《信息工程專業(yè)英語》課件第11章

Unit11InternetCommunication11.1Text11.2ReadingMaterials

11.1Text

InternetTelephony&VoIP

VoiceoverIP(VoIP,VoiceoverInternetProtocol)commonlyreferstothecommunicationprotocols,technologies,methodologies,andtransmissiontechniquesinvolvedinthedeliveryofvoicecommunicationsandmultimediasessionsoverInternetProtocol(IP)networks,suchastheInternet.OthertermscommonlyassociatedwithVoIP(Fig11.1)areIPtelephony,Internettelephony,voiceoverbroadband(VoBB),broadbandtelephony,IPcommunications,andbroadbandphone.Fig11.1VoIP

Internettelephonyreferstocommunicationsservices—voice,fax,SMS,and/orvoice-messagingapplications—thataretransportedviatheInternet,ratherthanthepublicswitchedtelephonenetwork(PSTN).ThestepsinvolvedinoriginatingaVoIPtelephonecallaresignalingandmediachannelsetup,digitizationoftheanalogvoicesignal,encoding,packetization,andtransmissionasInternetProtocol(IP)packetsoverapacket-switchednetwork.

Onthereceivingside,similarsteps(usuallyinthereverseorder)suchasreceptionoftheIPpackets,decodingofthepacketsanddigital-to-analogconversionreproducetheoriginalvoicestream.EventhoughIPTelephonyandVoIParetermsthatareusedinterchangeably,theyareactuallydifferent;IPtelephonyhastodowithdigitaltelephonysystemsthatuseIPprotocolsforvoicecommunication,whileVoIPisactuallyasubsetofIPTelephony.VoIPisatechnologyusedbyIPtelephonyasameansoftransportingphonecalls.

EarlyprovidersofvoiceoverIPservicesofferedbusinessmodels(andtechnicalsolutions)thatmirroredthearchitectureofthelegacytelephonenetwork.Secondgenerationproviders,suchasSkypehavebuiltclosednetworksforprivateuserbases,offeringthebenefitoffreecallsandconvenience,whiledenyingtheiruserstheabilitytocallouttoothernetworks.Thishasseverelylimitedtheabilityofuserstomix-and-matchthird-partyhardwareandsoftware.Thirdgenerationproviders,suchasGoogleTalkhaveadoptedtheconceptofFederatedVoIP-whichisacompletedeparturefromthearchitectureofthelegacynetworks.ThesesolutionstypicallyallowarbitraryanddynamicinterconnectionbetweenanytwodomainsontheInternetwheneverauserwishestoplaceacall.

VoIPsystemsemploysessioncontrolprotocolstocontroltheset-upandtear-downofcallsaswellasaudiocodeswhichencodespeechallowingtransmissionoveranIPnetworkasdigitalaudioviaanaudiostream.ThechoiceofcodecvariesbetweendifferentimplementationsofVoIPdependingonapplicationrequirementsandnetworkbandwidth;someimplementationsrelyonnarrowbandandcompressedspeech,whileotherssupporthighfidelitystereocodes.Somepopularcodesincludeu-lawanda-lawversionsofG.711,G.722whichisahigh-fidelitycodecmarketedasHDVoicebyPolycom,apopularopensourcevoicecodecknownasiLBC.

VoIPisavailableonmanysmartphonesandInternetdevicessothatusersofportabledevicesthatarenotphones,mayplacecallsorsendSMStextmessagesover3GorWi-Fi.

Technicalwordsandphrases

session

n.對話

originate

vi.發(fā)源；發(fā)生

subset

n.子集

mirror

vt.反射；反映

legacy

adj.舊的，傳統(tǒng)的

reverse

n.背面；相反

portable

adj.手提的，便攜式的；輕便的

federated

adj.聯(lián)邦的；聯(lián)合的；結(jié)成同盟的

arbitrary adj.任意的

implementation

n.實現(xiàn)；履行；安裝啟用

smartphone

智能手機

inthereverseorder

相反的順序

IPpackets

IP數(shù)據(jù)包

digital-to-analogconversion 數(shù)模轉(zhuǎn)換

voicestream

語音流

opensourcevoicecodec 開源語音編解碼器

portabledevices

便攜設(shè)備

VoIP(VoiceoverInternetProtocol)

