支持优先级服务的IEEE802．11无线局域网性能分析

在无线局域网(WLAN)中实现优先级服务需要媒体接入控制(MAC)层的支持。IEEE802．11标准的MAC层基于分布式协调功能(DCF)，不支持优先级服务。通过改变MAC协议参数，可对服务进行区分，但目前的研完尚未揭示MAC层协议参数与各优先级服务质量的数量关系，故而无法得到可商用的支持优先级服务的解决方案。本文提出一种分析模型，对无线站点的回退过程进行建模，从理论上解决了IEEE802．11MAC层协议参数与服务质量的定量关系问题。基于OPNET的仿真实验验证了分析模型的正确性。     

无线局域网实时业务优先的调度算法

该文在无线局域网的MAC协议上,基于时分多址技术,提出了分离缓冲器调度控制算法,将业务分为实时性要求的和非实时性要求两类,并给实时性业务以高的优先级.仿真结果表明该算法满足了实时性业务的失效概率要求,一定程度上保证了网络的QoS.     

无线局域网接入技术介绍

移动计算网络的解决方案可以分为两种：广域方案和局域方案。广域方案主要是依靠无线蜂窝数据通信网和卫星通信网作为移动计算的物理网;而局域方案WLAN由于具有更高的传输速率和更低的通信成本．可作为有线局域网LAN的扩展和替代,而显得格外的引人注目。WLAN都以多路复用信道作为通信的基础。这样与采用点到点连接的网络相比,存在一个关键的技术问题：当信道的使用产生竞争时,如何分配信道的使用权。在WLAN中完成该功能的是数据链路控制层DLC的媒体访问控制MAC子层。近年来,IEEE正式将无线局域网802、11协议标准化,来实现媒体共享。本文主要介绍了无线局域网接入技术并进行了比较。     

无线局域网实时业务优先的调度算法

该文在无线局域网的MAC协议上,基于时分多址技术,提出了分离缓冲器调度控制算法,将业务分为实时性要求的和非实时性要求两类,并给实时性业务以高的优先级。仿真结果表明该算法满足了实时性业务的失效概率要求,一定程度上保证了网络的QoS。     

IEEE802．11MAC协议中退避算法的研究

无线信道固有的特点,使得无线局域网面临比有线局域网更多的困难。发展合适的介质访问控制协议（MAC）是无线局域网一个关键,而其中退避算法又是MAC协议考虑的重点。本文介绍了退避机制在IEEE802．11MAC层中的作用．指出了二进制指数退避算法存在的不足。对当前已提出的多种典型退避算法进行了介绍,对各类算法的设计思想进行了分类。最后阐明了退避算法研究的发展趋势。     

基于IEEE 802.11bWLAN安全技术研究

随着无线局域网(WLAN)技术的迅速发展,其安全问题日益受到人们的关注.由于传输的特殊性,WLAN始终面临严峻的安全考验.无线网络攻击技术的不断翻新,基于IEE 802.11b的攻击技术会越来越多,复杂性将越来越高.所以与有线网路相比,保护WLAN安全的难度要远大于保护有线网络.文中介绍了基于IEEE 802.11b协议下的WLAN可能面临的非法登录、拒接服务、字典攻击等安全威胁,分析了如何提高WLAN下的几种安全机制及802.11协议下的802.11i安全标准.提出了现有无线设备使用中一些可提高WLAN安全性的解决措施,在一定程度上提高了WLAN的安全性.     

无线局域网MAC协议研究及实时业务QoS实现

随着无线网络的迅速发展,网络上的业务流量与日俱增,如何保证无线局域网实时业务服务质量的问题也越来越突出。本文介绍了提供QoS保障的IEEE802.11e协议,着重研究了一种基于标准PCF的增强QoS保证的媒体接入机制——动态PCF,讨论了它在802.11WLAN中支持IP话音及一般非实时数据业务的QoS控制流程。     

一种适应信道的WiMax分级调度架构

针对无线信道中与时间和位置相关性错误,简要介绍了IEEE80216d协议的QoS服务模型,在对WiMax的QoS机制和调度策略进行了深入的研究后,提出了一种新的MAC层分级分组调度架构。此架构主要由调度控制器、流量监控和两级分组调度组成,并给出了关键调度的算法模型。对算法的仿真表明,这个调度架构能够有效地区分对待实时业务、非实时业务和BE业务,满足不同类型业务的QoS需求,解决了无线信道特殊性带来的调度问题。     

