《信息工程专业英语》课件第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.对话

originate

vi.发源；发生

subset

n.子集

mirror

vt.反射；反映

legacy

adj.旧的，传统的

reverse

n.背面；相反

portable

adj.手提的，便携式的；轻便的

federated

adj.联邦的；联合的；结成同盟的

arbitrary adj.任意的

implementation

n.实现；履行；安装启用

smartphone

智能手机

inthereverseorder

相反的顺序

IPpackets

IP数据包

digital-to-analogconversion 数模转换

voicestream

语音流

opensourcevoicecodec 开源语音编解码器

portabledevices

便携设备

VoIP(VoiceoverInternetProtocol)

互联网协议语音

VoBB(VoiceoverBroadBand)

宽带语音

SMS(ShortMessageService) 短信通群法系统

PSTN(PublicSwitchedTelephoneNetwork)

公共交换电话网络

HD(HighDefinition) 高清

iLBC一种专为包交换网络通信设计的编解码，优于目前流行的G.729、G.723.1

11.1.1Exercises

1.PutthePhrasesintoEnglish

(1)通信协议； (2)媒体信道；

(3)数字化模拟语音信号；

(4)分组交换网络；

(5)数字电话系统； (6)第二代供应商；

(7)封闭的网络。

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.

11.1.2参考译文

IP语音(VoIP或互联网协议语音)通常指通信协议、技术、方法和传输技术，传输技术包括语音通信和通过因特网协议进行多媒体会话的传输，例如因特网。其他与VoIP相关的术语包括IP电话、互联网电话、宽带语音(VoBB)、宽带电话、IP通信和宽带电话。

网络电话是指通信服务—语音、传真、SMS和/或语音消息应用。即通过互联网传输的，而不是通过公共交换电话网(PSTN)。拨打VoIP(如图11.1所示)电话的步骤如下：信号和媒体信道的设定，数字化模拟语音信号，编码，分包，经过分组交换网络传输IP数据包。在接收端有类似的步骤(通常以相反的顺序)，比如接收IP数据包，解码，数模转化再现原始的语音流。即使IP电话和VoIP的术语可交替使用，但实际上它们是不同的；IP电话和数字电话系统有关，它使用IP协议进行语音通信，而VoIP实际上是IP电话的一个子集。VoIP用于作为传输电话呼叫方式之一的IP电话。

语音IP服务的早期供应商提出的业务模式(和技术解决方案)借鉴了旧的电话网络体系结构。第二代供应商，如Skype已经为私人用户群建立了封闭的网络，提供免费电话和便利条件，同时不让他们的用户能够调用其他的网络。这严格限制了用户混合和匹配第三方硬件和软件的能力。第三代供应商，如谷歌通话都采用了联邦网络电话的概念，完全背离了原有网络的结构。每当用户希望发出一个呼叫时，这些解决方案通常允许互联网上任何两个域之间的随机动态互联。

VoIP系统采用会话控制协议来控制设置和清除电话以及音频编解码器，语音编码允许数字音频通过音频流在IP网络上传输。依赖于应用需求和网络宽带VoIP的实现方式不同，编解码器的选择也不同；一些VoIP的实现依赖于窄频带和压缩语言，而其他的支持高保真立体声编解码器。一些流行的编解码器包括u-law和a-law版本的G.711、G.722，这些是高保真的编解码器，由宝利通公司作为高清语言销售。其中一个流行的开源语音编解码器称为iLBC。

许多智能手机和互联网设备都有VoIP，所以用户可以使用非电话的便携设备在许多地方通过3G或Wi-Fi拨打电话或发送短信。

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