一种基于网络拥塞程度阀值的退避算法

对IEEE802.11无线网络MAC层的技术所使用的DCF协议的退避算法进行了分析,针对二进制指数退避算法BEB存在的不足,提出了改进的退避算法。通过OPNET仿真,仿真结果表明,改进的退避算法能提高网络吞吐量,改善网络性能。     

无线网络预约退避算法的实现和分析

针对无线局域网IEEE 802.11MAC层二进制退避算法的不足,当前已经提出多种典型退避算法。主要针对预约发送类退避算法进行研究分析,并提出与EBA不同的实现措施,对NS2网络仿真软件的Mac802.11模块进行了修改和扩展,实现了NS2对改进的预约退避算法的实现。结果表明,与标准DCF相比,改进的预约退避算法也能对IEEE802.11无线局域网的吞吐量、时延等方面性能有所改善。为今后基于NS2对预约退避算法的进一步开发研究奠定基础。     

基于网络拥塞程度阈值的退避算法

对IEEE802.11无线网络MAC层技术所使用的DCF协议退避算法进行了分析,针对二进制指数退避算法BEB存在的不足,改进了从平均退避窗口取阈值的方案,基于这个阈值提出了适合Aol Hoc网络的NCT退避算法.OPNET仿真结果表明,改进的退避算法能提高网络吞吐量,改善网络性能.     

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。     

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。     

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。     

Ad Hoc网络退避算法分析及改进

IEEE 802.11 DCF中采用的标准退避算法是二进制指数(BEB)退避算法,为了提高该算法在网络中的效能,文中主要研究了BEB算法存在的缺陷,和改进后的线性减少(MILD)退避算法存在的优势,基于OPNET网络仿真平台,对两种算法的性能进行了仿真评估。仿真结果表明,改进的MILD算法对提高Ad Hoc网络的吞吐量性能和公平性有明显的效果。     

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。     

Ad Hoc中一种改进后退机制的基于多包接收的MAC算法

物理层多包接收技术的发展给利用物理层多包接收能力的媒体接入控制（MAC）协议的设计带来了挑战。IEEE802.11 DCF是目前WIAN最成熟的分布式MAC协议之一，对其在多包接收模型下进行性能改善将有很大的应用价值。在物理层具有多包接收能力的基础上，提出了一种改进的802.11 DCF协议，并将该协议应用于现有的基于802.11 DCF的多包接收MAC算法（MDCF），理论分析和NS-2仿真实验结果表明，该算法与IEEE 802.11 DCF和MDCF相比，在网络吞吐量和时延性能方面有很大的改善。     

通过自适应调整最小竞争窗口最大化IEEE 802.11 DCF的饱和吞吐量

最大化802.11 DCF的饱和吞吐量对充分利用无线局域网宝贵的带宽资源具有重要意义。该文在分析802.11 DCF的饱和吞吐量与最小竞争窗口、最大回退等级、网络中竞争信道的节点数的关系的基础上,推导了根据网络中竞争信道的节点数,计算最小竞争窗口的最佳值的简单公式。给出了估计竞争信道的节点数并据此动态调整最小竞争窗口的最佳值的自适应算法。同时,该文对估计竞争节点数的算法的准确性和计算最小竞争窗口最佳值的公式的准确性进行了仿真分析,并比较了改进后的802.11 DCF的饱和吞吐量与原802.11 DCF的饱和吞吐量的大小。仿真结果证明了上述公式、算法是准确和有效的。     

Optimal Constant-Window Backoff Scheme for IEEE 802.11 DCF in Single-Hop Wireless Networks Under Finite Load Conditions

