Performance Improvement of Distributed Coordination Function in IEEE 802.11 Wireless LAN

This paper proposes a modification to distributed coordination function （DCF） to improve the channel utilization in IEEE 802.11 wireless local area networks （ WLANs）. In the modified DCF, when a station has contended for the channel, it may transmit multiple data frames continuously to the same destination, which is called transmission burst（TB）. When the maximum number of data packets transmitted continuously in a TB is set to be 2, the performance is expected to be the best. The theoretical analysis and simulation results show that compared with the standard DCF, the modified DCF can increase the throughput and decrease the delay of the WLAN, and the modification does not introduce any additional control overhead.

Performance Improvement of DCF Supporting Multi-Rate in IEEE 802.11b Wireless LAN

This paper proposes a new channel access algorithm based on channel occupancy time （COT） fairness to guarantee fairness and improve the aggregate throughput of 802.11b multi-rate WLANs. In the algorithm, the COT is used as fairness index to analyze the fairness of WLANs instead of the channel access probability （CAP） used in the distributed coordination function （DCF）. The standard COT is given by access point （AP） and broadcasted to all wireless stations. The AP and wireless stations in the WLAN can achieve COT-based fairness by adjusting their packet length, sending the multiple back-to-back packets at one time, or giving up an opportunity to access the channel. Analysis and simulations show that our algorithm can provide COT-fairness. Compared with the CAP-based algorithm, the proposed algorithm leads to improvements in aggregate throughput of IEEE 802. lib multi-rate WLANs.

FER Performance in the IEEE 802.11 a/g/n Wireless LAN over Fading Channel

This paper explores and compares FER (Frame Error Rate) of a MAC (Medium Access Control) layer in the IEEE 802.11 a/g/n wireless LAN. It is evaluated under the fading wireless channel, using theoretical analysis method. It is analyzed by using the number of stations with both variable payload size and mobile speed on the condition that fading margin and transmission probability are fixed. Especially, in the IEEE 802.11n, A-MSDU (MAC Service Data Unit Aggregation) scheme is considered and the number of subframe is used as the variable parameter. In the IEEE802.11a/g wireless LAN, fixed wireless channel is assumed to be Rayleigh fading channel. Mobile wireless channel is assumed to be flat fading Rayleigh channel with Jake spectrum. The channel is in fading states or inter-fading states by evaluating a certain threshold value of received signal power level. If and only if the whole frame is in inter-fading state, there is the successful frame transmission. If any part of frame is in fading duration, the frame is received in error.

WLAN新安全标准IEEE 802.11i的网络构架和关键技术解析

就新一代无线局域网安全标准IEEE 802.11i的核心技术部分进行了系统分析和概述,指出了此标准仍存在的一些问题并对其应用前景作出了预测.

两种提高IEEE802.11b WLAN物理层数据传输速率的编码技术(英文)

本文提出两种将IEEE802.11b WLAN物理层数据速率提高到16.5Mbps的编码和译码方法。这两个方案都继续采用原IEEE802.11b 中对于5.5和11Mbps速率所采用的CCK编码方法。两个方案的编码过程变化不大,第一方案的译码还是采用5.5和11Mbps数据率所使用的快速WALSH变换(FWT),第二个方案的译码被修改成适合于16.5Mbps数据率的快速WALSH变换(FWT)。最后通过仿真比较了这两个方案在无多径干扰的AWGN信道上的性能,结果表明第二个方案优于第一个。

Influence of Error-prone Channel on the Performance of IEEE 802.11 WLAN Utilizing RTS/CTS Handshaking

A comprehensive study was presented for WLAN 802.11b using error-prone channel. It was theoretically and numerically evaluated the performance of three different network sizes with the bit rates that available in 802.11b protocol. Results show that throughput does not change with the size of the network for wide range of bit error rates (BERs) and the channel bit rates play a significant role in the main characteristics of the network. A comprehensive explanation has given for the phenomenon of the packet delay suppression at relatively high level of BERs in view of the size of the networks and the BERs. The effect length of the transmitting packets is also investigated.

