电子通信英语第三章.ppt

1、Chapter3 communications,Unit 14 Basic Knowledge of Communications Unit15 Mobile communication Unit 16 Broadband Communication Unit 17 Asynchronous Transfer Mode (ATM),Unit 14 Basic Knowledge of Communications,Communication System A generalized communication system has the following components (as sh

2、own in Fig. 3-1): (a)Information Source. This produces a message which may be written or spoken words, or some from of data. (b)Transmitter. The transmitter converts the message into a signal, the from of which is suitable for transmission over the communication channel.,下一页,返回,Unit 14 Basic Knowled

3、ge of Communications,(c)Communication Channel. The communication channel is the medium used transmit the signal, from the transmitter to the receiver. The channel may be a radio link or a direct wire connection. (d)Receiver. The receiver can be thought of as the inverse of the transmitter. It change

4、s the received signal back into a message and passes the message on to its destination which may be a loudspeaker, teleprinter or computer data bank. An unfortunate characteristic of all communication channels is that noise is added to the signal. This unwanted noise may cause distortions of sound i

5、n a telephone, or errors in a telegraph message or data.,上一页,下一页,返回,Unit 14 Basic Knowledge of Communications,Frequency Division Multiplexing Frequency Division Multiplexing(FDM) is one of analog techniques. A speech signal is 03kHz, Single sideband amplitude (SSB) modulation can be used to transfer

6、 speech signal to new frequency bands, four similar signals, for example, moved by SSB modulation to share the band from 5 to 20kHz. The gaps between channels are known as guard spaces and these allow for errors in frequency, inadequate filtering，etc in the engineered system.,上一页,下一页,返回,Unit 14 Basi

7、c Knowledge of Communications,Once this news base band signal, a “group” of 4 channels, has been formed it is moved around the trunk network as a single unit. A hierarchy can be set up with several channels forming a “group”, several groups a “supergroup” and several “supergroup” either a “mastergro

8、up” or “hypergroup” . Groups or supergroups are moved as single units by the communications equipment and it is not necessary for the radios to know many channels are involved. A radio can handle a supergroup provided sufficient bandwidth is available.,上一页,下一页,返回,Unit 14 Basic Knowledge of Communica

9、tions,The size of the group is a compromise as treating each channels individually involves far more equipment because separate filters, modulators and oscillators are required for every channels rather than each group. However the failure of one module will lose all of the channels associated with

10、a group.,上一页,下一页,返回,Unit 14 Basic Knowledge of Communications,Time Division Multiplexing It is possible, with pulse modulation systems, to use the between samples to transmit signals from other circuits. The technique is known as time division multiplexing(TDM).To do this it is necessary to employ s

11、ynchronized switches at each end of the communication link to enable samples to be transmitted in turn, from each of several circuits. Thus several subscribers appear to use the link simultaneously. Although each user only has periodic short time slots, the original analog signals between samples ca

12、n be reconstituted at the receiver. Typical TDM system is shown in Fig.3-2 .,上一页,下一页,返回,Unit 14 Basic Knowledge of Communications,Pulse Code Modulation In analog modulation, the signal was used to modulate the amplitude or frequency of a carrier, directly. However in digital modulation a stream of p

13、ulses, representing the original, is created. This stream is then used to modulate a carrier or alternatively is transmitted directly over a cable. Pulse Code Modulation(PCM) is one of the two techniques commonly used.,上一页,下一页,返回,Unit 14 Basic Knowledge of Communications,All pulse systems depend on

14、the analog waveform being sampled at regular intervals. The signal created by sampling our analog speech input is known as pulse amplitude modulation(as shown in Fig.3-3). It is not very useful in practice but is used as an intermediate stage towards forming a PCM signal. It will be seen later that

15、most of the advantages of digital modulation come from the transmitted pulses having two levels only, this being known as a binary system. In PCM the height of each sample is converted into a binary number. There are three steps in the process of PCM: sampling, quantising and coding.,上一页,返回,Unit15 M

16、obile communication,Mobile systems originally operated in analog mode in the 450 MHz band, moving later to 900 MHz with digital GSM and then to 1 800MHz with personal communication systems. The history of mobility can spilt into generations. The first generation systems were the advanced mobile phon

17、e systems (AMPS) in the US, total access communication system (TACS) in most of Europe and Nordic mobile telephone system (NMT); which were all analogue systems.,下一页,返回,Unit15 Mobile communication,The second generation (2G) is vary much dominated by the standard first set out in Europe by the group

