我眼中的下一代无线网络架构

WLAN技术的不断演变,将标准建立团体和互通性论坛紧密联系在一起,也将产品开发商和相应的半导体IC供应商紧密联系在一起,而Wi-Fi射频的演变过程及由此引发的对设计人员的挑战也越来越引起关注。Wi-Fi射频的演变历程在过去几年里,基于802.11b的产品已被证明具有低成本和简单易用的特点,能够确保与无线以太网和互联网的无线接入。目前,802.11b已经以多种封装方式被组装进了计算机、PDA、手机甚至游戏设备当中。后来,企业级应用对更高性能WLAN技术的追求导致了802.11a标准的诞生。尽管802.11a标准的有效覆盖范围比802.11b网络的范围小,…     

Multichannel 64-QAM transmission in AM/digital and all-digitalhybrid-fiber-coax based video dial-tone systems

A hybrid-fiber-coax (HFC) system is operated with a combination of 63 AM and 21 64-QAM subcarriers as well as with the 21 64-QAM subcarriers alone. The 64-QAM subcarriers are 6-MHz wide, decorrelated, and of the type intended for compressed digital video. The AM signal levels (over both the optical and coaxial links) are set to produce standard carrier-to-noise ratios (CNRs), composite second order distortion (CSO), and composite triple beat (CTB). The optical modulation depth (OMD) is independently varied and transmission tests are carried out over the optical link alone, as well as over the entire system (including two coaxial amplifiers, or actives). The relative degradation of the system due to the optical link versus the actives is investigated. For an OMD of 24% or less and with the raw bit-error ratio (BER) set to 10^-5 (as anticipated for use with forward error correction (FEC) systems), the performance is limited by the actives. There is an approximate 3.5 dB penalty in the necessary CNR for 64-QAM reception, due to the presence of the AM signals. On the other hand, when the system is operated with all-digital subcarriers, the optical modulation depth may be increased from 24% to 37% with no penalty in the required digital CNR     

A video conference network management system

Setting up either one or multiple conferences and managing the various devices can be a very difficult task. In this article a newly developed distributed video conference management system is explained and tested. © 1998 John Wiley & Sons, Ltd.     

A look at automatic testing

This survey reviews the current state of automation in the testing of electronic components, networks, and circuits. Elements and characteristics of typical test sytems, both hardware-controlled and computer-controlled, are described. Paths to be followed and pitfalls to be avoided in achieving automation are discussed in an effort to help the reader toward a better understanding of the subject and its broad applications. ``How to automate successfully,' a major theme in the report, places emphasis on economic justification.     

A new look at the antenna effect

It is a widely held belief that plasma process-induced damage (PID) is caused, among other things, by plasma nonuniformities. These nonuniformities are caused by either nonoptimum reactor design and operation or nonflat wafer topography, the latter giving rise to electron shading that selectively inhibits one charge specie over another causing local surface charging. This charging builds over many cycles. While these phenomena are of serious concern, it is the object of this work to point out that there is an additional way to conceptualize the mechanism for PID in ac glow discharges. In this model, it is proposed that the net charge flux in each power supply half cycle is enough to induce a surface potential of sufficient magnitude to damage thin dielectrics. Semiquantitative arguments are made that outline how this mechanism is capable, in principle, of explaining a wide range of published empirical data. Notable among these predictions is that plasma PID can depend upon the size and shape of the antenna, behaves as though the stress were from a current source, and is only proportional to antenna ratio (AR) under special circumstances     

交流声的快速测量

交流声测量是美国联邦通信委员会(FCC)要求每6个月进行一次的测试项目之一。FCC法规第76.605(a)(10)条中说:“由系统内产生的不希望有的低频干扰(交流声或重复性瞬变)或者由不适当的频率响应所造成的图像信号电平的峰—峰值变化不应大于图像信号电平的3％。” 事实上,在有线电视中被称为“交流声”干扰的问题,绝大部分是由于放大器电源供给器方面     

A critical look at the bathtub curve

This paper addresses some of the fundamental assumptions underlying the bathtub curve. It is shown to be unlikely that any practical hazard function is decreasing near zero. Great care should be taken in interpreting the hazard function, particularly in applying quality-control practices, such as burn-in or environmental-stress-screening to manufactured products.     

虚拟存储技术在视频网络中的应用

从对称式虚拟存储和非对称式虚拟存储两个方面，详细阐述了虚拟存储技术在视频网络中的应用。     

Dimensioning network links: a new look at equivalent bandwidth

One of the tasks of network management is to dimension the capacity of access and backbone links. Rules of thumb can be used, but they lack rigor and precision, as they fail to reliably predict whether the quality, as agreed on in the service level agreement, is actually provided. To make better predictions, a more sophisticated mathematical setup is needed. The major contribution of this article is that it presents such a setup; in this a pivotal role is played by a simple, yet versatile, formula that gives the minimum amount of capacity needed as a function of the average traffic rate, traffic variance (to be thought of as a measure of ?burstiness?), as well as the required performance level. In order to apply the dimensioning formula, accurate estimates of the average traffic rate and traffic variance are needed. As opposed to the average rate, the traffic variance is rather hard to estimate; this is because measurements on small timescales are needed. We present an easily implementable remedy for this problem, in which the traffic variance is inferred from occupancy statistics of the buffer within the switch or router. To validate the resulting dimensioning procedure, we collected hundreds of traces at multiple (representative) locations, estimated for each of the traces the average traffic rate and (using the approach described above) traffic variance, and inserted these in the dimensioning formula. It turns out that the capacity estimate obtained by the procedure, is usually just a few percent off from the (empirically determined) minimally required value.     