基于MAC帧分类匹配的WLAN入侵检测

在研究WLAN入侵检测技术的基础上,给出一种基于数据链路层的无线局域网入侵检测方法。该方法使用协议分析技术,采用MAC帧分类的方法匹配入侵特征,实现对WLAN的入侵检测。     

无线局域网中的认证机制

当前无线局域网（WLAN）主要的认证技术包括了IEEE802．11中的开放系统认证和共享密钥认证,IEEE802．11i中的802．1x认证协议,以及我国的无线局域网安全标准WAPI中的WAI认证机制。概述了以上各种认证机制,讨论了它们各自的优缺点,并根据其存在的安全漏洞,给出了两种改进方案。最后给出了WLAN认证机制的进一步研究方向。     

QoS guaranteed integration methodology for a WLAN-WiMAX heterogeneous network

Wireless technologies which serve multimedia applications have to guarantee the Quality of Service (QoS) demands along with seamless connectivity. The complexity in attaining such demands increases if the network is heterogeneous in nature. This paper proposes a QoS guaranteed integration methodology for a heterogeneous wireless local area network (WLAN) - worldwide interoperability for microwave access (WiMAX) network. An integrated protocol stack for the mobile terminal which could work in a WLAN-WiMAX heterogeneous network is proposed. The proposed integrated protocol stack, a blend of two modules namely generic virtual link layer (GVLL) and media independent handover (MIH) is placed above the media access control (MAC) layer which includes both WLAN and WiMAX MAC. The impact of GVLL in guaranteeing the QoS on QoS deterioration and the impact of MIH in attaining seamless handover are analyzed and the results are presented.     

Differentiation, QoS guarantee, and optimization for real-time traffic over one-hop ad hoc networks

Nodes having a self-centrically broadcasting nature of communication form a wireless ad hoc network. Many issues are involved to provide quality of service (QoS) for ad hoc networks, including routing, medium access, resource reservation, mobility management, etc. Previous work mostly focuses on QoS routing with an assumption that the medium access control (MAC) layer can support QoS very well. However, contention-based MAC protocols are adopted in most ad hoc networks since there is no centralized control. QoS support in contention-based MAC layer is a very challenging issue. Carefully designed distributed medium access techniques must be used as foundations for most ad hoc networks. In this paper, we study and enhance distributed medium access techniques for real-time transmissions in the IEEE 802.11 single-hop ad hoc wireless networks. In the IEEE 802.11 MAC, error control adopts positive acknowledgement and retransmission to improve transmission reliability in the wireless medium (WM). However, for real-time multimedia traffic with sensitive delay requirements, retransmitted frames may be too late to be useful due to the fact that the delay of competing the WM is unpredictable. In this paper, we address several MAC issues and QoS issues for delay-sensitive real-time traffic. First, a priority scheme is proposed to differentiate the delay sensitive real-time traffic from the best-effort traffic. In the proposed priority scheme, retransmission is not used for the real-time traffic, and a smaller backoff window size is adopted. Second, we propose several schemes to guarantee QoS requirements. The first scheme is to guarantee frame-dropping probability for the real-time traffic. The second scheme is to guarantee throughput and delay. The last scheme is to guarantee throughput, delay, and frame-dropping probability simultaneously. Finally, we propose adaptive window backoff schemes to optimize throughput with and without QoS constraints.     

Admission control scheme based on priority access for wireless LANs

In order to support the quality of service (QoS) requirements at the medium access control (MAC) layer, the enhanced distributed channel access (EDCA) has been developed in IEEE 802.11e standard. However, it cannot guarantee the stringent real-time constraints of multimedia applications such as voice and video without an efficient method of controlling network loads. In this paper, we propose a measurement-based admission control scheme, which is made up of two parts: priority access and admission control. First, in order to measure the channel status per traffic type, we propose a priority access mechanism in which each priority traffic is distinguished by a busy tone, and separately performs its own packet transmission operation. Then, admission control mechanism protects existing flows from new ones, and maintains the QoS of the admitted flows based on the measured channel status information. Performance of the proposed scheme is evaluated by simulation. Our results show that the proposed scheme is very effective in guaranteeing the QoS of multimedia applications as well as in avoiding the performance starvation of low priority traffics.     

无线网状网的QoS研究

作为下一代无线通信网络的关键技术,无线网状网能够融合异构网络,满足多类型的业务需求,因此必须提供一定的服务质量(QoS)保证.对目前各种QoS体系结构进行了分析,讨论了无线网状网的QoS体系结构.针对无线网状网网络层以下各层的QoS问题,对近年来国内外在功率控制、无线环境感知、支持QoS的MAC协议、QoS路由以及跨层QoS设计等方向所取得的研究成果进行了全面的概括总结和比较分析.最后对未来的研究发展趋势提出了自己的观点.     

