Unit9ComputerNetworksTextKeywordsTechnicaltermsExpressionsNotestothetextText

Inthe1960s,almostimmediatelyaftercomputersbecamewidespread,thedesiregrewtoconnectcomputerstogether.Originallytheaimwastoallowtheexchangeofinformation(programs,datafiles)betweentheusersofdifferentmainframes.Whensmallercomputersbecameavailableandspreadinlargenumberswithinasingleorganization,connectionsbetweenthesesmallcomputersbecameattractivefortheshareduseofresourcesaswell.Theresultingsystems,inwhichcomputersofpossiblydifferenttypeandsizeareconnectedbutfullyretaintheirautonomy,arecalledcomputernetworks.Computernetworks,world-widenetworksof(large)computersarereferredtoaswideareanetworks(WAN)andcampus-widenetworksof(small)computersarereferredtoaslocalareanetworks(LAN).

Toreducethedesigncomplexity,networkarchitectureisviewedasaseriesoflayerorlevels.Eachlayer(orlevel)consistsofprotocolsthatusefunctionsoflowerlayersandprovideservicestothehigherlayers.Eachlayerperformswell-definedfunctions,andcanbeoflimitedcomplexity.Togetherthelayersimplementthefacilitiesofthenetwork.

TheapproachtonetworkarchitectureandimplementationwasstandardizedintheInternationalStandardsOrganization’sOSIreferencemodelforOpenSystemsInterconnection(OSI).ThemodelcontainssevenlayersandisshowninFig.9-1.Fig.9-1ThemodelofOSI

Theincreasedcapabilitiesofthenetworksallowproviderstoofferawidervarietyofcommunicationservicesthanthetraditionalvoiceservices.Howweobtainnewsandentertainment,conductbusinessandeducation,andaccomplishscientificandtechnicalresearchwillallbeaffected.Networkscanbeanalog,inwhichinformationisrepresentedbycontinuoussmoothlyvaryingsignals,ordigital,inwhichinformationisrepresentedbydiscreteon/offsignals.

Therearethreemajorcategoriesofnetworksdistinguishedprimarilybygeographywide-areanetworks,metropolitan-areanetworks,andlocal-areanetworks.Thebenefitofnetworkisthatitcantransmitinformationrapidlyandeasilyamongwidelyseparatedpeople;peoplecansharehardwareandsoftwareresourcesonthenetwork;acomputernetworkcanalsoprovidehighreliabilitybyhavingalternativesourceofsupply.

Asthesenetworkshavegrowninpopularity,theyhavebeguntointerconnect,creatingaglobalcollectionofpeopleandcomputersthatareabletoelectronicallycommunicatewithoneanother.Keywords

retain vt.保持，保留

autonomy n.自治

complexity n.復(fù)雜(性)，復(fù)雜的事物

protocol n.草案，協(xié)議

implement vt.貫徹，實現(xiàn)；v.執(zhí)行

facility n.設(shè)備，工具

session n.會議，會話

transport vt.傳送，運輸

channel n.信道，頻道

popularity n.普及，流行

distinguish v.區(qū)別，辨別Technicalterms

wideareanetworks(WAN) 廣域網(wǎng)

localareanetworks(LAN) 局域網(wǎng)

InternationalStandardsOrganization

國際標準化組織

OpenSystemsInterconnection(OSI) 開放系統(tǒng)互連Expressions

beviewedas... 視為…

aseriesof 一系列的

berepresentedby...

用…表示

well-defined

明確的Notestothetext

1.

Howweobtainnewsandentertainment,conductbusinessandeducation,andaccomplishscientificandtechnicalresearchwillallbeaffected.

我們?nèi)绾潍@取新聞和娛樂信息、處理事務(wù)、進行教育以及完成科學(xué)與技術(shù)研究等將全都受到影響。句中，“how”引導(dǎo)的是主語從句；主語從句中的謂語“obtain”、“conduct”和“accomplish”是并列成分。

2.?Networkscanbeanalog,inwhichinformationisrepresentedbycontinuoussmoothlyvaryingsignals,ordigital,inwhichinformationisrepresentedbydiscreteon/offsignals.

網(wǎng)絡(luò)可以是模擬的，其中的信息用連續(xù)平穩(wěn)變化的信號表示；或者是數(shù)字的，其中的信息用離散的開關(guān)信號表示。主句中，“analog”與“digital”為并列表語；“inwhich”引導(dǎo)非限制性定語從句，前后兩個“which”分別指代“analog(networks)”和“digital(networks)”，省略“networks”是為了避免重復(fù)；“berepresentedby”為被動語態(tài)。

3.Thebenefitofnetworkisthatitcantransmitinformationrapidlyandeasilyamongwidelyseparatedpeople;peoplecansharehardwareandsoftwareresourcesonthenetwork;acomputernetworkcanalsoprovidehighreliabilitybyhavingalternativesourceofsupply.

網(wǎng)絡(luò)的優(yōu)勢在于它能在相距很遠的人們之間快速、便捷地傳遞信息；人們可以在網(wǎng)絡(luò)上共享硬件及軟件資源；由于資源可選，計算機網(wǎng)絡(luò)的可靠性也很高。句中，“that”引導(dǎo)表語從句，其后的三個并列分句均為表語成分；“byhavingalternativesourceofsupply”為并列表語從句中的方式狀語，“by”后需用動詞的現(xiàn)在分詞形式。

4.Asthesenetworkshavegrowninpopularity,theyhavebeguntointerconnect,creatingaglobalcollectionofpeopleandcomputersthatareabletoe