RTS/CTS对802.11e在多跳无线网络中的影响

IEEE802.11e的增强分布式信道接入机制(EDCA)对业务的服务质量提供支持,由于其增强高优先级信道竞争能力而提供服务区分的主要特点而应用于Ad Hoc网络中.本文探讨了802.11e应用于多跳无线网络在不同的侦听范围下RTS/CTS有效性问题,指出:网络仿真工具NS2中对于节点传输范围和侦听范围的默认设置使得MAC层使用了RTS/CTS后网络性能变差.通过理论分析,并对不同侦听范围、跳数增加、TCP最大窗口的减小在有无RTS/CTS时的不同性能表现进行仿真分析,最终确定结论的正确性.     

支持QoS的IEEE 802.11 EDCA性能研究

随着无线网络应用的不断发展,为了适应网络中不同类型业务的区分服务需要,IEEE 802.11工作组在IEEE 802.11 DCF(distributed coordination function)的基础上推出了支持QoS的IEEE 802.11 EDCA协议.针对EDCA(enhanced distributed channel access)协议的优先级区分信道接入特性,提出了一种基于Markov链的协议性能模型分析方法.与已有文献不同,该模型分析同时包含了EDCA主要的3个关键区分信道接入机制:Wmin/Wmax,AIFS(arbitration inter-frame space),TXOP(transmission opportunity).通过模型分析,获得了EDCA协议各优先级接入的传输吞吐量、信道接入延迟、数据丢失率等性能分析.不仅分析了饱和情形下的EDCA性能,而且还对非饱和情形下的EDCA性能进行了分析.仿真验证结果表明,模型分析结果和仿真结果取得了很好的一致性.利用给出的模型分析方法,提出了一种基于TXOP动态调整的D-TXOP(dynamic TXOP)接纳控制算法.算法根据网络中业务流的QoS要求,在动态调整各优先级的TXOP参数设置的基础上,对网络中新到业务流进行接纳控制,达到提高网络的业务承载能力的目的.     

基于802.11e EDCA的无线网络传输性能仿真与分析

首先从理论上对比分析了IEEE 802.11 DCF和IEEE 802.11e EDCA这两种MAC接入机制;然后运用NS2网络模拟软件,仿真网络中视频、FTP和背景流在不同MAC协议网络中的性能表现,证实了IEEE 802.11e EDCA能提供QoS支持,提升无线网络传输性能。     

基于OMNeT++的Ad Hoc网络跨层协议栈仿真

针对OMNeT++的MFW组件中物理层和MAC层模块的错误进行修正和功能改进,实现基于RBAB的IEEE 802.11b协议速率自适应数据传输,建立移动Ad Hoc网络跨层协议栈模型的网络仿真框架,实现将Ad Hoc网络MAC层的速率自适应机制与其路由协议联合优化设计的仿真。结果表明,在移动环境下,采用跨层设计的网络协议栈能大幅提高Ad Hoc网络系统的性能。     

AdHoc网络中考虑事件紧急度的跨层QoS保障机制

考虑Ad Hoc网络中事件紧急度,提出一种可以有效改善整体高优先级业务流时延和时延抖动性能的跨层QoS服务质量保障算法.根据时延要求及业务分组距离目的点的跳数信息,评估此业务的紧急度,并在调度传输时充分考虑各个事件的紧急度.在有实时要求的业务发生超时的情况下,通过合适的路由置换方法,改善业务流时延及网络吞吐量.本文重点讨论如何在IEEE802.11e协议基础上实现对指定优先级业务流的服务质量保障.GloMoSim仿真环境下的仿真结果显示该算法与原有IEEE802.11e协议相比,对于高优先级业务,在分组端到端时延、时延抖动以及网络吞吐量方面有较大的改善.     

