Chapter11–DistributedSystemsKEYCONCEPTSDatacommunicationLinkingterminalsandcomputersDatacommunicationsoftwareNetworksdistributed分布式的network网络DataCommunicationOnatime-sharedsystem,usersaccessacomputerthroughterminals.Someterminalsarelocal,linkeddirectlytoacomputerbycables,whileothersareremote,communicatingwithdistantcomputersovertelephonelinesorothertransmissionmedia.Remotecomputinginvolvesanumberofspecialproblems.Let’sinvestigate.local本地的、局部的remote远程的distanta.远处的distancen.距离、远方、一长段时间Withinacomputer,dataarerepresentedasdiscreteelectricalpulses–0sand1s.Sincethesystem’scomponentsarenormallywithinafewfeetofeachother,thesebitscaneasilybemovedfromcomponenttocomponent.However,whenweattempttotransmitelectricalpulsesoveradistance,severalthingshappen.First,thesignallosesintensityor“diesdown”becauseoftheresistanceofthewire(Fig.11.1).Atthesametime,itpicksupinterferenceornoise;thestaticinthebackgroundofadistantradiostationisagoodexample.discrete离散的、不连续的feet英尺diedown衰减resistance电阻、阻抗、抵抗picksup受到interference干扰staticn.静电噪声a.静态的Thesignalgrowsweakerandweakerasitmovesawayfromitssource,andthenoisebecomesmoreintenseuntileventuallythesignalisoverwhelmed.Ifdataaretobesentoveradistance,thenoisemustbefilteredoutandthesignalboostedoccasionally.DataareoftentransmittedinthecontextofacarriersignalsuchasthesinewaveofFig.11.2.Onecomplete“S-on-its-side”patterniscalledacycle.overwhelm淹没filterv.过滤n.过滤器occasionally有时候、偶尔boost推进、增强、放大inthecontextof在…环境下、关于carriersignal载波信号sinewave正弦波“S-on-its-side”pattern躺倒的S形Theheightofthewavefromthelowesttothehighestpointisitsamplitude.Thenumberofcyclespersecondisthewave’sfrequency.Becausethecarriersignalistransmittedwithaknownfrequencyandamplitude,itispossibletodesignequipmenttofilterandboostit.Howcanthesewavepropertiesbeusedtoencodeandtransmitbinarydata?Simple.Startwithastandardwaveofknownamplitudeandfrequency,andleteachcyclerepresentasinglebit.Totransmita1-bit,leavethewavealone;totransmita0-bit,varythefrequencyofonecycle(Fig.11.3).Ino