IEEE 802.11s Wireless Mesh Networks for Last-Mile Internet Access: An Open-Source Real-World Indoor Testbed Implementation

Due to their easy-to-deploy and self-healing features, WMNs (Wireless Mesh Networks) are emerging as a new promising technology with a rich set of applications. While the IEEE standardization of this new technology is still in progress, its main traits are already set, e.g., architecture and MAC routing. WMNs are attracting considerable research in academia and industry as well, but the lack of open-source testbeds is restricting such a research to simulation tools. The main problem with simulation tools is that they do not reflect the complexity of RF propagation, especially in indoor environments, of which IEEE 802.11s WMNs are an example. This paper presents an open-source implementation of an indoor IEEE 802.11s WMN testbed. The implementation is transparent, easy-to-deploy, and both the source code and deployment instructions are available online. The implementation can serve as a blueprint for the WMN research community to deploy their own testbeds, negating the shortcomings of using simulation tools. By delving into the testbed implementation subtleties, this paper is shedding further light on the details of the ongoing IEEE 802.11s standard. Major encountered implementation problems (e.g., clients association, Internetworking, and supporting multiple gateways) are identified and addressed. To ascertain the functionality of the testbed, both UDP and TCP traffic are supported and operational. The testbed uses the default IEEE 802.11s HWMP (Hybrid Wireless Mesh Protocol) routing protocol along with the default IEEE 802.11s Airtime routing metric.

A random adaptive method to adjust MAC parameters in IEEE802.11e WLAN

The IEEE 802.11e standard is proposed to provide QoS support in WLAN by providing prioritized differentiation of traffic. Since all the stations in the same priority access category (AC) have the same set of parameters, when the number of stations increases, the probability of different stations in the same AC choosing the same values will increase, which will result in collisions. Random adaptive MAC (medium access control) parameters scheme (RAMPS) is proposed, which uses random adaptive MAC differentiation parameters instead of the static ones used in the 802.11e standard. The performance of RAMPS is compared with that of enhanced distributed coordination access (EDCA) using NS2. The results show that RAMPS can reduce collision rate of the AC and improve the throughput by using adaptive random contention window size and inter-frame spacing values. RAMPS ensures that at any given time, several flows of the same priority have different MAC parameter values. By using the random offset for the inter-frame spacing value and the backoff time, RAMPS can provide intra-AC differentiation. The simulation results show that RAMPS outperforms EDCA in terms of both throughput and end-to-end delay irrespective of the traffic load.

Achieving Efficient Channel Utilization for Data Communications in IEEE 802.11 WLAN

In the IEEE g02. 11 protocol, the adoption of the exponential backoff technique leads to throughput performance strongly dependent on the initial contention window size and, most importantly, on the number of contending stations considered in the network. This paper proposes a simple but accurate method to dynamically estimate the number of contending stations in a wireless local area network （ WLAN ）. Based on estimation, all the mobile stations dynamically adjust the initial contention window in medium access control （ MAC ） layer to avoid collisions. The simulation results show that the proposed algorithm can achieve efficient channel utilization, higher system throughput, and better fairness performance.

基于IEEE 802.11标准的无线局域网组网方案

叙述了无线局域网的概念、基本原理及其网络标准的演变 ,并就目前无线局域网的研究热点—基于IEEE 80 2 .1

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。

非饱和状态下IEEE 802.11 DCF的性能分析

在非饱和状态下,针对具有多业务支持能力的IEEE 802.11分布式协调功能,提出了一种简单有效的性能分析模型.该模型中引入了“虚拟时隙”以及“虚拟时隙状态”的概念,近似地将无线终端的行为在时间上离散化.以此为基础用二维Markov链对系统的性能进行了建模,利用该模型得到了平均分组传输延迟的近似闭合解,并提出了控制所有通信业务的平均分组传输延迟方法,有助于进一步研究如何对系统性能进行预测与优化.

基于IEEE 802.11的长距离无线Mesh网络

基于IEEE 802.11的长距离无线Mesh网络(LDmesh)的单跳链路长度在几十到上百公里,具有带宽高、成本低、覆盖广等优点,可广泛应用于偏远乡村或人口分布稀疏地区的无线宽带接入等.LDmesh网络是点到点的链路,链路的干扰特性和传输特性与传统无线Mesh网络有显著不同,原有的基于CSMA的MAC协议不再适用,进而影响到上层协议的设计.分析了LDmesh网络的研究进展,从链路性能、MAC协议、路由协议及网络管理等方面深入论述了LDmesh网络面临的挑战和进一步研究方向.

