frequent route breakages in 802.11 based infrastructure wireless mesh networks. In these networks, if any intermediate node fails to successfully transmit a packet to the next hop node after a certain number of retransmissions, the link layer reports a transmission problem to the network layer. Reactive routing protocols systematically consider this as a link breakage (and therefore a route breakage). Transmission failures can be caused by a number of factors e.g. interference or noise and can be transient in nature. Frequent route breakages result in significant performance degradation. The proposed mechanism considers multiple factors to differentiate between links with transient transmission problems from those links which have permanent transmission problems and takes a coherent decision on link breakage. The proposed mechanism is implemented in AODV for single-radio single-channel mesh network and an extension is incorporated in multi-radio multi-channel scenarios. Simulation results show substantial performance improvement compared to classical AODV and local route repair schemes.     

Packet delay analysis on IEEE 802.11 DCF under finite load traffic in multi-hop ad hoc networks

In this paper, the average packet delay on IEEE 802.11 DCF under finite load traffic in multi-hop ad hoc networks is analyzed. We employ a Markov chain model to analyze the probability of transmission at each node in an arbitrary slot and derive the channel access delay. We model each node using an M/G/1 queue and derive the queueing delay. The model is extended from analyzing the single-hop average packet delay to evaluating the end-to-end packet delay in multi-hop ad hoc networks without assuming the traffic to be in a saturation state. To validate our analytic results, we have done extensive simulation. The analytic and the simulation results match very well.     

Performance model for IEEE 802.11s wireless mesh network deployment design

This paper presents a performance model developed for the deployment design of IEEE 802.11s Wireless Mesh Networks (WMN). The model contains seven metrics to analyze the state of WMN, and novel mechanisms to use multiple evaluation criteria in WMN performance optimization. The model can be used with various optimization algorithms. In this work, two example algorithms for channel assignment and minimizing the number of mesh Access Points (APs) have been developed. A prototype has been implemented with Java, evaluated by optimizing a network topology with different criteria and verified with NS-2 simulations. According to the results, multirate operation, interference aware routing, and the use of multiple evaluation criteria are crucial in WMN deployment design. By channel assignment and removing useless APs, the capacity increase in the presented simulations was between 230% and 470% compared to a single channel configuration. At the same time, the coverage was kept high and the traffic distribution fair among the APs.     

IEEE 802.11安全架构的网络性能分析

在无线局域网中引入安全机制常造成性能下降,其中部分原因决定于对加密、认证、密钥更新和数字证书等安全选项的设置.IEEE 802.11i (WPA2)及其在新硬件平台上的实现已显著地改善了性能问题.本文对IEEE 802.11i进行性能分析,并将结论与WEP、WPA的研究进行比较.研究显示安全选项虽本质上有差异,但实践中对性能影响的差异却很小,甚至忽略不计.分析结论,为有性能要求的安全无线网络的设计提供参考.     

存在隐藏终端的IEEE802.11e多跳无线网络模型分析

首次将802.11e的接入机制放入多跳环境中进行仿真分析和定量研究。提出一种新的多跳环境下802.11e网络模型,结合M/G/1/K排队模型,定量分析在隐藏终端影响下802.11e网络的MAC层吞吐率、MAC时延和帧丢失率,研究802.11e在多跳环境下性能表现的内在原因。经过仿真实验结果与数值分析结果的对比,验证了分析模型的准确性;通过有(无)隐藏终端影响下MAC层性能的对比分析,指出了802.11e在多跳无线网络中支持QoS的局限性:受隐藏终端的影响,802.11e对不同接入等级的业务提供QoS区分的性能明显降级。因而有必要研究隐藏终端问题的解决方案,提高802.11e在多跳环境下的性能。     

IEEE802．11无线局域网非饱和吞吐量分析

当前关于IEEE802．11无线局域网DCF性能分析研究通常假设网络工作在饱和条件下，但是对于实际的IEEES02．11局域网而言，非饱和工作状态更为普遍。本文针对非饱和条件下的IEEE802．11无线局域网，提出了一种基于二维马尔可夫链的分析模型，推导得到吞吐量与网络非饱和程度之间的定量数学表达式。结果表明，非饱和吞吐量可以超过饱和吞吐量，在一定的非饱和程度下，网络吞吐量可以最大化，在多数情况下RTS／CTS接入机制优于基本接入机制。     

802.11无线局域网标准与安全

介绍了无线局域网标准IEEE 802．11安全机制,描述了数据加密和用户身份认证机制与过程,以及802．11标准的安全问题。讨论了旨在提高802．11安全性的802．1x协议以及动态密钥管理方法,提出了使用无线局域网应采取的行之有效的安全措施。     

On the nominal capacity of multi-radio multi-channel wireless mesh networks

Wireless mesh networking is an emerging technology with a wide range of applications, from community to metropolitan area networking. Mesh networks consist of wireless routers and access points that make up the network backbone which is connected to the wired infrastructure. Client devices are located at the edge. The capacity of a wireless mesh network is an important performance indicator as well as design parameter. It depends on various factors such as network size and topology, expected traffic patterns, number of interfaces per node, number of channels, routing and channel assignment etc. In this paper, we present an analytical framework for estimating the capacity of wireless mesh networks, based on the concept of collision domains. The capacity of grid-oriented mesh networks is investigated for various scenarios to study the impact of different network parameters.     

