UnitFive Networking 
SectionA Networking 
Theneedtoshareinformationandresourcesamongdifferent 
computershasledtolinkedcomputersystems,callednetworks,in 
whichcomputersareconnectedsothatdatacanbetransferredfrom 
machinetomachine. 
Ⅰ.NetworkClassifications 
Acomputernetworkisoftenclassifiedasbeingeitheralocal 
areanetwork(LAN),a metropolitanareanetwork(MAN),ora 
wideareanetwork(WAN).Inrecentyears,withthedevelopment 
ofwirelesscommunicationtechnology,anew concept,personal 
areanetwork(PAN)isputforward.ALANnormallyconsistsofa 
collectionofcomputersinasinglebuildingorbuildingcomplex. 
Forexample,thecomputersonauniversitycampusorthoseina 
manufacturingplantmightbeconnectedbyaLAN.A MANisa 
network ofintermediate size,such as one spanning a local 
community.A WAN links machinesovera greaterdistance— 
perhapsinneighboringcitiesoronoppositesidesoftheworld.A 
network 
/.netw.:k/ 
n.网络
LAN 
局域网
metropolitan 
/.metrE.pXlItEn/ 
n.大都市的
MAN 
城域网
WAN 
广域网
PAN 
个域网

68 
PANisapersonalinformationnetwork,ascableradio orinfrared 
insteadofthetraditionalcable,torealizetheintelligentinternetof 
personalinformationterminal. 
Anothermeansofclassifyingnetworksisbasedonwhetherthe 
network..sinternaloperationisbasedondesignsthatareinthe 
public domain or on innovations owned and controlled by a 
particularentitysuchasanindividualoracorporation.Anetwork 
oftheformertypeiscalledanopennetwork;anetworkofthelatter 
typeiscalledaclosed,orsometimesaproprietarynetwork.The 
Internet is an open system.In particular, communication 
throughouttheInternetis governed by an open collection of 
standardsknownastheTCP/IPprotocolsuite. 
Stillanother way ofclassifying networksis based onthe 
topology ofthenetwork(Figure5A-1),whichreferstothepattern 
inwhichthe machinesareconnected.Twoofthe morepopular 
topologiesarethebus,inwhichthemachinesareallconnectedtoa 
commoncommunicationlinecalledabus,andthestar,inwhich 
onemachineservesasacentralfocalpointtowhichalltheothers 
areconnected.Thebustopologywaspopularizedinthe1990swhen 
itwasimplementedunderasetofstandardsknownasEthernet, 
andEthernetnetworksremainoneofthemostpopularnetworking 
systemsinusetoday.Thestartopologyhasrootsasfarbackasthe 
1970s.Itevolvedfromtheparadigm ofalargecentralcomputer 
servingmanyusers.Anothertopologiesaretheringtopology,tree 
topology,meshtopologyandhybridtopology. 
Figure5A-1 TheTopologyoftheNetwork 
cableradio 
无线电
infrared 
/.InfrE.red/ 
n.红外线
opennetwork 
开放网络
proprietary 
/prE.praIEtri/ 
adj.专有的
topology 
/tE.pXlEdZI/ 
n.拓扑学
bus 
/bVs/ 
n.总线
Ethernet 
/.i:TEnet/ 
n.以太网
mesh 
/meS/ 
n.网状;网格
hybrid 
/.haIbrId/ 
adj.混合的

69 
Ⅱ.CombiningNetworks 
Sometimesitisnecessarytoconnectexistingnetworkstoform 
anextendedcommunicationsystem.Thiscanbedonebyconnecting 
thenetworkstoformalargerversionofthesametypeofnetwork. 
InthecaseofbusnetworksbasedontheEthernetprotocols,itis 
oftenpossibletoconnectthebusestoformasinglelongbus.This 
isdonebymeansofdifferentdevicesknownasrepeaters,bridges, 
andswitches,thedistinctionsofwhicharesubtleyetinformative. 
Thesimplestoftheseistherepeater,whichislittlemorethana 
devicethatsimplypassessignalsbackandforthbetweenthetwo 
originalbuses(usually withsomeform ofamplification)without 
consideringthemeaningofthesignals. 
Abridgeissimilarto,butmorecomplexthan,arepeater. 
Likearepeater,itconnectstwobuses,butitdoesnotnecessarily 
passallmessagesacrosstheconnection.Instead,itlooksatthe 
destinationaddressthataccompanieseach messageandforwardsa 
messageacrosstheconnectiononlywhenthatmessageisdestined 
foracomputerontheotherside.Thus,twomachinesresidingon 
thesamesideofabridgecanexchangemessageswithoutinterfering 
withcommunicationtaking place onthe otherside.A bridge 
producesamoreefficientsystemthanthatproducedbyarepeater. 
Aswitchisessentiallyabridge with multipleconnections, 
allowingittoconnectseveralbusesratherthanjusttwo.Thus,a 
switchproducesanetworkconsistingofseveralbusesextending 
fromtheswitchasspokesonawheel.Asinthecaseofabridge,a 
switchconsidersthedestination addressesofall messagesand 
forwardsonlythosemessagesdestinedforotherspokes.Moreover, 
eachmessagethatisforwardedisrelayedonlyintotheappropriate 
spoke,thusminimizingthetrafficineachspoke. 
Sometimes,however,the networksto beconnected have 
incompatiblecharacteristics.Forinstance,thecharacteristicsofa 
WiFi① network are not readily compatible with an Ethernet 
network.Inthesecasesthenetworks mustbeconnectedin a 
mannerthatbuildsanetworkofnetworks,knownasaninternet, 
① WiFi:基于IEEE802.11b标准的无线局域网。
repeater 
/RI.Pi:tE(R)/ 
n.中继器
bridge 
/brIdZ/ 
n.网桥
switch 
/swItS/ 
n.交换机
subtle 
/.sVtl/ 
adj.微妙的
informative 
/In.fO:mEtIv/ 
adj.信息量大的
amplification 
/.QmplIfI.keISn/ 
n.放大
message 
/.mesIdZ/ 
n.报文
forward 
/.fO:wEd/ 
vt.转发

70 
in whichtheoriginalnetworks maintaintheirindividualityand 
continuetofunctionas autonomous networks.Theconnection 
betweennetworkstoformaninternetishandledbydevicesknown 
as routers, which are special purpose computers used for 
forwardingmessages. 
Asanexample,Figure5A-2depictstwo WiFistarnetworks 
andan Ethernet bus network connected by routers.When a 
machineinoneoftheWiFinetworkswantstosendamessagetoa 
machineintheEthernetnetwork,itfirstsendsthemessagetothe 
AP①initsnetwork.Fromthere,theAPsendsthemessagetoits 
associatedrouter,andthisrouterforwardsthe messagetothe 
routerattheEthernet.Therethemessageisgiventoamachineon 
thebus,andthatmachinethenforwardsthemessagetoitsfinal 
destinationintheEthernet. 
Figure5A-2 RoutersConnectingTwoWiFiandanEthernetBusNetwork 
Thepointatwhichonenetworkislinkedtoaninternetisoften 
calledagateway becauseitservesasapassagewaybetweenthe 
networkandtheoutsideworld.Gatewayscanbefoundinavariety 
offorms,andthusthetermisusedratherloosely.Inmanycasesa 
network..s gateway is merely the router through which it 
communicateswiththerestoftheinternet.Inothercasestheterm 
gateway maybeusedtoreferto morethanjustarouter.For 
① AP:AccessPoint,即(无线)访问接入点,是组建小型无线局域网最常用的设备。
autonomous 
/O:.tXnEmEs/ 
adj.自主的
router 
/.ru:tE/ 
n.路由器
gateway 
/.geItweI/ 
n.网关

71 
example,inmostresidentialWiFinetworksthatareconnectedto 
theInternet,theterm gatewayreferscollectivelyto boththe 
network..sAPandtherouterconnectedtotheAPbecausethesetwo 
devicesarenormallypackagedinasingleunit. 
Ⅲ.MethodsofProcessCommunication 
Thevariousactivities(orprocesses)executingonthedifferent 
computers within a network(or even executing on the same 
machineviatime-sharing/multitasking)mustoftencommunicate 
witheachothertocoordinatetheiractionsandtoperformtheir 
designatedtasks.Suchcommunicationbetweenprocessesiscalled 
interprocess communication. 
Apopularconventionusedforinterprocesscommunicationis 
theclient/servermodel.Thismodeldefinesthebasicrolesplayed 
bytheprocessesaseitheraclient,whichmakesrequestsofother 
processes,oraserver,whichsatisfiestherequestsmadebyclients. 
Todaytheclient/server modelis used extensivelyin network 
applications. 
However,theclient/server(C/S)modelisnottheonlymeans 
ofinterprocesscommunication.Anothermodelisthepeer-to-peer 
(P2P)model.Whereastheclient/servermodelinvolvesoneprocess 
(theserver)providingaservicetonumerousothers(clients),the 
peer-to-peermodelinvolvesprocessesthatprovideservicetoand 
receiveservicefromeachother(Figure5A-3).Moreover,whereasa 
servermustexecutecontinuouslysothatitispreparedtoserveits 
clients at any time,the peer-to-peer model usually involves 
processesthat execute on a temporary basis.For example, 
applicationsofthepeer-to-peermodelincludeinstantmessagingin 
whichpeoplecarryonawrittenconversationovertheInternetas 
wellassituationsin which people play competitiveinteractive 
games. 
Thepeer-to-peermodelisalsoapopularmeansofdistributing 
filessuchasmusicrecordingsandmotionpicturesviatheInternet. 
Inthiscase,onepeer mayreceiveafilefrom anotherandthen 
providethatfiletootherpeers.Thecollectionofpeersparticipating 
insuchadistributionissometimescalleda swarm.Theswarm 
approachtofiledistributionisincontrasttoearlierapproachesthat 
interprocess 
/.IntE.prXses/ 
n.进程间
client/server 
客户/服务器
peer-to-peer 
对等网;点对点
swarm 
/swO:m/ 
n.聚合;群

