一种无线网络中有效的MAC层信道竞争方案

在研究IEEE802．11DCF的基础上．提出了一种简单有效的无线局域网MAC层冲突解决机制（PBCR）．主要通过均匀分布站点的退避计数器值来达到减少冲突的目的．建模的计算结果和仿真的统计结果都表明PBCR在性能上，包括吞吐量，公平因子以及发送延迟等方面比DCF有显著的提高．     

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。     

IEEE 802.11性能评估和改进

IEEE802．11基本的媒体接入控制(MAC)协议是DCF，DCF基于CSMA／CA方案,采用二进制指数回退算法避免冲突．采用Bianchi等提出的模型对DCF的性能进行了理论分析和仿真研究,提出了DCF性能的改进方案TDCF，计算结果和仿真表明，TDCF性能优于DCF．     

IEEE 802.11 MAC层接入机制探讨

介绍IEEE 802.11 MAC层完成的主要功能,详细讨论MAC层的基本接入机制DCF.介绍了另一个可选的接入机制PCF的工作原理.简迷了DCF机制存在的问题以及目前相关研究的进展情况.     

IEEE802．11中MAC子层DCF＆PCF的研究与仿真

无线局域网络（WLAN）以其无可比拟的接入优点迅速得到普及,其传输性能好坏也成为关注的焦点。现在无线局域网络在通信过程中通常选用802．11中MAC子层的DCF＆PCF两种工作模式进行,但二者在选择中其传输性能还存在一些差别．在对IEEE802．11中DCF＆PCF工作原理和特点进行分析研究之后,以网络负载、接入延时、尝试重传的包数和吞吐量为权衡指标,进行网络仿真并给出了仿真结果。这为进一步分析和选择无线局域网络接入模式打下了基础,并具有一定的现实意义．     

一种功率自适应控制方案在移动自组网的应用

介绍了移动自组网的特点和802．11 DCF传输方式,论述了自组网的动态节能机制,提出了一种通过交换控制信息来预测最小发射功率的功率自适应调整MAC协议。仿真实验证明了该方案能有效降低系统的能量消耗,提高了网络的利用率。     

IEEE802.11e中基于无确认策略的EDCA机制研究

本文分析了IEEE802．11中DCF和IEEE802．11e中EDCA两种信道接入机制，研究了无确认策略在两种机制下对语音等实时业务QoS的支持能力。通过opnet软件仿真得出，相比于DCF，基于无确认策略的EDCA机制提高了实时业务的QOS，对当前WLAN网络中实时业务的传输有着重要的指导意义。     

WLAN基于竞争的信道接入机制分析研究

在文中,首先介绍了IEEE802.11 MAC层采用的DCF接入机制,然后介绍了IEEE802.11e MAC层采用的EDCA机制。对比分析DCF和EDCA接入机制,指出优点和不足,提出了下一步的研究方向。     

IEEE 802.11e基于竞争的信道访问机制性能分析

在回顾了IEEE802．11MAC协议的基础上，讨论并分析了即将定稿的IEEE802．11e的基于竞争的信道访问工作机制下MAC协议对QoS的支持。     

WLAN安全分析与解决方法

IEEE802．11协议在MAC层引入基于WEP算法的安全机制,WEP通过在RC4密码系统中采用CRC-32循环冗余校验的方式实现完整性验证。该文先分析了这种机制的若干缺点,然后重点介绍了802．11i和WAPI,说明后两者是如何弥补上述机制不足之处的。     

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.     

Design and evaluation of a novel MAC layer handoff protocol for IEEE 802.11 wireless networks

In recent years, the IEEE 802.11 wireless network family has become one of the most important set of standards in the wireless communications industry. IEEE 802.11 compliant devices are inexpensive and easier to configure and deploy than other wireless technologies. In an IEEE 802.11 wireless network, wireless terminals can move freely. As a result, when the wireless terminal moves away from its current access point, it must switch to another access point to maintain the active connection. This is known as the MAC layer handoff process. MAC layer handoff latency should be minimized to support real-time applications and to provide mobile devices with seamless roaming in IEEE 802.11 wireless networks. This paper proposes a novel MAC layer handoff protocol over IEEE 802.11 wireless networks by introducing advertisement messages sent from other mobile nodes and from which wireless terminals are able to receive the information of access points in their neighborhood. A mobile node can try to associate with access points based on the prediction before starting the probe process. The experimental results demonstrate that our solution can reduce MAC layer handoff latency to meet the requirements of real-time applications.     