互聯(lián)網(wǎng)協(xié)議語音

VoBB(VoiceoverBroadBand)

寬帶語音

SMS(ShortMessageService) 短信通群法系統(tǒng)

PSTN(PublicSwitchedTelephoneNetwork)

公共交換電話網(wǎng)絡(luò)

HD(HighDefinition) 高清

iLBC一種專為包交換網(wǎng)絡(luò)通信設(shè)計的編解碼，優(yōu)于目前流行的G.729、G.723.1

11.1.1Exercises

1.PutthePhrasesintoEnglish

(1)通信協(xié)議； (2)媒體信道；

(3)數(shù)字化模擬語音信號；

(4)分組交換網(wǎng)絡(luò)；

(5)數(shù)字電話系統(tǒng)； (6)第二代供應(yīng)商；

(7)封閉的網(wǎng)絡(luò)。

2.PutthePhrasesintoChinese

(1)voicecommunications;

(2)multimediasessions;

(3)apacket-switchednetwork;

(4)businessmodels;

(5)technicalsolutions;

(6)third-partyhardware;

(7)dynamicinterconnection;

(8)compressedspeech;

(9)highfidelitystereocodecs.

3.Translation

(1)ThestepsinvolvedinoriginatingaVoIPtelephonecallaresignalingandmediachannelsetup,digitizationoftheanalogvoicesignal,encoding,packetization,andtransmissionasInternetProtocol(IP)packetsoverapacket-switchednetwork.

(2)Secondgenerationproviders,suchasSkypehavebuiltclosednetworksforprivateuserbases,offeringthebenefitoffreecallsandconvenience,whiledenyingtheiruserstheabilitytocallouttoothernetworks.

(3)VoIPsystemsemploysessioncontrolprotocolstocontroltheset-upandtear-downofcallsaswellasaudiocodecswhichencodespeechallowingtransmissionoveranIPnetworkasdigitalaudioviaanaudiostream.

11.1.2參考譯文

IP語音(VoIP或互聯(lián)網(wǎng)協(xié)議語音)通常指通信協(xié)議、技術(shù)、方法和傳輸技術(shù)，傳輸技術(shù)包括語音通信和通過因特網(wǎng)協(xié)議進行多媒體會話的傳輸，例如因特網(wǎng)。其他與VoIP相關(guān)的術(shù)語包括IP電話、互聯(lián)網(wǎng)電話、寬帶語音(VoBB)、寬帶電話、IP通信和寬帶電話。

網(wǎng)絡(luò)電話是指通信服務(wù)—語音、傳真、SMS和/或語音消息應(yīng)用。即通過互聯(lián)網(wǎng)傳輸?shù)?，而不是通過公共交換電話網(wǎng)(PSTN)。撥打VoIP(如圖11.1所示)電話的步驟如下：信號和媒體信道的設(shè)定，數(shù)字化模擬語音信號，編碼，分包，經(jīng)過分組交換網(wǎng)絡(luò)傳輸IP數(shù)據(jù)包。在接收端有類似的步驟(通常以相反的順序)，比如接收IP數(shù)據(jù)包，解碼，數(shù)模轉(zhuǎn)化再現(xiàn)原始的語音流。即使IP電話和VoIP的術(shù)語可交替使用，但實際上它們是不同的；IP電話和數(shù)字電話系統(tǒng)有關(guān)，它使用IP協(xié)議進行語音通信，而VoIP實際上是IP電話的一個子集。VoIP用于作為傳輸電話呼叫方式之一的IP電話。

語音IP服務(wù)的早期供應(yīng)商提出的業(yè)務(wù)模式(和技術(shù)解決方案)借鑒了舊的電話網(wǎng)絡(luò)體系結(jié)構(gòu)。第二代供應(yīng)商，如Skype已經(jīng)為私人用戶群建立了封閉的網(wǎng)絡(luò)，提供免費電話和便利條件，同時不讓他們的用戶能夠調(diào)用其他的網(wǎng)絡(luò)。這嚴(yán)格限制了用戶混合和匹配第三方硬件和軟件的能力。第三代供應(yīng)商，如谷歌通話都采用了聯(lián)邦網(wǎng)絡(luò)電話的概念，完全背離了原有網(wǎng)絡(luò)的結(jié)構(gòu)。每當(dāng)用戶希望發(fā)出一個呼叫時，這些解決方案通常允許互聯(lián)網(wǎng)上任何兩個域之間的隨機動態(tài)互聯(lián)。

