岗位评估的原理与原则

ENG224INFORMATIONTECHNOLOGY–PartII5.IntroductiontoNetworking15.IntroductiontoNetworking中医免费资料–PartII5.IntroductiontoNetworking2References•HeldG.,“InternetworkingLANsandWANs–Concepts,TechniquesandMethods”,Wiley,2ndEd.,1998Comment:Goodatconcepts•StallingsW.,“DataandComputerCommunications”,PrenticeHall,7thEd.,2004Comment:Goodatconceptsandveryup-to-date•ForouzanB.A,“DataCommunicationsandNetworking”,McGraw-Hill,2ndEd.,2000Comment:Manygoodrevisionquestionsatendsofchapters•RedmondW.,“MCSETrainingKit:NetworkingEssentialsPlus”,MicrosoftPress,2000Comment:PracticalorientedENG224INFORMATIONTECHNOLOGY–PartII5.IntroductiontoNetworking3WhatisaNetwork?Anetworkconsistsof2ormorecomputersconnectedtogether,andtheycancommunicateandshareresources(e.g.information)ENG224INFORMATIONTECHNOLOGY–PartII5.IntroductiontoNetworking4WhyNetworking?•Sharinginformation—i.e.datacommunication•Doyoupreferthese?•Orthis?ENG224INFORMATIONTECHNOLOGY–PartII5.IntroductiontoNetworking5•Sharinghardwareorsoftware•Centralizeadministrationandsupport•E.g.printdocument•E.g.Internet-based,soeveryonecanaccessthesameadministrativeorsupportapplicationfromtheirPCsENG224INFORMATIONTECHNOLOGY–PartII5.IntroductiontoNetworking6HowmanykindsofNetworks?•Dependingonone’sperspective,wecanclassifynetworksindifferentways•Basedontransmissionmedia:Wired(UTP,coaxialcables,fiber-opticcables)andWireless•Basedonnetworksize:LANandWAN(andMAN)•Basedonmanagementmethod:Peer-to-peerandClient/Server•Basedontopology(connectivity):Bus,Star,Ring…::ENG224INFORMATIONTECHNOLOGY–PartII5.IntroductiontoNetworking7•Twomaincategories:–Guided―wires,cables–Unguided―wirelesstransmission,e.g.radio,microwave,infrared,sound,sonar•Wewillconcentrateonguidedmediahere:–Twisted-Paircables:UnshieldedTwisted-Pair(UTP)cablesShieldedTwisted-Pair(STP)cables–Coaxialcables–Fiber-opticcablesTransmissionMediaENG224INFORMATIONTECHNOLOGY–PartII5.IntroductiontoNetworking8•Ifthepairofwiresarenottwisted,electromagneticnoisesfrom,e.g.,motors,willaffectthecloserwiremorethanthefurtherone,therebycausingerrorsTwisted-PairCablesENG224INFORMATIONTECHNOLOGY–PartII5.IntroductiontoNetworking9UnshieldedTwisted-Pair(UTP)•Typicallywrappedinsideaplasticcover(formechanicalprotection)•AsampleUTPcablewith5unshieldedtwistedpairsofwiresMetalInsulatorENG224INFORMATIONTECHNOLOGY–PartII5.IntroductiontoNetworking10ShieldedTwisted-Pair(STP)•STPcablesaresimilartoUTPcables,exceptthereisametalfoilorbraided-metal-meshcoverthatencaseseachpairofinsulatedwiresENG224INFORMATIONTECHNOLOGY–PartII5.IntroductiontoNetworking11CategoriesofUTPCablesEIAclassifiesUTPcablesaccordingtothequality:•Category1―thelowestquality,onlygoodforvoice,mainlyfoundinveryoldbuildings,notrecommendednow•Category2―goodforvoiceandlowdatarates(upto4Mbpsforlow-speedtokenringnetworks)•Category3―atleast3twistsperfoot,forupto10Mbps(commoninphonenetworksinresidentialbuildings)•Category4―upto16Mbps(mainlyfortokenrings)•Category5(or5e)―upto100Mbps(commonfornetworkstargetedforhigh-speeddatacommunications)•Category6―moretwiststhanCat5,upto1GbpsENG224INFORMATIONTECHNOLOGY–PartII5.IntroductiontoNetworking12CoaxialCables•Ingeneral,coaxialcables,orcoax,carrysignalsofhigherfreq(100KHz–500MHz)thanUTPcables•OutermetallicwrappingservesbothasashieldagainstnoiseandasthesecondconductorthatcompletesthecircuitENG224INFORMATIONTECHNOLOGY–PartII5.IntroductiontoNetworking13Fiber-OpticCables•Lighttravelsat3108ms-1infreespaceandisthefastestpossiblespeedintheUniverse•Lightslowsdownindensermedia,e.g.glass•Refractionoccursatinterface,withlightbendingawayfromthenormalwhenitentersalessdensemedium•BeyondthecriticalangletotalinternalreflectionENG224INFORMATIONTECHNOLOGY–PartII5.IntroductiontoNetworking14•Anopticalfiberconsistsofacore(densermaterial)andacladding(lessdensematerial)•Simplestoneisamultimodestep-indexopticalfiber•Multimode=multiplepaths,whereasstep-index=refractiveindexfollowsastep-functionprofile(i.e.anabruptchangeofrefractiveindexbetweenthecoreandthecladding)•Lightbouncesbackandforthalongthecore•Commonlightsources:LEDsandlasersENG224INFORMATIONTECHNOLOGY–PartII5.IntroductiontoNetworking15AdvantagesandDisadvantagesNoiseresistance―externallightisblockedbyouterjacketLesssignalattenuation―asignalcanrunformileswithoutregeneration(currently,thelowestmeasuredlossisabout~4%or0.16dBperkm)Higherbandwidth―currently,limitsondataratescomefromthesignalgeneration/receptiontechnology,notthefiberitselfCost―OpticalfibersareexpensiveInstallation/maintenance―anycrackinthecorewilldegradethesignal,andallconnectionsmustbeperfectlyalignedENG224INFORMATIONTECHNOLOGY–PartII5.IntroductiontoNetworking16LANandWAN•LocalAreaNetwork(LAN)•Smallnetwork,shortdistance•Aroom,afloor,abuilding•Limitedbyno.ofcomputersanddistancecovered•UsuallyonekindoftechnologythroughouttheLAN•Serveadepartmentwithinanorganization•Examples:•NetworkinsidetheStudentComputerRoom•NetworkinsideCF502•NetworkinsideyourhomeENG224INFORMATIONTECHNOLOGY–PartII5.IntroductiontoNetworking17•WideAreaNetwork(W