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1、Unit7ComputerCommunicationsSectionATelecommunicationsandComputerTelecommunicationsallowspeoplearoundtheworldtocontactoneanother,toaccessinformationinstantly,andtocommunicatefromremoteareas.Telecommunicationsusuallyinvolvesasenderofinformationandoneormorerecipientsli
2、nkedbyatechnology,suchasatelephonesystem,thattransmitsinformationfromoneplacetoanother.Telecommunicationsdevicesconvertsdifferenttypesofinformation,suchassoundandvideo,intoelectronicsignals.Thesignalscanthenbetransmittedbymeansofmediasuchastelephonewiresorradiowaves
3、.Whenasignalreachesitsdestination,thedeviceonthereceivingendconvertstheelectronicsignalbackintoanunderstandablemessage,suchassoundoveratelephone,movingimagesonatelevision,orwordsandpicturesonacomputerscreen.Telecommunicationsenablepeopletosendandreceivepersonalmessa
4、gesacrosstown,betweencountries,andtoandfromouterspace.Italsoprovidesthekeymediumfornews,data,information,andentertainment.Telecommunicationsmessagescanbesentinavarietyofwaysandbyawiderangeofdevices.Themessagescanbesentfromonesendertoasinglereceiver(point-to-point)or
5、fromonesendertomanyreceivers(point-to-multipoint).Personalcommunications,suchasatelephoneconversationbetweentwopeopleorafacsimile(fax)message,usuallyinvolvepoint-to-pointtransmission.Point-to-multipointtelecommunicationsoftencalledbroadcasts,providethebasisforcommer
6、cialradioandtelevisionprogramming.Telecommunicationsbeginwithmessagesthatareconvertedintoelectronicsignals.Thesignalsarethensentoveramediumtoareceiver,wheretheyaredecodedbackintoaformthatthepersonreceivingthemessagecanunderstand.Therearevarietyofwaystocreateanddecod
7、esignals,andmanydifferentwaystotransmitsignals.Devicessuchasthetelegraphandtelephonerelaymessagesbycreatingmodulatedelectricalimpulses,orimpulsesthatchangeinasystematicway.Theseimpulsesarethensentbywires,radiowaves,orothermediatoareceiverthatdecodesthemodulation.The
8、telegraph,theearliestmethodofdeliveringtelecommunications,worksbyconvertingthecontacts(connectionsbetweentwoconductorsthatpermitaflowofcur
