Experiments in multipoint multimedia telecommunication

Five years of development and trials demonstrate that high-level primitives allow for the rapid creation of multipoint multimedia applications such as transcontinental computer-supported cooperative work. However, it remains unclear if the benefits justify the costs     

一种新的海底光缆监测系统的研究

建立海底光缆监测系统对保障已经成为当代洲际通信主要手段之一的海底光纤通信非常重要.文章提出了一种新的基于BOTDR和声纳的海底光缆事件监测系统,并讨论了其测量方法.     

Ultra-high-bit-rate networking: from the transcontinental backboneto the desktop

The advances in photonic device technologies are bringing ultra-high-bit-rate networking-at speeds towards 100 Gb/s and beyond-much closer to practical reality. It is increasingly likely that in the longer term ultrafast optical time-division techniques-together with wavelength multiplexing-will be used in networks at all levels, from the transcontinental backbone to the desktop. Examples of devices include a subpicosecond clock source packaged inside a laptop personal computer and an OTDM switch on a single semiconductor chip, both produced at HHI. Advances similar to these make it possible now to envisage the use of OTDM techniques, not just in the highest layers of national and international networks, but also much closer to the user-such as the world-first demonstrations at BT Laboratories of a 40 Gb/s TDMA LAN and a 100 Gb/s packet self-routing switch for multiprocessor interconnection. Ultrafast networks might even provide the interconnection backplane inside future desktop routers and servers with massive throughput     

A continent bound by wire [telegraph system]

The development of the transcontinental telegraph system is recounted. The Pony Express helped to establish routes for the system, leading to its own demise. In 1860, Western Union received a government subsidy to build an uninterrupted telegraph line from east to west. The logistics of this venture are described. On the eve of the Civil War, telegraph lines stretched through 53000 km, and by 1870, 100000 km. In 1890, they began to level off at over 400000 km, forming a widely accessible communications network that helped a growing business community expand beyond purely local operations     

Microwave Communications--An Historical Perspective

The history of microwave communications includes major discoveries of Morse, Maxwell, Hertz, Marconi, and other pioneers of the radio and electronics fields. This paper traces the early work which led to wireless communications and the long struggle to achieve practical microwave radio. Even though the first microwave line-of-sight systems were demonstrated and placed in service during the 1930's, it was not until the late 1940's and early 1950's that huge transcontinental microwave transmission systems were implemented. The 1960's and 1970's witnessed significant progress in the technology and application of line-of-sight microwave communication systems. Other microwave systems including troposcatter, satellite, and millimeter waveguide transmission systems were also developed during the 1960's and 1970's. The past 100 years have witnessed very significant breakthroughs in radio technology, particularly at microwave frequencies, that have had an enormous impact on the world's societies through improved communications for the populace, business, and governments.     

Airborne collision avoidance and other applications of time/frequency

Time/frequency technology provides a reliable aircraft collision avoidance system (CAS) that can operate in either synchronous or asynchronous modes. Precision time-ordered techniques of CAS provide both range and range-rate measurements in a one-way sense to all aircraft as well as ground stations within range of transmitted microwave signals. The cooperative system utilizes exact frequency references coupled with precise synchronization: control of frequency to 1 part in 10⁸and time to less than 1 µs. In addition to performing specific functions of protecting aircraft, the time/frequency CAS provides a means for wide dissemination of submicrosecond timing. Flying clocks, which are an integral part of the airborne CAS, have been providing transcontinental and intercontinental transfer of time since 1964. CAS ground stations can serve as depositories of time and frequency derived from flying clocks, satellites, Loran-C and Omega navigation systems, or television transfer referenced to national and international time/frequency standards. Airborne relay of CAS time provides a ready means for local transfer of time to users that are not within line of sight to a ground station. Thus time and frequency can be disseminated for applications other than collision avoidance. In the process of developing the technology and originating CAS, new systems for communication, position determination, navigation, and vehicle surveillance evolved. Such systems can be structured to operate at high data rates, provide improved accuracy and use less RF spectrum than existing techniques.     

信息技术与高职教务管理深度融合研究

教务管理是高职院校中的重要部分和核心环节,直接影响着一个学校的教学质量。近年来,由于高职学校的不断扩大,其教学性质也有别于其他高校,教务管理上应与时俱进,将信息技术与教务管理相结合,实现信息化教育,从而更好地提升教学质量和特色教学。文章对信息技术与高职教务管理深度融合进行了研究。     

高职院校自媒体对学生思想政治教育影响

随着网络技术的迅速发展和电子产品的广泛使用,自媒体深刻地改变着我们的生活,同时也给高职学生思想政治教育工作带来了不可忽视的影响和冲击.本文主要以高职学生思想政治教育为研究对象,分析自媒体对高职学生思想政治教育工作带来的积极和消极的影响,同时探索自媒体背景下加强高职学生思想政治教育的对策和方法.     

