10-11远程通信

1、湖南城市学院教师备课纸 第 次课题：Telecommunication Step I what is telecommunication The word telecommunication derives from the Greek word tele, meaning over a distance, plus the word communication. Thus, putting the two parts together, you get telecommunication- communication that spans a distance. An ancient exa

2、mple of telecommunication is the use of fire signals on hilltops those miles apart. These days, modern telecommunications is a vast field that encompasses everything from broadcasting to telephone technology to computer networks. TelecommunicationVoice Video DataWireline wireless wireline wireless w

3、ireline wireless Local+long cellular telephony video broadcast TV computers RF networksDistance conferencingTelephony digital cable radio broadcasting desktop video over networks satellite feeds LANs paging Intercom wireless local+long distance telephony cable TV WANs point-to-point services closed

4、circuit TV The internet Figure 1. Telecommunication services Three categories of telecommunication are: voice, video and data. Voice telecommunication refers to any means of using electrical signals to transmit human voice across a distance, such as telephone and audio broadcasts. Video telecommunic

5、ation refers to the electrically-based transmission of moving pictures and sound across a distance , such as TV broadcasting or distributing live feeds of an event to the screen of networked computers. Data telecommunication refers to use of electrical signals to exchange encoded information between

6、 computerized devices across a distance. Sending an e-mail to a colleague, retrieving a spread-sheet from your computers hard disk, and sending a message to your spouses pager are all examples of data telecommunications. Voice, video and data telecommunications all belong to the larger “data telecom

7、munication” categories that they rely on electrical signals. Step II Evolution of telecommunication networks The first phase The first phase in the evolution of communication networks is dominated by the wide spread of telephony and covers over 90 years. However, the telegraph preceded the telephone

8、 by more than 30 years and the teletypewriter by half a century. In 1844, Samuel Morse, the telegraphs inventor, sent the first telegraph message from Washington D.C, to his colleague Alfred Vail in Baltimore, Maryland. Shortly after the invention of the telephone by Alexander Grahan Bell in 1876, i

9、t was realized that telephone wires had to converge on central points where telephone-to-telephone connections could be made. These points were manual switch-boards, staffed by operators. 1889, Almond B. Strowger invented the first two-motion step-by-step switch. 1932, the first crossbar switch was

10、used in Sweden. 1938, the Bell System introduced the NO.1Crossbar switch. In the mid 1930s, analog point-to-point radio relay transmission was possible by extending high-frequency (HF) communication technology into the very-high-frequency range. During the World War 2, the operating bands of the ana

11、logue radio relay systems were extended into the ultra-high- frequency(UHF) range. The first link with over 100 repeater station was placed into commercial service in 1951 between New York and San Francisco, operating in the 4GHz band with a 20MHz band. By the 1980s, quadrature amplitude modulation

12、(QAM) methods were widely used as the modulation scheme for digital radio-relay systems. The second phase The second phase of networking, which occurs in the 1960s, included three majors milestones: software switching, digital transmission, and satellite deployment. In 1965, American Telephone and T

13、elegraph(ATT) introduced the first stored-program control(SPC) local switch, the 1 EES switching system. The principle of converting analog signals to digital signals became popular with the introduction of pulse code modulation (PCM); consequently, the known rate for telephone-quality speech (4 kHz

14、 bandwidth) is 64kbps. Satellite communication was first proposed in 1945 by British science fiction writer, Arthur C. Clarke. Satellite communication became a reality with the launching of the Russian satellite Sputnik and the American satellite Explorer. The ATT Telstar was the first experimental

15、satellite capable of relaying TV programs across the Atlantic Ocean. The third phase The third phase of the evolution of communication network occurred in the 1970s and is characterized by the introduction of data networks and packet switching technology. The concept of packet switching was first pu

16、blished in a 1964 report by Paul Baran of the Rand Corporation for the U.S. Air Force. In 1971, ARPANET was set up and was put into service. The American Standard Code for Information Interchange (ASCII) was the first standard, which was approved in 1964 and became the common method of coding data f

17、or communication. The second standard is the Electronics Industries Association (EIA) recommended standard (RS) RS-232D. Its first version, issued in 1969, specified how the encoded information would be transmitted by modem over a telephone network. To achieve compatibility between computers and pac

18、ket-switched Networks, the International Telegraph and Telephone Consultative Committee (CCITT) established a world-standard protocol, called X.25 in 1976. Working with CCITT, the International Organization of Standardization (ISO) in 1978 approved its seven-layer framework of protocols for data com

19、munication, called the Open System Interconnection (OSI) reference model. This phase of networking is also characterized by the introduction and wide acceptance of local area networking. (LANs). The oldest and best known of all LANs if the Ethernet . The invention of the LASER (which stands for ligh

