Tải bản đầy đủ - 0 (trang)
Chapter 1. A Short History of Computer Networking

Chapter 1. A Short History of Computer Networking

Tải bản đầy đủ - 0trang

refined.IfyouexaminetheOSISeven-LayerNetworking

ReferenceModel(seeAppendixA,"OverviewoftheOSISevenLayerNetworkingReferenceModel"),youcanseethatEthernet

worksatalowlevelinthemodel,andisusedtotransmit

packetsofinformationfromhigher-levelprotocols.TCPwasthe

firstmajorhigher-levelprotocolcreated.Oneofthefirst

refinementsofTCPwastobreakitintoseveralparts(orlayers),

whichiswhytheprotocol"suite"isknowntodayasTCP/IP.

TCP(theTransmissionControlProtocol)andIP(theInternet

Protocol)arethebasisoftheInternet.Anotherbreak-outfrom

TCPwastheUserDatagramProtocol(UDP).BothTCPandUDP

providedifferenttypesofservice,yetbothuseIPasthe

workhorseprotocolthatisusedtoroutepackets(ordatagrams)

ontheInternet,aswellasanyintranet.AndwhatdoesIPuse

tosenddataacrossthewire(ortheair,incaseofawireless

network)?Ethernetinmostcases.Learnmoreabouthow

wirelessnetworksworkwithEthernetinPartVofthisbook,

"WirelessNetworkingProtocols."

ItisimportanttokeepinmindthatEthernetandotherwirelevelprotocolssimplyprovidethemeanstoframedatacreate

discreteunitsofdatafortransmissionandthenuseaspecified

methodtosenddataacrossthenetworkmedia.Forexample,

simplyvaryingthevoltageonawirecanbeusedtosenddata

fromonepointtoanother.Anearlytransmissionmethod,called

non-returntozero(NRZ)encoding,usedjustthismethod.A

highvaluewasusedtospecifythebitvalueofone,andalow

valuewasusedforzero.Adrawbacktothisencodingschemeis

thatalongstreamofeitheronesorzeroscanbedifficultto

decipheratthereceivingend.Theterm"clock"isusedtomean

thateachendofthetransmissionunderstandswhereabit

startsandwhereitendsduringthetransmission.Becauseit

wouldbeveryexpensivetohaveaphysicalclockateachend

thatcouldpreciselytimeeachbittransmission,thisencoding

methoddoesnotscalewell.

EarlyEthernetnetworksusedatechniquecalledManchester



encoding.Thismethoddoesprovideaclockingmechanismthat

isbuiltintothecodingschemeitself.Insteadofusingahigh-or

low-voltagestatetoindicateaspecificbit,Manchesterencoding

usesthechangefromonestatetoanother,duringaspecific

interval.

WhileTCP/IPcontinueditsdevelopmentontheARPANET,

computervendorsbegantorecognizetheimportanceof

networking,andmanyproprietaryprotocolsweredeveloped.

DigitalEquipmentCorporation(DEC)createdDECnet(and

numerousotherprotocols),whichwasusedtoconnectitsPDP

computersand,later,VAXandAlphaServercomputers.Today

DECnetisstillused,althoughTCP/IPhasprettymuchreplaced

itformostinstallations.Forashorttimeinthe1980s,however,

DECoperatedthelargestcomputernetworkintheworld,short

oftheInternet.Whatprotocolwasused?DECnet,ofcourse.

Duringthatsameperiod,theOSImodelwascreated,and

Digitalincorporatedtheconceptsofthatmodel,aswellasthe

protocolsthatweredevelopedbyISObasedontheOSImodel,

intoDECnet.BecausetheVMS(VirtualMemorySystem)

operatingsystemusedonDEC'sVAXcomputerswasadopting

theseopenstandards,thenameoftheoperatingsystemwas

changedtoOpenVMS.However,fewothervendorschoseto

incorporatethehigh-overheadconceptsoftheseopen

protocols,andthisfirstattempttostandardizenetworking

protocolsbetweendifferentcomputersfailedtocomeabout.

Othercomputermanufacturersalsoproducedtheirown

proprietarynetworkprotocols.Forexample,IBM'sworkinthis

directionresultedinSNA,whichcombinednetworkingprotocols

fromthehighend(application)downtothelowend(wire

protocols).Asnetworkingbegantobecomeanimportantpartof

thecomputingworld,othervendors,suchasXerox(XNS),also

cameupwiththeirownprotocols.

Theresultwasthatifyouwantedtocreateanetworkof

computersforyourbusiness,youhadtostickwithasingle

vendor.Proprietaryprotocols,then,werenotagoodsolutionto



theproblemofexchangingdatabetweencomputers.

DuringtheearlydaysofPCs,thesamesortofsituation

occurred.AlthoughPCswerebasicallythesamewhenitcame

totheoperatingsystem(DOSatthestart),youcouldbuya

networksetupfrommanydifferentvendors.Onethatcomesto

mindisNetWare(whichisstillaroundtoday,althoughinrecent

yearsTCP/IPhasreplacedtheproprietaryIPX/SPXNetWare

protocols).LANManagerwasMicrosoft'sentryintothefield,

withalegacyofNetBEUIandNetBIOSstilllurkingaroundon

manyWindowscomputerspriortoWindows2000.WhenDEC

startedtobuilditsownPCs,itlicensedLANManagertechnology

andcreatedPathworks.YoumightstillfindPathworksinsome

networks,although,likeLANManager,itisconsideredhistory

today.