18、special mobile (GSM) committee, which was designed as a global mobile communication system. Four systems are in use now: D-AMPS, GSM, CDMA, and PDC. The third generation mobile communication system (3G) currently being developed in Europe is intended to integrate all the different services of second

19、 generation systems and cover a much wider range of broadband services (voice, data, video and multimedia) consistent and compatible with technology developments taking place within the fixed telecommunication networks.,上一页,下一页,返回,Unit15 Mobile communication,The ideal mobile telephone system would o

20、perate within a limited assigned frequency band and would serve an almost unlimited number of users in unlimited areas. Three major approaches to achieve the ideal are: 1.Single-sideband (SSB), which divides the allocated frequency band into maximum numbers of channels; 2.Cellular, which reuses the

21、allocated frequency band in different geographic locations; 3.Spread spectrum, frequency-hopped, which generates many codes over a wide frequency band.,上一页,下一页,返回,Unit15 Mobile communication,Here we introduce some popular mobile telephone systems: 1. Cellular Mobile Telephone System On Jan. 4, 1979,

22、 the FCC authorized Illinois Bell Telephone Co. to conduct a developmental cellular system in the Chicago area and make a limited commercial offering of its cellular service to the public. In addition, American Radio Telephone Service Inc. (ARTS) was authorized to operate a cellular system in the Wa

23、shington, D.C.-Baltimore, Md., area. These first systems showed the technological feasibility of cellular service.,上一页,下一页,返回,Unit15 Mobile communication,A basic cellular system consists of mobile stations, base stations and a mobile switching center (MSC). The mobile station contains a transceiver,

24、 an antenna, and control circuitry, and may be mounted in a vehicle or used as a portable hand-held unit. The base stations consist of several transmitters and receivers which simultaneously handle full duplex communications and generally have towers which support several transmitting and receiving

25、antennas.,上一页,下一页,返回,Unit15 Mobile communication,The base station serves as a bridge between all mobile users in the cell and connects the simultaneous mobile calls via telephone lines or microwave links to the MSC. The MSC coordinates the activities of all of the base stations and connects the enti

26、re cellular system to the PSTN. A typical MSC handles 100 000 cellular subscribers and 5 000 simultaneous conversations at a time, and accommodates all billing and system maintenance functions, as well. In large cities, several MSCs are used by a single carrier.,上一页,下一页,返回,Unit15 Mobile communicatio

27、n,2.GSM -Global System for Mobile Communications The success of mobile systems across the world is a sign that communication is moving towards a more personalized, convenient system. The GSM system is based on a cellular communications principle, but the use of digital radio transmission and the adv

28、anced handover algorithms between radio cells in GSM network allows for significantly better frequency usage than in analogue cellular systems, thus increasing the number of subscribers that can be served4. Since GSM provides common standard, cellular subscribers will also be able to use their telep

29、hones over the entire GSM service area.,上一页,下一页,返回,Unit15 Mobile communication,Roaming is fully automatic between and within all countries covered by GSM system. In addition to international roaming, GSM provides new services, such as high-speed data communication, facsimile and short message servic

30、e. The GSM technical specifications are designed to work in concert with other standards, e.g. ISDN (the integrated services digital network). Interworking between the standards is in this way assured. In the long term perspective cellular systems, using a digital technology will become the universa

31、l method of telecommunication.,上一页,下一页,返回,Unit15 Mobile communication,The GSM system operates in a burst transmission mode with 124 radio channels in the 900 MHz band, and these bursts can carry different types of information. The first type of information is speech, which is coded at 6.5 kbit/s or

32、13 kbit/s. The second type is data, which can be sent at 3.6 kbit/s, 6 kbit/s or 12.6 kbit/s. These two forms of transmission are the useful parts of the transmission, but have to be supported by overhead information which is sent in control channels.,上一页,下一页,返回,Unit15 Mobile communication,3.CDMA -C

33、ode Division Multiple Access CDMA is completely different from AMPS, D-AMPS, and GSM. Instead of dividing the allowed frequency range into a few hundred narrow channels, CDMA allows each station to transmit over the entire frequency spectrum all the time. Multiple simultaneous transmissions are sepa

34、rated using coding theory. CDMA also relaxes the assumption that colliding frames are totally garbled. Instead, it assumes that multiple signals add linearly.,上一页,下一页,返回,Unit15 Mobile communication,The key to CDMA is to be able to extract the desired signal while rejecting everything else as random

