车载自组织网络中基于邻居节点数估计的最小竞争窗口调整算法

针对基于IEEE 802.11p的车载自组织网络(VANET)中控制信道上周期性广播的可扩展性问题,提出了一种基于邻居节点估计的最小竞争窗口调整算法.首先,在IEEE 802.11广播退避的马尔可夫模型的基础上,以最小化碰撞概率为目的,推导出最小竞争窗口(CWmin)和活跃节点数n之间的关系;然后,利用周期性广播的beacon消息的特点对节点的邻居节点数进行实时估计,并根据估计的邻居节点数动态调整最小竞争窗口;最后,对提出的算法和IEEE 802.11p固定CWmin方法进行仿真比较分析.结果表明,提出的算法在广播接收率上优于原始方法.     

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。     

一种最小竞争窗口自适应调整的802.11退避算法

该文在对原有的IEEE 802.11 DCF研究的基础上,提出了一种基于最小竞争窗口自适应调整的退避算法(Minimum Contention Window Self-adaptive Adjusting, MCWSA)。该算法的思想是每个站点根据网络中当前的时隙利用率和理论上最优时隙利用率进行比较,周期性动态调整自身的最小竞争窗口,以适应不同的网络拥塞状况。仿真表明,该算法提高了IEEE 802.11局域网在拥塞情况下的性能,在饱和吞吐量和时延上都有不同程度的改善。     

SCWGF:802.11 DCF竞争窗口增长因子 自适应调整算法

 802.11 DCF(Distributed Coordination Function)性能的分析和改进是MAC(Medium Access Control)中一个重要的研究领域.本文在该协议的基础上提出了一种竞争窗口增长因子自适应调整算法—SCWGF(Self-adaptive algorithm of adjusting Contention Window’s Growth Factor),通过理论分析得到不同竞争节点数下竞争窗口增长因子的最优解和最优冲突时间占用率,提出了使用自适应算法使竞争窗口增长因子收敛到最优解,仿真结果表明SCWGF提高了网络的饱和吞吐量.     

IEEE 802.11移动自组织网络节点竞争窗口长度的概率分布

 在基于IEEE 802.11的移动自组织网络中,MAC(Medium Access Control)层提供了DCF(Distributed Coordinate Function)以控制节点对无线信道的争用.DCF包括了BEB (Binary Exponential Backoff)算法.该文对BEB的重要参数——竞争窗口CW(Contention Window)进行研究,通过随机建模,导出了竞争窗口长度的概率分布,并进行数值分析.研究结果可应用于IEEE 802.11移动自组织网络.     

IEEE802.11MAC协议中一种改进的退避解决算法

IEEE802.11无线局域网,在共享信道中经历碰撞的网络节点需要随机退避一段时间,这段时间是从竞争窗口中均匀选取,竞争窗口大小由BEB机制动态控制,一些文献研究表明,BEB机制在重负载的情况下,突现出公平性问题和低的吞吐量,本文基于MILD退避机制,提出一种适用于分布式协调功能改进算法。该算法通过修改802.11的MAC层中的DCF(DistributedCoordinationFunction)子协议,改善了IEEE802.11无线局域网在拥塞情况下的性能,提高了网络吞吐量。     

新的改进IEEE 802.11 DCF性能的退避机制

分布式协调功能DCF是IEEE802.11标准最基本的媒体接入方法,它的核心是载波检测多址接入/冲突避免（CSMA/CA）机制,通过退避算法,减少碰撞的概率。提出了一种新的退避机制改进IEEE802.11DCF饱和吞吐量性能,建立了三维马尔可夫链网络模型详细研究分析,同时利用NS2对所提出的机制进行仿真,比较了改进后的802.11DCF饱和吞吐量与原802.11DCF的饱和吞吐量的大小,仿真结果证明了算法的准确有效。     

Ad hoc网络中DCF公平性分析与改进

在考虑节点的物理载波检测范围大于通信范围的情况下,该文分析了多跳Adhoc网络中物理载波检测机制对IEEE 802.11DCF协议公平性的影响。针对载波干扰给IEEE802.11DCF协议带来的严重不公平问题,提出了一种基于冲突和干扰感知的退避(CIAB)算法。仿真证明,该算法能有效地改善IEEE802.11DCF协议的公平性,并且没有引起网络吞吐量的严重下降。     

