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1、Exercise1Fiber-opticlineswillformmostofthebackboneoftheinformationhighway,justastheydoforthephonesystemtoday.Fiber-opticcableismadeoflong,thinkstrandsofglassratherthanwire,andittransmitsinformationintheformofdigitizedpulsesoflaserlightratherthantheradio
2、wavesusedbycoaxialcable.Becauselightpulseshaveshorterwavelengthsthanradiowaves,engineerscancrammuchmoredataintofiber-opticlinesthanintootherkindsofcablesandwires.Asinglefiber,forexample,canhandleamind-boggling5,000videosignalsormorethan500,000voiceconve
3、rsationssimultaneously.Thishugecapacityallowsittotransmitallsignalsdigitally.Sonoiseorstaticeasilycanbefilteredout.Finally,becauseglassisaninherentlymoreefficientmediumfortransmittinginformationthanothermaterials,afiber-opticlinecantransmitasignalthousa
4、ndsofmileswithoutmuch“signalloss”.Fiber-opticcable,simplyput,isthemethodofchoicefortransmittingmassivequantitiesofinformationoverlongdistances.Anotherkeyis“digitalcompression”–avarietyofmethodsforreducingtheamountofdigitalcode(streamsofonesandzeros)need
5、edtorepresentapieceofinformation–whetheritisadocument,astillpicture,amovieorasound.Digitalcompressionismostcriticalfortransmittingvideo,becausedigitizedvideoconsumesenormousamountsofspace.Justfoursecondsofadigitizedfilm,forexample,wouldcompletelyfilla10
6、0-megabyteharddrive.Afeaturefilmoftypicallength,ifuncompressed,wouldoccupymorethan350ordinarycompactdiscs.Compressiontechniquesachievetheirginsbyrecordingonlythechangesfromoneframetothenext.Thebackgroundimageinamoviescene,forexample,typicallydoesnotchan
7、gemuchfromoneframetoanother.Inadigitalcompressionscheme,thebackgroundwouldberecordedonlyonce;afterthat,onlytheactors’movementswouldbecaptured.Oneresultismorechoices–hundredsofchannelscomingthroughyourcableTVlineinsteadofdozens.Digitalcompressionalsomake
8、siteasiertopiggybackdataontomediathatwerenotdesignedwithdatainmind:inparticular,phonelines.译文：光纤电缆构成了今日电话系统的主干线，同样，它也将构成未来信息公路的主干线。光纤电缆的制造材料是细长的玻璃丝而不是金属导线，它的信息传输方式是激光数字脉冲而不是同轴电缆使用的无线电波。由于光脉冲的波长比无线电波的波长短，因而工程师可以输入光纤电缆的信息量之大令其它类型的电
