共查询到20条相似文献，搜索用时 31 毫秒
1.
Inrecent years ,IEEE802.11 wirelesslocal area net-work (WLAN) has emerged as a prevailing technology forthe (indoor) broadband wireless access to internet for themobile/portable devices . In IEEE 802.11 standards ,802.11b has become the most widely used v… 相似文献
2.
3.
Jázef LUBZCZ 《浙江大学学报(A卷英文版)》2008,9(10):1309-1317
This paper presents an analytical saturation throughput model of IEEE 802.11DCF （distributed coordination function） with basic access in ad hoc mode. The model takes into account freezing of the backofftimer when a station senses busy channel. It is shown that taking into account this feature of DCF is important in modeling saturation throughput by yielding more accurate and realistic results than models known from literature. The proposed analytical model also takes into account the effect of transmission errors. All essential features of the proposed analytical approach are illustrated with numerical results. The presen- tation of the model is proceeded by an overview of approaches to IEEE 802.11 network performance evaluation presented in the literature. 相似文献
4.
Conditioning-specific reflex modification occurs when an unconditioned response is modified in theabsence of the conditioned stimulus as a result of pairings of the conditioned stimulus and an unconditioned stimulus. In two experiments,
we assessed conditioning-specific reflex modification in either a novel context (Experiment 1) or a context different from,
but equally familiar in relation to, the training context (Experiment 2). Conditioning-specific reflex modification did not
demonstrate sensitivity to a novel context but did demonstrate sensitivity to a change in familiar context. The data cannot
be explained by unconditioned stimulus preexposure, overtraining, or context insensitivity. The results suggest that conditioning-specific
reflex modification models normal stress and may be used to evaluate theories of and treatments for posttraumatic stress disorder. 相似文献
5.
首先研究了b-family方程在临界空间中的局部适定性.在参数为s=3/2的临界Besov空间Bs2,r(该空间是Sobolev空间Hs的一种推广形式)中,采用Littlewood-Paley分解方法,得到当初值u0(x)∈B3/22,1为临界正则时,存在最长时间T=T(u0)0,使得b-family方程有唯一解u(t,x)∈C[0,T];B3/22,()1∩C1([0,T];B12,1),且解u(x,t)是连续依赖于初值u0(x).进一步,在合适的Besov尺度空间E中,运用抽象的CauchyKow alevski定理研究b-family方程解的解析性,证得:当初值是解析的,则该方程解在全空间和局部时间内也是解析的. 相似文献
6.
XU Ou-guan SU Hong-ye MU Sheng-jing CHU Jian 《浙江大学学报(A卷英文版)》2006,7(11):1932-1941
INTRODUCTION Residua catalytic cracking (RCC) is a process in which crude oil fractions are cracked into more valuable lighter products in a riser reactor with flu- idized catalyst. The complexity of charge stocks make it extremely infeasible to characterize and de- scribe at a molecular level, so attempts have been made to lump large numbers of chemical compounds into a kinetic species to describe the complex reac- tions in the process (Wei and Kuo, 1969; Kuo and Wei, 1969). Some early… 相似文献
7.
UrbanareapollutionofShijiazhuangcityisnowacauseofmajorconcernduetorapidindustrializationandin creasedgrowthofautomobiles .Theambientairqualityhasdeterioratedtosuchanextentthatitposesaserioushealthhazard .Considerableeffortsarebeingtakenbyregu latorya genciestomonitorthequalityofairandtomaintainpollutionlevelswithinpermissiblelimits .Sothelong term (to 2 0 10 )andshort term (to 2 0 0 5 )controlstrategieshavebeende signed . Thelong termcontrolstrategyhastwoschemes.Thefirstschemeincludesthefo… 相似文献
8.
孙博 《连云港职业技术学院学报》2007,20(4):31-33,36
随着便携式计算机(包含笔记本和PDA)的快速普及,无线局域网IEEE 802.1 1成为目前短距离无线通讯技术的主流.无线局域网IEEE 802.11中采用DCF、PCF的接入机制,IEEE802.11e中采用EDCF、HCF的接入机制.对这四种接入机制的接入效率和服务质量 (QoS)进行分析,在此基础上提出了两种改进接入效率的方式,即碰撞导向工作模式与工作站数量导向工作模式.通过这两种方式使网络资源的分配更具有弹性,并满足具有不同优先级的工作站对效率的需求. 相似文献
9.
