《信息工程专业英语》课件第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.以太网

interfaces n.界面；接口

protocol n.协议；草案

configuration

n.配置；结构；外形

backbone n.主干网

coaxial adj.同轴的，共轴的

mandatory

adj.强制的；托管的

preclude vt.排除；妨碍；阻止

collision n.冲突

gigabit

n.千兆比特

twisted-pair

双绞线

sharedaccesstoprinters

打印机共享

bepluggedinto

附属于

independentrepeaters

单独的中继器

lowleveldata

低层数据

fiber-opticcabling

光纤的布线网络

discretepackets

离散的数据包

LAN(LocalAreaNetworks) 局域网

WAN(WideAreaNetworks) 广域网

IU(IndianaUniversity) 印第安纳大学

CRC(CyclicRedundancyCheck) 循环冗余码校验

UDP(UserDatagramProtocol)

用户数据报协议

ICMP(InternetControlMessagesProtocol)

网间控制报文协议

3.1.1Exercises

1.PutthePhrasesintoEnglish

(1)打印机共享； (2)主要布线层；

(3)骨干链路； (4)离散的数据包；

(5)中央控制设备。

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.

3.1.2参考译文

局域网是一种计算机网络，覆盖范围小到一个办公室的几台计算机，大到几个建筑物的百台甚至千台计算机。它的功能是连接计算机以实现打印机共享、文件共享等。局域网可以附属于更大的计算机网，比如更大的局域网或者广域网，用于一个组织的计算机相互联系或者连接到互联网。星型结构局域网如图3.1所示。

由于建设局域网的技术是多种多样的，所以只能用最普通的方式来描述它们。一般的组成元素包括连接计算机的物理媒介和媒介连接的个人界面，通过网络传输数据的协议，协商、解释、管理网络和设备的软件。很多局域网仍然包括单独的中继器、桥接器或者路由器，特别是大型的或者与其他网络连接的局域网。

运行一个局域网所需的管理水平取决于所涉及的设备的种类、配置和数量，但在某些情况是需要考虑的。

以太网(IEEE802.3CSMA/CD的常用名称)是局域网(LAN)中最主要的布线层，运用的是低层数据传送技术。以太网卡如图3.2所示。以下是以太网的功能：

以太网传输数据的速度高达千万位(即10兆比特)每秒(Mbps)。快速以太网支持高达100Mbps，千兆以太网卡支持高达1000Mbps，万兆以太网的最高速度就跟它的名字一样(即每秒10千兆比特或者说是10000Mbps)。

美国印第安纳大学的大楼都使用1Gbps以太网连接到校园主干网。在国际单位中，主要用10Gbps连接骨干链路，虽然有些连接数据中心的系统的速度也是如此。

以太网支持双绞线(10BaseT)、细电缆或者粗同轴电缆(10BASE2、10BASE5)，以及光纤(10BaseF)的布线，百兆和千兆以太网可支持双绞线(100/1000BaseT)和光纤电缆(100BaseF/1000BaseLR)布线。目前，建筑物内主机最常用的以太网是10BaseT和100BaseT。

数据在网络上传输是通过离散的数据包(帧)实现的，帧的长度在64～1518字节之间(其中46～1500是字节的数据，再加上一个强制性的18字节的报头和CRC信息)。

每一台以太网上的设备都独立平等地运作，这样，就不需要中央控制设备了。

以太网支持广泛的协议，最常见的包括TCP/IP、UDP、ICMP(PING)。为防止数据丢失，当两个或多个设备试图同时发送数据报时，以太网会检测到冲突。所有的设备立即停止传输，并等待一个随机确定的时间段，然后再次尝试发送。

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