Figure5A-
3 
TwoPopularModelsofInterproces 
Communication 

applied the client/server model by establishing a central 
ditiuincnetesrerihcinsdlaeie.

srbtoetr(hevr)fomwhcletownoddfls

Exercises 

Ⅰ.Filintheblankswiththeinformationgiveninthetext. 

1.Thenedtoshareinformationand amongdiferentcomputershasled 
tolinkedcomputersystems,caled ,inwhichcomputersareconnectedsothat 
datacanbetransferedfrommachinetomachine. 

2.Acomputernetworkisoftenclasifiedasbeingeithera ,aMAN,a 
WAN,ora . 
3.TheInternetisan system,whichisgovernedbyanopencolectionof 
standardsknownasthe protocolsuite. 
4.Basedonthetopologyofthenetwork,twoofthemorepopulartopologiesare 
,and . 

5.A isesentialyabridgewithmultipleconnections,alowingitto 
connectseveralbusesratherthanjusttwo. 
6.Theconnectionbetwennetworkstoformaninternetishandledbydevices 
knownas . 
7.Thepointatwhichonenetworkislinkedtoaninternetisoftencaleda 
becauseitservesasapasagewaybetwenthenetworkandtheoutsideworld. 
8.InmostresidentialWiFinetworksthatareconnectedtotheInternet,theterm 
gatewayreferscolectivelytoboththenetwork..s andthe connected 
totheAPbecausethesetwodevicesarenormalypackagedinasingleunit. 

9.Thepopularconventionusedforinterprocescommunicationisthe 
modeland model. 

72 


Ⅱ.TranslatethefolowingtermsorphrasesfromEnglishintoChinese. 

LAN MAN 

WAN PAN 

cableradio opennetwork 

proprietarynetwork bustopology

Ethernetnetwork ringtopology

tretopology meshtopology

hybridtopology repeater 

bridge switch 

router WiFi 

AP gateway

client/server(C/S) per-to-per(P2P) 

Ⅲ.TranslatethefolowingpasagefromEnglishintoChinese. 

TheGenerationsofWireles 
Telephones 

Inthepastdecademobilephonetechnologyhasadvancedfromsimple,single 
purpose,portabledevicestocomplex,multifunctionhand-heldcomputers.Thefirst 
generationwirelestelephonenetworktransmitedanalogvoicesignalsthroughtheair, 
muchliketraditionaltelephonesbutwithoutthecopperwirerunningthroughthewal. 
Inretrospect,wecaltheseearlyphonesystems“1G”,whichusingFDMA.Thesecond 
generationmainlyusingTDMA,canprovidevoiceandlow-speddigitalservicessuchas 
textmesaging.Thirdgeneration(3G)phonenetworkwhichusingCDMAtechnologyis 
characterizedbyuserpeakrateof2Mb/storeachtensofMb/s,alowingformobile 
videocalsandothermultimediadataservices.4GOFDMAtechnologyasthecore,the 
userpeakrateofupto100Mb/s~1Gb/s,cansupportavarietyofmobilebroadband 
dataservices.5Gkeycompetenciesricherthanpreviousgenerationsofmobile 
communications,userexperience,sped,densityofconnections,endtoenddelay,the 
peakrateandmobilityandsowilbethe5Gkeyperformanceindicators. 