Ryan J. Kettler Michael C. Rodriguez Daniel M. Bolt Stephen N. Elliott Peter A. Beddow Alexander Kurz 《教育实用测度》2013,26(3):210-234
Federal policy on alternate assessment based on modified academic achievement standards (AA-MAS) inspired this research. Specifically, an experimental study was conducted to determine whether tests composed of modified items would have the same level of reliability as tests composed of original items, and whether these modified items helped reduce the performance gap between AA-MAS eligible and ineligible students. Three groups of eighth-grade students (N?=?755) defined by eligibility and disability status took original and modified versions of reading and mathematics tests. In a third condition, the students were provided limited reading support along with the modified items. Changes in reliability across groups and conditions for both the reading and mathematics tests were determined to be minimal. Mean item difficulties within the Rasch model were shown to decrease more for students who would be eligible for the AA-MAS than for non-eligible groups, revealing evidence of differential boost. Exploratory analyses indicated that shortening the question stem may be a highly effective modification, and that adding graphics to reading items may be a poor modification. 相似文献
10.
为了提高低信噪比条件下再生伪码测距系统中的复合伪码时钟恢复性能,采用了一种新型的码片跟踪环.此码片跟踪环是一种改进型的数据转换跟踪环,两者之间的区别在于,码片跟踪环的Q路输出直接乘上了时钟分量,而数据转换跟踪环乘上了I路的转换检测器输出.在准方波伪码测距信号的情况下,详细分析了码片跟踪环的跟踪(均方定时抖动)性能.分析对比了不同码片信噪比条件下的码片跟踪环和数据转换跟踪环跟踪性能.分析表明,码片跟踪环和数据转换跟踪环在大信噪比条件下性能一致;而在小信噪比条件下,码片跟踪环性能明显好于数据转换跟踪环. 相似文献
11.
Behavior was studied in 15 rhesus monkeys mother-infant pairs, 5 of whom were exposed to delta-9-tetrahydrocannabinol continuously throughout gestation and the nursing period via drug treatment of mother (2.4 mg/kg/day). The drug-exposed mother-infant pairs were similar to nontreated controls in the amount and types displayed at 10 and 90 days of age, and adequate maternal care was demonstrated by all mothers. Drug-exposed pairs, however, showed no increase in interaction patterns that signal the onset of mother-infant independence at about 3 months of age. This finding suggests that mother-infant attachment may be a behavioral system that is particularly sensitive to modification by perinatal exposure to psychoactive drugs. 相似文献
12.
钱冬云 《浙江工贸职业技术学院学报》2006,6(1):88-92,87
在数据挖掘算法中，Apriori算法是关联规则的经典算法。文章在分析经典Apriori算法的基础上，提出了改进型的Apriori算法。改进后的算法对事务数据库进行两次压缩，减少事务数据库的平均长度，提高了数据挖掘的效率，具有一定的实用性。 相似文献
13.
刘松明 《漯河职业技术学院学报》2008,7(2):57-58
WLAN作为一种无线接入网络,在许多场合下正扮演着越来越重要的角色。本文介绍了无线局域网标准和无线局域网QoS的相关情况,重点介绍了DCF机制,对DCF进行理论分析,通过编写C语言程序在OPNET里的仿真,得到了影响节点的吞吐量和延时的参数,最后对参数进行了分析。 相似文献
14.
A modification of the nonlinear curve-fitting procedure proposed by Wetherington and Lucas (1980) was used to assess how well Herrnstein’s (1970) equation for the rates of responding during concurrent schedules described performance. The equation fitted some results very well, accounting for 80% or more of the variance in the data in studies that used moderate-duration changeover delays and provided the same positive reinforcers, operanda, and simple schedules in the two components. The equation fitted the data poorly in other studies. The k parameter changed with several variables; it was not as constant as Herrnstein (1974) suggested. R0 did not fit Herrnstein’s interpretation as reinforcement from unprogrammed sources. Forty percent of all values of R0 were negative, and another 23% were unreasonably large (greater than 50 reinforcers/h). The data suggest that Herrnstein’s equation is not a general theory of concurrent-schedule responding, and that Herrnstein’s interpretation of k and R0 should be modified. 相似文献
15.
