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Wireless địa phương vòng - lý thuyết và ứng dụng P13
In the telephone networks, the circuit between the subscriber's equipment (e.g. telephone set) and the local exchange in the central office is called the `subscriber loop' or `local loop'. Traditionally, the copper wire has been used as the medium for local loop to provide voice and voice-band data services. Since 1980's, the demand for communications services has increased explosively. There has been a great need for the basic telephone service, i.e. the plain old telephone service (POTS) in developing countries. On the other hand, in the industrialized countries, the demand for high-speed data and multimedia services at home and/or office...
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Lý thuyết mạng không dây P1
Although it has history of more than a century, wireless transmission has found widespread use in communication systems only in the last 15–20 years. Currently the field of wireless communications is one of the fastest growing segments of the telecommunications industry. Wireless communication systems, such as cellular, cordless and satellite phones as well as wireless local area networks (WLANs) have found widespread use and have become an essential tool in many people’s every-day life, both professional and personal. To gain insight into the wireless market momentum, it is sufficient to mention that it is expected that the number of worldwide...
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Lý thuyết mạng không dây P2
Wireless networks, as the name suggests, utilize wireless transmission for exchange of information. The exact form of wireless transmission can vary. For example, most people are accustomed to using remote control devices that employ infrared transmission. However, the dominant form of wireless transmission is radio-based transmission. Radio technology is not new, it has a history of over a century and its basic principles remain the same with those in its early stage of development. In order to explain wireless transmission, an explanation of electromagnetic wave propagation must be given. A great deal of theory accompanies the way in which electromagnetic...
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Lý thuyết mạng không dây P3
As mentioned in Chapter 1, the first public mobile telephone system, known as Mobile Telephone System (MTS), was introduced in 1946. Although it was considered a big technological breakthrough at that time, it suffered many limitations such as (a) the fact that transceivers were very big and could be carried only by vehicles, (b) inefficient way of spectrum usage and (c) manual call switching. IMTS was an improvement on MTS offering more channels and automatic call switching. However, the era of cellular telephony as we understand it today began with the introduction of the First Generation of cellular systems (1G...
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Lý thuyết mạng không dây P4
As was mentioned in the previous chapter, the era of mobile telephony began with the development and operation of the First Generation (1G) of cellular systems in the late 1970s. Although these systems have found widespread use and are still used nowadays, the evolution of technology has enabled the industry to move to Second Generation (2G) systems, the successors of 1G systems. 2G systems overcome many of the deficiencies of 1G systems mentioned in the previous chapter. Their increased capabilities stem from the fact that, contrary to 1G systems, 2G systems are completely digital. ...
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Lý thuyết mạng không dây P5
The big success of first (1G) and second-generation (2G) wireless cellular systems can be attributed to the user need for voice communication services, a need that follows the 3A paradigm: Anywhere, Anytime, with Anyone. By dialing a friend or colleague’s mobile phone number, one is able to contact him/her in a variety of geographical locations, thus overcoming the disadvantage of fixed telephony. For more than a decade, the 2G systems presented in the previous chapters (GSM, IS-136, IS-95) have performed very well as far voice communication is concerned. ...
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Lý thuyết mạng không dây P6
By looking back to the history of wireless systems, one can reach the conclusion that the industry follows a ten-year cycle. First generation systems were introduced in 1981 followed by the deployment of second generation systems in 1991, ten years later. Moreover, third generation systems are due for deployment in 2001–2002. From the point of view of services, 1G systems offered only voice services, 2G systems also offered support for a primitive type of low-speed data services and 3G systems will enable a vast number of advanced voice and high-speed data services. The trend is towards support for even advanced...
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Lý thuyết mạng không dây P7
The first reference to satellite communication systems was made in the mid-1940s by Arthur Clarke [1]. In this paper, Clarke described a number of fundamental issues relating to the building of a satellite network that entirely covers the earth including issues related to spectrum use, the power needed to run the network and the way of bringing the satellites to orbit. Clarke also introduced the concept of geostationary satellites, which – as explained later – orbit the earth in a radius that allows them to appear stationary from the earth’s surface....