73 

74 
SectionB DistributedSystem 
Ⅰ.Introduction 
With the success of networking technology,interaction 
between computers via networks has become common and 
multifaceted.Many modern software systems,such as global 
information retrieval systems, company-wide accounting and 
inventorysystems,computergames,andeventhesoftwarethat 
controlsanetwork..sinfrastructureitselfaredesignedasdistributed 
systems,meaningthattheyconsistofsoftwareunitsthatexecute 
asprocessesondifferentcomputers. 
Adistributedsystem(seeFigure5B-1)consistsofacollection 
ofautonomous computers,connected through a network and 
distribution middleware,whichenablescomputerstocoordinate 
theiractivitiesandtosharetheresourcesofthesystem,sothat 
usersperceivethesystemasasingle,integratedcomputingfacility. 
Thecertain common characteristicscan be usedto assess 
distributedsystems: 
. ResourceSharing 
. Openness 
. Concurrency 
. Scalability 
. FaultTolerance 
. Transparency 
Middlewareisaclassofsoftwaretechnologiesdesignedtohelp 
multifaceted 
/.mVltI.fQsItId/ 
adj.多方面的
distributed 
/dI.strIbju:tId/ 
adj.分布式的
middleware 
/.mIdlweE/ 
n.中间件
concurrency 
/kEn.kVrEnsI/ 
n.并发性
tolerance 
/.tXlErEns/ 
n.容忍
transparency 
/trQns.pQrEnsI/ 
n.透明性

75 
Figure5B-1 GeneralStructureofaDistributedSystem 
managethecomplexityandheterogeneity inherentindistributed 
systems.Itisdefinedasalayerofsoftwareabovetheoperating 
systembutbelowtheapplicationprogramthatprovidesacommon 
programmingabstractionacrossadistributedsystem. 
Ⅱ.ApplicationofDistributedSystem 
Severaltypes of distributed computing systems are now 
common. 
. ClusterComputing 
Clustercomputing(Figure 5B-2)is a type of parallel or 
Figure5B-2 ClusterComputing 
distributedprocessingsystem,whichconsistsofacollectionof 
interconnected stand-alone computers cooperatively working 
togetherasasingle,integratedcomputingresource.Thecostof 
theseindividualmachines,plusthehigh-speednetworktoconnect 
them,canbelessthanahigher-pricedsupercomputer,butwith 
higherreliabilityandlower maintenancecosts.Such distributed 
systemshasthekey operationalbenefitsofhigh performance, 
expandabilityandscalability,highthroughputandhighavailability. 
heterogeneity 
/.hetErE.dZI.nIEtI/ 
n.异构性
inherent 
/In.hIErEnt/ 
adj.固有的;内在的
cluster 
/.klVstE/ 
n.集群
throughput 
/.TRU:P.t/ 
n.吞吐量

76 
Clustercomputingismainlyusedingrandchallengingapplications 
such as weather forecasting, quantum chemistry, molecular 
biologymodeling,data-miningwebservers,etc. 
. GridComputing 
Gridcomputingisaformofdistributedcomputinginwhichan 
organization usesitsexistingcomputers(desktopand/orcluster 
nodes)tohandleitsownlong-runningcomputationaltasks(see 
Figure5B-3).Gridcomputingaimsto“enableresourcesharingand 
coordinatedproblemsolvingindynamic,multi-institutionalvirtual 
organizations”.ExamplesincludeUniversityofWisconsin..sCondor 
system①,or Berkeley..s Open Infrastructure for Network 
Computing(BOINC)②.Bothofthesesystemsareofteninstalledon 
computersthatareusedforotherpurposes,suchasPCsatworkor 
athome,thatcanthenvolunteer computingpowertothegrid. 
EnabledbythegrowingconnectivityoftheInternet,thistypeof 
voluntary,distributedgridcomputinghasenabledmillionsofhome 
PCstoworkonenormouslycomplex mathematicalandscientific 
problems.
Figure5B-3 ADistributedGridTaskSchedulingSystem 
. CloudComputing 
Cloudcomputing(Figure5B-4)isalarge-scale distributed 
①
② 
Wisconsin..sCondorsystem:威斯康星大学研发的Condor系统,是一个开源的高吞
吐量计算软件框架,用于计算密集型任务的粗粒度分布式并行计算。
BOINC:伯克利开放式网络计算,由美国加利福尼亚大学伯克利分校空间科学实验
室研发的用于志愿者和网格计算的一个开源的中间件系统。
grid 
/grid/ 
n.网格
volunteer 
/.vXlEn.tIE(R)/ 
v.自愿去做/ 
n.自(志)愿者
voluntary 
/.vXlEntri/ 
n.自愿; 
adj.自愿的