01-04-2020-Oxford_English_for_Infomation_Technology (1171844), страница 14
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25,modem, or otherservicesThis diagram is basedon the ISO seven-layermodel for networkstructure. It shows theupward path throughsoftware andhardware that makesTCP/IP a viable meansof data transferbetween dissimilarmachines linked in anetwork.UNIT 12 The Internetmust know each other's Internet addresses if theyare to communicate.
A machine acting as agateway connecting different TCP/IP networkswill have a different Internet address on eachnetwork. Internal look-up tables and softwarebased on another standard - called ResolutionProtocol - are used to route the data through agateway between networks.Another piece of software works with the IP-layerprograms to move information to the rightapplication on the receiving system. This softwarefollows a standard called the User DatagramProtocol (UDP). You can think of the UDPsoftware as creating a data address in the TCP/IPmessage that states exactly what application thedata block is supposed to contact at the addressthe IP software has described. The UDP softwareprovides the final routing for the data within thereceiving system.The Transmission Control Protocol (TCP) part ofTCP/IP comes into operation once the packet isdelivered to the correct Internet address andapplication port.
Software packages that followthe TCP standard run on each machine, establisha connection to each other, and manage thecommunication exchanges. A data-deliverysystem like Ethernet doesn't promise to deliver apacket successfully. Neither IP nor UDP knowsanything about recovering packets that aren'tsuccessfully delivered, but TCP structures andbuffers the data flow, looks for responses andtakes action to replace missing data blocks. Thisconcept of data management is called reliablestream service.After TCP brings the data packet into acomputer, other high-level programs handle it.Some are enshrined in official US governmentstandards, like the File Transfer Protocol (FTP)and the Simple Mail Transfer Protocol (SMTP).
Ifyou use these standard protocols on differentkinds of computers, you will at least have ways ofeasily transferring files and other kinds of data.Conceptually, software that supports the TCPprotocol stands alone. It can work with datareceived through a serial port, over a packetswitched network, or from a network system likeEthernet. TCP software doesn't need to use IP orUDP, it doesn't even have to know they exist. Butin practice TCP is an integral part of the TCP/IPpicture, and it is most frequently used with thosetwo protocols.[Adapted from 'How TCP/IP Links Dissimilar Machines',PC Magazine, September 1989]B87Re-read the text to find the answers tothese questions.1 Match the terms in Table A with thestatements in Table B.Table Aa Internet addressbResolution Protocolc Look-up tabledGatewayeUser Datagram Protocolf Transmission Control ProtocolTable Bi Standard used for software that routesdata through a gatewayii Standard used by software that movesinformation to the correct application onthe receiving system of a networkiii Standard used by software that managescommunication exchanges betweencomputers on the Internetiv A 32-bit number identifying a node on anIP networkv Stored information used to route datathrough a gatewayvi A device for connecting dissimilarnetworks2 Mark the following statements as True orFalse:a Internet addresses are an integral part of theIP protocol.b Internet addresses can be written as a seriesof numbers.c UDP software provides the final routing fordata within the receiving system.d UDP recovers packets that aren'tsuccessfully delivered.e TCP only works with packet-switchednetworks.f TCP only works when it is combined with IP.UNIT 13The World Wide WebStudy this URL (Uniform Resource Locator).STARTERhttp: //www.
hw. ac. uk/libWWW/irn/irn. htmlprotocolprefixWebFig 1Uniform Resource LocatordomaindomainnameextensioncountrycodedirectorypathdocumentWhich part of the address tells you:1234567the company is in the UKthis is the webpagethe type of transmission standard your browser must use toaccess the datathis points to the computer where the webpage is storedthis is where the webpage is stored in the computerthis is a companythis is a Web fileStudy these approved domain name extensions and theirmeanings.
Then match these suggestions for new extensions to theirmeanings.ExtensionMeaning. aero. biz. com (. co in UK). coop. edu(. ac in UK). gov. info. int. mil. museum. name. net. org. proaviation industrybusinessescommercialcooperativeseducational and researchgovernmentgeneral useinternational organisationmilitary agencymuseumsindividualsgateway or hostnon-profit organisationprofessionalsLISTENINGSuggested extensionMeaning1234567abcdefg.
firm. store. web. arts. rec. info. nominformativecultural or entertainmentpersonalfirm or agencyonline retail shopWeb-relatedrecreationalStudy this diagram which illustrates how your browser findsthe webpage you want. Label these items:abcdefRouterDomain Name System (DNS) serverRemote Web serverBrowser PCURLInternet Protocol addressFig 2How your browser finds the page you want90 UNIT 13 The World Wide WebNow listen to this recording which explains how theprocess works and take brief notes on each stage. For example:Stage 1Click on a webpage hyperlink or URL.The browser sends the URL to a DNS server.LANGUAGE WORKTime clausesWhat is the relationship between each ofthese pairs of actions?We can use once in place of when toemphasise the completion of the first action.It often occurs with the Present perfect. Forexample:Once the DNS server has found the IP address,it sends the address back to the browser.1 a You click on a URL.b Your browser sends it to a DNS server.2 a The packets are passed from router torouter.b They reach the Web server.3 a The packets may travel by differentroutes.b They reach the Web server.4 a The individual packets reach the Webserver.b They are put back together again.We can use until to link an action and thelimit of that action:Each pair of actions is linked in time.