一种具有业务感知的多路径QoS路由策略

在基于IEEE 802.11e协议的无线Mesh网络多路径数据传输过程中,为了有效区分不同类型的多媒体数据,802.11e协议的EDCA机制采用了4个具有固定信道竞争参数的AC队列来实现不同业务类型数据包的存储转发,这种参数预设的竞争机制在多跳环境下不能提供有效的业务区分与QoS性能保障.同时,传统的多路径路由大多因为采用了最小跳数为其路由度量,未能反映实时的链路质量和节点状态,因而无法为不同类型的业务流提供其所需的高质量传输路径.为此,文中提出一种基于具有业务感知的多路径QoS路由策略TA2 P(Traffic-Aware AOMDV Protocol),该策略对静态EDCA信道竞争参数分配机制进行了改进,并提出了适用于不同业务类型数据传输的路由判据,以便TA2P能及时地根据MAC层反馈来的统计数据进行高质量路由的选择.仿真结果表明,该策略有效地实现了不同类型业务流之间对信道及无线链路的公平使用.     

AIFS 区分的IEEE 802.11e EDCA 吞吐率分析模型与自适应MAC 算法

针对IEEE 802.11e EDCA(enhanced distributed channel access)支持业务区分服务的特点,提出了一个基于AIFS(arbitration inter-frame space)区分的信道吞吐率分析模型,该模型将不同接入等级的业务统一到一个信道模型中进行分析.通过数值计算结果与仿真实验结果的对比,验证了该模型的准确性,尤其是在分析信道吞吐率方面更优于Xiao 的Markov 链模型.根据提出的分析模型,研究了近似优化条件,使得各类优先级业务的发送概率平衡虚拟发 送时间段中空闲时间与冲突持续时间对系统性能的影响,实现靠近最优的信道吞吐率,从而使计算复杂度大为减小. 利用数值分析的方法验证了近似优化条件实现靠近最优信道吞吐率的可行性.最后,提出的DPS(dynamicparameter-tuning scheme)算法根据负载情况自适应地调整不同级别业务的相应参数,使得系统时时满足优化条件,在各种场景下都能实现最大信道吞吐率,同时又满足EDCA 支持QoS 区分的要求.仿真结果验证了DPS 算法不仅能够根据竞争节点的数目变化对信道吞吐率进行优化,而且其性能也明显优于标准的IEEE 802.11e EDCA 机制.     

基于CRAHN网络的MAC层竞争时段帧突发机制的研究

近年来,关于无线电Ad Hoc网络(Cognitive Radio Ad Hoc Networks,CRAHN)接入的研究较多。该文研究了基于竞争型(CP)和时隙分配型(CFP)两种机制混合的混合型MAC层接入机制,在EDCA机制的前提下,给出了在CP时段的帧突发机制(Frame Bursting EDCA,F-EDCA),通过仿真验证了所提基于帧突发机制的F-EDCA算法的优越性。该算法可提高系统性能,尤其是音频业务的Qo S性能。     

动态时延的非均匀Ad Hoc网络MAC设计和分析

针对密度非均匀Ad Hoc网络,提出了一种基于预留的时隙混合类MAC——RTV协议。该协议将业务区分为预留和非预留,以提供不同质量的接入传输服务。通过时延调整预留算法来满足多种预留业务的不同时延要求。同时,通过预留机制解决了 TDMA 技术不适应于密度非均匀网络的问题。最后,通过数学建模分析得到了协议的时隙利用率和系统吞吐量。仿真实验结果表明：在密度非均匀的大型Ad Hoc网络中,RTV协议可以为混合业务的传输提供时延保障,并在时隙利用率和吞吐量方面呈现出较好的性能。     

移动Ad hoc网络路由协议的研究

介绍了当前Ad Hoc(移动自组网络)的网络路由协议研究现状,对Ad Hoc网络路由协议分类,并就传统的按需距离矢量路由(Ad-hoc On-Demand Distance Vector routing,AODV)的特征、优点和缺点给出分析,提出了一个新的基于AODV的改进路由协议--N-AODV(Neighborhood Routing Protocol based on AODV).采用NS-2系统仿真工具搭建了移动Ad Hoc网的仿真平台,在此平台的基础上,实现了N-AODV路由算法.通过仿真比较传统的AODV路由算法和改进的AODV路由算法的网络性能,验证了N-AODV算法的可行性,为实际系统的设计和规划提供了参考算法、参考模型和参考数据.最后,提出了一个基于Ad Hoc网络应用的系统架构.     