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Lý thuyết mạng không dây P8
Fixed Wireless Access Systems The goal of this chapter is to review the main techniques used for Wireless Local Loop (WLL) including the Multichannel Multipoint Distribution Service (MMDS), and the Local Multipoint Distribution Service (LMDS). We also present the main aspects, advantages and disadvantages, and applications of these techniques. The chapter also deals with the wireless local loop subscriber terminals, Wireless Local Loop Interfaces to the Public Switched Telephone Network (PSTN), and the IEEE 802.16 standards on Broadband Wireless Access. Then a final section is given to summarize the main points presented in this chapter. ...
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Lý thuyết mạng không dây P9
The growth of Wireless Local Area Network (WLANs) commenced in the mid-1980s and was triggered by the US Federal Communications Commission (FCC) decision to authorize the public use of the Industrial, Scientific and Medical (ISM) bands. This decision eliminated the need for companies and end users to obtain FCC licenses to operate their wireless products. Since then, there has been a substantial growth in the area of WLANs. Lack of standards, however, enabled the appearance of many proprietary products thus dividing the market into several, possibly incompatible parts. Consequently, the need for standardization in the area appeared. ...
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Lý thuyết mạng không dây P10
Recently, considerable research effort has been put into the direction of integrating the broadband wired ATM [1] and wireless technologies. In 1996 the ATM Forum approved a study group devoted to wireless ATM, WATM. WATM [2–4] aims to provide end-to-end ATM connectivity between mobile and stationary nodes. WATM can be viewed as a solution for next-generation personal communication networks, or a wireless extension of the B-ISDN networks, which will support guaranteed QoS integrated data transmission. WATM will combine the advantages of freedom of movement of wireless networks with the statistical multiplexing (flexible bandwidth allocation) and QoS guarantees supported by traditional...
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Lý thuyết mạng không dây P11
Introduction to PAN Technology and Applications 11.1.1 Historical Overview The concept of a Personal Area Network (PAN) differs from that of other types of data networks (e.g. LAN, MAN, WAN) in terms of size, performance and cost (Figure 11.1). PANs are the next step down from LANs and target applications that demand short-range communications inside the Personal Operating Space (POS) of a person or device. The term POS is used to define the space in the near vicinity of a person or device and can be thought of as a bubble that surrounds him. As the person goes through his regular daily...
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Lý thuyết mạng không dây P12
Security Issues in Wireless Systems The issue of security of computer systems and networks, especially security of wireless networks and systems has become essential, given the dependence of people on these systems in their daily life. This chapter presents the main issues for wireless networks and the need to secure access to such systems; any breach to such systems may entail loss of money, loss of national security information, or leak of such information and secrets to unwanted parties including competitors and enemies (see Section 12.1)....
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Lý thuyết mạng không dây P13
Simulation of Wireless Network Systems This chapter deals with simulation of wireless network systems. We introduce the basics of discrete-event simulation as it is the simulation technique that is used for simulating wireless networks. We then review the main characteristics of the commonly used stochastic distributions used for the simulation of wireless networks. The techniques used to generate and test random number sequences are investigated. Then, we introduce the techniques used to generate random variates followed by performance metrics considerations. The chapter concludes with cases studies on the simulation of some wireless network systems. ...
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Lý thuyết mạng không dây P14
Economics of Wireless Networks The field of mobile wireless communications is currently one of the fastest growing segments of the telecommunications industry. Wireless devices have nowadays found extensive use and have become an indispensable tool on the everyday life of many people, both the professionally and personally. To gain insight into the momentum of the growth of the wireless industry, it is sufficient to state the tremendous growth in the number of worldwide subscribers of wireless systems. This figure has risen from only one mobile subscriber per 100 inhabitants worldwide in 1990 to 26 subscribers per 100 inhabitants in 1999 and...