WDM对终端业务和高层协议的影响

主要从基于 WDM网络的网络层及其高层出发 ,描述了以下问题 :业务准备算法以便最小化电复用的代价 ;波长分配算法 ;光层和电层联合保护和恢复 ;光网络拓扑对高层协议 ,如 IP选路、TCP流量控制和多层交换对光网络拓扑的影响。     

Method dealing with the experimental determination of the hyperabrupt-junction varactor characteristics

An experimental method used for the accurate determination of the differential capacitance C of the diffusion voltage ? and of the exponent ? of a varactor is proposed. The data processing of the experimental results in a numerical way allows independence from the parasitic capacitance. The accuracy which has been achieved is higher than 2 × 10?3 for the determination of C, and higher than 4 × 10?2 for the determination of ?.     

Effect of enhancing the Raman gain factor on the linearity of Raman fibre amplifiers

The effect of enhancing the Raman gain factor on the linearity characteristics of Raman fibre amplifiers is analysed. In addition to providing higher Raman gain per unit pump power, a higher value for the Raman gain factor also results in a lower value for the maximum gain, and therefore the maximum output power, available from the amplifier if it is to operate in the linear regime. This has important implications for optimising the design of Raman fibre amplifiers.<>     

Calculating the optical properties of multidimensionalheterostructures: Application to the modeling of quaternary quantum welllasers

A method for calculating the electronic states and optical properties of multidimensional semiconductor quantum structures is described. The method is applicable to heterostructures with confinement in any number of dimensions: e.g. bulk, quantum wells, quantum wires and quantum dots. It is applied here to model bulk and multiquantum well (MQW) InGaAsP active layer quaternary lasers. The band parameters of the quaternary system required for the modeling are interpolated from the available literature. We compare bulk versus MQW performance, the effects of compressive and tensile strain, room temperature versus high temperature operation and 1.3 versus 1.55 pm wavelength operation. Our model shows that: compressive strain improves MQW laser performance. MQW lasers have higher amplification per carrier and higher differential gain than bulk lasers, however, MQW performance is far from ideal because of occupation of non-lasing minibands. This results in higher carrier densities at threshold than in bulk lasers, and may nullify the advantage of MQW lasers over bulk devices for high temperature operation     

利用扩频技术减少高速时钟电路的电磁干扰

数字电子设备的运行速度越来越快,其时钟频率就越来越高,时钟电路产生的辐射干扰就越为严重。本文介绍了一种对抑制时钟电路辐射干扰较为有效的方法-时钟扩频技术。     

A comparison of the GaAs MESFET and the AlGaAs/GaAs heterojunctionbipolar transistor for power microwave amplification

The study is normalized by constraining the devices to specific application: 1-W RF output power at 10 GHz. It is shown that the power gain and thermal resistance are higher and input impedances lower for the heterojunction bipolar transistor (HBT). Because of the higher thermal resistance, the operating temperature is significantly higher for the HBT, limiting the CW power output from the device. If the device area is increased to reduce the power density, then the input impedance (common emitter) will be proportionally reduced, making input matching much more difficult     

On the Performance of Digital Phase Locked Loops in the Threshold Region

Extended Kalman filter algorithms are used to obtain a digital phase lock loop structure for demodulation of angle modulated signals. It is shown that the error variance equations obtained directly from this structure enable one to predict threshold if one retains higher frequency terms. This is in sharp contrast to the similar analysis of the analog phase lock loop, where the higher frequency terms are filtered out because of the low pass filter in the loop. Results are compared to actual simulation results and threshold region results obtained previously.     

借用高数浅识城市生活污水处理的流程

本文是基于作者在一次实践锻炼的过程中了解到有关城市生活污水处理的具体流程,学习了很多教学课本之外的相关知识,并用高等数学知识对其进行简单的分析和研究.该文充分体现了高等数学做为工具在生活中无处不在,进一步揭示数学来源于生活,生活离不开数学.     

信息技术背景下高职计算机网络技术专业应用技能型人才 培养研究

随着互联网技术的发展,互联网技术人才的需求越来越高,如何培养出满足社会需求的互联网技术人才值得深思和考虑.本文从由当前计算机网络技术的快速发展和不断拓展而造成对计算机网络技术应用和开发的人才需求增多的背景引入,指出目前高职院校计算机网络技术专业教学中存在的问题,提出要培养计算机网络技术应用型人才,并给出了相应的方案.     

Effects of channelling on the electrical properties of donor implanted GaAs

We have measured the electrical effects of implanting tin or tellurium ions into GaAs in random and channelling directions. Channelled implants produced higher electrical activities and broader electron concentration profiles than did random direction implants. To avoid channelling, samples should be carefully misaligned from a low-index direction for implant energies of less than about 100 keV, while implantations at higher energies require less stringent control of the implant direction.     

Effect of the field dependence of the diffusion constant on small-signal behaviour of bulk negative-mobility devices

Modification of the small-signal behaviour of bulk-semiconductor devices, due to the effective drift velocity resulting from the field dependence of the diffusion constant, is discussed. It is effectively less important at lower doping levels and at higher frequencies.     

Convergence analysis of the cascade second-order adaptive line equalizer

The high-speed data transmission on a twisted pair cable suffers from frequency-dependent loss that causes significant inter-symbol interference. Although a first-order adaptive equalizer based on pole-zero cancellation is sufficient to compensate the limited bandwidth for relatively low data rate, a higher order equalizer is necessary for higher data rate. This brief proposes a cascade second-order adaptive equalizer structure and provides its convergence analysis.