IEEE 802.11 DCF的一种优化退避算法

为了提升无线网络的整体性能,通过IEEE 802.11 MAC(媒体访问控制)层DCF(分布式协调功能)在基本工作模式下所使用的二进制退避算法和CW(竞争窗口)的大小对网络整体性能的影响进行了分析。采用了增大初始 CW、取消信道从忙碌变为空闲后继续退避需要等待的DIFS (分布式帧间间隔)和设置一个中间值作为阀值使得节点发送失败或成功后采用不同的退避算法进行退避这3种措施来优化DCF方式下的退避机制。经 OPNET仿真验证,改进算法能够有效地降低网络接入时延并能提高网络吞吐量。     

基于IEEE 802.11的多信道多址接入协议性能分析

 本文对一种基于IEEE 802.11 DCF协议多信道多址接入方案的性能进行了分析,在该接入方案中,节点在公共的控制信道上预约数据信道,随后在预约成功的数据信道上,进行数据分组的传输.本文采用马尔科夫链构建了控制信道的预约模型,给出了多信道吞吐量的理论分析算法.最后,通过仿真验证了理论分析结果与仿真结果的一致性,并给出了信道吞吐量与信道数、数据分组长度、节点密度、信道切换时延等参数之间的数值关系.     

一种基于竞争节点估计的改进DCF机制

对于多跳Mesh网络特有的树状拓扑，距离网关不同跳数的路由器所承载的网络流量不尽相同，与周围节点的竞争情况也不一致，由此传统802．11的DCF机制并不适用。提出了一种基于竞争节点估计的竞争窗调整机制，即利用节点分布式地获取各自的条件碰撞概率，并由此估算其竞争节点个数，在此基础上由最优最小竞争窗与竞争节点个数的关系函数，调整竞争窗大小来提高Mesh网络的总体性能。采用OPNET仿真结果表明，优化后的饱和系统吞吐量可提高70～80％，饱和时延可降低50％以上。     

An effective medium contention method to improve the performance of IEEE 802.11

In this paper, we propose an effective medium access mechanism to enhance performance of the IEEE 802.11 distributed coordination function (DCF). One of the primary issues of 802.11 is a contention-based medium access control (MAC) mechanism over a limited medium, which is shared by many mobile users. In the original 802.11 DCF, the binary exponential backoff algorithm with specific contention window size is employed to coordinate the competition for shared channel. Instead of binary exponential increase, we adopt linear increase for the contention window that is determined according to the competing number of nodes. We also assume that the access point can broadcast the number of mobile nodes to each station through management frames. An analytical model is developed for the throughput performance of the wireless medium. Using simulation results from the NS2 simulator, we show that our model can accurately predict the system saturation throughput, and can obtain better performance in terms of throughput, fairness, and packet drop.     

Defining and maximizing PPT—a novel performance parameter for IEEE 802.11 DCF

Much research has been conducted on saturation throughput of IEEE802.11 DCF, and has led to some improvement. But increasing the successful transmission probability of packet is also important for saving stations' battery energy and decreasing the packet delay. In this paper, we define a new performance parameter, named Product of successful transmission Probability and saturation Throughput (PPT), for 802.11 DCF, which binds successful transmission probability and saturation throughput together. An analysis is given to maximize PPT. An expression of optimal minimum contention windows (CW_min) is obtained analytically for maximizing PPT. For simplicity, we give a name DCF‐PPT to the 802.11 DCF that sets its CW_min according to this expression. The performance of DCF‐PPT is simulated with different stations in terms of saturation throughput, successful transmission probability and PPT. The simulation results indicate that, compared to 802.11 DCF, DCF‐PPT can significantly increase the PPT and successful transmission probability (about 0.95) on condition that the saturation throughput is not decreased. Copyright © 2006 John Wiley & Sons, Ltd.     

