《信息工程專業(yè)英語》課件第3章

Unit3ComputerNetwork3.1Text3.2ReadingMaterials

3.1Text

WhatisLAN&WhatisEthernetWhatisLAN

Localareanetworks(LANs)arecomputernetworksranginginsizefromafewcomputersinasingleofficetohundredsoreventhousandsofdevicesspreadacrossseveralbuildings.Theirfunctiontolinkcomputerstogetheristoprovidesharedaccesstoprinters,fileservers,andotherservices.

LANsinturnmaybepluggedintolargernetworks,suchaslargerLANsorwideareanetworks(WANs),connectingmanycomputerswithinanorganizationtoeachotherand/ortotheInternet.Y-TopologyofLANisshowninFig3.1.

Fig3.1Y-TopologyofLAN

ThetechnologiesusedtobuildLANsareextremelydiverse,soitisimpossibletodescribethemexceptinthemostgeneralway.Universalcomponentsconsistofthephysicalmediathatconnectdevices,interfacesontheindividualdevicesthatconnecttothemedia,protocolsthattransmitdataacrossthenetwork,andsoftwarethatnegotiates,interpretsandadministersthenetworkanditsservices.ManyLANsalsoincludeindependentrepeatersandbridgesorrouters,especiallyiftheyarelargeorconnecttoothernetworks.

ThelevelofmanagementrequiredtorunaLANdependsonthetype,configuration,andnumberofdevicesinvolved,butinsomecasesitcanbeconsiderable.

WhatisEthernet

Ethernet(thenamecommonlyusedforIEEE802.3CSMA/CD)isthedominantcablingandlowleveldatadeliverytechnologyusedinlocalareanetworks(LANs).EthernetcardisshowninFig3.2.FollowingaresomeEthernetfeatures:

Ethernettransmitsdataatuptotenmillionbits(i.e.,10megabits)persecond(10Mbps).FastEthernetsupportsupto100Mbps,GigabitEthernetsupportsupto1,000Mbps,and10GbpsEthernetsupportsupspeedsmatchingitsname(10gigabitspersecondor10,000Mbps).

BuildingsatIndianaUniversityareconnectedtothecampusbackboneusing1GbpsEthernet.AtIU,10Gbpsconnectivityisprimarilyusedforbackbonelinks,thoughsomesystemsintheDataCentersareconnectedatthisspeedaswell.

Fig3.2Ethernetcardbefore1990

Ethernetsupportsnetworksbuiltwithtwisted-pair(10BaseT),thinandthickcoaxial(10Base2and10Base5,respectively),andfiber-optic(10BaseF)cabling.FastandGigabitEthernetscanbebuiltwithtwisted-pair(100/1000BaseT)andfiber-optic(100BaseF/1000BaseLR)cabling.Currently,10and100BaseTEthernetsarethemostcommonforhostswithinbuildings.

Dataistransmittedoverthenetworkindiscretepackets(frames)whicharebetween64and1,518bytesinlength(46to1,500bytesofdata,plusamandatory18bytesofheaderandCRCinformation).

EachdeviceonanEthernetnetworkoperatesindependentlyandequally,precludingtheneedforacentralcontrollingdevice.

Ethernetsupportsawidearrayofprotocols,themostcommonincludingTCP/IP,UDP,andICMP(Ping).

Topreventthelossofdata,whentwoormoredevicesattempttosendpacketsatthesametime,Ethernetdetectscollisions.Alldevicesimmediatelystoptransmittingandwaitarandomlydeterminedperiodoftimebeforetheyattempttotransmitagain.

Technicalwordsandphrases

Ethernet

n.以太網(wǎng)

interfaces n.界面；接口

protocol n.協(xié)議；草案

configuration

n.配置；結(jié)構(gòu)；外形

backbone n.主干網(wǎng)

coaxial adj.同軸的，共軸的

mandatory

adj.強(qiáng)制的；托管的

preclude vt.排除；妨礙；阻止

collision n.沖突

gigabit

n.千兆比特

twisted-pair

雙絞線

sharedaccesstoprinters

打印機(jī)共享

bepluggedinto

附屬于

independentrepeaters

單獨(dú)的中繼器

lowleveldata

低層數(shù)據(jù)

fiber-opticcabling

光纖的布線網(wǎng)絡(luò)

discretepackets

離散的數(shù)據(jù)包

LAN(LocalAreaNetworks) 局域網(wǎng)

