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分析了IEE802.11无线局域网MAC层快速冲突分解算法,修改和扩展了ns-2网络仿真软件的IEE802.11无线局域网仿真模块,实现了ns-2对快速冲突分解算法的仿真。仿真结果表明,快速冲突分解算法提高了网络的平均吞吐量、减小了平均时延,但网络的时延抖动和公平性却更加恶化。 相似文献
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在802.11协议中,DCF(Distributed Coordination Function)机制是节点共享无线信道进行数据传输的基本接入方式,为了解决无线网络中隐藏节点问题,使用RTS/CTS机制减少冲突,然而当网络节点数增加时,节点传输的冲突次数亦增加,从而使网络性能明显下降。因此,需要设计新的MAC协议,以适应当前Ad Hoc网络应用的快速发展。在IEEE 802.11的分布式协调功能访问机制(DCF)基础上,本文设计新的节点合作式的网络协议(C-MAC)。C-MAC节点通过控制帧获得本节点2跳内的邻接节点信息,并且根据邻接节点的信息设计调度算法,使节点以轮询的合作方式传输数据,有效地避免冲突。仿真实验表明,在改变节点速率、帧长度、网络节点数等参数情况下,分别以吞吐量、单帧传输时间和公平性为指标,对DCF和C MAC协议进行性能比较。在节点传输速率为11Mbps时,C MAC协议吞吐量比标准DCF最多可增加50%。 相似文献
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周晓彬 《计算机技术与发展》2012,(10)
文中主要分析了IEEE802.11的MAC协议中的分布式协调工作模式(Distribute Coordination Function,DCF).使用DCF方式进行访问控制时,二进制指数退避算法在竞争周期内冲突增加会使信道分配不合理,这样严重影响网络的通信吞吐量.针对影响网络吞吐量这个问题,文中首先结合了马尔科夫(Markov)的改进模型,根据节点的实时状态,设计了节点在初始竞争成功发送数据后和发生冲突时的退避算法,优化了DCF的退避机制,动态地调整了节点在不同状态时的信道获取能力,提高了节点发送数据的网络吞吐量.最后以OPNET为仿真平台证明其在吞吐量方面有所改善. 相似文献
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研究非饱和状态下分布式协调功能(DCF)协议的性能,针对非饱和状态下的无线局域网,引入虚拟空闲时间定量描述系统的忙碌程度,对二维马尔科夫链模型进行改进.在理想信道条件下,使用基本接人方式,利用改进模型研究虚拟空闲时间及站点数对DCF性能(系统吞吐量和分组传输平均时延)的影响.理论推导及仿真结果表明,在不同的虚拟空闲时间下,随着站点数的增加,系统非饱和吞吐量为先快速上升后缓慢下降的过程,分组传输时延则是平稳上升的过程,与选取的对比模型相比,吞吐量在站点数目较小时有明显改善,时延性能有较大提高. 相似文献
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退避算法的设计对基于竞争的IEEE 802.11协议影响重大,而退避的前提取决于冲突的发生和正确判断。本文在DCF协议的基础上提出了一种基于全网冲突的自适应调整竞争窗口的新型退避算法(CWN-BEB),CWN-BEB算法通过统计全网冲突次数(即整个网络所有节点发生冲突的总次数),使全网冲突对节点透明,并引入一个新的变量全网冲突概率来自适应改变竞争窗口大小。此算法未引入额外开销,可以很好地与802.11 DCF协议兼容,实现复杂度低。仿真结果表明,在低负载情况下,CWN-BEB算法可以较好地向DCF协议收敛;在高负载情况下,CWN-BEB的时延和吞吐量等性能明显优于802.11 DCF协议。 相似文献
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无线自组织网络(Ad hoc)在MAC层中使用IEEE 802.11 DCF的接入机制来解决节点传输竞争问题,为了解决高负载环境下的网络拥塞状况问题,在退避算法中加入了动态阈值和岭型函数来降低数据传输的冲突概率并利用NS2对改进的退避算法进行网络仿真。仿真结果表明,相对于传统的退避算法,在负载较重的网络中改进后的退避算法大概能够提高网络性能20%左右,优化后网络的时延、吞吐量和公平性均得到明显改善。 相似文献
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RSAD: A Robust Distributed Contention-Based Adaptive Mechanism for IEEE 802.11 Wireless LANs
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YongPeng Shi-DuanCheng Jun-LiangChen 《计算机科学技术学报》2005,20(2):0-0
Previous researches have shown that Distributed Coordination Function (DCF) access mode of IEEE 802.11 has lower performance in heavy contention environment. Based on the in-depth analysis of IEEE 802.11 DCF, NSAD (New Self-adapt DCF-based protocol) has been proposed to improve system saturation throughput in heavy contention condition. The initial contention window tuning algorithm of NSAD is proved effective in error-free environment. However, problems concerning the exchanging of initial contention window occur in error-prone environment. Based on the analysis of NSAD's performance in error-prone environment, RSAD is proposed to further enhance the performance. Simulation in a more real shadowing error-prone environment is done to compare the performance of NSAD and RSAD and results have shown that RSAD can achieve further performance improvement as expected in the error-prone environment than NSAD (i.e., better goodput and fairness index). 相似文献