35、noise. CDMA also has many other complicating factors that have been glossed over here. Nevertheless, CDMA is a clever scheme that is being rapidly introduced for wireless mobile communication. It normally operates in a band of 1.25 MHz (versus 30 kHz for D-AMPS and 200 kHz for GSM ), but it supports

36、 many more users in that band than either of the other systems. In practice, the bandwidth of CDMA available to each user is at least as good as that of GSM and often much better.,上一页,返回,Unit 16 Broadband Communication,As can be inferred from the examples of video phone and HDTV, the evolution of fu

37、ture communications will be via broadband communication centered around video signals. The associated services make up a diverse set of high-speed and broadband services ranging from video services such as video phone, video conferencing, video surveillance, cable television (CATV) distribution, and

38、 HDTV distribution to the high-speed data services such as high-resolution image transmission, high-speed data transmission, and color facsimile.,下一页,返回,Unit 16 Broadband Communication,The means of standardizing these various broadband communication services so that they can be provided in an integr

39、ated manner is no other than the broadband integrated services digital network (B-ISDN) . Simple put, therefore, the future communications network can be said to be a broadband telecommunication system based on the B-ISDN.,上一页,下一页,返回,Unit 16 Broadband Communication,For realization of the B-ISDN, the

40、 role of several broadband communication technologies is crucial. Fortunately, the remarkable advances in the field of electronics and fiber optics have led to the maturation of broadband communication technologies. As the B-ISDN becomes possible on the optical communication foundation, the relevant

41、 manufacturing technologies for light-source and passive devices and for optical fiber have advanced to considerable levels. Advances in high-speed device and integrated circuit technologies for broadband signal processing are also worthy of close attention. There has also been notable progress in s

42、oftware, signal processing, and video equipment technologies. Hence, from the technological standpoint, the B-ISDN has finally reached a realizable state.,上一页,下一页,返回,Unit 16 Broadband Communication,On the other, standardization activities associated with broadband communication have been progressing

43、. The Synchronous Optical Network (SONET) standardization centered around the T1 committee eventually bore fruit in the form of the Synchronous Digital Hierarchy (SDH) standards of the International Consultative Committee in Telegraphy and Telephony (CCITT), paving the way for synchronous digital tr

44、ansmission based on optical communication.,上一页,下一页,返回,Unit 16 Broadband Communication,The standardization activities of the integrated services digital network (ISDN), which commenced in early 1980s with the objective of integrating narrowband services, expanded in scope with the inclusion of broadb

45、and services, leading to the standardization of the B-ISDN in late 1980s and establishing the concept of asynchronous transfer mode (ATM) communication in process.,上一页,下一页,返回,Unit 16 Broadband Communication,In addition, standardization of various video signals is becoming finalized through the coope

46、ration among such organizations as CCITT, the International Radio-communications Consultative Committee (CCIR), and the International Standards Organization (ISO), and reference protocols for high-speed packet communication are being standardized through ISO, CCITT, and the Institute of Electrical a

47、nd Electronics Engineer (IEEE).,上一页,下一页,返回,Unit 16 Broadband Communication,Various factors such as these have made broadband communication realizable. Therefore, the 1990 s is the decade in which matured broadband communication technologies will be used in conjunction with broadband standards to rea

48、lize broadband communication networks. In the broadband communication network, the fiber optic network will represent the physical medium for implementing broadband communication, while synchronous transmission will make possible the transmission of broadband service signals over the optical medium.

49、,上一页,下一页,返回,Unit 16 Broadband Communication,Also, the B-ISDN will be essential as the broadband telecommunication network established on the basis of optical medium and synchronous transmission and ATM is the communication means that enables the realization of the B-ISDN. The most important of the b

50、roadband services to be provided through the B-ISDN are high-speed data communication services and video communication services.,上一页,返回,Unit 17 Asynchronous Transfer Mode (ATM),Demand for rich media services such as Internet access, video on demand, digital television and voice over IP grows more cl

51、amorous every day. So, too, does the need for high-performance distribution technology. To meet this demand, service providers are turning to ATM technologya flexible, scalable way of moving high-speed voice, video and data across networks. ATMs sophisticated bandwidth utilization capabilities enabl

52、e providers to efficiently transport large, complex video packets without taxing a network.,下一页,返回,Unit 17 Asynchronous Transfer Mode (ATM),The majority of traffic ported over the ATM infrastructure is voice and data. Video will soon be as prominent and will drive the need for more high-capacity ATM networks. The basis of ATM technology is a high-efficiency, low-latency switching and multiplexing mechanism ideally suited to an environment in which there are specific bandwidth limitations. ATM allocate