三维网络拓扑结构的水声传感器网络MAC协议

水声传感器网络Underwater Acoustic Sensor Network(UWASN)由于其广阔的应用前景,近年来逐渐被人们关注。然而由于水下环境的限制性,水声传感器网络具有其一些独特性。例如,与二维的陆地传感器网络不同,水声传感器网络是三维的。如今,现有的介质访问控制Medium Access Control(MAC)协议大多是针对二维无线传感网络,很少有基于三维水声传感器网络的 MAC 协议。针对水下三维网络,提出了基于三维网络拓扑结构的水声传感器网络 MAC 协议。该协议将网络中的节点生成树结构,并利用子节点与父节点之间的关系,通过动态节点算法实现三维动态的水声传感器网络。此外,详细分析了节点间的碰撞并有效解决各种碰撞,从而大大提高了信道利用率。通过仿真软件对比不同协议在同一网络拓扑中的实验结果,证实文中协议能够有效节约大量能源。     

采用渐进式预留机制的Ad Hoc网络MAC协议

实时业务传输的服务质量(QoS)保障问题一直是限制Ad Hoc网络发展的瓶颈.在分布式网络环境下,分组冲突十分普遍,节点很难在限定的时间内成功接入信道.现有很多算法(如FPRP)虽能很好的消除冲突,却需要非常多的控制分组,同样造成过高的接入时延.在研究动态时隙分配类MAC协议的基础上,设计了一种采用渐进式预留机制的MAC协议,协议通过分级预留、协同竞争和空闲时隙的时隙重构来达到分组冲突的充分化解,信道资源的高效利用,以及不同优先级业务的有效接入.仿真表明,与现有时隙类协议相比,新协议可以显著减少分组冲突,提高信道利用率,实现较低的分组接入延迟,并且能够较好的支持数据报业务.     

水声传感器网络MAC协议的能效分析与仿真

为了研究水下传感器网络多址接入(MAC)协议的能量损耗,在集中式拓扑结构下,对ALOHA和MACAW两种协议的能量效率进行了理论分析,并通过Matlab进行了仿真。结果表明:通过改进的MA-CAW协议在网络节点数较多,数据到达率较高,数据包较大的网络中比ALOHA协议具有更高的能量效率。     

水下声音网络中基于簇的按需时间共享MAC协议

水下声音网络中基于调制解调器的商用系统具有传输率低、前同步码长两大特征,严重降低了媒介访问控制协议(MAC)的性能;针对上述问题,分析了调制解调器特征对当前水下声音网络两种主流MAC协议(随机访问MAC,握手MAC)的影响,提出了一种基于簇的按需时间共享MAC协议(COD-TS);该协议采用簇技术实现了局部调度和按需响应特性;文章首先给出单跳网络的COD-TS协议的含义,提出相应的吞吐量模型;然后,实现方案多跳场景的扩展,仿真结果表明,相比于已有的Slotted ALOHA和RTS/CTS等协议而言,本文方案的吞吐量更优。     

一种高效的水下传感器网络MAC协议

水声信道具有窄带宽、高传播延时等特点,为有效减少水声信道冲突,信道预约型MAC协议成为水下传感器网络的首要选择.通过分析信道预约型MAC协议节点在高负载网络条件下的RTS-CTS控制包的交换过程,给出了多个节点对同时取得信道使用权成立条件,设计了一种在一次RTS-CTS握手后,拥有信道使用权的多个节点对依次进行无碰撞数据传输的MAC协议.最后通过仿真实验验证协议的性能,结果表明:该协议具有更高的网络吞吐量和更低的网络时延.     

无线传感器网络MAC协议研究

媒体访问控制协议是保证无线传感器网络高效通信的关键网络协议之一,传感器网络的性能如吞吐量、延迟性能等完全取决于所采用的MAC协议.与传统网络的MAC协议不相同的是,无线传感器网络的MAC协议首要考虑的因素就是节省能量.针对无线传感器网络的要求,研究人员提出了很多MAC协议,本文中将现有的MAC协议分为三类:随机竞争类MAC协议,时分复用MAC协议,混合型MAC协议.并分别就其中的典型协议进行了分析.     

