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1.
The standardized IEEE ,802. II distributed coordination function （ DCF） provides a contention-based distributed channel access mechanism for mobile stations to share the wireless medium. However, when stations are mobile or portable units, power consumption becomes a primary issue since terminals are usually battery driven. This paper proposes an analytical model that calculates the energy efficiency of both the basic and the RTS/CTS access mechanisms of the IEEE 802. II protocol. The model is validated with simulation results using NS-2 simulation package. The effects of the network size, the average packet length, the initial contention window and maximum backoff stages on the energy efficiency of both access mechanisms are also investigated. Results show that the basic scheme has low energy efficiency at large packet length and large network size, and depends strongly on the number of stations and the backoff procedure parameters. Conversely, the RTS/CTS mechanism provides higher energy efficiency when the network size is large, and is more robust to variations in the backoff procedure parameters. 相似文献
2.
钱善刚 《安徽教育学院学报》2008,26(2)
胡适从辨析<淮南子>的性质入手,用唯科学主义的角度省视其得失,认为"道"是一个无从证实的假设.其实,从人文主义的角度看,毋宁把"道"看作是一个不必求证的本体论承诺. 相似文献
3.
An analytical model for straight hemming was developed based on minimum energy method to study the effect of flanging die
corner radius on hemming qualities. In order to calculate plastic strain and strain energy more exactly, the neutral layer
of specimen corner after hemming is assumed to be a half ellipse with its major semi-axis unknown. Isotropic hardening rule
is adopted to describe bending and reverse bending processes neglecting Bauschinger effect. The model takes into account the
material property parameters in order to satisfy a wide application range of different materials. Specimen profile, creepage/growing
(roll-in/roll-out) and maximum equivalent strain are predicted, which are greatly influenced by the flanging die corner radius.
Experimental facilities were designed and hemming experiments were undertaken. The predicted results of the present analytical
model were compared to experimental data as well as finite element (FE) simulation results. It was confirmed that they are
in good agreement, and the model can be used to evaluate whether the material used as an outer panel for hemming is appropriate
and to optimize process parameters when the material used for hemming is changed. 相似文献
4.
Dynamic response analysis of damper connected adjacent multi-story structures with uncertain parameters is carried out. A formula of the multi degree of freedom （MDOF） for the structure-damper system with stochastic parameters is derived. The uncertainties of mass and stiffness are taken into consideration firstly. The ground acceleration is represented by Kanai-Tajimi filtered non-stationary process. The mean square random responses of structural displacement and story drift are chosen as the optimization objective. The variations of mean square responses of top floor displacements and bottom story drifts in neighboring structures with the damper stiffness and damping coefficient are analyzed in detail. Through the parametric study, the acquiring optimum parameters of damper are regarded as numerical results. Then, a reducing order model of the MDOF system for adjacent structures with mean parameters is presented. The explicit expressions for determining optimal parameters of Kelvin model-defined damper which is used to connect adjacent single degree of freedom （SDOF） structures subjected to a white-noise excitation are employed to achieve the appropriate damper parameters, which are called theory results. Through a comparative study, it can be found that the theory values of damper parameters are consistent with the results based on extensive parametric studies. The analytical results can be obtained by using the first natural frequencies and the total mass of the adjacent deterministic structures with mean parameters. The analytical formulas can be used to find appropriate parameters of damper between adjacent structures for engineering applications. The performance of damper is investigated on the basis of mitigations of mean square random responses of inter-story drifts, displacements and accelerations in adjacent structures. The numerical results demonstrate the robustness of coupled building control strategies. 相似文献
5.
人性假设是教育的重要理论基础。教育实践中出现的一些问题,可能与我们对教育所持的不恰当的人性假设有关。人性的实质是趋利避害、自我保存,它本无所谓善恶。教育应在此认识基础上进行,才能真正促进人的发展,培养社会所需要的人。 相似文献
6.
This paper attempts to provide an overview of the key assumptions underpinning the Whole School
Evaluation (WSE) inspection policy developed in Ireland since 2003. Beginning with a documentary analysis the paper argues that the
capacity to generate useful self evaluative data in schools was seen as being at the heart of the model of school evaluation
proposed by the Department of Education and Science. It further suggests that while the rhetoric of self evaluative capacity
building has been key to the emerging system the lack of a meaningful structural response within schools means that this has
remained aspirational. The latter part of the paper seeks to test this contention, examining the research base in the area
of school evaluation and inspection in Ireland and conducting a number of targeted focus groups with school leaders. For the
most part the initial contention is confirmed although there is a sense that there may be significant new pressures emerging
in the near future that could cause the whole system to be revisited and perhaps be radically overhauled. An initial indicator
of these potential changes can be seen in the emergence of a refined WSE Management, Leadership and Learning inspection policy
which has yet to be widely implemented. 相似文献
7.
