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COMMUNICATIONCHANNELSCommunicationchannelsareanessentialelementofeverycommunicationsystem.Thesechannelsactuallycarrythedatafromonecomputertoanother.Therearetwocategoriesofcommunicationchannels.Onecategoryconnectssendingandreceivingdevicesbyprovidingaphysicalconnectionsuchasawireorcable.Theothercategoryiswireless.沟通渠道是一个至关重要的元素的每个通信系统。这些渠道实际上把数据从一台计算机到另一个。有两个类别的沟通渠道。一个类别连接发送和接收设备通过提供一个物理连接导线或电缆等。另一类是无线。PHYSICALCONNECTIONSPhysicalconnectionsuseasolidmediumtoconnectsendingandreceivingdevices.Theseconnectionsincludetelephonelines(twistedpair),coaxialcable,andfiber-opticcable.物理连接使用固体介质连接发送和接收设备。这些连接包括电话线路(双绞线)、同轴电缆和光纤电缆。Telephonelinesyouseestrungonpolesconsistoftwisted-paircable,whichismadeupofhundredsofcopperwires.Asingletwistedpairculminatesinawalljackintowhichyoucanplugyourphoneandcomputer.(SeeFigure9-3.)Telephonelineshavebeenthestandardtransmissionmediumforyearsforbothvoiceanddata.However,theyarenowbeingphasedoutbymoretechnicallyadvancedandreliablemedia.Coaxialcable,ahigh-frequencytransmissioncable,replacesthemultiplewiresoftelephonelineswithasinglesolid-coppercore.(SeeFigure9-4.)Intermsofthenumberoftelephoneconnections,acoaxialcablehasover80timesthetransmissioncapacityoftwistedpair.Coaxialcableisusedtodelivertelevisionsignalsaswellastoconnectcomputersinanetwork..电话线路你看到两极串在由双绞线,是由数以百计的铜线。一个双绞线的顶端是一墙杰克,你可以插你的手机和电脑。电话线被标准的传输介质多年的语音和数据。然而,他们现在被淘汰的技术更先进、可靠的媒体。同轴电缆、高频传输电缆,取代了多个连接的电话线和一个固体铜核心。在数量方面的电话连接,一个同轴电缆有超过80倍的双绞线传输容量。同轴电缆是用于提供电视信号以及连接网络的电脑..Fiber-opticcabletransmitsdataaspulsesoflightthroughtinytubesofglass.(SeeFigure9-5.)Intermsofthenumberoftelephoneconnections,fiber-opticcablehasover26,000timesthetransmissioncapacityoftwisted-paircable.Comparedtocoaxialcable,theyarelighterandmorereliableattransmittingdata.Theytransmitinformationusingbeamsoflightatlightspeedsinsteadofpulsesofelectricity,makingthemfarfasterthancoppercable.Fiber-opticcableisrapidlyreplacingtwisted-paircabletelephonelines.光纤电缆传输数据的光脉冲,通过微小的玻璃管。在数量方面的电话连接,光纤电缆已经超过26000倍的双绞线传输容量。同轴电缆相比,它们更轻和更可靠的传输数据。他们使用光束传输信息以光的速度而不是的电脉冲,使他们远远快于铜电缆。光纤电缆正在迅速取代双绞线电缆电话线路。WIRELESSCONNECTIONSWirelessconnectionsdonotuseasolidsubstancetoconnectsendingandreceivingdevices.Rather,theyusetheairitself.Primarytechnologiesusedforwirelessconnectionsareinfrared,broadcastradio,microwave,andsatellite.无线连接不使用固体物质连接发送和接收设备。相反,他们使用空气本身。主要技术用于无线连接红外、广播无线电、微波和卫星。Infraredusesinfraredlightwavestocommunicateovershortdistances.Itissometimesreferredtoasline-of-sightcommunicationbecausethelightwavescanonlytravelinastraightline.Thisrequiresthatsendingandreceivingdevicesmustbeinclearviewofoneanotherwithoutanyobstructionsblockingthatview.OneofthemostcommonapplicationsistotransferdataandinformationfromaportabledevicesuchasanotebookcomputerorPDAtoadesktopcomputer.红外使用红外光波在短距离通信。它有时被称为视距通信因为光波只能以直线的方式行进。这就要求发送和接收设备必须在清楚彼此的观点没有任何障碍物阻止这一观点。一个最常见的应用是用来传送数据和信息从一个便携式设备如笔记本电脑或掌上电脑桌面计算机。Broadcastradiousesradiosignalstocommunicatewithwirelessdevices.Forexample,cellulartelephonesandmanyWeb-enableddevicesusebroadcastradiotoplacetelephonecallsand/ortoconnecttotheInternet.SomeendusersconnecttheirnotebookorhandheldcomputerstoacellulartelephonetoaccesstheWebfromremotelocations.MostoftheseWeb-enableddevicesfollowastandardknownasWi-FI(wirelessfidelity).ThiswirelessstandardiswidelyusedtoconnectcomputerstoeachotherandtotheInternet.广播电台使用无线电信号与无线设备。例如,移动电话和许多网络设备使用广播电台将电话和/或连接到互联网。一些终端用户连接他们的笔记本或者掌上电脑到移动电话访问网络从远程位置。大多数的这些网络设备遵循一个标准的称为wi-fi(无线保真度)。这个无线标准被广泛用于连接电脑到对方和因特网。Microwavecommunicationuseshigh-frequencyradiowaves.Likeinfrared,microwavecommunicationprovidesline-of-sightcommunicationbecausemicrowavestravelinastraightline.Becausethewavescannotbendwiththecurvatureoftheearth,theycanbetransmittedonlyoverrelativelyshortdistances.Thus,microwaveisagoodmediumforsendingdatabetweenbuildingsinacityoronalargecollegecampus.Forlongerdistances,thewavesmustberelayedbymeansofmicrowavestationswithmicrowavedishesorantennas.(SeeFigure9-6.)Thesestationscanbeinstalledontowers,highbuildingsandmountaintops.微波通信使用高频无线电波。如红外、微波通信提供视距通信因为微波以直线的方式行进。因为海浪不能弯曲曲率的地球,他们可以传播只在相对较短的距离。因此,微波是一个很好的媒介建筑物之间发送数据在一个城市或一个大的大学校园。时间的距离,海浪必须转发通过微波站与微波碗或天线。这些站可以安装在大厦,高层建筑,山顶。Bluetoothisashort-rangewirelesscommunicationstandardthatusesmicrowavestotransmitdataovershortdistancesofuptoapproximately33feet.Unliketraditionalmicrowaves,Bluetoothdoesnotrequireline-of-sightcommunication.Rather,itusesradiowavesthatcanpassthroughnearbywallsandothernonmetalbarriers.Itisanticipatedthatwithinthenextfewyears,thistechnologywillbewidelyusedtoconnectavarietyofdifferentcommunicationdevices.蓝牙是一种短距离的无线通信标准,使用微波传输数据在短的距离大约33英尺。与传统的微波,蓝牙不需要视距通信。相反,它使用无线电波可以通过附近的墙壁和其他非金属壁垒。预计在未来几年中,这种技术将被广泛用于连接各种不同的通信设备。Satellitecommunicationusessatellitesorbitingabout22,000milesabovetheearthasmicrowaverelaystations.(SeeFigure9-7.)ManyoftheseareofferedbyIntelsat,theInternationalTelecommunicationsSatelliteConsortium,whichisownedby114governmentsandformsaworldwidecommunicationsystem.Satellitesrotateataprecisepointandspeedabovetheearth.Theycanamplifyandrelaymicrowavesignalsfromonetransmitteronth