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Mạng lưới giao thông và đánh giá hiệu suất P8
High-quality traf®c measurements indicate that actual traf®c behavior over highspeed networks shows self-similar features. These include an analysis of hundreds of millions of observed packets on several Ethernet LANs [7, 8], and an analysis of a few million observed frame data by variable bit rate (VBR) video services [1]. In these studies, packet traf®c appears to be statistically self-similar [2, 11].
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Mạng lưới giao thông và đánh giá hiệu suất P9
Several recent measurement studies have concluded that classical Poisson-like traf®c models do not account for time dependencies observed at multiple time scales in a wide range of networking applications, for example, Ethernet LANs [13, 20, 33], variable bit rate (VBR) traf®c [3, 15], Web traf®c [6], and WAN traf®c [31].
8/29/2018 6:16:59 PM +00:00
RF và mạch lạc lò vi sóng P1
Scientists and mathematicians of the nineteenth century laid the foundation of telecommunication and wireless technology, which has affected all facets of modern society. In 1864, James C. Maxwell put forth fundamental relations of electromagnetic ®elds that not only summed up the research ®ndings of Laplace, Poisson, Faraday, Gauss, and others but also predicted the propagation of electrical signals through space. Heinrich Hertz subsequently veri®ed this in 1887 and Guglielmo Marconi successfully transmitted wireless signals across the Atlantic Ocean in 1900...
8/29/2018 6:13:57 PM +00:00
RF và mạch lạc lò vi sóng P2
Modern communication systems require radio frequency and microwave signals for the wireless transmission of information. These systems employ oscillators, mixers, ®lters, and ampli®ers to generate and process various kinds of signals. The transmitter communicates with the receiver via antennas placed on each side. Electrical noise associated with the systems and the channel affects the performance. A system designer needs to know about the channel characteristics and system noise in order to estimate the required power levels....
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RF và mạch lạc lò vi sóng P3
Transmission lines are needed for connecting various circuit elements and systems together. Open-wire and coaxial lines are commonly used for circuits operating at low frequencies. On the other hand, coaxial line, stripline, microstrip line, and waveguides are employed at radio and microwave frequencies. Generally, the lowfrequency signal characteristics are not affected as it propagates through the line. However, radio frequency and microwave signals are affected signi®cantly because of the circuit size being comparable to the wavelength...
8/29/2018 6:13:57 PM +00:00
Kỹ thuật xung - số
Tham khảo sách 'kỹ thuật xung - số', kỹ thuật - công nghệ, kĩ thuật viễn thông phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả
8/29/2018 6:13:52 PM +00:00
Giáo trình kỹ thuật xung - số
Các tín hiệu điện áp hay dòng điện biến đổi theo thời gian được chia thành 2 loại cơ bản là tín hiệu liên tục và tín hiệu rời rạc (gián đoạn). Tín hiệu liên tục còn gọi là tín hiệu tuyến tính hay tương tự. Tín hiệu rời rạc gọi là tín hiệu xung hay số Tiêu biểu cho tín hiệu liên tục là tín hiệu sin, như hình 1, với tín hiệu sin ta có thể tính được biên độ của tín hiệu tại từng thời điểm khác nhau....
8/29/2018 6:13:39 PM +00:00
Tuyến cáp quang theo quỹ công suất và thời gian lên trong hệ thống thông tin sợi quang, ch 4
Giới thiệu chương Trong những năm gần đây, công nghệ thông tin quang đã đạt được những thành tựu rất lớn trong đó phải kể đển kỹ thuật ghép kênh quang, nó thực hiện việc ghép các tín hiệu ánh sáng để truyền trên sợi dẫn quang và việc ghép kênh sẽ không có một quá trình biến đổi về điện nào. Mục tiêu của việc ghép kênh cũng nhằm tăng dung lượng kênh truyền dẫn và tạo ra các tuyến thông tin quang có dung lượng cao. Khi tốc độ đạt tới một mức độ nào đó thì người...
8/29/2018 6:13:35 PM +00:00
thiết kế tuyến cáp quang theo quỹ công suất và thời gian lên trong hệ thống thông tin sợi quang, chương 5
Giới thiệu chương Các hệ thống thông tin quang được ứng dụng có hiệu quả nhất trong lĩnh vực truyền dẫn số. Do vậy trong tính toán, thiết kế ta xem xét hệ thống truyền dẫn số IM-DD (Intensity Modulation-Direct Detection) thì những điều kiện bắt buộc về kỹ thuật và tính kinh tế đóng một vai trò quan trong trong tất cả các tuyến thông tin sợi quang. Người thiết kế phải chọn cẩn thận từng công đoạn để đảm bảo sao cho cả hệ thống trong suốt thời gian phục vụ đều hoạt động tốt. 5.2 Các khái...