研究了移动Ad Hoc网络中的媒体接入控制问题. 首先, 基于IEEE 802. 11媒体接入控制协议, 提出了应用于MANET网络媒体接入控制的2个机制: 自适应回退窗转发优先机制(FPF)和多跳链邀请发送机制(MFCTI). FPF机制根据业务优先级自适应调整竞争窗大小, 而MFCTI机制则充分利用了网络层路由信息和无线传播的广播特性. 在此基础上对所提出的机制进行了仿真研究并与原IEEE 802. 11媒体接入控制进行了比较. 结果表明, 所提机制可以有效地提高网络吞吐率, 降低端对端迟延, 缓解网络局部拥塞, 从而提高了MANET网络的性能. 其另一优点是只需对IEEE 802. 11媒体接入控制作少量的补充和改动. 相似文献
8.
Marie Sandy 《Interchange》2011,42(3):261-285
In this paper I consider how Hans-Georg Gadamer’s philosophical hermeneutics can complement the pragmatic theory that has
informed the field of service-learning, and with its emphasis on community and respect for others, can offer an orientation
to further the work of service-learning and community engagement in a mutually satisfying way for scholars, practitioners,
and community partners. In keeping with Gadamer’s contention that mythological thinking has its rightful place alongside analytical
thinking, I provide an interpretation of the myth of Hermes for emancipatory education practice, and then invite readers to
consider some implications of philosophical hermeneutics through traditional philosophical exposition. I posit there is much
to be gained by framing community engagement as a civic art or practical beauty with a distinct epistemological foundation that values conversation, participation, and openended, collective processes to
work for the common good. This orientation would require us to reconsider the purposes and reorient the values of this field
of educational practice. 相似文献
9.
A Second-Order Conditionally Linear Mixed Effects Model With Observed and Latent Variable Covariates
A conditionally linear mixed effects model is an appropriate framework for investigating nonlinear change in a continuous latent variable that is repeatedly measured over time. The efficacy of the model is that it allows parameters that enter the specified nonlinear time-response function to be stochastic, whereas those parameters that enter in a nonlinear manner are common to all subjects. In this article we describe how a variant of the Michaelis-Menten (M-M) function can be fit within this modeling framework using Mplus 6.0. We demonstrate how observed and latent covariates can be incorporated to help explain individual differences in growth characteristics. Features of the model including an explication of key analytic decision points are illustrated using longitudinal reading data. To aid in making this class of models accessible, annotated Mplus code is provided. 相似文献
10.
在基于固定窗口宽度滑动窗口模型的基础上。提出了一种基于回归参数存储的预测模型,该模型设置了计算区、数据区和参数区。计算区用于获得最近一个滑动窗口中的数据。数据区用于接收新数据,参数区存储最近若干组滑动窗口数据所计算得到的线性回归参数值,作为计算预测结果的原始数据集。按照这种模型的处理思路。提出了一种基于数据平滑技术的回归预测算法,随着窗口的滑动。对数据区中的数据进行回归分析,获得前面若干组滑动窗口数据的回归函数并存入参数区中,检验当前窗口中数据获得的回归函数预测效果。实验分析表明。通过修正当前回归函数的参数。可以使预测函数的预测精度得到很大程度的提高。 相似文献
11.
研究目的：预测保护层开裂的时间以及分析锈胀参数
研究方法：基于混凝十的各向异性损伤,建立考虑钢筋腐蚀产物混凝十三者不同力学性能的钢筋锈胀导致保护层开裂的数学模型。模型考虑了腐蚀产物对钢筋混凝土界面区的孔隙和混凝十开裂裂缝的填充效应,采用了非线性分析算法,预测了开裂过程中每一时刻混凝土构件的应变与位移场以及混凝土保护层开裂时间,最后将模型预测值与试验值进行对比。
1.当混凝土出现裂缝之后,随着腐蚀产物对裂缝的填充,混凝土的环向拉应变的增长速率减缓;2.选定钢筋的型号、直径以及混凝土的强度之后,可通过增大保护层的厚度来减小钢筋锈胀开裂的风险。 相似文献
12.
站在教师的立场解读其社会文化、职业文化和个体需求是理解教师的最佳方式,是解读教师精神世界的有效途径。教师管理可以从中找寻到人本管理的基本依据和思路,从而构建高效的教师管理模式。 相似文献
13.