VoIP系統(tǒng)采用會話控制協(xié)議來控制設(shè)置和清除電話以及音頻編解碼器，語音編碼允許數(shù)字音頻通過音頻流在IP網(wǎng)絡(luò)上傳輸。依賴于應(yīng)用需求和網(wǎng)絡(luò)寬帶VoIP的實現(xiàn)方式不同，編解碼器的選擇也不同；一些VoIP的實現(xiàn)依賴于窄頻帶和壓縮語言，而其他的支持高保真立體聲編解碼器。一些流行的編解碼器包括u-law和a-law版本的G.711、G.722，這些是高保真的編解碼器，由寶利通公司作為高清語言銷售。其中一個流行的開源語音編解碼器稱為iLBC。

許多智能手機和互聯(lián)網(wǎng)設(shè)備都有VoIP，所以用戶可以使用非電話的便攜設(shè)備在許多地方通過3G或Wi-Fi撥打電話或發(fā)送短信。

11.2ReadingMaterials

11.2.1E-mail

Electronicmail,alsoknownasEmailorE-mail,isamethodofexchangingdigitalmessagesfromanauthortooneormorerecipients.ModernemailoperatesacrosstheInternetorothercomputernetworks.Someearlyemailsystemsrequiredthattheauthorandtherecipientbothbeonlineatthesametime,incommonwithinstantmessaging.

Today'semailsystemsarebasedonastore-and-forwardmodel.Emailserversaccept,forward,deliverandstoremessages.Neithertheusersnortheircomputersarerequiredtobeonlinesimultaneously;theyneedconnectonlybriefly,typicallytoanemailserver,foraslongasittakestosendorreceivemessages.

AnInternetemailmessageconsistsofthreecomponents,themessageenvelope,themessageheader,andthemessagebody.Themessageheadercontainscontrolinformation,including,minimally,anoriginator'semailaddressandoneormorerecipientaddresses.Usuallydescriptiveinformationisalsoadded,suchasasubjectheaderfieldandamessagesubmissiondate/timestamp.

Originallyatext-only(7-bitASCIIandothers)communicationsmedium,emailwasextendedtocarrymulti-mediacontentattachments,aprocessstandardizedinRFC2045through2049.Collectively,theseRFCshavecometobecalledMultipurposeInternetMailExtensions(MIME).

ElectronicmailpredatestheinceptionoftheInternet,andwasinfactacrucialtoolincreatingit,butthehistoryofmodern,globalInternetemailservicesreachesbacktotheearlyARPANET.Standardsforencodingemailmessageswereproposedasearlyas1973(RFC561).ConversionfromARPANETtotheInternetintheearly1980sproducedthecoreofthecurrentservices.Anemailsentintheearly1970slooksquitesimilartoabasictextmessagesentontheInternettoday.

Network-basedemailwasinitiallyexchangedontheARPANETinextensionstotheFileTransferProtocol(FTP),butisnowcarriedbytheSimpleMailTransferProtocol(SMTP),firstpublishedasInternetstandard10(RFC821)in1982.Intheprocessoftransportingemailmessagesbetweensystems,SMTPcommunicatesdeliveryparametersusingamessageenvelopeseparatefromthemessage(headerandbody)itself.

11.2.2Videoconferencing

Videoconferencingusesaudioandvideotelecommunicationstobringpeopleatdifferentsitestogether.Thiscanbeassimpleasaconversationbetweenpeopleinprivateoffices(point-to-point)orinvolveseveral(multipoint)sitesinlargeroomsatmultiplelocations.Besidestheaudioandvisualtransmissionofmeetingactivities,alliedvideoconferencingtechnologiescanbeusedtosharedocumentsanddisplayinformationonwhiteboards.

Simpleanalogvideophonecommunicationcouldbeestablishedasearlyastheinventionofthetelevision.Suchanantecedentusuallyconsistedoftwoclosed-circuittelevisionsystemsconnectedviacoaxcableorradio.AnexampleofthatwastheGermanReichPostzentralamt(postoffice)videotelephonenetworkservingBerlinandseveralGermancitiesviacoaxialcablesbetween1936and1940.