8/29/2018 6:13:35 PM +00:00
Thiết kế tuyến cáp quang theo quỹ công suất và thời gian lên trong hệ thống thông tin sợi quang, ch 3
Giới thiệu chương Trong chương này sẽ trình bày một cách khá chi tiết về thiết bị phát quang như LED, LD hay thiết bị thu PIN, APD cũng như nguyên tắc hoạt động của nó để từ đó chúng ta có thể lựa chọn được thiết bị phù hợp với hệ thống và yêu cầu thiết kế. 3.2 Thiết bị phát quang 3.2.1 Cơ chế phát xạ ánh sáng Giả thuyết có một điện tử đang nằm ở mức năng lượng thấp ( E1 ), không có điện tử nào nằm ở mức năng lượng mức cao hơn (...
8/29/2018 6:13:35 PM +00:00
Bài tập lập trình chuyên dụng trong điện tử - viễn thông
Tài liệu tham khảo gồm ngân hàng câu hỏi môn kỹ thuật lập trình trong điện tử - viễn thông. Đây là 1 số câu hỏi bài tập trong môn lập trình chuyên dụng cho ngành điện tử viễn thông, mời các bạn cùng tham khảo ôn tập.
8/29/2018 6:13:32 PM +00:00
Dịch vụ mạng thế hệ kế tiếp P1
THE EVOLUTION OF CIRCUIT SWITCHING The current circuit switched network concept has remained essentially unchanged from the original electromechanical Strowger exchange (see the Preface for an explanation of how this exchange came by its name). At its most basic level the telephone network comprises transmission paths and switching nodes. The design of a circuit switch is based on the ability to physically create a path (circuit) from one network element to another and to hold this path open for the duration of the interaction (call)....
8/29/2018 6:13:31 PM +00:00
Dịch vụ mạng thế hệ kế tiếp P2
The Transmission Infrastructure We pass by transmission on the way to the Intelligent Network (IN) because it is important to understand how all the switching and signalling nodes (not to mention voice links) are connected together. Like the last chapter, we will pass briefly through analogue transmission first as a means of exposing the desires for digital transmission. Then move on to digitisation of speech and look at how large volumes of calls are economically carried across the world. Analogue voice signals can suffer from a number of interference problems ranging from degradation due to distance through to external signalling inducing...
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Dịch vụ mạng thế hệ kế tiếp P3
Intelligent Networks The Intelligent Network (IN) standards are divided into the Telcordia standardisation called Advanced Intelligent Networks (AIN) and the International Telecommunications Union telecommunications (ITU-T) IN standards capability sets. The Telcordia standards are used mainly in North America, whilst the ITU-T standards are relevant in pretty much the rest of the world.
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Dịch vụ mạng thế hệ kế tiếp P4
Mobile Networks Mobile telephony networks have been a phenomenal success story since their introduction in the mid- to late 1980s. This success is being built upon, and a number of operators (old and new) around the world are looking to provide the latest generation of mobile network technology labelled 3G for third generation.
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Dịch vụ mạng thế hệ kế tiếp P5
Packet Switched Technologies In the same way we started with the transmission infrastructure for circuit switched technologies, packet switching has a similar underlying infrastructure. At this stage, it is worth reviewing the International Standards Organisation’s Open Systems Interconnect model (ISO-OSI) that was used in Chapter 1 as a framework for the telephony signalling protocols (Figure 5.1). In the case of packet-based networks such as the Internet, since this is the one of specific interest to us, the bottom two layers of the OSI stack are populated with technologies that differ based on their geographic boundary: local area, campus or metropolitan area...
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Dịch vụ mạng thế hệ kế tiếp P6
Access Technologies Access is arguably the most important part of the network infrastructure, since it is the point at which all revenue-generating customers come into contact with the network. It is also an area of great diversity from basic telephone access through private exchange and Automatic Call Distributor (ACD) connections, not to mention HFC cable Integrated Access Device (IAD) protocol stack TV, to leased data service connections and wireless connectivity in the form of wireless Local Area Networks (LANs) and of course the now near ubiquitous mobile phone (in Europe at least)....
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Dịch vụ mạng thế hệ kế tiếp P7
Voice and Data Convergence The subject of this book is all about the convergence of voice and data services into a new world of advanced (and hopefully useful) applications that will enrich the way people work and communicate. So a specific chapter entitled voice and data convergence could jar a little with the reader, however, this chapter was prompted by the words in the International Telecommunications Union telecommunications (ITU-T) I.121 specification ‘‘The need to integrate both circuit- and packet-transfer mode into one universal broadband network’’....
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Dịch vụ mạng thế hệ kế tiếp P8
Representing Information Throughout computing history information has been represented in various forms from basic text through rich-text formats, postscript to binary encoding. More recently information and the way it should be presented with its roots in publishing has found its way to the top in the form of markup languages. The markup language most people will be familiar with (perhaps unknowingly) is Hypertext Markup Language (HTML). If you use a web browser, you’re using HTML and in future its successor Extensible Hypertext Markup Language (XHTML)....