WAN(WideAreaNetworks) 廣域網(wǎng)

IU(IndianaUniversity) 印第安納大學(xué)

CRC(CyclicRedundancyCheck) 循環(huán)冗余碼校驗(yàn)

UDP(UserDatagramProtocol)

用戶數(shù)據(jù)報(bào)協(xié)議

ICMP(InternetControlMessagesProtocol)

網(wǎng)間控制報(bào)文協(xié)議

3.1.1Exercises

1.PutthePhrasesintoEnglish

(1)打印機(jī)共享； (2)主要布線層；

(3)骨干鏈路； (4)離散的數(shù)據(jù)包；

(5)中央控制設(shè)備。

2.PutthePhrasesintoChinese

(1)linkcomputerstogether;

(2)fileservers;

(3)physicalmedia;

(4)independentrepeaters;

(5)fiber-opticcabling.

3.Translation

(1)LANsinturnmaybepluggedintolargernetworks,suchaslargerLANsorWideAreaNetworks(WANs),connectingmanycomputerswithinanorganizationtoeachotherand/ortotheInternet.

(2)FastEthernetsupportsupto100Mbps,GigabitEthernetsupportsupto1,000Mbps,and10GbpsEthernetsupportsupspeedsmatchingitsname(10Gigabitspersecondor10,000Mbps).

(3)EachdeviceonanEthernetnetworkoperatesindependentlyandequally,precludingtheneedforacentralcontrollingdevice.

3.1.2參考譯文

局域網(wǎng)是一種計(jì)算機(jī)網(wǎng)絡(luò)，覆蓋范圍小到一個(gè)辦公室的幾臺(tái)計(jì)算機(jī)，大到幾個(gè)建筑物的百臺(tái)甚至千臺(tái)計(jì)算機(jī)。它的功能是連接計(jì)算機(jī)以實(shí)現(xiàn)打印機(jī)共享、文件共享等。局域網(wǎng)可以附屬于更大的計(jì)算機(jī)網(wǎng)，比如更大的局域網(wǎng)或者廣域網(wǎng)，用于一個(gè)組織的計(jì)算機(jī)相互聯(lián)系或者連接到互聯(lián)網(wǎng)。星型結(jié)構(gòu)局域網(wǎng)如圖3.1所示。

由于建設(shè)局域網(wǎng)的技術(shù)是多種多樣的，所以只能用最普通的方式來描述它們。一般的組成元素包括連接計(jì)算機(jī)的物理媒介和媒介連接的個(gè)人界面，通過網(wǎng)絡(luò)傳輸數(shù)據(jù)的協(xié)議，協(xié)商、解釋、管理網(wǎng)絡(luò)和設(shè)備的軟件。很多局域網(wǎng)仍然包括單獨(dú)的中繼器、橋接器或者路由器，特別是大型的或者與其他網(wǎng)絡(luò)連接的局域網(wǎng)。

運(yùn)行一個(gè)局域網(wǎng)所需的管理水平取決于所涉及的設(shè)備的種類、配置和數(shù)量，但在某些情況是需要考慮的。

以太網(wǎng)(IEEE802.3CSMA/CD的常用名稱)是局域網(wǎng)(LAN)中最主要的布線層，運(yùn)用的是低層數(shù)據(jù)傳送技術(shù)。以太網(wǎng)卡如圖3.2所示。以下是以太網(wǎng)的功能：

以太網(wǎng)傳輸數(shù)據(jù)的速度高達(dá)千萬位(即10兆比特)每秒(Mbps)。快速以太網(wǎng)支持高達(dá)100Mbps，千兆以太網(wǎng)卡支持高達(dá)1000Mbps，萬兆以太網(wǎng)的最高速度就跟它的名字一樣(即每秒10千兆比特或者說是10000Mbps)。

美國印第安納大學(xué)的大樓都使用1Gbps以太網(wǎng)連接到校園主干網(wǎng)。在國際單位中，主要用10Gbps連接骨干鏈路，雖然有些連接數(shù)據(jù)中心的系統(tǒng)的速度也是如此。

