電子信息與通信工程專業(yè)英語課件chapter10

電子信息與通信工程專業(yè)英語Chapter10OpticalFiber

Communications10.1TheBasicCommunicationsSystem1Abasiccommunicationssystemconsistsof2atransmitter,areceiver,andaninformationchannel.Atthetransmitter,themessageisgeneratedandputintoaformsuitablefortransferovertheinformationchannel.Theinformationtravelsfromthetransmittertothereceiveroverthischannel.Informationchannelscanbedividedinto3twocategories:unguidedchannel,andguidedchannel.Theatmosphereisanexampleofanunguidedchanneloverwhichwavescanpropagate.Systemsusingatmosphericchannelsincludecommercialradioandtelevisionbroadcastsandmicrowaverelaylinks.Guidedchannelsincludeavarietyofconductingtransmissionstructures.Therearetwo-wireline,coaxialcable,andrectangularwaveguide.Guidedlinescostmoretomanufacture,install,andservicethando4atmosphericchannels.Guidedchannelshavetheadvantagesofprivacy,weatherindependence,andtheabilitytoconveymessageswithin,under,andaroundphysicalstructures.Fiberwaveguideshavetheseadvantagesandothers.Atthereceiver,themessageisextractedfromtheinformationchannelandputintoitsfinalform.Amoredetailed,butstillquitegeneralblockdiagramappearsinFigure10.1.Abriefdiscussionofeachblockinthisfiguregivesusagoodfeel5forthemainelementsofacommunicationssystem.Descriptionsoftheseelementsemphasizethosesuitableforfibersystems.Manyoftheconcisedescriptionsgiveninthissectionareexpandedlater.Fornow6,wepresentanoverviewofthesubject.Figure10.1Ageneralizedfiberopticcommunicationssystem10.1.1MessageOriginThemessageoriginmaytakeseveralphysicalforms.Quiteoftenitisatransducerthatconvertsanonelectricalmessageintoanelectricalsignal.Commonexamplesincludemicrophonesforconvertingsoundwavesintocurrentsandvideo(TV)camerasforconvertingimagesintocurrents.Insomecases,suchasdatatransferbetweencomputersorpartsofacomputer,themessageisalreadyinelectricalform.Thissituationalsoariseswhenafiberlinkconstitutesaportionofsomelargersystem.Examplesincludefibersusedinthegroundportionofasatellitecommunicationssystemandfibersusedinrelayingcabletelevisionsignals.Inanycase7,theinformationmustbeinelectricalformbeforetransmissionforeitherelectronicoropticalcommunications.10.1.2ModulatorThemodulatorhastwomainfunctions.First,itconvertstheelectricalmessageintotheproperformat.Second,itimpressesthissignalontothewavegeneratedbythecarriersource.Twodistinctcategoriesofmodulationformatareanaloganddigital.Ananalogsignaliscontinuousandreproducestheformoftheoriginalmessagequitefaithfully.Forexample,supposeasoundwavecontainingasingletoneistobetransmitted.Theelectricalcurrentproducedwhenamicrophonepicksupthiswavehasthesameshapeasthewaveitself.Inthiscase,themodulatorneednotchangetheformatofthesignal.Itcouldbeappropriatetoamplifythissignal,however,sothatthesignalwillbestrongenoughtodrivethecarriersource.Digitalmodulationinvolvestransmittinginformationindiscreteform.Thesignaliseitheronoroff.Theonstaterepresentsadigital1,theoffstateadigital0.Thesestatesarethebinarydigits(orbits)ofthedigitalsystem.Thedatarateisthenumberofbitspersecond(b/s)transmitted.Thesequenceofonandoffpulsesmightbeacodedversionofananalogmessage.Ananalog-to-digitalconverterdevelopsthedigitalsequencefromtheanalogmessage.Thereverseprocessoccursatthereceiver,wherethedigitalsignalisreturnedtoitsanalogform.Toimpressadigitalsignalontoacarrier,themodulatorneedonlyturnthesourceonoroffattheappropriatetime.Theeaseofconstructingdigitalmodulatorsmakesthisformatveryattractiveforfibersystems.10.1.3CarrierSourceThecarriersourcegeneratesthewaveonwhichtheinformationistransmitted.Thiswaveiscalledthecarrier.Thecarrierisproducedbyanelectronicoscillatorinradio-frequencycommunicationsystems.Forfiberopticsystems,alaserdiode(LD)oralight-emittingdiode(LED)isused.Thesetwodevicescancorrectlybecalledopticoscillators.Ideally,theyprovidestable,single-frequencywaveswithsufficientpowerforlong-distancepropagation.Actuallaserdiodesandlight-emittingdiodesdiffersomewhatfromthisideal.Theyemitarangeoffrequenciesandgenerallyradiateonlyafewmilliwatts8ofaveragepower.Thispowerissufficientinmanycases,becausereceiversaresosensitive.However,transmissionlossescontinuallydecreasethepowerlevelalongthefiber,sothelackofasufficientsourcepowerlimitsthelengthofanycommunicationslink.Thelackofatruesingle-frequencysourcealsodegradesthesystem.Thisdegradationlimitstheamountofinformationthatcanbecarriedalongagivenpathlength.LEDsandlaserdiodesaresmallandlightandconsumeonlymoderateamountsofpower.Itisrelativelyeasytomodulatethem—thatis,toimpresstheinformationontheirradiation.Bothofthesedevicesareoperatedbypassingcurrentthroughthem.Theamountofpowerthattheyradiatecanbemadeproportionaltothiscurrent.Inthisway,theopticoutputpowertakestheshapeoftheinputcurrentcomingfromthemodulator.Itshouldbeemphasizedthattheinformationbeingtransmittediscontainedinthevariationoftheopticpower.Thisiscalledintensitymodulation(IM).Althoughthesignalcurrenthasbothpositiveandnegativeparts,theoutputpowerofalightemitterisalwayspositive.Toachievelinearity,theactualmodulationcurrentinananalogsystemmustbeentirelypositive.ADCbiascurrent,addedtothedesiredinformationsignal,plishesthisresult.Similarly,themodulatingcurrentforadigitalsystemisalwayspositive.Becausealaserdiodedoesnotturnon(thatis,itdoesnotradiate)untilsomethresholdcurrentisapplied,themodulationcurrentcouldincludeaDCoffsetequaltothisthresholdvalue9.Thepresenceofabinary1drivesthecurrentbeyondthresholdandmakesthediodeemitlight.Abinary0leavesthecurrentatthreshold,wherenoradiationoccurs.AnLEDdoesnothaveathresholdandturnsonwheneverpositivecurrentflowsthroughit.LaserdiodesandLEDshavebeenconstructedthatradiateatfrequencieswhereglassfibersareefficienttransmittersoflight—thatis,wherefibershavelowattenuation.Thisisindeedfortunate,becausesuitablesourcesemittingatarbitraryfrequenciesaredifficulttoobtain.Withoutthismatchbetweensourcefrequencyandfiberlow-lossregion,fiberopticalcommunicationswouldnotexist.10.1.4ChannelCouplerNextweconsiderthecoupler,whichfeedspowerintotheinformationchannel.Inaradioortelevisionbroadcastingsystem,thiselementisanantenna.Theantennatransfersthesignalsfromthetransmitterontotheinformationchannel—inthiscase,theatmosphere.Inaguidedsystemusingwires,suchasatelephonelink,thecouplerneedonlybeasimpleconnectorforattachingthetransmittertothetransmissionlineusedastheinformationchannel.Foranatmosphericopticsystem,thechannelcouplerisalensusedforcollimatingthelightemittedbythesourceanddirectingthislighttowardthereceiver.Inourfibersystem,thecouplermustefficientlytransferthemodulatedlightbeamfromthesourcetotheopticfiber.Unfortunately,itisnoteasytoplishthistransferwithoutrelativelylargereductionsinpowerorsomewhatcomplicatedcouplerdesigns.Onedifficultyarisesbecauseofthesmallsizeofconventionalfibers,whichhavediametersoftheorderof50millionthsofameter.However,thelargelossbasicallyoccursbecauselightsourcesemitoveralargeangularextent.Fiberscapturelightonlywithinmorelimitedangles.Thelightemitterismerelybuttedagainstthefiber.Asindicated,evenifthefiberisbigenoughtointerceptallthelightraysemittedbythesource,thelightwillnotbeentirelycollected,becauseofthedifferencebetweentheradiationandacceptanceconeangles.Moreefficient,butalsomorecomplex,couplerscanbeconstructed.Thechannelcouplerisanimportantpartofthedesignofafibersystembecauseofthepossibilityofhighlosses.10.1.5InformationChannelTheinformationchannelreferstothepathbetweenthetransmitterandreceiver.Infiberopticcommunications,aglass(orplastic)fiberisthechannel.Desirablecharacteristicsoftheinformationchannelincludelowattenuationandalargelight-acceptance-coneangle.Lowattenuationandefficientlightcollectionareparticularlynecessaryfortransmissionoverlongpathlengths.Althoughhighlysensitivereceiversareavailable,thepowerdeliveredtothereceivermustbeabovesomelimitingvaluetoconveythedesiredmessagewithappropriateclarity.Opticalamplifiersboostthepowerlevelsofweaksignals.Theyareneededinverylonglinks(hundredsandthousandsofkilometers)toprovidesufficientpowertothereceiver.Repeaters(alsoknownasregenerators)convert10weakanddistortedopticalsignalstoelectricalonesandthenregeneratetheoriginaldigitalpulsetrainsforfurthertransmission.Repeaterscanbeusedonlyfordigitalsystems;opticalamplifiersaresuitableforbothanaloganddigitalsignals.Opticalamplifierscompensateonlyforsignalattenuation,butrepeatersreconstituteboththeamplitudeandwaveshapeofadigitalsignal.Inalongsystem,numerousamplifiersandsomerepeatersmaybeutilized.Beingmuchmorecomplex,repeatersaremoreexpensivethanamplifiers.Anotherimportantpropertyoftheinformationchannelisthepropagationtimeofthewavestravelingalongit.Ingeneral,thetraveltimedependsonthelightfrequencyandonthepathtakenbythelightrays.Asignalpropagatingalongafibernormallycontainsarangeofopticfrequencies(becauseopticsourcesemitarangeoffrequencies)anddividesitspoweralongseveralraypaths.Thisresultsinadistortionofthepropagatingsignal.Inadigitalsystem,thisdistortionappearsasaspreadinganddeformingoftheonpulses.Thespreadingincreaseswiththedistancetraveled.Eventually,thespreadingissogreatthatadjacentpulsesbegintooverlapandeunrecognizableasseparatebitsofinformation.Errorsinthetransmissionnowresult.Tokeepthisfromoccurring,pulsesmustbetransmittedlessfrequently.This,ofcourse,limitstherateatwhichthepulsescanbesent.Thewave-velocitydependenceonfrequencyandpathresultsinalimitationontheinformationrate,whetherthemodulationisdigitaloranalog.Therequirementsforlargelight-acceptanceangleandlowsignaldistortionarecontradictory.Practicalfibersrepresentadesigncompromisebetweenthesetwoqualities.Forsystemshavingmoderatepathlengthsandinformationrates,fiberswithsuitablevaluesofacceptanceangleandsignaldistortioncanbeobtained.Inanatmosphericelectroniccommunicationssystem,anantennacollectsthesignalfromtheinformationchannelandroutesittotherestofthereceiver.Inthefibersystem,theoutputcouplermerelydirectsthelightemergingfromthefiberontothelightdetector.Thislightisradiatedinapatternidenticaltothefiber'sacceptancecone.Becausecommonphotodetectorshavelargesurfaceareasandlargeacceptanceangles,lightcanbeefficientlycoupledfromthefiberbyasimplebuttconnection.10.1.6DetectorTheinformationbeingtransmittedmustnowbetakenoffthecarrierwave.Inanelectronicsystem,thisistheprocessofdemodulatingthesignalandisperformedbytheproperelectroniccircuit.Inthefibersystem,theopticwaveisconvertedintoanelectriccurrentbyaphotodetector.Semiconductorphotodiodesofvariousdesignsaremostcommonlyused.Thecurrentdevelopedbythesedetectorsisproportionaltothepowerintheincidentopticwave.Becausetheinformationiscontainedintheopticpowervariation,thedetectoroutputcurrentcontainsthisinformation.Thiscurrentisareplicaofthatusedtodrivethecarrierlightsource.Importantpropertiesofphotodetectorsincludesmallsize,economy,longlife,lowpowerconsumption,highsensitivitytoopticsignals,andfastresponsetoquickvariationsintheopticpower.Fortunately,lightdetectorshavingthesecharacteristicsarereadilyavailable.10.1.7SignalProcessorForanalogtransmission,thesignalprocessorincludesamplificationandfilteringofthesignal.Inadditiontofilteringoftheconstantbias,anyotherundesiredfrequencies,shouldbeblockedfromfurthertravel.Anidealfilterpassesallfrequenciescontainedinthetransmittedinformationandrejectsallothers.Thisimprovestheclarityoftheintendedtransmission.Properfilteringmaximizestheratioofsignalpowertounwantedpower.Randomfluctuationsinthereceivedsignalarereferredtoasnoise.Noiseispresentinallcommunicationsystems.Foradigitalsystem,theprocessorcanincludedecisioncircuitsinadditiontoamplifiersandfilters.Thedecisioncircuitdecideswhetherabinary1or0wasreceivedduringthetimeslotofanyindividualbit.Becauseofunavoidablenoise,therewillalwaysbesomeprobabilityoferrorinthisprocess.Thebit-errorrate(BER)shouldbeverysmallforqualitycommunications.Thedigital-signalprocessormustalsodecodetheingsequenceof0sand1siftheoriginalmessagewasanalog.Thisisdonebyadigital-to-analogconverter,whichrecreatestheoriginalelectricalformoftheinformation.Ifthecommunicationswerebetweenmachines,thenthedigitalformmightbesuitableforusewithoutdigital-to-analogconversion.10.1.8MessageOutputWeareconcernedwithtwodifferentsituationsatthispoint.Inonecase,themessageispresentedtoaperson,whoeitherhearsorviewstheinformation.Toachievethis,theelectricalsignalmustbetransformedintoasoundwaveoravisualimage.Suitabletransducersforplishingthistransformationaretheloudspeakerforaudiomessagesandthecathoderaytube(similartotheonesusedinatelevisionset)forpictures.Inthesecondcase,theelectricalformofthemessageemergingfromthesignalprocessorisdirectlyusable.Thissituationoccurs,forexample,whencomputersorothermachinesareconnectedthroughafibersystem.Italsooccurswhenthefibersystemisonlyapartofalargernetwork,asinafiberlinkbetweentelephoneexchangesorafibertrunklinecarryinganumberoftelevisionprograms.Intheselasttwosystems,theprocessingincludesdistributionoftheelectricalsignalstotheproperdestinations.Themessageoutputdeviceissimplyanelectricalconnectorfromthesignalprocessortothesucceedingsystem.Weareinterestedonlyperipherallyinsignal-processingcircuitsandmessage-outputdevices,becausethesecomponentsarethesameasthosealreadydevelopedforsystemsthatdonotuseoptics.Newwordsandphraseselectrical [i?lektrik?l] adj. 與電有關(guān)的，電學(xué)的，電的microphone [?maikr?f?un] n. 麥克風(fēng)；話筒；傳聲器current [?k?r?nt] adj.n. 現(xiàn)在的，現(xiàn)行的；通用的，通行的，被普遍接電流portion [?p?:??n] n.vt. 一部分，一份把……分成份額，分配proper [?pr?p?] adj. 