OtherpioneeringnetworkingpackagesincludedBanyanVines

andArtisoftLANtastic,neitherofwhichisstillonthemarket.

However,BanyanVineshelpedestablishtheideaofa

nameservice,StreetTalk,whichhelpedpioneertheideaofa

distributeddatabasefortrackingnetworkresources.LANtastic

wasanearlyadvocateofpeer-to-peernetworking,whichhas

ledtothecurrentpopularityofSOHOwiredandwireless

networks.

TheInternetchangedtheentirelandscape.AsTCP/IPcontinued

tomatureintothestableprotocolsuitethatitistoday,thePC

landscape,aswellasmainframeandminicomputers,beganto

adoptTCP/IP.AlthoughtheISOfirstattemptedtodefineopen

protocolssothatcomputersfromdifferentvendorscould

interacttoexchangedata,itturnsoutthatTCP/IPisthewinner

intheend.AndwhenIPv6(IPversion6)finallyreachesfrom

theinnercoreoftheInternettotheedge,youwillfindthat

TCP/IPcontinuestoaddnewfeatures,enhancesecurity,and

providemorerobustfeatures.

Otherprotocols,suchasATMandFrameRelay,areusedfor

long-distancetransferofdata,andcanencapsulateother



protocolssuchasTCP/IP.FibreChannelisawireprotocolthatis

themostwidelyusedprotocolinStorageAreaNetworkstoday

(seeChapter11,"NetworkAttachedStorage(NAS)andStorage

AreaNetworks(SAN)").

TheoldstandardEthernethasitselfcontinuedtobeenhanced

tokeepupwiththeneedforspeed.Earlyversionsoperatedat

25Mbps,andmostdesktopstodayuse100BASE-T,or100Mbps

Ethernet.Gigabitand10GigabitEthernetarenowonthe

market,althoughthesenewerversionsdonotusethesame

signalingtechniquesasearlierversions.Thecapabilityto

providebackwardcompatibilitywithearlierversions,however,

isanotherimportantfactorforthecontinueduseofEthernet.

Todayyouwillfindthatmostdesktopcomputersinacompany's

LANuseTCP/IP.Althoughotherprotocolsmayencapsulate

TCP/IPfortransmissionoveralongdistance,theTCP/IP

protocolisstillthedefactostandardforcomputer-to-computer

communications.TCP/IPisalsosupportedbynetworked

printersandwirelesscommunications.

Becauseofthisstandardization,pricesforequipmentthat

supportEthernetandTCP/IParedramaticallylessthana

decadeago.Networkadaptersthemselvesmaybecomehistory

becausemanycomputermotherboardmanufacturersare

startingtoincorporatethatfunctionalityforbothwiredand

wirelessconnectionsdirectlyontothemotherboard.

Sowhatdoesthisallmean?Itmeansthatwhetheryouoperate

abusinessorahomenetwork,orifyoujustconnecttothe

Internetfromhome,ithasbeenalongprocesstogettowhere

wearetoday.Inthisbookyouwillfindtopicsthatcovermany

oftheimportantprotocolsinusetoday,aswellastopicson

newerdevelopments.







Chapter2.OverviewofNetwork

Topologies

SOMEOFTHEMAINTOPICSINTHISCHAPTERARE

LANTopologies12

BuildingandCampusTopologies23

Multi-TieredNetworkTopology26

Beforeyoucanbegintoupgradeandrepairyournetwork,you

needtounderstandhowit'slaidout,howitfunctions,andhow

thevariouspartsarerelatedtooneanother.Knowinghowyour

networkcomponentsarerelatedmakestheextension,

expansion,andtroubleshootingofyourenvironmentmore

focusedandproductive.Becausenetworkuptimeisrelated

directlytoproductivity,asolidgraspofnetworkconceptsisa

necessitywhenyou'refacingatroubledLAN.

Inthischapter,youwillreviewthetopologiesinusetodayand

learnthestrengthsandweaknessesofeach.







LANTopologies

Severaluniquenetworktechnologieshavebeendevelopedover

thepastthreedecades.Differenttypesofnetworkshave

differentdesigncriteriaand,thus,varioustopologieshavecome

intouse.Oneimportantdistinctionneedstobemadebeforewe

enterintoaseriousdiscussionontopology:physicaltopology

versuslogicaltopology.Thephysicaltopologydescribesthe

layoutofanetworkmedia(suchascopperandfiber-optic

cablesand,morerecently,wirelessequipment)andthedevices

thatconnecttoit.Thelogicaltopologyisconcernednotwiththe

actualphysicalconnectionsbutwiththelogicalpaththrough

thenetworkthatdatacantakefromoneplacetoanother.The

differenceswillbemoreevidentasthedifferenttopologiesare

discussed.

ThebasictopologiesyouwillfindinmostLANstodayinclude

thefollowing:

Bus

Star

Ring

Mesh

Hybrids



BusTopology

Thesimplebustopologystructurewasthefirsttypeusedin

Ethernetnetworks.Thetypicalbusphysicaltopologyconsistsof



Tài liệu bạn tìm kiếm đã sẵn sàng tải về

Chapter 1. A Short History of Computer Networking

Tải bản đầy đủ ngay(0 tr)

×