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Cơ bản về xây dựng mạng truyền thông
Mục đích xây dựng MMT: Nhu cầu chia sẻ tài nguyên, thông tin và dịch vụ. Mô hình tập trung được thay thế bằng mô hình mới: mạng truyền
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Giáo trình kỹ thuật chuyển mạch
Mạng viễn thông là phương tiện truyền đưa thông tin từ đầu phát tới đầu thu. Mạng có nhiệm vụ cung cấp các dịch vụ cho khách hàng. Mạng viễn thông bao gồm các thành phần chính: thiết bị chuyển mạch, thiết bị truyền dẫn, môi trường truyền và thiết bị đầu cuối.
8/29/2018 6:24:46 PM +00:00
Điện thoại di động vô tuyến điện - Tuyên truyền Channel P8
Sounding, Sampling and Simulation In the earlier chapters we discussed the characteristics of mobile radio channels in some detail. It emerged that there are certain parameters which provide an adequate description of the channel and it remains now to describe measuring equipment (channel sounders) that can be used to obtain experimental data from which these parameters can be derived. It is often of interest to make measurements which shed some light on the propagation mechanisms that exist in the radio channel but engineers are usually more interested in obtaining parameters that can be used to predict the performance, or the...
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Điện thoại di động vô tuyến điện - Tuyên truyền Channel P9
The performance of any communication system depends on the characteristics of the transmission medium and can often be improved by using techniques which successfully exploit these characteristics, for example by using an optimum modulation method. The important characteristics for the communications engineer are the frequency and time responses of the channel, and the magnitude and nature of the noise. The channel responses have been discussed in earlier chapters; we now deal with the problem of noise. There are two basic reasons for a study of noise. Firstly there is a need to understand the nature of the noise in order...
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Điện thoại di động vô tuyến điện - Tuyên truyền Channel P10
Mitigation of Multipath Eects We have seen in Chapters 4 and 5 that buildings and other obstacles in built-up areas act as scatterers of the signal, and because of the interaction between the various incoming component waves, the resultant signal at the mobile antenna is subject to rapid and deep fading. The fading is most severe in heavily built-up areas such as city centres, and the signal envelope often follows a Rayleigh distribution over short distances in these heavily cluttered regions. As the degree of urbanisation decreases, the fading becomes less severe; in rural areas it is often only serious...
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Điện thoại di động vô tuyến điện - Tuyên truyền Channel P11
Planning Radio Networks In earlier chapters we discussed the characteristics of the radio propagation channel in some detail. We introduced methods for predicting the mean signal level within a small area in rural, suburban and urban environments and it became clear that this is a complicated process involving a knowledge of several factors, including the details of the terrain, the building clutter and the extent of foliage along the radio path. Most importantly perhaps, it became apparent that signal strength prediction is not an exact science; the mean signal in a small area can be predicted using any of the methods...
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Cẩm nang dữ liệu không dây P1
A SHORT HISTORY OF DATA RADIO IN THE BEGINNING In 1899, four years after Marconis first wireless telegraph, the British Navy converted to data radio.1 The Czars Navy quickly followed. By 1905 the Japanese had mastered the key techniques and began to intercept messages from the Russian Vladivostok fleet cruising secretly south of Tokyo Bay. Japans victorious Battle of Tsushima followed. Driven by continued military demands, wireless data technology leaped forward. In 1914 the hapless Russians lost the Battle of Tannenburg because of German intercepts of their land-based data radio communications and in 1917 the British successfully...
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Cẩm nang dữ liệu không dây P2
DATA NETWORK TYPES A ROUGH SORT OF TWO-WAY SYSTEMS In North America terrestrial, two-way wireless data networks can initially be sorted into private versus public systems. Private system examples include a few very large users, such as .ederal Express, and many smaller systems for public safety (police and fire departments), taxi/limo dispatch, and utilities. In private systems the infrastructure is owned, and the spectrum is licensed, by the user. Public systems are for hire. The end user does not own the infrastructure and has no special rights to the spectrum that carries the traffic. 2.2 PRIVATE SYSTEMS 2.2.1 Overview Private...
