无线局域网MAC层DCF方式的仿真研究

针对无线局域网MAC层DCF方式模型建立难度较高,性能参数较难得到的问题,采用一种基于时间窗口分析的方法,建立DCF基本模型.根据IEEE802.11提供的二进制指数退避算法,应用OPNET仿真工具进行实验设计.实验结果表明,系统的网络吞吐量指标随着节点个数的增加而减少,时延指标则随时间的增加而线性增长.     

矿业权评估的实物期权方法研究初探

矿业权评估的准确与否直接影响矿业权市场的交易和国家的利益 ,矿业投资领域的矿产品价格波动大 ,不确定性高 ,传统的折现现金流法由于预期现金流的不准、对折现率的选取困难、以及不能反映管理的灵活性而容易低估项目的价值 ,导致对项目的误评。矿业权评估的实物期权方法优于传统的DCF法 ,能克服DCF法的弱点 ,更合理地评估矿业权的价值。     

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.     

Adaptive Power Saving Mechanisms for DCF in IEEE 802.11

Distributed Coordination Function (DCF) is the basis of IEEE 802.11 MAC sublayer. To improve energy efficiency, IEEE 802.11 has specified power saving mechanism (PSM) for DCF. However, the performance of PSM degrades seriously when load is heavy. In this paper we first analyze the reason of performance degradation, and then propose two adaptive mechanisms: PSM+ and fairPSM+. Numerous simulation results have shown that these two mechanisms can achieve high performance with heavy load and still get the same performance as PSM when load is light.Shihong Zou was born in 1978. He received his Bacchelor of Engineering degree in Computer Engineering from Nanjing University of Posts and Telecommunications (Nanjing, China) in 1999, and his Ph.D. degree in communication and information systems from Beijng University of Posts and Telecommunications (BUPT) in 2004. He is currently a lecturer in BUPT. His research interests include IP QoS, WLAN, mobile ad hoc networks and network security.Haitao Wu was born in 1976. He received his Bachelor of Engineering (B.E.) degree in Communication Engineering from Beijing University of Posts and Telecommunications (BUPT) in 1998, and his Ph.D. degree in communication and information systems from BUPT in 2003. He is currently a Post-doctor in Microsoft Research Asia (MSRA), wireless and networking group. His research interests include IP QoS, TCP/IP, wireless networks and peer-to-peer networks.Shiduan Cheng graduated in communication engineering at the Beijing University of Posts and Telecommunications (BUPT) in 1963. Since then she has been working at BUPT. From 1984 to 1987 and in 1994 she twice joined Alcatel Bell, Belgium as a visiting scholar, where she was involved in R&D work on ISDN and B-ISDN networks and systems. She became vice dean of the computer department of BUPT in 1987. From 1992 to 1999 she was the director of The National Key Laboratory of Switching Technology and Telecommunication Networks of BUPT. In the same duration she was also the head of The Switching and Networking Expert Group in 863 program, a national high-tech R&D plan organized by The Ministry of Science and Technology of China. She has published more than 100 papers and several books in the field of telecommunications. She is currently the vise director of Standing Committee of Science of BUPT. Her research interests cover ISDN, ATM, TCP/IP, switching software, protocol engineering, traffic engineering, network performance, QoS, security and survivability. Currently she is working on QoS control, measurement and management for the next generation Internet.     

无线局域网中的自适应节能机制

为节省能量,无线局域网标准IEEE 802．11在分布式协调功能（DCF）的基础上定义了节能机制PSM,但该机制在重负载情况下性能严重劣化,为提高该机制在重负载情况下的性能,该文对现有PSM进行研究分析后提出自适应调整接入概率的PSM＋和fairPSM＋．仿真结果表明PSM＋和fairPSM＋在轻负载下性能与PSM相当,而在重负载下较大地提高了吞吐量及能量有效性．fairPSM＋在PSM＋的基础之上考虑了系统的稳定性和公平性,因此具有最好的性能.     

高速无线局域网DCF机制下的链路自适应算法研究

随着高速无线局域网物理层的制定,动态选择物理层传输模式的链路自适应被认为是一种有效提高系统吞吐量的方法。本文给出了两种基于IEEE802．11a DCF机制下的链路自适应算法,并对它们进行了理论分析与数值仿真,结论表明,基于MPDU的链路自适应算法优于基于MSDU的算法。在分析MPDU算法的基础上,我们给出了一种简单的通过查表来来选择最适合物理层传输模式的方法,并深入地讨论了此物理层传输模式表的建立过程。     

在无线网络中一种分组公平调度的混合策略

设计了一个集中式和分布式的分组公平排队混合机制来解决无线分组网络中的资源公平调度问题．集中式公平控制采用了传统算法WFQ和SBFA的混合策略;分布式公平控制是在通信协议IEEE802．1l中分布式协调功能(DCF)的基础上,加载公平调度机制来实现．混合调度策略可以保证在无线分组网络中,对多个移动主机上的多个分组流,在短期和长期、局部和全局的范围内信道资源的公平使用和共享．     

A generic methodology to evaluate economics of hydrogen production using energy from nuclear power plants

Hydrogen, the deemed future transportation fuel can be produced from nuclear assisted energy sources. Assessment of economics of hydrogen production using energy from nuclear power plants is vital for asserting its competitiveness with competing technologies. A generic method is presented in this paper to evaluate Levelised Hydrogen Generation Cost, based on the discounted cash flow analysis. The method is illustrated by consideration of a typical case of hydrogen production via conventional electrolysis using electrical energy supplied from a pressure tube type boiling light water cooled heavy water moderated reactor concept.     

室内氡子体有效剂量转换系数的影响因素分析

室内氡子体有效剂量转换系数受室内环境参数的影响。为了理解和评价室内氡子体有效剂量转换系数随换气率、气溶胶浓度和气溶胶粒径分布的变化关系,从室内氡子体模型出发,结合前人实测的室内环境参数,计算了典型室内环境氡子体剂量转换系数值,并重点分析了室内环境参数中换气率、气溶胶浓度、气溶胶粒径分布对室内氡子体剂量转换系数的影响。在换气率为0.55 h-1,气溶胶浓度1.0×104 cm-3,AMTD为1.0 nm,AMAD为200 nm的典型室内环境,氡子体有效剂量转换系数为28.4 nSv.(Bq.h.m-3)-1,考虑到室内环境参数的变化范围,该值可以在19.9~33.9 nSv.(Bq.h.m-3)-1范围内变化。     

Enhanced binary exponential backoff algorithm for fair channel access in the ieee 802.11 medium access control protocol

The medium access control protocol determines system throughput in wireless mobile ad hoc networks following the ieee 802.11 standard. Under this standard, asynchronous data transmissions have a defined distributed coordination function that allows stations to contend for channel usage in a distributed manner via the carrier sensing multiple access with collision avoidance protocol. In distributed coordination function, a slotted binary exponential backoff (BEB) algorithm resolves collisions of packets transmitted simultaneously by different stations. The BEB algorithm prevents packet collisions during simultaneous access by randomizing moments at stations attempting to access the wireless channels. However, this randomization does not eliminate packet collisions entirely, leading to reduced system throughput and increased packet delay and drop. In addition, the BEB algorithm results in unfair channel access among stations. In this paper, we propose an enhanced binary exponential backoff algorithm to improve channel access fairness by adjusting the manner of increasing or decreasing the contention window based on the number of the successfully sent frames. We propose several configurations and use the NS2 simulator to analyze network performance. The enhanced binary exponential backoff algorithm improves channel access fairness, significantly increases network throughput capacity, and reduces packet delay and drop. Copyright © 2013 John Wiley & Sons, Ltd.