As a raw material,kaolin was modified to prepare a high-performance ammonium ion-exchange material.According to cation-exchange capacity (CEC) measurement,the prepared ammonium ion-exchange material has an ammonium ion-exchange capacity greater than 75mg/g and can be used to remove ammonia nitrogen in water treatment.A pharmaceutical extruder-rounder was used to study the effects of granulation process.Polyethylene glycol6000 (PEG-6000) was used as the pore-forming agent and calcining temperature of 650℃ was used as the optimal condition.NMR data indicate that 27 Al is mainly converted from hexa-coordinated Al to tetra-coordinated Al during the modification.Compared with 29 Si in the original kaolin,29 Si in the modified kaolin does not have an obvious change.A comparison of the data with Langmuir and Freundlich isotherm models shows a good correlation.The Freundlich model describes the process more accurately,and the adsorption process of ammonia nitrogen in water with the ammonium ion-exchange material closely matches the pseudo-second-order reaction. 相似文献
16.
拓宽公民有序的政治参与渠道是构建和谐社会的重要前提。近年来昆明市在这方面取得了良好的成效,但现有的参与渠道仍满足不了市民的政治参与需求。因此,要进一步完善昆明市公民有序政治参与的基本制度;健全和规范公民有序政治参与的社会组织渠道;加强网络政治参与平台建设;转变相关部门工作作风,保证现有政治参与渠道畅通;继续开辟新的政治参与渠道;等等。 相似文献
17.
ZHOU Jie YANG Yong-rong REN Xiao-hong STAPF Siegfried 《浙江大学学报(A卷英文版)》2006,7(8):1440-1446
INTRODUCTION Waste tires cannot degrade in the short term; they may cause accidental fire and emit poisonous gases rich in dioxins. How to recycle and utilize them effectively, and prevent secondary pollution to the environment has become a new issue that the reuse of resources is faced with. Pyrolysis is an environ-ment-friendly process for recycling of used tires. Three products are typically obtained from the pyrolysis of waste tires: gas, oil and char (Gonzalez etal., 2001). Pyrolyt… 相似文献
18.
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
19.
Wang Gaoying Yuan Jintao Luo Ji Ocansey Dickson Kofi Wiredu Zhang Xu Qian Hui Xu Wenrong Mao Fei 《Journal of Zhejiang University. Science. B》2022,23(3):173-188
The onset of inflammatory bowel disease (IBD) involves many factors, including environmental parameters, microorganisms, and the immune system. Although research on IBD continues to expand, the specific pathogenesis mechanism is still unclear. Protein modification refers to chemical modification after protein biosynthesis, also known as post-translational modification (PTM), which causes changes in the properties and functions of proteins. Since proteins can be modified in different ways, such as acetylation, methylation, and phosphorylation, the functions of proteins in different modified states will also be different. Transitions between different states of protein or changes in modification sites can regulate protein properties and functions. Such modifications like neddylation, sumoylation, glycosylation, and acetylation can activate or inhibit various signaling pathways (e.g., nuclear factor-κB (NF-κB), extracellular signal-regulated kinase (ERK), and protein kinase B (AKT)) by changing the intestinal flora, regulating immune cells, modulating the release of cytokines such as interleukin-1β (IL-1β), tumor necrosis factor-α (TNF-α), and interferon-γ (IFN-γ), and ultimately leading to the maintenance of the stability of the intestinal epithelial barrier. In this review, we focus on the current understanding of PTM and describe its regulatory role in the pathogenesis of IBD.
相似文献20.
在移动自组织网络中,还没有实现多信道多接口的功能。通过修改NS2中移动节点结构引入多信道多接口,使得路由协议能够灵活调用多信道多接口功能。对于新增加的信道需要进行合理分配,基于路由协议AODVPlus的实现机制,提出一种多信道多接口的信道分配策略,并通过仿真模拟与单信道进行比较分析。多信道多接口在一定程度上改善了网络互通的性能,但有效的多信道分配策略显得尤为重要。 相似文献