IEEE 802.11 DCF协议性能分析模型

提出了一种改进的二维Markov链模型用于分析IEEE 802.11 DCF协议.该模型不仅考虑实际应用中饱和业务量和非饱和业务量的情况,而且考虑DCF协议的退避计时器的冻结状态和有限的重传次数.通过对该Markov链模型稳定状态的求解并结合M/M/1/K排队模型,导出了DCF系统性能的理论模型.仿真结果表明,该理论模型能够精确地预测广义IEEE 802.11 DCF协议的系统性能.

IEEE802.11无线局域网OPNET建模与性能测试

讨论了IEEE 802.11无线局域网的建模和性能问题。基于对IEEE 802.11协议和目前功能最强大的网络仿真工具OPNET[1]软件的研究,首先分析了无线局域网的各种行为,分别对这些行为单独建模后,通过有线状态机将它们集成而形成IEEE 802.11协议支持模块。给出了模型输入输出接口的详细说明,并对网络的性能进行了分析比较。

IEEE 802.11无线网络的两步指数退避算法

IEEE 802.11标准引入二进制指数退避(Binary Exponential Backoff,BEB)算法以降低节点发送数据包碰撞的概率。然而,BEB存在着不足之处,当数据包碰撞概率较大时,节点的竞争窗口长度会出现振荡,即节点每次发送数据包之前,需要多次加倍扩大竞争窗口长度,而在发送成功之后又把竞争窗口长度缩小到最小值,这个过程反复出现。为了克服竞争窗口振荡问题以增加吞吐量,该文提出两步指数退避(Two-step BEB,TBEB)算法,利用2维马尔可夫链进行建模,导出TBEB算法中节点的退避状态概率分布、平均竞争窗口长度、平均退避次数、每发送一个数据帧所耗时间以及吞吐量等指标,并通过仿真进行验证。通过求解一个简单的优化问题,TBEB可以获得最优竞争窗口长度复位值,使吞吐量达到最优。

一种利用信道侦听的IEEE802.11自适应优化算法

提出一种适用于DCF(distributed coordination function)机制的自适应优化算法.该算法基于网络节点侦听信道得到的网络状态信息进行相关参数的自适应调整以获得最优的网络性能,称为CSB(channel sensing backoff)算法.算法采用了对节点的信道接入请求以概率参数P_T进行过滤的方法控制节点竞争接入信道的激烈程度.不同于已有的DCF机制优化方法,CSB算法的特点在于,在优化调整过程中不需要进行计算复杂的网络节点数量估计,并且可以在不同网络状态下始终围绕确定的优化目标进行参数优化调整.仿真实验结果表明,算法能够针对网络节点数量和分组大小改变等网络状态变化作出自适应的网络优化调整,并获得了系统吞吐量、碰撞概率、延迟、延迟抖动、公平性等多方面的性能改善.

一种提高IEEE 802.11吞吐量和公平性的自适应优化算法

该文提出了一种针对IEEE 802.11 DCF网络增强其吞吐量和公平性性能的自适应优化算法,算法基于网络节点侦听信道得到的网络状态信息进行竞争发送的自适应调整以获得最优的网络性能,称之为CSCC(Channel Sensing Contention Control)算法。算法采用了对节点的信道接入请求以概率参数P_T进行过滤的方法控制节点竞争接入信道的激烈程度,其主要特点在于在优化调整过程中不需要进行计算复杂的网络节点数量估计,并且可以在不同网络状态下围绕始终确定的优化目标进行参数优化调整。仿真实验结果表明,算法能够适应不同节点数量和不同数据大小的网络进行自适应的网络优化调整,并获得了系统吞吐量、碰撞概率、延迟、延迟抖动、公平性等多方面的性能改善。

基于效用最大化的IEEE 802.11 DCF性能分析及改进

针对802.11DCF在系统负载较大时不能有效利用带宽资源的缺点,该文提出一种基于效用函数的DCF优化机制(U-DCF)。通过设置站点吞吐量的对数效用函数,将带宽资源的有效利用问题建模为系统效用最大化问题;应用最优化理论将此系统问题等效为可分布式求解的用户问题,即各站点只须独立选择最大化其净效用的竞争参数(CWmin),则系统整体效用也获得最大化。仿真结果表明:与标准DCF相比,U-DCF通过预估系统的当前平均分组长度和竞争站点数来调整竞争参数CWmin,能够显著提高系统的饱和吞吐量,减小分组发送时延和丢帧率。