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1、Incomputer,storagedevicestypicallyrefertomassstoragedevicesthatarecapableofstoringlargequantityofdata,evenafterelectricalpowertothecompteristurnoff.Makingordownloadingunauthorizedcopiesofsoftwareisbreakingthelaw,nomatterhowmanycopiesorpeopleareinvolved.制作或者下载未经授权的软件复制件都是违法
2、的，无论涉及多少复制件或人员。在计算机中，存储设备通常是指那些即使是在计算机关机断电之后，也可以存储大量数据的大容量存储设备。ComputernetworksToday,networksareeverywhereandnetworktechnologyisevolvingrapidly.Justwhenyouthinkyouhavedial-upundercontrol,alongcomescableaccessandWi-Fi.Justasyougetcomfortablewithyourschool’scampus-widenetwor
3、k,everyoneisissuednotebookcomputerthatusesomesortofwirlessconnections.EvolutionofComputerNetworksComputernetworksemergedinthelate1960s.Theyhaveinheritedmanyusefulpropertiesformtheirpredecessors,namely,olderandmorewidelyadoptedtelephonenetworks.Thisisnotsurprising,sinceboth
4、computersandtelephonesareuniversalinstrumentsofcommunications.however,computernetworkshavebroughtsomethingnewintotheworldofcommunications,namely,thepracticallyinexhaustiblestoreofinformationaccumulatedbyhumancivilizationduringtheseveralthousandyearsofitsexistence.Thisbecam
5、eespeciallynoticeableinthemid-1990s,whentherapidgrowthoftheInternetclearlydemonstratedthatfreeandanonymousaccesstoinformationandinstant,writtencommunicationswerehighlyvaluedbymostindividuals.CommunicationBasicsThedatatransmittedcanbevoice,text,video,images,sound,oracombina
6、tionofthem.Fortransmittingthedata,therearetwotypesofsignals:analoganddigital.Analogsignals:acontinuouselectricalsignalintheformofawavecalledacarrierwave.Telephones,radios,andtelevisions---theolderformsofcommunications---weredesignedtoworkwithananalogsignal.Digitalsignals:u
7、seson/offelectricalpulsesindiscrete,ratherthanacontinuouswave.Thistwo-statesignalrepresentsthetwo-statebinarylanguageof0and1thatcomputeruse.DirectionoftransmissionTherearethreetypesoftransmissionmodes:Simplex:thecommunicationisunidirectional.Onlyoneofthetwostationonalinkca
8、ntransmit;theothercanonlyreceive.Forexample,aradiostationisasimplexchannelbecauseitalways