基于IEEE 802.11无线局域网的VoIP性能分析

对无线局域网的两种标准IEEE 802.11b和IEEE 802.11g的差异进行了研究,后者在前者的基础上采用了正交频分复用(OFDM)技术,通过设计一个针对这两种标准的无线局域网上的VoIP性能测试的试验,使用Chariot网络工具进行仿真,对延迟、抖动、丢包率进行对比分析,结果表明IEEE 802.11g标准比IEEE 802.11b标准具有明显的性能优势.     

An effective scheduling scheme for multi-hop multicast in wireless mesh networks

With the utilization of concurrent transmission strategy, a throughput-enhanced scheduling scheme is devised for multicast service in wireless multi-hop mesh networks. Since the performance of a multicast mechanism is constrained in a wireless setting due to the interference among local wireless transmissions, the interference relationships are first characterized by introducing a graph transformation method. Based on the graph transformation, the multicast scheduling problem is converted to the graph coloring problem, and then a capacity greedy algorithm is designed to provide concurrent transmission scheduling so that the demanded multicast transmission rate can be achieved. Moreover, the necessary and sufficient conditions of multicast schedulable feasibility are derived. Through corresponding simulations, it is shown that the proposed strategy can enhance the throughput of wireless multi-hop multicast systems significantly.     

IEEE 802.11DCF的不公平性及改进方法综述

首先介绍了几种关于公平性的定义,讨论了IEEE802.11标准中定义的分布式协调功能DCF产生不公平性现象的原因以及由此带来的不利后果;对当前已提出的各种主要的IEEE802.11公平性改进方法作了系统地介绍和评述,并对它们的特点进行了比较;最后,对公平性研究的发展趋势作了展望。     

IEEE 802.11 WLAN最佳负载长度的仿真研究

无线网络的高误码率严重影响IEEE802.11的吞吐率。利用NS-2仿真,研究IEEE802.11网络中,负载长度对信道吞吐率的影响。研究表明,位误码率大于的时候,负载的长度明显影响802.11的吞吐率;负载长度存在一个最佳范围(负载极值段),负载长度在极值段内变化时,吞吐率无明显变化,且接近极大值。根据负载极值段的范围选择负载长度,可以在高误码的情况下充分利用信道。极值段的长度和极值段的中值受位误码率、发送站点数、信道发送速率这3个因素的影响。     

无线IEEE 802.11网络多信道MAC性能建模与分析

IEEE 802.11动态频谱接入网络是当前协议和模型在无线网络研究中一直受到广泛的关注.虽然它的物理层支持多信道,但其MAC层对多信道的支持仍面临挑战.目前的多信道MAC研究大多基于仿真实验,缺乏性能分析模型.本文设计了一种简单通用的信道切换机制,将IEEE802.11MAC扩展成为一种多信道MAC.本文提出了三维马尔可夫链分析模型描述多信道MAC性能,刻画单信道内重传次数和多信道间切换对性能的影响,支持基本和RTS/CTS两种接入方式.仿真结果表明,该多信道MAC模型能够很好地预测系统的饱和吞吐量,系统性能随着重传次数的增加能得到提高,而切换信道数量的增加并不能总是带来性能的提高.     

基于IEEE 802.11e EDCA的优先级可调机制

针对IEEE 802.11e中不同优先级数据流所获得的服务相差很大、低优先级数据流获得服务量很少的问题,提出一个可以提高低优先级竞争能力的EDCA改进机制.低优先级数据流在重传m次后使其优先级增加1,并用马尔可夫链模型对此改进机制进行了吞吐量性能分析.通过数学分析和仿真结果的对比,验证了此改进机制可提高总体吞吐量性能.     

MAC protocols over wireless mesh networks: problems and perspective

The wireless mesh network (WMN) has been an emerging technology in recent years. Because the transmission medium used in networking backhaul access points (APs) is radio, the wireless mesh network is not only easy and cost effective in deployment, but also has good scalability in coverage area and capacity. This paper provides an overview of distributed medium access control (MAC) protocols, discusses their features and suitability for WMNs and identifies potential challenges and open research issues. Specifically, we overview the technical medium access control of the new standard for WMNs, the mesh deployments of the IEEE 802.16 MAC and we focus on the coordinated distributed scheduling (CDS) scheme of this. Moreover, we assess the fitness of the IEEE 802.11 over wireless mesh networks. Through the study of existing solutions, we analyze the previous work and sketch the contours of the directions to achieve the throughput and the latency that must be guaranteed in WMNs. In this context we suggest future enhancements to the standard that could increase throughput, while also increase the robustness, without complexity increases and propose and evaluate a new distributed scheduling scheme (DSS). We compared the CDS and the DSS and simulations studies document and confirm the positive characteristics of the proposed scheme. Furthermore, we proposed an analytical model to assess a lower bound in terms of delay.