Duringthefirstmannedspaceflights,NASAusedtworadio-frequency(UHForVHF)videolinks,oneineachdirection.TVchannelsroutinelyusethistypeofvideotelephonywhenreportingfromdistantlocations.Thenewsmediaweretobecomeregularusersofmobilelinkstosatellitesusingspeciallyequippedtrucks,andmuchlaterviaspecialsatellitevideophonesinabriefcase.

Thistechniquewasveryexpensive,though,andcouldnotbeusedforapplicationssuchastelemedicine,distanceeducation,andbusinessmeetings.Attemptsatusingnormaltelephonynetworkstotransmitslow-scanvideo,suchasthefirstsystemsdevelopedbyAT&T,firstresearchedinthe1950s,failedmostlyduetothepoorpicturequalityandthelackofefficientvideocompressiontechniques.Thegreater1MHzbandwidthand6Mb/sbitrateofthePicturephoneinthe1970salsodidnotachievecommercialsuccess,mostlyduetoitshighcost,butalsoduetoalackofnetworkeffect—withonlyafewhundredPicturephonesintheworld,usershadextremelyfewcontactstheycouldactuallycallto,andinteroperabilitywithothervideophonesystemsdidnotexist.

Itwasonlyinthe1980sthatdigitaltelephonytransmissionnetworksbecamepossible,suchaswithISDNnetworks,assuringaminimumbitrate(usually128kilobits/s)forcompressedvideoandaudiotransmission.Duringthistime,therewasalsoresearchintootherformsofdigitalvideoandaudiocommunication.Manyofthesetechnologies,suchastheMediaspace,arenotaswidelyusedtodayasvideoconferencing(Fig11.2)butwerestillanimportantareaofresearch.ThefirstdedicatedsystemsstartedtoappearinthemarketasISDNnetworkswereexpandingthroughouttheworld.OneofthefirstcommercialvideoconferencingsystemssoldtocompaniescamefromPictureTelCorp.,whichhadanInitialPublicOfferinginNovember,1984.

Fig11.2Videoconferencing

In1984ConceptCommunicationintheUnitedStatesreplacedthethen-100pound,US$100,000computersnecessaryforteleconferencingwitha$12,000circuitboardwhichdoubledthevideoframeratefromto30framespersecond,andwhichwasreducedtheequipmentinsizetoacircuitboardthatfitintostandardpersonalcomputers.Thecompany'sfounder,WilliamJ.Tobinalsosecuredapatentforacodecforfull-motionvideoconferencing,firstdemonstratedatAT&TBellLabsin1986.

Videoconferencingsystemsthroughoutthe1990srapidlyevolvedfromveryexpensiveproprietaryequipment,softwareandnetworkrequirementstoastandardsbasedtechnologythatisreadilyavailabletothegeneralpublicatareasonablecost.

Finally,inthe1990s,IP(InternetProtocol)basedvideoconferencingbecamepossible,andmoreefficientvideocompressiontechnologiesweredeveloped,permittingdesktop,orpersonalcomputer(PC)-basedvideoconferencing.In1992CU-SeeMewasdevelopedatCornellbyTimDorceyetal.In1995thefirstpublicvideoconferencebetweenNorthAmericaandAfricatookplace,linkingatechnofairinSanFranciscowithatechno-raveandcyberdeliinCapeTown.AttheWinterOlympicsopeningceremonyinNagano,Japan,SeijiOzawaconductedtheOdetoJoyfromBeethoven’sNinthSymphonysimultaneouslyacrossfivecontinentsinnear-realtime.

Whilevideoconferencingtechnologywasinitiallyusedprimarilywithininternalcorporatecommunicationnetworks,oneofthefirstcommunityserviceusagesofthetechnologystartedin1992throughauniquepartnershipwithPictureTelandIBMCorporationswhichatthetimewerepromotingajointlydevelopeddesktopbasedvideoconferencingproductknownasthePCS/1.Overthenext15years,ProjectDIANE(DiversifiedInformationandAssistanceNetwork)grewtoutilizeavarietyofvideoconferencingplatformstocreateamultistatecooperativepublicserviceanddistanceeducationnetworkconsistingofseveralhundredschools,neighborhoodcenters,libraries,sciencemuseums,zoosandparks,publicassistancecenters,andothercommunityorientedorganizations.

