10-11远程通信

1、课题：TelecommunicationStep I what is telecommunicationThe word telecom muni cati on derives from the Greek word tele, meaning over a dista nee, plus the word com muni cati on. Thus, putt ing the two parts together, you get telecom muni cati on- com muni cati on that spa ns a distanee.An an cie nt exam

2、ple of telecom mun icatio n is the use of fire sig nals on hilltops those miles apart. These days, moder n telecom muni cati ons is a vast field that en compasses everythi ng from broadcasti ng to teleph one tech no logy to computer n etworks.Telecom mun icatio nwireli newirelessVoiceVideoData一 Loca

3、l+long-cellular telephony_ videoJroadcast TVcomputersDistanceconferencingTelephonyradio broadcasting一 desktop videodigital cableover networks.satellite feeds一 LANsIntercom1wireless local+longdistance telephonycable TVWANsclosed circuit TVhe internetWireli newirelesswireli ne_RF networks pagingLpoint

4、-to-point servicesFigure 1. Telecommunication serviceswirelessThree categories of telecom muni catio n are: voice, video and data.Voice telecommunication refers to any means of using electrical sig nals to tran smit huma n voice across a dista nee, such as teleph one and audio broadcasts.Video telec

5、ommunication refers to the electrically-based transmission of moving pictures and sound across a distanee , such as TV broadcast ing or distribut ing live feeds of an eve nt to the scree n of networked computers.Data telecommunication refers to use of electrical signals to exchange encoded informati

6、on between computerized devices across a distanee. Sending an e-mail to a colleague, retrieving a spread-sheet from your computer hard disk, and sending a message to your spous pager are all examples of data telecommunications.Voice, video and data telecom muni cati ons all bel ong to the larger dat

7、a telecom muni catior” categories that they rely on electrical sig nals. Step II Evolutio n of telecom mun icatio n networks狐 The first phaseThe first phase in the evoluti on of com muni cati on n etworks is domin ated by the wide spread of teleph ony and covers over 90 years. However, the telegraph

8、 preceded the teleph one by more tha n 30 years and the teletypewriter by half a century.In 1844. Samuel Morse, the telegraph inven tor, sent the first telegraph message from Wash ington D.C, to his colleague Alfred Vail in Baltimore, Maryland.Shortly after the invention of the telephone by Alexande

9、r Grahan Bell in 1876, it was realized that teleph one wires had to conv erge on central points where telephone-to-telephone connections could be made.These points were manual switch-boards, staffed by operators.1889, Alm ond B. Strowger inven ted the first two-moti on step-by-step switch.1932, the

10、first crossbar switch was used in Sweden.1938, the Bell System in troduced the NO.1Crossbar switch.In the mid 1930s, analog point-to-point radio relay transmission was possible by exte nding high-freque ncy (HF) com muni cati on tech no logy into the very-high-freque ncy ran ge. During the World War

11、 2, the operati ng bands of the an alogue radio relay systems were exte nded in to the ultra-high- freque ncy(UHF) range.The first link with over 100 repeater station was placed into commercial service in 1951 betwee n New York and San Fran cisco, operating in the 4GHz band with a 20MHz band.By the

12、1980s, quadrature amplitude modulati on (QAM) methods were widely used as the modulation scheme for digital radio-relay systems.狐 The second phaseThe second phase of networking, which occurs in the 1960s, in cluded three majors milestones: software switching, digital tran smission, and satellite dep

13、loyment.In 1965, America n Teleph one and Telegraph(ATT) in troduced the first stored-program control(SPC) local switch, the 1 EES switching system.The prin ciple of converting an alog sig nals to digital sig nals became popular with the introduction of pulse code modulation (PCM); consequently, the

14、 known rate for telephone-quality speech (4 kHz ban dwidth) is 64kbps.Satellite com muni cati on was first proposed in 1945 by British scie nee ficti on writer, Arthur C. Clarke. Satellite com muni catio n became a reality with the laun chi ng of the Russia n satellite Sput nik and the America n sat

15、ellite Explorer. The ATT Telstar was the first experime ntal satellite capable of relayi ng TV programs across the Atla ntic Ocea n.狐 The third phaseThe third phase of the evolutio n of com muni catio n n etwork occurred in the 1970s and is characterized by the in troducti on of data n etworks and p

16、acket switch ing tech no logy. The con cept of packet switch ing was first published in a 1964 report by Paul Bara n of the Rand Corporati on for the U.S. Air Force.In 1971. ARPANET was set up and was put in to service.The American Standard Code for Information Interchange (ASCII) was the first stan

17、dard, which was approved in 1964 and became the com mon method of codi ng data for com muni catio n. The sec ond standard is the Electr onics In dustries Associati on (EIA) recomme nded standard (RS) RS-232D. Its first version, issued in 1969, specified how the encoded information would be transmitt

18、ed by modem over a telephone network.To achieve compatibility betwee n computers and packet-switchedNetworks, the Intern ati onal Telegraph and Telepho ne Con sultative Committee (CCITT) established a world-sta ndard protocol, called X.25 in 1976. Working with CCITT, the International Organization o

19、f Standardization (ISO) in 1978 approved its seven-layer framework of protocols for data com muni cati on, called the Ope n System Interconnection (OSI) referenee model.This phase of n etworki ng is also characterized by the in troductio n and wide accepta nee of local area n etwork ing. (LANs) . Th

20、e oldest and best known of all LANs if the Ether net .The invention of the LASER (which stands for light amplification by simulated emissi on of radiati on) in 1959 led to major tech nical developme nts in the field of optical com muni cati on. In 1970, Cori ng Glass Works reported on the first low-