以太網(wǎng)支持雙絞線(10BaseT)、細(xì)電纜或者粗同軸電纜(10BASE2、10BASE5)，以及光纖(10BaseF)的布線，百兆和千兆以太網(wǎng)可支持雙絞線(100/1000BaseT)和光纖電纜(100BaseF/1000BaseLR)布線。目前，建筑物內(nèi)主機(jī)最常用的以太網(wǎng)是10BaseT和100BaseT。

數(shù)據(jù)在網(wǎng)絡(luò)上傳輸是通過離散的數(shù)據(jù)包(幀)實(shí)現(xiàn)的，幀的長度在64～1518字節(jié)之間(其中46～1500是字節(jié)的數(shù)據(jù)，再加上一個(gè)強(qiáng)制性的18字節(jié)的報(bào)頭和CRC信息)。

每一臺(tái)以太網(wǎng)上的設(shè)備都獨(dú)立平等地運(yùn)作，這樣，就不需要中央控制設(shè)備了。

以太網(wǎng)支持廣泛的協(xié)議，最常見的包括TCP/IP、UDP、ICMP(PING)。為防止數(shù)據(jù)丟失，當(dāng)兩個(gè)或多個(gè)設(shè)備試圖同時(shí)發(fā)送數(shù)據(jù)報(bào)時(shí)，以太網(wǎng)會(huì)檢測到?jīng)_突。所有的設(shè)備立即停止傳輸，并等待一個(gè)隨機(jī)確定的時(shí)間段，然后再次嘗試發(fā)送。

3.2ReadingMaterials

3.2.1GlobalOpenEthernet(GOE)Technology

ThebasicGOEconceptistoprovideasimpleandcost-efficientEthernetVPNorvirtualprivateLANservice(VPLS)byperformingthesamefunctionsasEoMPLSbasedonasimpleextensionofQ-in-QVLAN-tagging.Fig3.3showsaGOE-basedcarriernetwork,whichconsistsofGOEcoreandedgenodes.

Fig3.3andFig3.4alsoshowtheframeformatofincominguserframesandGOE-encapsulatedtransportedframes(inunicastandinbroadcast)withinthecarriernetwork.TheGOEedgenode,receivinguserEthernetframeswithorwithoutIEEE802.1DVLAN-tag,pushesaGOEheaderimmediatelyafterEthernetsourceMACaddressfield.TheGOEheaderisaflexibleandextensiblevariable-lengthheaderstructure;theheaderconsistsofmandatorytags(forwardingtagorsourcetag)andoptionaltags(customerIDtagandoptionaltags),ifnecessary.

TheformatofeachtagusesTPIDfield(16bit)(802.1Q)forspecifyingtagtypeandTCIfield(16bit)(802.1Q)forspecifyingitsvalue,asthisisthewaythecurrentVLANtaginformationisencoded.Optionaltagsarereservedforfutureuse.NotethatinordertomakeaGOEnodeintemperateeasilywithlegacyEthernetswitches,mandatorytagsusethesameTPIDaslegacyVLAN/VMANtagswithspecifyingdifferenttypesoftagsattheunusedprioritybitfieldofTCI.

Fig3.3GOEforwardingtaggedframeformatFig3.4GOEbroadcasttaggedframeformat

TheforwardingtagandthesourcetagisakeycomponentofGOEforunicastroutingandforbroadcastroutinginthenetwork,respectively,whereasthecustomerIDtagisusedforuniquelyidentifyingcustomersandforapplyingcustomer-specifictrafficprocessing.Decouplingforwardingandcustomerinformationintodifferenttagfieldscanseparatenetworkandcustomermanagement,whichleadstosimpleandscalablenetworkoperation.

AsopposedtothecurrentVLANtagusage,theGOEforwardingtagusesthedestinationnodeaddress,sothattheforwardingtablescreatedbyGOEformunidirectionalpaths,whicharesimilartoMPLSpaths.AlthoughMPLSusesthesamevirtualcircuitmechanism(i.e.,theconnection-orientedparadigm)asasynchronoustransfermode(ATM)network,GOEforwardingisbasedonthedestinationnodeaddress(i.e.,theconnectionlessparadigm),whichissimilartoInternetprotocol(IP)networks.