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Distributed Coordination Function (DCF)is the basis of the IEEE 802.11 WLAN MAC protocols .This paper proposes a scheme named DCF ,whic can be regarded as an option to DCF,to enhance the performance of reliable transport protocol over WLAN.To analyze the performance of DCF and DCF ,this paper also introduces and analytical model to compute the saturated throughput performance of WLAN.Compared with other models,this model is more accurate ,which is verified by elaborate simulations.Moreover,DCF is shown to be able to improve the performance of TCP over WLAN by both modeling and simulations. 相似文献
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This paper proposes an algorithm named WLFVC, which is a fully distributed wireless packet scheduling algorithm based on LFVC (Leap Forward Virtual Clock), to provide streams with fairness, especially in the short time scale. This algorithm is able to allocate band-width to streams proportional to their weights. Thus, better delay and jitter performance can be achieved. Since collisions and wireless link error degrade the performance of MAC protocols in the WLAN environment, WLFVC also takes them into consideration. Finally,this algorithm works in a fully distributed way and it can coexist with other stations that only support DCF (Distributed Coordination P-hnction). 相似文献
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在WRR(WeightedRoundRobin,带权重的循环)算法的基础上,提出了一种适应无线局 域网特点的实时业务的调度算法。该算法工作在数据链路层,和无线局域网的介质访问控制(MAC) 层协议DCF(DistributedCoordinateFunction)紧密结合,有效避免了对列头部阻塞问题,提高了信道利 用率。通过采用补偿机制,近似达到了长期公平性。使用NS(NetworkSimulator)仿真进行性能评价, 结果表明该算法能有效提高实时业务的端端吞吐率,降低端端延迟和丢包率,延迟抖动也较小,可以 在一定程度上保证实时业务的服务质量。 相似文献
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Random-access mechanisms play an important role in wireless networks, and have been extensively studied in recent years. Although many previous studies have proposed enhanced algorithms, each one has only considered either throughput or fairness. In this paper, we propose an efficient random-access mechanism called Multi-round Collision Avoidance (MrCA) that considers throughput and fairness together. The key idea in MrCA is to avoid collisions through multiple contentions, each with a smaller sized contention window. With this simple modification, we can significantly reduce the collision probability as well as the access delay, in addition to increasing fairness index. We find the collision probability and throughput analytically. Through simulation, we validate our analytical model and find appropriate parameters for achieving good performance. We also demonstrate that, compared to the IEEE 802.11 DCF, MrCA makes the collision probability extremely low, so that it increases throughput by 25% as well as short-term fairness by 50% with 50 contending nodes. When MrCA and 802.11 DCF schemes are combined with the auto rate fallback scheme, the performance gain of MrCA over 802.11 DCF increases because MrCA lowers the collision probability significantly, which makes channel error estimation more accurate. We also discuss the issues of implementation and backward compatibility. 相似文献
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在基于IEEE 802.11的无线局域网(WLAN)中,所有节点只能和接入点(AP)直接通信,即使在直接通信范围内的无线节点,其相互通信也要通过接入点进行转发,从而导致无线局域网的吞吐率大大降低。针对这一问题,提出一种分布协调功能(DCF)扩展算法——ExDCF算法:当两个通信节点在同一传输范围内时,两节点间的数据传输不需要接入点转发;即使两个节点不在一个通信范围内,两节点间数据传输需要接入点转发,但是接入点不需要再次竞争信道。该算法可以将节点间的吞吐率至少提高2倍。理论分析和仿真结果表明该算法能有效地提高无线网络的整体性能,并且能够与标准DCF很好地兼容。 相似文献