21、loss optical fiber that achieved 20dB/km loss.狐 The fourth phaseThe fourth phase of com muni cation networking, which bega n in 1980, is characterized by the availability of the integrated services digital n etwork (ISDN) and mobile communication.Mobile communication entered a new era with the intro

22、duction ofthe cellular con cept. I n 1981, the Federal Communi catio n Commissi on(FCC) allocated 50MHz of the spectrum to mobile radio cellular systems.The four phase of com muni cati on n etworksTeleph onyDigital n etworkData n etworkIn tegrated digital n etworkYear1880s1960s1970s1980sType of traf

23、ficvoicevoicedataVoice, data, video. image etc.Switchi ng tech no logyCircuit switchi ng(a nalog)Circuit switchi ng(a nalog)Packet switchi ngCircuit, packet, Fast packet switchi ngTran smissi on mediasmicrowaveCopper, ,satellitemicrowave, Copper, satelliteCopper, microwaveCopper, microwave, satellit

24、e, fiber opticsStep HI The devicesTelecom muni catio ns are the tran smissi on of words, soun ds, images, or data in the form of electronic or electromagnetic signals or pulses. Tran smissi on media in elude the telepho ne, radio, televisio n, microwave, and satellite. Data com muni catio n, the fas

25、test grow ing field of telecom muni cati on, is the process of tran smitti ng data in digital form by wire or radio. HardwareHardware usually con sists of a tra nsmitter and a cable in terface, or if the teleph one is used as a tran smissi on line, a modulator/demodulator, or modem.A tran smitter pr

26、epares in formatio n for tran smissi on by converting it from a form that the device uses to a form that the transmission line uses. Most tran smitters are an in tegral eleme nt of the sending device.A cable in terface, as the n ame in dicates, conn ects a device to a cable. It converts the tran smi

27、tted sig nals 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 telepho ne line to the demodulated form that the device itself requires. Modems tran smit data through a teleph one line at various spe

28、eds, which are measures in bits per sec ond (bps) or sig nals per sec ond (baud). SoftwareAmong the different kinds of software are file-transfer, host and n etwork programs. File-tra nsfer software is used to tran smit a data file from one device to ano ther. Host software inden tifies a host compu

29、ter as such and controls the flow of data among devices connected to it. Network software allows devices in a computer network to transmit in formati on to one other. Applicati onO Host-terminalO File tran sferO Computer network ServicesO NetworkO In formatio n retrievalO Electro nic mailO Bullet in

30、 boardStep IV Eleme nts of telecom mun icatio n systemAny com muni cati on system moves in formatio n from a source to a destination through a channel. Figure 1 illustrates this very simple idea. The in formatio n from the source will gen erally n ot be in a form that can travel through the cha nn e

31、l, so a device called a tran smitter will be employed at one end and a receiver at the other.Source i: transmitterreceiver- destinationFigure .1 Simple com muni cati on systemi The sourceThe source of i nformatio n sig nal can be an alog or digital. Com mon examples are an alog audi o, vide o, sig n

32、als and digital data. Source are often described in terms of thefreque ncy range that they occupy.e.g. teleph on e-quality an alog voice sig nal 300Hz3KHzan alog high-fidelity music20HZ 20KHzVideo requires a much large freque ncy range tha n audio. An an alog video sig nal of televisi on-broadcast q

33、uality n eeds a freque ncy range from 6MHz to about 8MHz.Digital source can be derived from audio or video signals or can con sist of data. Digital sig nals can have almost any ban dwidth depe nding on the number of bits transmitted per second, and the methods used to convert binary and zeros into e

34、lectrical signals.ii The channelA com muni cati on cha nnel can be almost any thi ng: a pair of con ductors, an optical fiber or a free space that we live. Sometimes a cha nnel can carry the in formatio n sig nal directly. For example, an audio signal can be carried directly by atwisted-pair telepho

35、ne cable .On the other hand, a radio link through free spacecannot be used directly for voice sig nals. Such situationsrequirethe useof acarriersignal whosefrequencyis suchthat itwilltravel,or propagate,through the cha nn el. This carrier wave will be altered, or modulated, by the in formati on sig

36、nal in such a waythat thein formatio ncan be recovered at thedest in ati on. Whe n a carrier is used, the in formati on sig nal is also known as the modulat ing sig nal. The three terms information sig nal, modulating sig nal, and baseba nd are equivale nt in com muni catio n schemes involving modul

37、ated carrier.iii Types of modulationModulati on is tech nique for process ing sig nals in which two waves are comb ined to produce a wave that processes characteristics of both and can be decoded to separate these characteristics.A systems of modulation are variations on a small number of possibilit

38、ies. A carrier is gen erated at a freque ncy much higher tha n the highest baseband frequency. The carrier is a sine wave. The instantaneous amplitude of the basebandsignal is used to vary some parameter of the carrier.A general equation for a sine-wave carrier is:wheree(t)=i nsta nta neous voltage

39、as a fun cti on of time;Ec二peak voltage; wc二freque ncy in radia ns per sec ond;t=time in seconds；8=phase.angle in . radians .In modulati on, the parameters that can be cha nged are amplitude c, frequency wc, and phase. comb in ati ons are also possible.The modulati on is done at the tran smitter. An inv erse process, called demodulation or detection, takes place at the receiver to restore the original baseba nd signal.iv Signal bandwidthAn unmodulated sine-wave carrier would exist at one frequency and so wo