Revealing the problems with 802.11 medium access control protocol in multi-hop wireless ad hoc networks

The IEEE 802.11 medium access control (MAC) protocol is a standard for wireless LANs, it is also widely used in almost all test beds and simulations for the research in wireless mobile multi-hop ad hoc networks. However, this protocol was not designed for multi-hop networks. Although it can support some ad hoc network architecture, it is not intended to support the wireless mobile ad hoc network, in which multi-hop connectivity is one of the most prominent features. In this paper, we focus on the following question: can IEEE 802.11 MAC protocol function well in multi-hop networks? By presenting several serious problems encountered in transmission control protocol (TCP) connections in an IEEE 802.11 based multi-hop network, we show that the current TCP protocol does not work well above the current 802.11 MAC layer. The relevant problems include the TCP instability problem found in this kind of network, the severe unfairness problem, and the incompatibility problem. We illustrate that all these problems are rooted in the MAC layer. Furthermore, by revealing the in-depth cause of these problems, we conclude that the current version of this wireless LAN protocol does not function well in multi-hop ad hoc networks. We thus doubt whether the current WaveLAN based system is workable as a mobile multi-hop ad hoc test bed. All the results shown in this paper are based on NS2 simulations, and are compatible with the results from the OPNET simulations.     

A context-aware cross-layer broadcast model for ad hoc networks

The standard 802.11 medium access control (MAC) performs poorly for heavy broadcast traffic. We present our context-aware cross-layer (CACL) broadcast model as an alternative. The basic CACL model uses only contextual data available to the 802.11 MAC and so is usable by any routing protocol that uses the 802.11 MAC. CACL fits the total broadcasts in any two-hop neighborhood to wireless channel capacity. We compare collision rates for CACL and the 802.11 MAC and conclude that, for a wide range of network conditions, CACL offers superior single-hop transmission rates. We also present a geographically constrained extension to CACL, CACL-G and compare it against CACL in vehicular scenarios of varying node density. Our results show that CACL-G offers increasingly superior performance over the basic CACL model as node density increases.     

无线自组织网络中TCP稳定性的分析及改进

在无线自组织网络中,基于IEEE 802.11的TCP流存在严重的不稳定性,其原因与MAC协议、路由协议和TCP本身均有一定的关系,但最根本之处在于MAC协议的不公平性以及假的链路失效消息导致了不必要的耗时的路由发现过程.结合IEEE802.11的MAC协议和DSR路由协议,对这些原因进行了深入的理论分析和仿真实验,并提出了针对MAC协议和路由协议的改进算法.仿真结果证明,提出的改进算法不仅能基本上避免TCP流的不稳定性,还能够极大地提高TCP流的平均吞吐量.     

无线局域网IEED 802.11的业务质量机制策略探讨

本文介绍了IEEE802.11标准中所采纳的改进分布协调功能的分布式媒体访问控制方法,它可以为其具有业务质量(QoS)要求的应用提供带有优先权的媒体访问支持,通过与802.11标准的MAC层进行对比分析,指出了82.11标准对MAC层所做的改进.     

802．11MAC层切换过程的代数验证

802．11MAC层的切换过程是其协议的一个重要组成部分，它影响着网络的动态拓扑结构。利用π演算理论来进行分析，可以严格而规范地描述其切换过程。在本文中，首先建立了切换的π演算模型。然后通过推导，证明了模型的一致性。这将为802．11MAC层的研究提供一定的 参考和分析价值。     

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。     

A performance analysis of block ACK scheme for IEEE 802.11e networks

The demand for the IEEE 802.11 wireless local-area networks (WLANs) has been drastically increasing along with many emerging applications and services over WLAN. However, the IEEE 802.11 medium access control (MAC) is known to be limited in terms of its throughput performance due to the high MAC overhead, such as interframe spaces (IFS) or per-frame based acknowledgement (ACK) frame transmissions. The IEEE 802.11e MAC introduces the block ACK scheme for improving the system efficiency of the WLAN. Using the block ACK scheme can reduce the ACK transmission overhead by integrating multiple ACKs for a number of data frames into a bitmap that is contained in a block ACK frame, thus increasing the MAC efficiency.In this paper, we mathematically analyze the throughput and delay performance of the IEEE 802.11e block ACK scheme in an erroneous channel environment. Our extensive ns-2 simulation results validate the accuracy of our analytical model and they further demonstrate that the block ACK scheme enhances the MAC throughput performance at the cost of the resequencing delay at the receiving buffer.     

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。