Existing backoff scheme’s optimization of IEEE 802.11 DCF MAC protocol consider only saturated networks or asymptotic conditions. In real situations, traffic is bursty or streamed at low rates so that stations do not operate usually in saturated regime. In this work, we propose and analyze a backoff enhancement for IEEE 802.11 DCF that requires information only about the network size and that is quasi-optimal under all traffic loads. We first analyze the performance of DCF multiple access scheme under general load conditions in single-hop configuration and we provide an accurate delay statistics model that consider the self-loop probability in every backoff state. We prove then the short-term unfairness of the binary exponential backoff used in IEEE 802.11 by defining channel capture probability as fairness metric. Motivated by the results on fairness, we introduce the constant-window backoff scheme and we compare its performance to IEEE 802.11 DCF with Binary exponential backoff. The quasi-optimality of the proposed scheme is proved analytically and numerical results show that it increases, both the throughput and fairness, of IEEE 802.11 DCF while remaining insensitive to traffic intensity. The analysis is then extended to consider the finite queuing capacity at nodes buffers using results from the delay analysis. NS2 simulations validate the obtained results. Institut Eurecom’s research is partially supported by its industrial members: BMW Group Research & Technology—BMW Group Company, Bouygues Telecom, Cisco Systems, France Telecom , Hitachi Europe, SFR, Sharp, STMicroelectronics, Swisscom, Thales.     

Distributed throughput optimization for heterogeneous IEEE 802.11 DCF networks

For IEEE 802.11 DCF networks in ad-hoc mode, how to achieve the maximum throughput in a distributed manner draws much attention in previous studies. The problem becomes challenging for partially-saturated heterogeneous networks with multiple groups, as the optimal access parameters not only depend on the group size of saturated groups but also the aggregate input rate of all the unsaturated groups, both of which are hard to obtain without a central controller. In this paper, a novel distributive scheme is proposed for partially-saturated heterogeneous IEEE 802.11 DCF networks to achieve the maximum network throughput. With the proposed scheme, each saturated transmitter can obtain the optimal initial backoff window size distributively by two estimation rounds. In each estimation round, each saturated transmitter only needs to count the number of busy intervals and ACK frames on the channel. For fully-saturated networks, only one estimation round is needed. It is shown by extensive simulations that the proposed scheme can achieve the maximum network throughput in a distributive manner.     

Performance analysis of the IEEE 802.11 distributed coordinationfunction

The IEEE has standardized the 802.11 protocol for wireless local area networks. The primary medium access control (MAC) technique of 802.11 is called the distributed coordination function (DCF). The DCF is a carrier sense multiple access with collision avoidance (CSMA/CA) scheme with binary slotted exponential backoff. This paper provides a simple, but nevertheless extremely accurate, analytical model to compute the 802.11 DCF throughput, in the assumption of finite number of terminals and ideal channel conditions. The proposed analysis applies to both the packet transmission schemes employed by DCF, namely, the basic access and the RTS/CTS access mechanisms. In addition, it also applies to a combination of the two schemes, in which packets longer than a given threshold are transmitted according to the RTS/CTS mechanism. By means of the proposed model, we provide an extensive throughput performance evaluation of both access mechanisms of the 802.11 protocol     

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.     

An accurate two dimensional Markov chain model for IEEE 802.11n DCF

According to the amendment 5 of the IEEE 802.11 standard, 802.11n still uses the distributed coordination function (DCF) access method as mandatory function in access points and wireless stations (essentially to assure compatibility with previous 802.11 versions). This article provides an accurate two dimensional Markov chain model to investigate the throughput performance of IEEE 802.11n networks when frame aggregation and block acknowledgements (Block-ACK) schemes are adopted. Our proposed model considered packet loss either from collisions or channel errors. Further, it took anomalous slots and the freezing of backoff counter into account. The contribution of this work was the analysis of the DCF performance under error-prone channels considering both 802.11n MAC schemes and the anomalous slot in the backoff process. To validate the accuracy of our proposed model, we compared its mathematical simulation results with those obtained using the 802.11n DCF in the network simulator (NS-2) and with other analytical models investigating the performance of 802.11n DCF. Simulation results proved the accuracy of our model.     

EBA: an enhancement of the IEEE 802.11 DCF via distributed reservation

The IEEE 802.11 standard for wireless local area networks (WLANs) employs a medium access control (MAC), called distributed coordination function (DCF), which is based on carrier sense multiple access with collision avoidance (CSMA/CA). The collision avoidance mechanism utilizes the random backoff prior to each frame transmission attempt. The random nature of the backoff reduces the collision probability, but cannot completely eliminate collisions. It is known that the throughput performance of the 802.11 WLAN is significantly compromised as the number of stations increases. In this paper, we propose a novel distributed reservation-based MAC protocol, called early backoff announcement (EBA), which is backward compatible with the legacy DCF. Under EBA, a station announces its future backoff information in terms of the number of backoff slots via the MAC header of its frame being transmitted. All the stations receiving the information avoid collisions by excluding the same backoff duration when selecting their future backoff value. Through extensive simulations, EBA is found to achieve a significant increase in the throughput performance as well as a higher degree of fairness compared to the 802.11 DCF.     