20、t amplification by simulated emission of radiation) in 1959 led to major technical developments in the field of optical communication. In 1970, Coring Glass Works reported on the first low-loss optical fiber that achieved 20dB/km loss. The fourth phase The fourth phase of communication networking, w

21、hich began in 1980, is characterized by the availability of the integrated services digital network (ISDN) and mobile communication. Mobile communication entered a new era with the introduction of the cellular concept. In 1981, the Federal Communication Commission (FCC) allocated 50MHz of the spectr

22、um to mobile radio cellular systems. The four phase of communication networks TelephonyDigital networkData networkIntegrated digital networkYear1880s1960s1970s1980sType of trafficvoicevoicedataVoice, data, video. image etc.Switching technologyCircuit switching(analog)Circuit switching(analog)Packet

23、switchingCircuit, packet,Fast packet switchingTransmission mediasmicrowave Copper, ,satellitemicrowave,Copper, satelliteCopper, microwaveCopper, microwave, satellite, fiber opticsStep III The devices Telecommunications are the transmission of words, sounds, images, or data in the form of electronic

24、or electromagnetic signals or pulses. Transmission media include the telephone, radio, television, microwave, and satellite. Data communication, the fastest growing field of telecommunication, is the process of transmitting data in digital form by wire or radio. Hardware Hardware usually consists of

25、 a transmitter and a cable interface, or if the telephone is used as a transmission line, a modulator/demodulator, or modem. A transmitter prepares information for transmission by converting it from a form that the device uses to a form that the transmission line uses. Most transmitters are an integ

26、ral element of the sending device. A cable interface, as the name indicates, connects a device to a cable. It converts the transmitted signals from the form required by the device to the form required by the cable. A modem converts digital signals to and from the modulated from required by the telep

27、hone line to the demodulated form that the device itself requires. Modems transmit data through a telephone line at various speeds, which are measures in bits per second (bps) or signals per second (baud). Software Among the different kinds of software are file-transfer, host and network programs. F

28、ile-transfer software is used to transmit a data file from one device to another. Host software indentifies a host computer as such and controls the flow of data among devices connected to it. Network software allows devices in a computer network to transmit information to one other. Application Hos

29、t-terminal File transfer Computer network Services Network Information retrieval Electronic mail Bulletin board Step IV Elements of telecommunication system Any communication system moves information from a source to a destination through a channel. Figure 1 illustrates this very simple idea. The in

30、formation from the source will generally not be in a form that can travel through the channel, so a device called a transmitter will be employed at one end and a receiver at the other. Source transmitter receiver destination Figure .1 Simple communication system i The source The source of informatio

31、n signal can be analog or digital. Common examples are analog audio, video, signals and digital data. Source are often described in terms of the frequency range that they occupy. e.g. telephone-quality analog voice signal 300Hz3KHz analog high-fidelity music 20HZ20KHz Video requires a much large fre

32、quency range than audio. An analog video signal of television-broadcast quality needs a frequency range from 6MHz to about 8MHz. Digital source can be derived from audio or video signals or can consist of data. Digital signals can have almost any bandwidth depending on the number of bits transmitted

33、 per second, and the methods used to convert binary and zeros into electrical signals. ii The channel A communication channel can be almost anything: a pair of conductors, an optical fiber or a free space that we live. Sometimes a channel can carry the information signal directly. For example, an au

34、dio signal can be carried directly by a twisted-pair telephone cable. On the other hand, a radio link through free space cannot be used directly for voice signals. Such situations require the use of a carrier signal whose frequency is such that it will travel, or propagate, through the channel. This

35、 carrier wave will be altered, or modulated, by the information signal in such a way that the information can be recovered at the destination. When a carrier is used, the information signal is also known as the modulating signal. The three terms information signal, modulating signal, and baseband ar

36、e equivalent in communication schemes involving modulated carrier. iii Types of modulation Modulation is technique for processing signals in which two waves are combined to produce a wave that processes characteristics of both and can be decoded to separate these characteristics. All systems of modu

37、lation are variations on a small number of possibilities. A carrier is generated at a frequency much higher than the highest baseband frequency. The carrier is a sine wave. The instantaneous amplitude of the baseband signal is used to vary some parameter of the carrier. A general equation for a sine

38、-wave carrier is: e(t)=Ecsin(ct+) where e(t)=instantaneous voltage as a function of time; Ec=peak voltage; c=frequency in radians per second; t=time in seconds; =phase angle in radians In modulation, the parameters that can be changed are amplitude Ec, frequency c, and phase . combinations are also possible. The modulation is done at the transmitter. An inverse process, called demodulation or detection, takes place at the receiver to restore the original baseband signal. iv Signal bandwidth An unmodulated sine-wave carrier would exist at one frequency