VoWLAN系统的MAC层改进机制

IEEE 802.11b的DCF接入方式是为无线数据业务提供接入而设计的,不能有效地支持诸如语音之类的实时业务。提出了两种改进的MAC层机制来提高无线局域网上传输语音（VoWLAN）的系统容量和有效性。分析和仿真结果表明,改进机制显著提高了系统容量,并保证了传输延迟、抖动和平均丢包率等服务质量（QoS）要求。     

Channel time allocation scheme based on feedback information in IEEE 802.11e wireless LANs

The IEEE 802.11e standard defines a set of quality of service (QoS) enhancements for wireless local area network (WLAN) applications such as voice and streaming multimedia traffic. In the standard, a new medium access control (MAC) protocol is called the hybrid coordinator function (HCF), and also a channel access scheme to transmit multimedia traffic is called the HCF controlled channel access (HCCA). In the IEEE 802.11e WLANs, to satisfy the stringent real-time constraints of multimedia services it is very important to provide an efficient method of allocating network resources. In this regard, we propose a feedback-assisted dynamic channel time allocation scheme considering the application layer information in order to achieve better performance of multimedia traffic over IEEE 802.11e HCCA under the constrained QoS requirements. Performance of the proposed scheme is investigated by simulation. Our results show that under typical channel error conditions, the proposed scheme is very effective regardless of the variation of station numbers and service intervals. Also, it yields high performances while guaranteeing the delay bound.     

Tuning the EDCA parameters in WLANs with heterogeneous traffic: A flow-level analysis

The need to support multimedia services in WLANs has motivated the research on traffic differentiation mechanisms at the MAC layer. The most common approach is the tuning of the different MAC parameters of the heterogeneous traffic profiles in order to provide different channel access probabilities. The benefits of these mechanisms in terms of throughput and transmission delay, as well as their traffic differentiation and QoS capabilities, have been thoroughly studied. However, there are very few results on how the tuning of the MAC parameters impacts on the flow-level metrics, such as blocking probability or average flow duration. In this article, several EDCA-based tuning algorithms have been evaluated by comparing their flow-level response in presence of rigid (e.g. VoIP) and elastic (e.g. P2P) flows. Results show that those algorithms which adapt better to the changing WLAN state (number and type of active flows), and that are designed under multiple objectives, provide significantly higher performance and QoS than static and single-objective configurations.     

MPLS VPN技术在WLAN接入分组域中的应用与研究

以WLAN为代表的无线局域网技术,已成为蜂窝网络数据流量的有效分流手段.通过WLAN访问运营商的分组域业务以成为研究的热点.MPLS VPN为利用多协议标签交换技术组建的虚拟专用网,管理维护简单,安全性高且支持QoS和流量工程.负载均衡是MPLS流量工程的一项重要应用,本文通过MPLS VPN在WLAN接入分组域组网中的应用,给出了VPN的组网方式,同时提出了基于多LSP的网络优化方案,建立了相应的优化模型并研究了在实际网络中流量分配的方式.     

一种新的基于HCCA的流量调度算法

IEEE802.11e中MAC协议传送方式采用了混合协调功能(HCF),其中混合式协调控制信道访问(HCCA)是基于集中轮询方式接入业务,它提供了参数化的QoS接入,着重讨论变比特率数据流的接入控制,提出了一种新的根据网络的实际流量、动态调整服务间隔和传输机会,来灵活地分配网络资源的算法.仿真结果表明,该算法能有效提高网络的吞吐量和降低丢包率,大大提高系统性能.     

Network selection in a WiMAX–WiFi environment

The tremendous growth of wireless technologies has introduced the potential of continuous service adaptation to the users’ needs by giving them the ability to be able to select and access the proper network based on different criteria. Moreover, next generation wireless networks have been designed to provide support for multimedia services, with different traffic characteristics and different Quality of Service (QoS) guarantees. However, the expansion of Wireless Local Area Networks (WLANs) and Worldwide Interoperability for Microwave Access (WiMAX) networks poses new research era in the decision of the access network selection. In this paper, the existing access network selection schemes are classified into three categories: the network-centric, the user-centric and the collaborative schemes, and are analyzed respectively. Moreover, we propose a multicriteria access network selection algorithm applied in a WiMAX–WLAN environment, in order to facilitate the provision of high quality services and at the same time to satisfy different types of user Service Level Agreements (SLAs).