A new per-class flow fixed proportional differentiated service for multi-service wireless LAN

In this paper, we propose a new per-CLAss Flow fixed proportional differentiated service model (CLAF) and a companioning medium access control scheme for multi-service wireless LANs (WLANs), based on the IEEE 802.11 framework. Different from conventional relative differentiated service, in CLAF, a fixed bandwidth quality ratio is guaranteed on per-class per-flow basis regardless of the traffic load of each service class. Specifically, each service class is assigned a number of separate coordination periods, proportional to the policy-based bandwidth quality ratio for class isolation. Each class is associated with its own contention window size which is dynamically adjusted in accordance with the number of flows in the class in such a way to minimize collision probability between flows of the same class. Simulations results of the CLAF performance as well as a comparison with IEEE 802.11e EDCA including the support of QoS-sensitive VoIP applications are presented. The results show that the proposed scheme outperforms EDCA and achieves better resource utilization efficiency. CLAF can provide users a more predictive, affirmative service guarantees than conventional relative differentiated service like IEEE 802.11e EDCA.     

Throughput analysis of IEEE 802.11e EDCA under heterogeneous traffic

The amended version of IEEE 802.11e defines the enhanced distributed channel access (EDCA) for quality of service (QoS) connections. EDCA provides a priority scheme to classify different traffic categories by the arbitration inter-frame space (AIFS), and the initial and maximum contention window sizes. Most previous Markov chain analyses of EDCA have concentrated on the effect of contention window size, they neglected the effect of AIFS. In this paper, we consider of AIFS events among each back-off procedure and evaluate the saturation throughput of the IEEE 802.11e EDCA under heterogeneous traffic scenarios. The analytical model is based on the differentiated AIFS and uses the discrete time slot to analyses the external collision time. We take advantage of extensive simulation studies to validate the accuracy of the proposed model. Our models accurately estimate the effects of varied back-off window size and AIFS under heterogeneous traffic.     

Saturated throughput analysis of IEEE 802.11e EDCA

IEEE 802.11e standard has been published to introduce quality of service (QoS) support to the conventional IEEE 802.11 wireless local area network (WLAN). Enhanced distributed channel access (EDCA) is used as the fundamental access mechanism for the medium access control (MAC) layer in IEEE 802.11e. In this paper, a novel Markov chain based model with a simple architecture for EDCA performance analysis under the saturated traffic load is proposed. Compared with the existing analytical models of EDCA, the proposed model incorporates more features of EDCA into the analysis. Firstly, we analyze the effect of using different arbitration interframe spaces (AIFSs) on the performance of EDCA. That is, the time interval from the end of the busy channel can be classified into different contention zones based on the different AIFSs used by different sets of stations, and these different sets of stations may have different transmission probabilities in the same contention zone. Secondly, we analyze the possibility that a station’s backoff procedure may be suspended due to transmission from other stations. We consider that the contention zone specific transmission probability caused by the use of different AIFSs can affect the occurrence and the duration of the backoff suspension procedure. Based on the proposed model, saturated throughput of EDCA is analyzed. Simulation study is performed, which demonstrates that the proposed model has better accuracy than those in the literature.     

Extended EDCA for delay guarantees in wireless local area networks

Despite their very broad diffusion, IEEE 802.11 Wireless Local Area Networks (WLANs) are not able to provide service differentiation and to support real-time multimedia applications, due to their channel access methods. To overcome these limitations, the 802.11e working group has proposed the Enhanced Distributed Coordination Access (EDCA) scheme, which achieves service differentiation on a statistical basis by properly mapping user Quality of Service (QoS) requirements to channel contention parameters. Such a scheme will be included in the emerging 802.11n standard and in the revision of the 802.11 standard. However, it has been widely demonstrated that, especially at high network loads, EDCA does not provide an effective usage of the channel capacity. In particular, it is unable to provide a bounded delay service to all kinds of multimedia flow because flows with lower channel access priorities are starved to advantage only those with the highest priority. To fix this undesired behavior and improve wireless LAN performance, this paper proposes a new Extended EDCA (E²DCA) scheme, that is compliant with 802.11e specifications. By exploiting a closed-loop control algorithm, E²DCA performs a distributed dynamic bandwidth allocation, providing guarantees on average/absolute delays to real-time media flows, regardless of their priorities. Moreover, an innovative Call Admission Control (CAC) procedure has been developed. Using the ns-2 simulator, the effectiveness of the algorithm has been investigated in realistic network scenarios, involving a mix of audio, video, and FTP flows, at several network loads and with random losses. Results have shown that the proposed scheme is able to provide a bounded delay service to multimedia flows in a wide range of network loads and frame loss ratios.     