適合的；適當(dāng)?shù)腸ontinuous [k?n?tinju?s] adj. 連續(xù)的，沒有中斷的appropriate [??pr?upri?t] adj. 合適的，適合的，適宜的converter [k?n?v?:t?] n. 轉(zhuǎn)換器；變頻器；整流器；變流器occur [??k?:] vi. 發(fā)生；舉行；存在attractive [??tr?ktiv] adj. 有吸引力的oscillator [??sileit?] n. 振蕩器sufficient [s??fi??nt] adj. 足夠的；充足的propagation [pr??p??ɡei??n] n. 繁殖；增殖radiate [?reidieit] vt. 發(fā)射出moderate [?m?d?rit] adj. 中等的，適度的，不是很大的arbitrary [?ɑ:bitr?ri] adj. 任意的；武斷的；隨心所欲的region [?ri:d??n] n. 地區(qū)；地帶；區(qū)域；范圍lens [lenz] n. 透鏡，鏡頭；鏡片Notes1.本節(jié)涉及光纖通信領(lǐng)域，標(biāo)題可譯為：光纖通信。2.consistsof由……組成，由……構(gòu)成。此句譯為：基本通信系統(tǒng)由發(fā)送機(jī)、接收機(jī)和信道構(gòu)成。同義詞組：form;makeupof，compose，formation，composition，configuration。3.bedividedinto被分成。此句譯為：信道可以分為兩類，即非導(dǎo)向信道和導(dǎo)向信道。例句：Theseimprovementscanbedividedintogeneralfeaturecategories:synchronization,thestorageengine,andthequeryprocessor.這些改進(jìn)可劃分為以下常規(guī)功能類別：同步、存儲引擎和查詢處理器。4.do用于加強(qiáng)語氣和倒裝語序的句中。此句譯為：在制造、鋪設(shè)和服務(wù)方面，被導(dǎo)引線路要比大氣信道的成本更高。例句：Theaideadded:"Itdependsonhowyouseebenefits:aretheyarightoraprivilege?Sanctionslikethisdosendastrongmessage."該顧問補(bǔ)充說：“這取決于你如何看待福利：它們是一種權(quán)利還是一種特權(quán)？這種制裁措施確實(shí)傳遞了一條強(qiáng)有力的信息。”5.goodfeel深入的，很好的。此句譯為：通過對圖中的每個(gè)模塊進(jìn)行簡單討論，可以對通信系統(tǒng)的主要單元有很好的了解。例句：Trialanderror,andagoodfeelfortheproblem,arethebestwaysaroundthisissue.嘗試、出錯，以及深入的思考是解決這個(gè)問題的最佳途徑。6.fornow眼下，目前，暫時(shí)。此句譯為：現(xiàn)在就對該內(nèi)容做概要介紹。例句：Fornow,thedegreetowhichtabletsarecannibalizingPCsalesisamatteroffiercedebate.目前為止，平板計(jì)算機(jī)擠占個(gè)人計(jì)算機(jī)市場的程度成為激烈爭論的焦點(diǎn)。7.inanycase無論如何，總之。同類詞語：whateverhappens,comewhatmay,anyway,ineithercase。此句譯為：在任何情況下，無論是在電通信還是在光通信中，信息在傳輸之前都必定是電形態(tài)的。8.milliwatts【電】毫瓦(特)。milli，千分之一。例如：millisecond毫秒，millimeter毫米,millinile毫反應(yīng)性單位。9.Becausealaserdiodedoesnotturnon(thatis,itdoesnotradiate)untilsomethresholdcurrentisapplied,themodulationcurrentcouldincludeaDCoffsetequaltothisthresholdvalue.turnon開啟，turnoff關(guān)閉。DC=directcurrent的縮寫，直流電。thresholdvalue，門限值。此句譯為：半導(dǎo)體激光器在未達(dá)到閾值電流以前是不會開啟的(也就是說，不輻射)，調(diào)制電流中應(yīng)該包括一個(gè)約等于這個(gè)閾值的直流偏置。10.convert…to…，將……轉(zhuǎn)換為……。此句譯為：中繼器(也就是所謂的再生器)將微弱的并且已失真的光信號轉(zhuǎn)換成電信號，然后還原成原來的數(shù)字脈沖串，以便進(jìn)行進(jìn)一步傳輸。10.2AdvantageofFibers1Wearenowreadytodiscusstheadvantagesofopticfibers.Beforedoingso2,letusmentionafewwordsofcaution.Fibersystemsarenotperfect.Theyhavetechnicalandeconomiclimitations.Foranydesiredsystem,therelativemeritsofguidedchannelversusunguidedchannelandmetallicconductorversusfibermustbeevaluated.Thefollowingdiscussionofdesirablefiberpropertiescanbeusefulinthatevaluation.Thebasicmaterialforglassfibersissilicondioxide,whichisplentiful.Someopticfibersaremadeoftransparentplastic,anotherreadilyavailablematerial.Costsareoftenthemostimportantconsiderationinasystem.Comparisonsbetweenfiberandmetalliccablesmustbedonewithcare.Therearemanyfibercablesavailable,someofwhicharecheaperthantheirwireequivalents.Thesavingsmayeparticularlyapparentwhenthecomparisonismadeonthebasisofcostperunitofinformationtransfer.Forexample,avalidcomparisonforatelephonelinkwouldbeonthebasisofcostper3meterpertelephonechannel,ratherthanjustcostpermeter.Thisconsiderationarisesbecausefibershavegreaterinformation-carryingcapacitiesthandometallicchannels.Economiccomparisonsshouldalsoincludethecostsofinstallation,operation,andmaintenance.Somegeneralitiesabouttheseconcernsareworthpresenting.Forlongpaths,fibercablesarecheapertotransportandeasiertoinstallthanmetalcables.Thisisbecausefibersaresmallerandlighter.Onecabledesignhasafiber125μmindiameterenclosedinaplasticsheathof2.5mminouterdiameter.Theweightofthiscableis6kg/km;thelossis5dB/km.LetuscomparethiscablewiththeRG-19/Ucoaxialcable,whichhasanattenuationof22.6dB/kmwhencarryinga100MHzsignal.Itsouterdiameteris28.4mm,anditsweightis1110kg/km.Smallerandlightercoaxialcablesareavailable,buttheyhavehigherlossesthantheRG-19/U.Thesignificantsizeandweightadvantagesoffibercablesareapparentfromthisexample.Therearenogreatdifferencesbetweentheoperationoffibersystemsandthatofmetallicsystems.Thecostshereshouldbethesame.Maintenanceoffibercablesdoesdiffer,however.Ifalineisbroken,asaresultofeitheranaccidentorasystemmodification,splicesmustbemadeornewconnectorsattached.Theseoperationsrequiremoretimeandskillforfibersthanforwires.Asaresult,maintenancecostsshouldbeconsideredwhendesigningasysteminwhichmanychangesarelikelytobemade.Fibersandfibercableshaveturnedouttobesurprisinglystrongandflexible.Somefibersaresoslenderthattheydonotbreakwhenwrappedaroundcurvesofonlyafewcentimetersradius.Fibersareoftenstoredandtransportedwhiletightlywrappedaroundspoolshavingthissmallcurvature4.Fiberflexibilityisattractiveforinstallationscontainingmanyturnsalongthetransmissionpath.Foralarge-radiusbend,fibersguidelightwithnegligibleloss.Thereissomelossataverytightbend,however.Whenafiberisprotected—forexample,byencasingitinaplasticsheath—itisdifficulttobendthecableintoaradiussmallenoughtobreakthefiber.Fibersembeddedincablesdonotbreakeasily.Theadditionofaplasticsheathincreasesthetensilestrengthofafibertransmissionline.Steelrodscanbeplacedinsidetheplasticcabletoaddfurtherstrength,ifneeded.AnotherstrengtheningmaterialisKevlar,asyntheticpolymerfiberwithgreattensilestrength.Despitetheapparentlyfragilenatureofglass,opticfibercablesareveryruggedandserviceable.Techniqueshavebeendevelopedfortheproductionoffiberswithverylowtransmissionlosses.Manyfiberdesignsexist,butanattenuationof4dB/kmistypicalofcommercialglassfiberswhenoperatedatawavelengtharound0.82μm.Thisrepresentsatransmissionefficiencyof40%fora1kmlength.Thisdegreeoftransparencycouldnotbeachievedbefore1970.Now,fiberswithlossesofonlyafewtenthsofadB/kmareavailableforusearound1.3μmand1.55μm.Verylongcommunicationslinkscanbeconstructedbecauseoftheavailabilityoflow-lossfibers.Amplifiers,neededtoamplifyweaksignals,canbelocatedatlargeintervals.Thelossesofwiretransmissionlinesincreaserapidlywithfrequency.Athighfrequencies,linklengthsandamplifierspacingswouldbesignificantlysmallerforwiresystemsthanforfibersystems.Oneofthemostimportantadvantagesoffibersistheirabilitytocarrylargeamountsofinformationandtodosoineitherdigitaloranalogform.Forexample,asinglefiberofthetypedevelopedfortelephoneservicecanpropagatedataattheT3rate,44.7Mbps.Thisfibertransmits672voicechannels.Fiberswithevengreatercapacitiesareavailable.Althoughpulsespreadinglimitsthemaximumrate,fibercapabilitiesmeettherequirementsofmostdata-handlingsystemsandexceedthecapabilitiesofconductingcables.Intheanalogformat,modulationratesofhundredsofmegahertz,ormore,canpropagatealongfibers.Aswiththedigitalsystems,therateislimitedbydistortionoftheopticsignal.Adramaticcomparisoncanbemadebetweenastandardwiretelephonecableandafibercable.Themetalcablecontains900twisted-wirepairs,anditsdiameteris70mm.Eachpaircarries24voicechannels(T1rate),sothecablecapacityis21600calls.Onefibercabledevelopedfortelephoneapplicationshasa12.7mmdiameterandcontains144fibers,eachoperatingattheT3rate(672channels).Thiscablehasatotalcapacityof96768calls.Thefibercablehasnearly4.5timesasmuchcapacityasthewirecableandhasacross-sectionalareathatis30timesless5.Stillmoreremarkablecomparisonscanbemadeifoperationatahigherrate(say2.5Gbpsor10Gb/s)isassumed.Opticfibers,glassorplastic,areinsulators.Noelectriccurrentsflowthroughthem,eitherowingtothetransmittedsignalorowingtoexternalradiationstrikingthefiber.Inaddition,theopticwavewithinthefiberistrapped,sononeleaksoutduringtransmissiontointerferewithsignalsinotherfibers.Conversely,lightcannotcoupleintothefiberfromitsside.Weconcludethatafiberiswellprotectedfrominterferenceandcouplingwithothercommunicationschannels,whethertheyareelectricaloroptic.Forthosereasonsnoted,fibershaveexcellentrejectionofradio-frequencyinterference(RFI)andofelectromagneticinterference(EMI).RFIreferstointerferencecausedbyradioandtelevisionstations,radar,andothersignalsoriginatinginelectronicequipment.EMIincludesbothsuchsourcesofradiationandthosecausedbynaturalphenomena(suchaslightning)orcausedunintentionally(suchassparking).Ifnotrejected,thentheseundesiredsignalscouldincreasethesystemnoiselevelbeyondacceptablelimits.Afiberexcelsatrejectingexternallycausedbackgroundnoise.Theabilityofafibertoisolateitselffromitsenvironmentallowsustopacknumerousfiberstogetherinacabletotransmitmanychannelsofinformationalongasinglepath.Nocrosstalkoccurs.Becausefibersareinsulators,theywillnotpickuporpropagateelectromagneticpulses(EMP)causedbynuclearexplosions,whichcaninducemillionsofvoltsinaconductingtransmissionline.Thevoltagepulsecantravelmanymilesalongthewireandeventually(becauseofitsstrength)destroytheelectronicsattheendofthepath.Theinsulatingnatureofafiberhasseveralotherpracticalconsequences.Inanenvironmentinwhichhigh-voltagelinesarepresent,awirecommunicationslinkcouldpossiblyshort-circuitthelinesbyfallingacrossthem,causingconsiderabledamage.Sparkingoccurringinthisprocesscouldignitecombustiblegasesinthearea.Thisproblemdisappearswithfibers.Anotheradvantageisthatopticcouplingeliminatestheneedforacommongroundbetweenafibertransmitterandreceiver.Troublesomegroundloopsarenotformed.Additionally,itispossibletorepairthefiberwhilethesystemisonwithoutthepossibilityofshort-circuitingtheelectronicsatthetransmitterorreceiver.Thisproblemmightoccurwhenrepairingametalliccable.Fibersofferadegreeofsecurityandprivacy.Becausefibersdonotradiatetheenergywithinthem,itisdifficultforanintrudertodetectthesignalbeingtransmitted.Thefiberwouldhavetobephysicallyviolatedtoobtainthesignal.Breakingthefiber,orfusinganewfibertothetransmittingfiber,wouldprovideaccesstotheopticbeam.Duringsuchmodificationofthelink,thepowerreachingthereceiver,woulddrop.Asensitivereceivercanmeasurethisloss,providingknowledgethatanintrusionisoccurring.Toimprovethesuccessofdetection,thesystemwouldhavetobemonitoredcontinuously.Electroniccommunicationssystemsincludeprocessingofinformationbeforeitisdeliveredtotheinformationchannelandafteritreachesthereceivingterminal.Opticsystemsincludeprocessingthatisverysimilar.Thissimilarityallowsincorporationoffibersin