Enhancing channel utilization by improving media access coordination in wireless local area networks

Distributed coordination function (DCF) is the basis protocol for IEEE 802.11 standard wireless local area networks. It is based on carrier sense multiple access with collision avoidance (CSMA/CA) mechanism. DCF uses backoff process to avoid collisions on the wireless channel. The main drawback with this process is that packets have to spend time in the backoff process which is an additional overhead in their transmission time. The channel is rendered idle when all the stations defer their transmissions due to their backoff process. Therefore, the channel utilization and the total throughput on the channel can be improved by reducing the average time spent by the packets in the backoff process. In this paper, we propose a new media access coordination function called proposed media access protocol (PMAP) that will improve the channel utilization for successful packet transmission and therefore, the total achievable throughput. In addition, we propose an analytical model for PMAP under saturated conditions. We use this model to analyze the performance of PMAP under saturated conditions. To substantiate the effectiveness of our model, we have verified the model by simulating PMAP in NS‐2. Simulation and analytical results show that under saturated conditions, PMAP shows profound improvement in the throughput performance compared to DCF. In addition, the throughput performance of PMAP under unsaturated conditions is presented. We have also presented the delay performance of PMAP and DCF through simulation in both saturated and unsaturated conditions. Simulation results show that the average delay experienced by the packets is less in PMAP compared to DCF. Further, the variance in the packet delay is same for both PMAP and DCF protocols under unsaturated conditions. From the performance results obtained for PMAP under both saturated and unsaturated conditions, it can be concluded that PMAP is superior in performance compared to DCF. Copyright © 2006 John Wiley & Sons, Ltd.     

Performance Analysis of IEEE 802.11 DCF in Imperfect Channels

IEEE 802.11 is the most important standard for wireless local area networks (WLANs). In IEEE 802.11, the fundamental medium access control (MAC) scheme is the distributed coordination function (DCF). To understand the performance of WLANs, it is important to analyze IEEE 802.11 DCF. Recently, several analytical models have been proposed to evaluate the performance of DCF under different incoming traffic conditions. However, to the best of the authors' knowledge, there is no accurate model that takes into account both the incoming traffic loads and the effect of imperfect wireless channels, in which unsuccessful packet delivery may occur due to bit transmission errors. In this paper, the authors address this issue and provide an analytical model to evaluate the performance of DCF in imperfect wireless channels. The authors consider the impact of different factors together, including the binary exponential backoff mechanism in DCF, various incoming traffic loads, distribution of incoming packet size, queueing system at the MAC layer, and the imperfect wireless channels, which has never been done before. Extensive simulation and analysis results show that the proposed analytical model can accurately predict the delay and throughput performance of IEEE 802.11 DCF under different channel and traffic conditions.     

A Multichain Backoff Mechanism for IEEE 802.11 WLANs

The distributed coordination function (DCF) of IEEE 802.11 standard adopts the binary exponential backoff (BEB) for collision avoidance. In DCF, the contention window is reset to an initial value, i.e., CWmin, after each successful transmission. Much research has shown that this dramatic change of window size may degrade the network performance. Therefore, backoff algorithms, such as gentle DCF (GDCF), multiplicative increase–linear decrease (MILD), exponential increase–exponential decrease (EIED), etc., have been proposed that try to keep the memory of congestion level by not resetting the contention window after each successful transmission. This paper proposes a multichain backoff (MCB) algorithm, which allows stations to adapt to different congestion levels by using more than one backoff chain together with collision events caused by stations themselves as well as other stations as indications for choosing the next backoff chain. The performance of MCB is analyzed and compared with those of 802.11 DCF, GDCF, MILD, and EIED backoff algorithms. Simulation results show that, with multiple backoff chains and collision events as reference for chain transition, MCB can offer a higher throughput while still maintaining fair channel access than the existing backoff algorithms.