Inthe2000s,videotelephonywaspopularizedviafreeInternetservicessuchasSkypeandiChat,webpluginsandon-linetelecommunicationprogramswhichpromotedlowcost,albeitlow-quality,videoconferencingtovirtuallyeverylocationwithanInternetconnection.

InMay2005,thefirsthighdefinitionvideoconferencingsystems,producedbyLifeSizeCommunications,weredisplayedattheInteroptradeshowinLasVegas,Nevada,abletoprovide30framespersecondata1280by720displayresolution.Polycomintroduceditsfirsthighdefinitionvideoconferencingsystemtothemarketin2006.Currently,highdefinitionresolutionhasnowbecomeastandardfeature,withmostmajorsuppliersinthevideoconferencingmarketofferingit.

RecenttechnologicaldevelopmentsbyLibrestreamhaveextendedthecapabilitiesofvideoconferencingsystemsbeyondtheboardroomforusewithhand-heldmobiledevicesthatcombinetheuseofvideo,audioandon-screendrawingcapabilitiesbroadcastinginreal-timeoversecurenetworks,independentoflocation.Mobilecollaborationsystemsallowmultiplepeopleinpreviouslyunreachablelocations,suchasworkersonanoff-shoreoilrig,theabilitytoviewanddiscussissueswithcolleaguesthousandsofmilesaway.Traditionalvideoconferencingsystemmanufacturershavebegunprovidingmobileapplicationsaswell,suchasAVerInformation’sVCLinkappwhichallowsforliveandstillimagestreaming.

11.2.3Server

Inmostcommonuse,aserverisaphysicalcomputer(acomputerhardwaresystem)dedicatedtorunningoneormoreservices(asahost),toservetheneedsoftheusersofothercomputersonthenetwork.Dependingonthecomputingservicethatitoffersitcouldbeadatabaseserver,fileserver,mailserver,printserver,webserver,gamingserver,orsomeotherkindofserver.IntelserverisshowninFig11.3.

Fig11.3Intelserver

Inthecontextofclient-serverarchitecture,aserverisacomputerprogramrunningtoservetherequestsofotherprograms,the“clients”.Thus,the“server”performssomecomputationaltaskonbehalfof“clients”.Theclientseitherrunonthesamecomputerorconnectthroughthenetwork.

InthecontextofInternetProtocol(IP)networking,aserverisaprogramthatoperatesasasocketlistener.

Serversoftenprovideessentialservicesacrossanetwork,eithertoprivateusersinsidealargeorganizationortopublicusersviatheInternet.

Thetermserverisusedquitebroadlyininformationtechnology.Despitethemanyserver-brandedproductsavailable(suchasserverversionsofhardware,softwareoroperatingsystems),intheoryanycomputerisedprocessthatsharesaresourcetooneormoreclientprocessesisaserver.Toillustratethis,takethecommonexampleoffilesharing.Whiletheexistenceoffilesonamachinedoesnotclassifyitasaserver,themechanismwhichsharesthesefilestoclientsbytheoperatingsystemistheserver.

Similarly,considerawebserverapplication(suchasthemultiplatform“ApacheHTTPServer”).Thiswebserversoftwarecanberunonanycapablecomputer.Forexample,whilealaptoporpersonalcomputerisnottypicallyknownasaserver,theycaninthesesituationsfulfilltheroleofone,andhencebelabelledasone.Itis,inthiscase,themachine’srolethatplacesitinthecategoryofserver.

Inthehardwaresense,thewordservertypicallydesignatescomputermodelsintendedforhostingsoftwareapplicationsundertheheavydemandofanetworkenvironment.Inthisclient–serverconfigurationoneormoremachines,eitheracomputeroracomputerappliance,shareinformationwitheachotherwithoneactingasahostfortheothers.

Whilenearlyanypersonalcomputeriscapableofactingasanetworkserver,adedicatedserverwillcontainfeaturesmakingitmoresuitableforproductionenvironments.ThesefeaturesmayincludeafasterCPU,increasedhigh-performanceRAM,andincreasedstoragecapacityintheformofalargerormultipleharddrives.Serversalsotypicallyhavefaulttolerantfeatures,suchasredundancyinpowersupplies,storag