无线传感器网络中一种竞争窗口自适应MAC协议

媒体访问控制(MAC)协议是保证无线传感器网络(WSNs)高效通信的关键网络协议之一。MAC层协议设计的是否合理将严重影响网络的性能。介绍了现有MAC协议分类和主要MAC协议。分析了WSNs中典型的S-MAC协议,针对S-MAC协议在载波侦听时采用固定竞争窗口的弊端,提出了一种新型的能够根据流量变化对竞争窗口进行动态调整的新的MAC协议ASMAC,利用NS2对ASMAC进行了仿真,证明了新的ASMAC不仅能够显著地提高吞吐量,降低时延,还能有效提高能量效率,在提高网络性能的同时达到节能的目的。     

基于能量的无线传感器网络MAC协议研究

无线传感器网络采用能量有限的电池供电,能量决定了传感器网络的生命期,传感器节点的能耗主要集中在介质访问控制层协议(MAC)上.如何降低传感器网络MAC层协议的能耗,成了解决能量问题的关键.文章从减少数据流量,增加射频模块休眠时间和冲突避免3个方面,深入剖析了当前经典的MAC协议,通过分析得出,在节省能耗上,应该综合考虑各种MAC协议的运用.     

解决Ad Hoc入侵终端问题的MAC层协议

针对目前Ad Hoc网络多信道环境下MAC层存在的入侵终端问题,研究并提出了MAC层协议的改进方案.即在原有的动态信道分配MAC层协议基础上,增加了对入侵终端发生情况下信道重新分配机制和链路重新建立机制.使得Ad Hoc网络在入侵终端冲突发生的情况下,各节点得以迅速重新恢复通信.最后,对改进后协议的正确性和有效性进行仿真验证.     

一种应用于分布式网络的新型私有多信道UWB MAC协议设计

目前,单信道MAC协议在网络负载较重时的冲突和退避,造成信道带宽浪费而使得协议效率不高.本文主要针对UWB应用在无线个域网络中的特定场景,提出了一种新的应用于分布式网络的私有多信道UWB MAC协议,既可以解决隐藏终端和暴露终端问题,又可以有效地分配信道资源,提高网络性能.     

European Community policy and the market

Abstract This paper starts with some reflections on the policy considerations and priorities which are shaping European Commission (EC) research programmes. Then it attempts to position the current projects which seek to capitalise on information and communications technologies for learning in relation to these priorities and the apparent realities of the marketplace. It concludes that while there are grounds to be optimistic about the contribution EC programmes can make to the efficiency and standard of education and training, they are still too technology driven.     

一种自适应子融合集成多分类器方法

融合集成方法已经广泛应用在模式识别领域,然而一些基分类器实时性能稳定性较差,导致多分类器融合性能差,针对上述问题本文提出了一种新的基于多分类器的子融合集成分类器系统。该方法考虑在度量层融合层次之上通过对各类基多分类器进行动态选择,票数最多的类别作为融合系统中对特征向量识别的类别,构成一种新的自适应子融合集成分类器方法。实验表明,该方法比传统的分类器以及分类融合方法识别准确率明显更高,具有更好的鲁棒性。     

Explanation and prediction: an architecture for default and abductive reasoning

Although there are many arguments that logic is an appropriate tool for artificial intelligence, there has been a perceived problem with the monotonicity of classical logic. This paper elaborates on the idea that reasoning should be viewed as theory formation where logic tells us the consequences of our assumptions. The two activities of predicting what is expected to be true and explaining observations are considered in a simple theory formation framework. Properties of each activity are discussed, along with a number of proposals as to what should be predicted or accepted as reasonable explanations. An architecture is proposed to combine explanation and prediction into one coherent framework. Algorithms used to implement the system as well as examples from a running implementation are given.     

Three Process Perspectives: Organizations, Teams, and People

This paper provides the author's personal views and perspectives on software process improvement. Starting with his first work on technology assessment in IBM over 20 years ago, Watts Humphrey describes the process improvement work he has been directly involved in. This includes the development of the early process assessment methods, the original design of the CMM, and the introduction of the Personal Software Process (PSP)^SM and Team Software Process (TSP){^SM}. In addition to describing the original motivation for this work, the author also reviews many of the problems he and his associates encountered and why they solved them the way they did. He also comments on the outstanding issues and likely directions for future work. Finally, this work has built on the experiences and contributions of many people. Mr. Humphrey only describes work that he was personally involved in and he names many of the key contributors. However, so many people have been involved in this work that a full list of the important participants would be impractical.     