A new look at the Barankin lower bound

This paper is primarily tutorial in nature and presents an alternative derivation of the Barankin estimator. The approach presented here places clearly in evidence the significance of the choice of parameter points that one must make in obtaining a Barankin lower bound. Specifically, for any finite set of parameter points, the corresponding Barankin bound is the variance of the locally best estimator that is constrained to be unbiased at the chosen parameter points. In addition, the derivation of the Barankin theory presented here provides a new interpretation of the Cramér-Rao and Bhattacharyya bounds.     

A new look at the generalized distributive law

In this paper, we develop a measure-theoretic version of the junction tree algorithm to compute desired marginals of a product function. We reformulate the problem in a measure-theoretic framework, where the desired marginals are viewed as corresponding conditional expectations of a product of random variables. We generalize the notions of independence and junction trees to collections of /spl sigma/-fields on a space with a signed measure. We provide an algorithm to find such a junction tree when one exists. We also give a general procedure to augment the /spl sigma/-fields to create independencies, which we call "lifting." This procedure is the counterpart of the moralization and triangulation procedure in the conventional generalized distributive law (GDL) framework, in order to guarantee the existence of a junction tree. Our procedure includes the conventional GDL procedure as a special case. However, it can take advantage of structures at the atomic level of the sample space to produce junction tree-based algorithms for computing the desired marginals that are less complex than those GDL can discover, as we argue through examples. Our formalism gives a new way by which one can hope to find low-complexity algorithms for marginalization problems.     

无线局域网--展望未来

符合802．11b标准的无线局域网为市场上无线网络的发展搭建了稳定的平台。WiFi标准可保证不同厂商产品的互操作性，从而确保最终消费者投资的安全。随着与802．11b同在2．4GHz带宽上运行的更新的802．11g网络的推出，无线网络得到进一步发展。它们既能支持更高速率(最大54Mb/秒)，又具有向下兼容的特性。此外，消费者也可以选择在5GHz带宽的速率上运行802．11a     

A closer look at VLSI multiplication

In this paper we analyze and compare several algorithms for integer multiplication with respect to their suitability for implementation using VLSI technology. Based on general characteristics of the basic building blocks the VLSI complexities are determined without neglecting constant factors or lower order terms. Thus our analysis goes beyond the usual study of the asymptotic behaviour. Within certain limits the obtained results are technology independent. If a concrete underlying technology (CMOS, nMOS,…) is assumed, they allow to derive statements on the practical relevance of the algorithms in view by using concrete layouts for the basic building blocks. We consider the behaviour in nMOS-technology as an example.     

新视点之一辩证地看待网络成本

由于运营商从目前的GSM网络升级到WCDMA需要耗费巨额资金,因此使很多2G运营商在相当长时间内一直望而却步."但事实上,所谓网络成本是由覆盖成本和容量成本两部分组成,运营商对3G的成本应该综合考虑."华为公司副总裁余承东指出.     

平板显示器的未来新星--纳米平板显示器

美国加州物理学教授,诺贝尔奖得主费曼(Richard P.Feynman)曾经对纳米科技发表他的看法,他说:"在这微小的世界里,蕴藏着无穷的奥妙与机会".     

A new look at systems engineering

The system idea?solution of a complete problem in its full environment by systematic assembly and matching of parts to solve the whole problem in the context of the lifetime use of the system, considering all aspects?is one of the most important ideas of modern times. It has made possible the solution of complicated problems that previously could not be touched. However, there exists a rising tide of problems and general disgruntlement (expressed by many people) that indicates trouble. Therefore, the author speaks as an ``engineering critic' (intended in the sense of a music critic: ``Are we turning in a good performance?') and, contrary to the many who are taking the positive side of systems engineering, offers the negative side of the controversy.     

A critical look at the coordinate measuring technique

The article presents a critical review of the coordinate measuring technique. It is shown that significant measurement errors may occur due to imprecise formulations in the ISO 10360-2 standard (simplified record related to the best match method, simplified acceptance test, reverification test and interim check), limitation regarding the use of measuring equipment (CMM, PC, IEEE 745, software) and accepted calibration procedure (it is only possible to calibrate half the ball tip probe against half the reference sphere). The article also points out the problems of adequate correction of the ball tip radius, simplifications of the computation procedures, limitations of the correction software (Computer Aided Accuracy), ensuring stable temperature and creating a new system of coordinates (Workpiece Coordinate System). The article discusses the weak points of the coordinate measuring technique which have the largest impact on the measurement results and suggests improvements. The material refers only to contact measurements.     

A quick look at rapid prototyping

A recent experience with rapid prototyping at Sandia National Laboratories, using stereolithography and selective laser sintering equipment, as part of a technology transfer program to produce a bicycle crank arm for a local bike shop is described. A process for creating a wax model in only five days, versus several months by traditional methods, is sketched. An experience at Sundstrand Aerospace's Electric Power Systems Division with rapid prototyping of part of an aircraft's variable-speed, constant-frequency electric system is also cited