ThebasicideaisthataVLAN-IdisassignednottoanEthernetport(asitisusuallydone),buttoanEthernetswitch,sothattheVLAN-tagisusedasaroutingaddress.Inotherwords,VLAN-IdbecomesakindofanIPaddress,whichEthernetswitchescanprocessinawaysimilartothatofIProuters.VPNpathscanbeestablishedinaconnectionlessmannerbySTPasdescribedbelow.TosupportevenlargescaleVPNs,ahierarchicaladdress,representedbymultiplestackedVLANs(asshowninFig3.3),isassignedtoeachEthernetswitchsothatmultilevelhierarchiescanbeeasilycreated.Inparticular,forlargescaleVPNnetworks,hierarchicalmultiplerapidspanningtreesareperformedbyacombinationofIEEE802.1sand802.1wprotocolsateachlevel.

3.2.2WhatisWLAN

AWirelessLocalAreaNetwork(WLAN)isaLocalAreaNetwork(LAN)thatdoesn’trelyonwiredEthernetconnections.AWLAN(Fig3.5)canbeeitheranextensiontoacurrentwirednetworkoranalternativetoit.

Fig3.5WLAN

WLANshavedatatransferspeedsrangingfrom1to54Mbps,withsomemanufacturersofferingproprietary108Mbpssolutions.The802.11nstandardcanreach300to600Mbps.

Becausethewirelesssignalisbroadcastsoeverybodynearbycanshareit,severalsecurityprecautionsarenecessarytoensureonlyauthorizeduserscanaccessyourWLAN.

AWLANsignalcanbebroadcasttocoveranarearanginginsizefromasmallofficetoalargecampus.Mostcommonly,aWLANaccesspointprovidesaccesswithinaradiusof65to300feet.

WLANtypes

PrivatehomeorsmallbusinessWLAN

Commonly,ahomeorbusinessWLANemploysoneortwoaccesspointstobroadcastasignalarounda100-to200-footradius.YoucanfindequipmentforinstallingahomeWLANinmanyretailstores.

Withfewexceptions,hardwareinthiscategorysubscribestothe802.11a,b,orgstandards(alsoknownasWi-Fi);somehomeandofficeWLANsnowadheretothenew802.11nstandard.Also,becauseofsecurityconcerns,manyhomeandofficeWLANsadheretotheWi-FiProtectedAccess2(WPA2)standard.

EnterpriseclassWLAN

AnenterpriseclassWLANemploysalargenumberofindividualaccesspointstobroadcastthesignaltoawidearea.TheaccesspointshavemorefeaturesthanhomeorsmallofficeWLANequipment,suchasbettersecurity,authentication,remotemanagement,andtoolstohelpintegratewithexistingnetworks.Theseaccesspointshavealargercoverageareathanhomeorsmallofficeequipment,andaredesignedtoworktogethertocoveramuchlargerarea.Thisequipmentcanadheretothe802.11a,b,g,ornstandard,ortosecurity-refiningstandards,suchas802.1xandWPA2.

3.2.3WhatisVLAN

HaveyoueverwonderedwhataVirtualLAN(orVLAN)isorbeenunclearastowhyyouwouldwantone?Ifso,Ihavebeeninyourplaceatonetimetoo.Sincethen,IhavelearnedalotaboutwhataVLANisandhowitcanhelpme.Inthisarticle,Iwillsharethatknowledgewithyou.

AsIsaid,aVLAN(Fig3.6)isavirtualLAN.Intechnicalterms,aVLANisabroadcastdomaincreatedbyswitches.Normally,itisaroutercreatingthatbroadcastdomain.WithVLAN’s,aswitchcancreatethebroadcastdomain.

Fig3.6VLAN

Thisworksby,you,theadministrator,puttingsomeswitchportsinaVLANotherthan1,thedefaultVLAN.AllportsinasingleVLANareinasinglebroadcastdomain.

Becauseswitchescantalktoeachother,someportsonswitchAcanbeinVLAN10andotherportsonswitchBcanbeinVLAN10.BroadcastsbetweenthesedeviceswillnotbeseenonanyotherportinanyotherVLAN,otherthan10.Ho