We canshow how actions are linked in time by usingtime clauses. For example:3 The packets may travel by different routesbefore they reach the Web server.We can use when to show that one actionhappens immediately after another action:1When you click on a URL, your browsersends it to a DNS server.2 The packets are passed from router torouter until they reach the Web server.We can use before to show that one actionprecedes another:If the subjects are the same in both actions,we can use a participle:The packets may travel by different routesbefore reaching the Web server.We can use as to link two connected actionshappening at the same time:4 As the individual packets reach the Webserver, they are put back together again.Link each pair of actions using a time clause.abababYou use a search engine.It provides a set of links related to your search.With POP3, email is stored on the server.You check your email account.You have clicked on a hyperlink.You have to wait for the webpage to be copied to yourcomputer.UNIT 13 The World Wide Web 914 ab5 ab6 ab7 ab8 ab9 ab10 abYou listen to the first part of a streamed audio file.The next part is downloading.The graphics can be displayed gradually.The webpage is downloaded.You receive an email message.You can forward it to another address.You click on a hyperlink.The browser checks to see if the linked webpage is stored inthe cache.You can bookmark a webpage to make it easier to find in thefuture.You find a webpage you like.You type in a Web address.You should press the Enter key.You click on the Home button.The browser displays your starting webpage.Fill in the gaps in this description of buffering, a way ofensuring that Web video runs smoothly.The player feedsaudio or videodata into thebuffer.When connectionslowdowns orinterruptionsoccur, the amountof data in thebuffer decreases,but as long assome remains,playback isuninterrupted.Data goes out tothe player'sdecompressionand playbackroutines.Fig 3Video bufferingStreaming is a way of dealing with1bandwidth problemsyoudownload video from the Internet.
One keyto successful streaming is the process of2bufferingyou download amovie, the video player stores part of the3movie in memoryplaying it.Imagine the buffer as a container filled from4the top as shown in Fig 3thecontainer is full, the player sends data onfor playback from the bottom. Data keeps5coming ina clip plays. Theuser can view the beginning of the movie6the rest of the clip downloads.7connection slowdowns orinterruptions occur, the amount of data inthe buffer decreases but as long as someremains, playback is uninterrupted.Playback continues at a steady rate8the buffer is empty.92UNIT 13 The World Wide WebPROBLEM-SOLVINGSearch engines Study these tips for conducting searchesusing AltaVista. Then decide what you would type into the searchbox to find this data. Compare your answers with others in yourgroup and together decide what would be the best search.
Restrictsites to English language.Don't use simple keywords. Typing inthe word football is unlikely to helpyou to find information on yourfavourite football team. Unless special operators areincluded, AltaVista assumes the default operator isOR. If, for example, the search query is Americanfootball, AltaVista will look for documents containingeither American or football although it will list higherthose documents which contain both.AltaVista is specifically case sensitive.If you specify apple as your searchterm, AltaVista will return matchesfor apple, Apple and APPLE. However, if you useApple or apPle, AltaVista will only match Apple andapPle respectively.AltaVista supports natural languagequeries.
If you really aren't sure whereto start looking, try typing a naturallanguage query in the search box. The questionWhere can I find pages about digital cameras? willfind a number of answers but at least it will give yousome idea of where to start.UNIT 13 The World Wide Web 93Try using phrase searching. This iswhere you place quotation marksaround your search term, e. g.'alternative medicine'. This will search for alldocuments where these two words appear as aphrase.Attaching a + to a word is a way ofnarrowing your search. It means thatword must be included in yoursearch.
For example, if you were looking forinformation on cancer research, use +cancer+research instead of just cancer.Attaching a - to a word or using NOTis another way of narrowing yoursearch. This excludes the search itemfollowing the word NOT or the - sign. For example,science NOT fiction or science -fiction will excludesites in which these two words occur together.Use brackets to group complexsearches, for example: (cakes ANDrecipes) AND (chocolate OR ginger)will find pages including cakes and recipes and eitherchocolate or ginger or both.12345678910You can refine your search by doing afield search.
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