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Cẩm nang dữ liệu không dây P3
KEY PUBLIC NETWORK CHARACTERISTICS The variety of public network choices cited in the preceding chapter lead some potential users to despair at what seems a plethora of alternatives. Actually, specific user requirements can quickly eliminate many network candidates. This permits a more straightforward assessment of realistic alternatives. A few of those salient user characteristics are discussed here. 3.1 COVERAGE While the scale is continuous, ranging from international down to a single truck depot, the user coverage requirements discussed in this book fall into three broad categories: 1. Nationwide. 2. Regional: Examples of this might include the field sales company that works a...
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Cẩm nang dữ liệu không dây P4
PUBLIC TERRESTRIAL PACKET SWITCHED NETWORKS MESSAGING/DISPATCH ORIENTATION Amateur packet radio has been active in the 2- and 20-meter bands since the early 1980s. But recognizable pay-for-service packet switched offerings began in Chicago in 1986 with Motorolas DRN. With coverage in only three cities and weak marketing, DRN struggled for three years. In 1990 it was folded into a joint venture that also embraced IBMs .ield Service System. The result was ARDIS, a truly nationwide public packet data network. The formation of ARDIS was quickly followed by an alternative carrier: RAM Mobile Data. With extremely limited geographic coverage, RAMs early struggles were...
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Cẩm nang dữ liệu không dây P5
PUBLIC TERRESTRIAL CIRCUIT SWITCHED NETWORKS REPRESENTATIVE NATIONWIDE NETWORKS Nationwide circuit switched networks exist in vehicular, mobile applications; the successful ones use the existing voice cellular network as a last mile solution. One well-known system is that of United Parcel Service (UPS), which has no voice capability in its devices. During 1998 UPS began its move to a packet switched approach via a public network: ARDIS. Other circuit switched systems are also beginning to experiment with packet alternatives, from hybrid CS-CDPD through Cellemetry. The prominent cellular alternative, Nextels digital network, is nationwide by pricing and advertising but has huge coverage gaps....
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Cẩm nang dữ liệu không dây P6
PUBLIC SATELLITE NETWORKS Satellites have been carrying communications since the early 1960s, serving as relays for voice, video, or data. They have long been dominated by the military and, in their direct broadcast form, by large television communications companies. In 1989 the Gartner Group believed that the mobile satellite industry will be one of the great growth markets of the next decadesurpassing cellular telephones and digital paging, and in the same league as personal computers.1 Gartner believed that there would be 800,000 satellite data terminals installed by 1993, growing to 2 million by 1995. A flood of vendors rushed toward this...
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Cẩm nang dữ liệu không dây P7
HYBRID NETWORKS TERRESTRIAL PACKET/SATELLITE Trucking company Boyle Transport makes it crystal clear: you should be able to pick a radio or cellular based system for regional service and satellite for long haul, but be able to use the same provider.1 This is the goal of most hybrid offerings, though satellite may well be avoided in favor of an alternative technique such as circuit switched cellular. 7.1.1 AMSC/ARDIS Multimode System In March 1990 Rockwell announced that it was developing a satellite-based vehicle location and two-way data system.2 Rockwells plan was to lease L-band capacity from AMSC, which was leasing time from InMarSat....
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Cẩm nang dữ liệu không dây P8
FITTING APPLICATIONS TO PUBLIC OFFERINGS NETWORK PRICE POSITIONING We are always interested in simple ways to classify complex choices. An initial application fit technique for terrestrial, two-way wireless data networks is shown in .igure 8-1. If traffic activity is brisk but message lengths are short, public packet switched networks are a reasonable business choice. If traffic activity is modest but message lengths are long, data over cellular channels are a better fit. This category includes pure (image) facsimile....
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Cẩm nang dữ liệu không dây P9
SUBSCRIBER GROWTH: HISTORY AND BARRIERS APPROACH Each public service provider has a different philosophy on reporting subscribers. Not unexpectedly the most successful, OmniTRACS, is also the most forthright about both customer names and device shipments, though one must take care to discriminate between U.S. and worldwide totals. ARDIS is good at overall subscriber counts, a bit more guarded on customer names and their specific active devices. .or years RAM was very guarded in both respects, but in 1996 it began to reveal gross subscriber count summaries. User names, and especially quantities for those users, were more difficult to obtain....
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