基于复小波噪声方差显著修正的SAR图像去噪

提出了一种基于复小波域统计建模与噪声方差估计显著性修正相结合的合成孔径雷达(Synthetic Aperture Radar,SAR)图像斑点噪声滤波方法。该方法首先通过对数变换将乘性噪声模型转化为加性噪声模型,然后对变换后的图像进行双树复小波变换(Dualtree Complex Wavelet Transform,DCWT),并对复数小波系数的统计分布进行建模。在此先验分布的基础上,通过运用贝叶斯估计方法从含噪系数中恢复原始系数,达到滤除噪声的目的。实验结果表明该方法在去除噪声的同时保留了图像的细节信息,取得了很好的降噪效果。     

How do children do mathematics with LOGO?

Abstract  This paper considers some results of a study designed to investigate the kinds of mathematical activity undertaken by children (aged between 8 and 11) as they learned to program in LOGO. A model of learning modes is proposed, which attempts to describe the ways in which children used and acquired understanding of the programming/mathematical concepts involved. The remainder of the paper is concerned with discussing the validity and limitations of the model, and its implications for further research and curriculum development.     

Editorial: Special issue on recent advances in hybrid control theory and applications

正The demands of a rapidly advancing technology for faster and more accurate controllers have always had a strong influence on the progress of automatic control theory.In recent years control problems have been arising with increasing frequency in widely different areas,which cannot be addressed using conventional control techniques.The principal reason for this is the fact that a highly competitive economy is forcing systems to operate in regimes where     

Information for Authors

正Aim The Journals of Zhejiang University-SCIENCE(A/B/C)areedited by the international board of distinguished Chinese andforeign scientists,and are aimed to present the latest devel-opments and achievements in scientific research in China andoverseas to the world’s scientific circles,especially to stimulateand promote academic exchange between Chinese and for-eign scientists everywhere.     

Enhancing a leaf radiative transfer model to estimate concentrations and in vivo specific absorption coefficients of total carotenoids and chlorophylls a and b from single-needle reflectance and transmittance

The relative concentrations of different pigments within a leaf have significant physiological and spectral consequences. Photosynthesis, light use efficiency, mass and energy exchange, and stress response are dependent on relationships among an ensemble of pigments. This ensemble also determines the visible characteristics of a leaf, which can be measured remotely and used to quantify leaf biochemistry and structure. But current remote sensing approaches are limited in their ability to resolve individual pigments. This paper focuses on the incorporation of three pigments—chlorophyll a, chlorophyll b, and total carotenoids—into the LIBERTY leaf radiative transfer model to better understand relationships between leaf biochemical, biophysical, and spectral properties.Pinus ponderosa and Pinus jeffreyi needles were collected from three sites in the California Sierra Nevada. Hemispheric single-leaf visible reflectance and transmittance and concentrations of chlorophylls a and b and total carotenoids of fresh needles were measured. These data were input to the enhanced LIBERTY model to estimate optical and biochemical properties of pine needles. The enhanced model successfully estimated reflectance (RMSE = 0.0255, BIAS = 0.00477, RMS%E = 16.7%), had variable success estimating transmittance (RMSE = 0.0442, BIAS = 0.0294, RMS%E = 181%), and generated very good estimates of carotenoid concentrations (RMSE = 2.48 µg/cm², BIAS = 0.143 µg/cm², RMS%E = 20.4%), good estimates of chlorophyll a concentrations (RMSE = 10.7 µg/cm², BIAS = − 0.992 µg/cm², RMS%E = 21.1%), and fair estimates of chlorophyll b concentrations (RMSE = 7.49 µg/cm², BIAS = − 2.12 µg/cm², RMS%E = 43.7%). Overall root mean squared errors of reflectance, transmittance, and pigment concentration estimates were lower for the three-pigment model than for the single-pigment model. The algorithm to estimate three in vivo specific absorption coefficients is robust, although estimated values are distorted by inconsistencies in model biophysics. The capacity to invert the model from single-leaf reflectance and transmittance was added to the model so it could be coupled with vegetation canopy models to estimate canopy biochemistry from remotely sensed data.     

Mr. Twinn’s bombes

This article discusses the history and design of the special versions of the bombe key-finding machines used by Britain’s Government Code & Cypher School (GC&CS) during World War II to attack the Enigma traffic of the Abwehr (the German military intelligence service). These special bombes were based on the design of their more numerous counterparts used against the traffic of the German armed services, but differed from them in important ways that highlight the adaptability of the British bombe design, and the power and flexibility of the diagonal board. Also discussed are the changes in the Abwehr indicating system that drove the development of these machines, the ingenious ways in which they were used, and some related developments involving the bombes used by the U.S. Navy’s cryptanalytic unit (OP-20-G).