Novel Adaptive DCF Protocol Using the Computationally-Efficient Optimization with the Feedback Network Information for Wireless Local-Area Networks

In this paper, we design a novel computationallyefficient linear programming (LP) algorithm to maximize the throughput with respect to the minimum contention window size for the IEEE 802.11 Distributed Coordination Function (DCF) protocol. Based on our LP scheme, a new DCF protocol which can select the best access mode and the optimal size of the minimum contention window is proposed by considering the channel condition and the number of competing stations jointly. The numerical results demonstrate that our proposed DCF protocol significantly outperforms the conventional method.     

Impact of bursty error rates on the performance of wireless local area network (WLAN)

With an increasing popularity of DCF based wireless LAN, the modeling of 802.11 distributed coordination function (DCF) has attracted lots of research attention. Existing analysis of 802.11 DCF has been focused on the determination of the throughput and the packet delay under saturated traffic and ideal channel conditions. Although some recent papers address the saturated performance under a simple uniform error model, they can hardly capture the impact of bursty characteristics of wireless fading on the performance of 802.11 DCF. This paper presents exact formulae for the throughput and the delay in DCF for various traffic conditions when either saturated or unsaturated traffic load is present. A two-state Markov channel model is incorporated to present the bursty characteristics of channel errors. With our analysis, the impact of bursty channel error on unsuccessful transmission probability and the DCF performance can be determined. The results of our analytical framework reveal that the four-way handshaking scheme does not improve throughput substantially for light traffic load. However, for heavy traffic load, the four-way handshaking scheme is advantageous as compared to the basic access scheme. Also, extensive simulation is done to substantiate the accuracy of our analytical model.     

A backoff algorithm based on self-adaptive contention window update factor for IEEE 802.11 DCF

The binary exponential backoff (BEB) mechanism is applied to the packet retransmission in lots of wireless network protocols including IEEE 802.11 and 802.15.4. In distributed dynamic network environments, the fixed contention window (CW) updating factor of BEB mechanism can’t adapt to the variety of network size properly, resulting in serious collisions. To solve this problem, this paper proposes a backoff algorithm based on self-adaptive contention window update factor for IEEE 802.11 DCF. In WLANs, this proposed backoff algorithm can greatly enhance the throughput by setting the optimal CW updating factor according to the theoretical analysis. When the number of active nodes varies, an intelligent scheme can adaptively adjust the CW updating factor to achieve the maximal throughput during run time. As a result, it effectively reduces the number of collisions, improves the channel utilization and retains the advantages of the binary exponential back-off algorithm, such as simplicity and zero cost. In IEEE 802.11 distributed coordination function (DCF) protocol, the numerical analysis of physical layer parameters show that the new backoff algorithm performance is much better than BEB, MIMD and MMS algorithm.     

IEEE802.11无线局域网中一种支持业务区分的回退算法

支持QoS的MAC机制是WLAN支持QoS的关键所在.我们已经给出了一种低分组碰撞概率的MAC层回退机制——RWBO+BEB.该文进一步讨论如何让RWBO+BEB支持业务区分的问题.首先提出了一个Markov链模型,分析如何根据无线终端的带宽比率设置最小竞争窗口,然后给出了一种新的支持业务区分的回退算法——DS-RWBO,仿真结果表明,DS-RWBO能根据局域网中每个终端的带宽比率分配无线信道的带宽资源.     

A k-Round Elimination Contention Scheme for WLANs

The contention resolution scheme is a key component in carrier-sense-based wireless MAC protocols. It has a major impact on MAC'S performance metrics such as throughput, delay, and jitter. The IEEE 802.11 DCF adopts a simple contention resolution scheme, namely, the binary exponential backoff (BEB) scheme. The BEB scheme achieves a reasonable performance for transmitting best-effort packets in small-sized wireless networks. However, as the network size increases, it suffers from inefficiency because of the medium contention, which leads to reduced performance. The main reason is that the BEB mechanism incurs an ever- increasing collision rate as the number of contending nodes increases. We devise a novel contention resolution scheme, a k-round elimination contention (k-EC) scheme. The k-EC scheme exhibits high efficiency and robustness during the collision resolution. More importantly, it is insensitive to the number of contending nodes. This feature makes it feasible for use in networks of different sizes. Simulation results show that the k-EC scheme offers a powerful remedy to medium contention resolution. It significantly outperforms the IEEE 802.11 DCF scheme in all the MAC'S performance metrics and also exhibits better fairness.