IEEE 802.11e EDCA机制的一种参数调节策略

研究了IEEE 802.11e EDCA机制的原理和无线信道利用模型,阐明了EDCA参数设置对其性能的重要性。在此基础上,提出了一种简单有效的参数调节策略。仿真结果表明,该参数调节机制在保证实时业务要求的同时,可以显著提高数据流的吞吐率,优化了EDCA机制的运行性能。     

A Novel Hybrid Slot Allocation Mechanism for 802.11e EDCA Protocol

This letter proposes a novel Hybrid Slot Allocation Mechanism (HSAM) to solve the high collision problem among four Access Categories (ACs) introduced by IEEE 802.11e EDCA protocol. In the HSAM, three time slots are assembled into a super-slot, each slot in the super slot is allocated to a particular AC according to its priority. The main idea of HSAM is to stagger the transmission slots of different ACs to reduce collisions, while maintaining the service differentiation. Extensive results from simulations have demonstrated good performance of HSAM comparing with EDCA protocol in terms of throughput, packet loss and MAC delay.     

基于节点区分的IEEE802.11e无线局域网中的VoIP容量分析与优化

基于因特网协议的语音(VoIP)是无线局域网(WLAN)的一个具有广阔前景的应用,但VoIP容量在WLAN中却受到了较大的限制.为了增强对VoIP等实时业务的支持,IEEE 802.11e EDCA协议提供了竞争窗口(CW)、仲裁帧间间隔(AIFS)和传输机会(TXOP)3个参数来实现对业务流的区分,但没有考虑如何优化这些参数以达到对WLAN中的接入点和终端节点进行区分服务,而WLAN中的接入点却恰恰是制约其VoIP容量的一个重要因素.提出了一个语音容量分析模型,引入了协议的3个参数以区分接入点和终端节点,并考虑到语音流的ON-OFF模型和语音流的上行与下行时延约束.基于给出的分析模型,分别进行了单参数独立区分与多参数联合区分优化分析,在不同的语音编码与语音间隔情况下,定量地给出了利用CW,AIFS和TXOP进行独立区分时和联合区分时的优化值,从而达到提高VoIP容量的目的.仿真结果表明,利用这些参数的优化值为接入点和终端节点提供区分服务是正确的和有效的.     

An analytic study of tuning systems parameters in IEEE 802.11e enhanced distributed channel access

In this paper, we derive, based on the analytical model developed by Cali et al., a multi-class model to study how to adaptively tune parameters in IEEE 802.11e EDCA and support service differentiation in WLANs. Through analytical modeling, we demonstrate that by assigning appropriate different attempt probabilities (or contention window sizes) to stations of different classes, it is feasible to provide (proportional) service differentiation and achieve pre-specified targeted throughput ratios among different classes, while at the same time, maximizing the total system capacity. We also extend the derived theoretical model to analyze the role of AIFS and TXOP values on service differentiation perceived by different traffic classes. We show that, to achieve QoS guarantees (i.e., throughput differentiation) and high channel utilization, it may not be desirable to allow tuning of multiple parameters (e.g., both the contention window sizes and the AIFS values). Instead, the design dimension should be kept small by turning only one set of parameters, while keeping the other two sets of parameters for all the access categories fixed (i.e., setting the AIFS values of all access categories to 2, which is equivalent to AIFS = DIFS).We also elaborate on how to incorporate our derived theoretical results into IEEE 802.11e. These include (i) how to reduce the computational complexity and practically calculate results on-line, (ii) how to convert the optimal parameters derived in the model that characterizes the p-persistent version of IEEE 802.11e to those in IEEE 802.11e (which is based on the notion of the contention window to determine whether or not to transmit in a slot), and (iii) how to on-line measure parameters needed for calculating the best value of the contention window size. Both the analytical models and the proposed approaches for practically incorporating theoretical findings into IEEE 802.11e EDCA are validated through detailed ns-2 simulations and empirical experimentation on a Linux-based MADWifi driver for wireless LAN devices with the Atheros chipset.