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Dịch vụ mạng thế hệ kế tiếp P9
Directories – More Than Just Information Storage Directories are rapidly becoming the cornerstone to delivering flexibility for advanced services, both as means of storing location information, user configuration and resource privileges and as a means of storing servicespecific information allowing for rapid provisioning of customers and rapid modification of services. One of the key issues that directories will resolve for the next-generation networks is the dynamic nature of the future services and the desire to reach services based on useful textbased naming....
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Dịch vụ mạng thế hệ kế tiếp P10
Intelligent Network Services In Part 1 of this book, we examined the functional and physical characteristics of circuit switched based Intelligent Networks (INs). In this chapter, we are going to explore what these elements do by way of offering services to customers and giving carriers a flexible means of delivering new services. The IN service model was the first step to releasing service control from the hands of switch manufacturers and as such presented telecoms operators with a new vehicle for realising services that enhanced the basic call control capabilities of stored program control switches. ...
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Dịch vụ mạng thế hệ kế tiếp P11
Call Centres Call centres are the place where technology advances seem to be taken up with the most gusto and where the saying ‘time is money’ is so true. A call centre environment is a pressure bottle where agents are constantly trying to serve the customer, whilst keeping their interaction times to a bare minimum, they have been labelled the sweat-shops of the 1990s and gained a bad reputation for staff morale.
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Dịch vụ mạng thế hệ kế tiếp P12
Internet-Based Services It all started with Internet Service Providers (ISPs) offering to host a company’s website and email. ISPs can offer economies of scale and could cost effectively maintain a permanent connection to the Internet and offer dial-up solutions to their customers. The customers in return get a permanent presence on the Internet for their website and permanent email delivery service. In the corporate environments the move from centralised server and mainframe environments to peer-to-peer networks with local workgroup servers had started the return to centrally managed servers....
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Dịch vụ mạng thế hệ kế tiếp P13
Bringing it all Together – the New Network Architecture In Part 2 of the book we have explored some of the key existing services and how they may progress. In this chapter I would like to take you a little way into the future, for a view of what the network may look like and explore how a service or group of services may fit together to form a useful example of where next-generation networks will take us. Let us go over the components of the next-generation network:
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Dịch vụ mạng thế hệ kế tiếp P14
Expectation and Realisation The last decade has seen massive acceleration in the introduction of new technologies and what seemed to be an insatiable demand for IT. Just look at the changes that we have seen: WAP, GPRS, xDSL, DWDM, VoIP, xSPs and hosted services, Bluetooth, B2B and B2C, XML, Java, and others. The thirst for new things has made the market fickle; just look what’s happened to Wireless Application Protocol (WAP). WAP was/is basically a good idea with a good foundation just oversold (some might argue under-delivered?)....
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Đo lường quang học P1
Before entering into the different techniques of optical metrology some basic terms and definitions have to be established. Optical metrology is about light and therefore we must develop a mathematical description of waves and wave propagation, introducing important terms like wavelength, phase, phase fronts, rays, etc. The treatment is kept as simple as possible, without going into complicated electromagnetic theory.
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Đo lường quang học P2
Gaussian Optics Lenses are an important part of most optical systems. Good results in optical measurements often rely on the best selection of lenses. In this chapter we develop the relations governing the passage of light rays through imaging elements on the basis of the paraxial approximation using matrix algebra. We also mention the aberrations occurring when rays deviate from this ideal Gaussian behaviour. Finally we go through some of the standard imaging systems.
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Đo lường quang học P3
Interference The superposition principle for electromagnetic waves implies that, for example, two overlapping fields u1 and u2 add to give u1 + u2 . This is the basis for interference. Because of the slow response of practical detectors, interference phenomena are also a matter of averaging over time and space. Therefore the concept of coherence is intimately related to interference. In this chapter we will investigate both topics. A high degree of coherence is obtained from lasers, which therefore have been widely used as light sources in interferometry. In recent years, lack of coherence has been taken to advantage in a...
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Đo lường quang học P4
Diffraction When light passes an edge, it will deviate from rectilinear propagation. This phenomenon (which is a natural consequence of the wave nature of light) is known as diffraction and plays an important role in optics. The term diffraction has been conveniently defined by Sommerfeld as ‘any deviation from rectilinear paths which cannot be interpreted as reflection or refraction’. A rigorous theory of diffraction is quite complicated. Here we develop expressions for the diffracted field based on Huygens’ principle of secondary spherical wavelets. It is a very fortunate coincidence that the aperture and the diffracted far field are connected by a...
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Đo lường quang học P5
Light Sources and Detectors The most important ‘hardware’ in optical metrology is light sources and detectors. To appreciate the various concepts of these devices, we first introduce the different units and terms for the measurement of electromagnetic radiation. Then the laser is given a relatively comprehensive treatment. The description of detectors involves some understanding of semiconductor technology. Therefore a brief introduction to semiconductors is given in Appendix E. Because of the increasing use of the CCD camera in optical metrology, this device is described separately in Section 5.8. ...
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