A Second-Order Conditionally Linear Mixed Effects Model With Observed and Latent Variable Covariates
Jeffrey R. Harring Nidhi Kohli Rebecca D. Silverman Deborah L. Speece 《Structural equation modeling》2013,20(1):118-136
A conditionally linear mixed effects model is an appropriate framework for investigating nonlinear change in a continuous latent variable that is repeatedly measured over time. The efficacy of the model is that it allows parameters that enter the specified nonlinear time-response function to be stochastic, whereas those parameters that enter in a nonlinear manner are common to all subjects. In this article we describe how a variant of the Michaelis–Menten (M–M) function can be fit within this modeling framework using Mplus 6.0. We demonstrate how observed and latent covariates can be incorporated to help explain individual differences in growth characteristics. Features of the model including an explication of key analytic decision points are illustrated using longitudinal reading data. To aid in making this class of models accessible, annotated Mplus code is provided. 相似文献
14.
The structural circumferential periodicity of inertial excitation produced by concentrated mass was utilized to establish the mathematical model of thin circular plate carrying eccentric concentrated mass and to analyze its transverse vibration. The fundamental frequency coefficient, natural frequency and mode shape function are determined by this method. A clamped thin circular plate was taken as an example to study the mass effect on the vibrating system.Comparison between the present results and published ones exhibits excellent agreement, which shows that the analytical method in this paper can be used to predict the transverse vibration parameters accurately. 相似文献
15.
提出了一种两阶段多功能光伏逆变器(QMFI),有功功率借助于该结构可以在一个功率转换器内从光伏阵列直接传输到电网或负载,从而提高转换效率。此外,考虑QMFI最大功率点跟踪(MPPT)的实现和非有功电流的补偿,提出了QMFI的数学模型,从而获得有效的控制参数。在求解过程中,采用基于旋转坐标系的分析模型,并推导了QMFI的空间矢量脉冲宽度调制策略,直观地揭示了非有功电流补偿对两阶段有功潮流的影响。最后,通过3 kVA样机验证了所提方案的可行性和有效性。 相似文献
16.
后金融危机下的“民工荒”问题研究 总被引：1，自引：0，他引：1
蒯鹏州 《淮阴师范学院学报(哲学社会科学版)》2010,32(1):47-50
受金融危机的影响,国内制造业的“民工荒”现象暂时被压制,民工就业难问题凸显。但随着金融危机影响的消退,发达地区再次出现“民工荒”征兆。基于刘易斯模型和托达罗模型的分析框架,从剩余劳动力的供给和需求两方面来看：供给方面,农村仍存有大量剩余劳动力,剩余劳动力的受教育结构依然偏低;需求方面,产业升级使城市部门对劳动力人力资本存量的要求发生了变化。由于需求发生了持续性的变化而供给却未有调整,剩余劳动力市场的供求关系出现结构性失衡,民工就业难与“民工荒”并存的现象随之出现。 相似文献
17.
Andrew Fisher 《Educational Philosophy and Theory》2020,52(7):780-790
AbstractThe philosophy of trust is a relatively small subfield. Nonetheless, it contains within it many important insights. Our contention in this paper is that careful study of this subfield can bring with it many insights that can and should be used to reconsider a variety of arguments that have been brought forward in the literature on the philosophy of education and other areas of educational research. Our focus is largely destructive in section 2 of the paper. We object to two recent papers from the philosophy of education literature arguing that they fail to recognise important insights from the philosophical literature on trust. In section 3 we say something more positive, noting the routes forward. 相似文献
18.
INTRODUCTION The convenience of wireless communications has led to increasing use of wireless networks for civilian and critical applications. With the advent of Internet technology and the increasing popularity of wireless data devices, the wireless industry is evolv- ing its core networks toward IP-based networks. It will be necessary to integrate the wireless networks and the existing wired networks into the Internet. TCP has been the predominant transport protocol used in the wired … 相似文献
19.
周洪伟 《南京晓庄学院学报》2006,22(6):1-4,25
为了进一步探求复杂网络的形成机制,文章提出了一种B A(Barabási-A lbert)模型的扩展模型,考虑网络节点增加的同时,网络内部演化对网络发展的影响.该模型不仅包含加点、加边两种外部演化;而且还包含重连和删边两种内部演化,在每个时间步的操作中,新引入结点的度、重连的边数都是随机的.针对单偏好依附网络模型,运用连续性理论及随机分析理论证明,如果适当选取模型参数,这种网络自演化为无标度网络.而且验证了分析的结果与计算机模拟具有很好的一致性. 相似文献