Unit7-Computer Network

1、Section A Network Fundamentals課文課文A 網絡基本原理網絡基本原理Unit 7 Computer Network第第7單元單元 計算機網絡計算機網絡劉曉璐 講師Unit7 Computer Network2 The need to share information and resources among different computers has led to linked computer systems, called networks, in which computers are connected so that data can be trans

2、ferred from machine to machine. 在不同計算機之間共享信息和資源的需在不同計算機之間共享信息和資源的需要，導致了相互連接的計算機系統(tǒng)的產生。這要，導致了相互連接的計算機系統(tǒng)的產生。這種相互連接的計算機系統(tǒng)被稱為網絡。在網絡種相互連接的計算機系統(tǒng)被稱為網絡。在網絡中，計算機連接在一起，從而數據可以從一臺中，計算機連接在一起，從而數據可以從一臺計算機傳輸到另一臺計算機計算機傳輸到另一臺計算機。Unit7 Computer Network3In these networks, computer users can exchange messages and share

3、resourcessuch as printing capabilities, software packages, and data storage facilitiesthat are scattered throughout the system. 在網絡中，計算機用戶可以在網絡中，計算機用戶可以交換信息交換信息，并，并共享共享分散在整個網絡系統(tǒng)的分散在整個網絡系統(tǒng)的資源資源，如打印，如打印能力、軟件包以及數據存儲設備。能力、軟件包以及數據存儲設備。插入語插入語定語從句，修飾定語從句，修飾resourcesUnit7 Computer Network4The underlying sof

4、tware required to support such applications has grown from simple utility packages into an expanding system of network software that provides a sophisticated network wide infrastructure. underlying,ndlaadj.潛在的;基礎的;表面下的sophisticatedsfstketdadj.(機器、裝置等)高級的,精密的;(方法)復雜的infrastructurenfrstrkt(r)n.基礎設施；基礎

5、建設支持這類應用所需的基本軟件，已經從簡單的支持這類應用所需的基本軟件，已經從簡單的實實用軟件包用軟件包發(fā)展成一個不斷擴展的網絡軟件系統(tǒng)，發(fā)展成一個不斷擴展的網絡軟件系統(tǒng)，該系統(tǒng)提供了一個復雜的網絡范圍的基礎結構。該系統(tǒng)提供了一個復雜的網絡范圍的基礎結構。Unit7 Computer Network5In a sense, network software is evolving into a network wide operating system.evolve into逐漸發(fā)展(進化、演變)成從某種意義上說，網絡軟件正在演變成一個網絡范圍的操作系統(tǒng)。Unit7 Computer Netw

6、ork6I. Network Classifications A computer network is often classified as being either a local area network (LAN), a metropolitan area network (MAN), or a wide area network (WAN). metropolitan,metrpltnadj.大都會的；大城市的；宗主國的；大主教教區(qū)的一、網絡分類 計算機網絡常常劃分為局域網、城域網和廣域網。Unit7 Computer Network7A LAN normally consists

7、 of a collection of computers in a single building or building complex. For example, the computers on a university campus or those in a manufacturing plant might be connected by a LAN. complex綜合建筑群;綜合大樓plant工廠;成套設備局域網通常由一幢建筑物或一個建筑群中的若干計算機組成。例如，大學校園里的計算機或制造工廠中的計算機可以用局域網連接。Unit7 Computer Network8A MAN

8、 is a network of intermediate size, such as one spanning a local community. A WAN links machines over a greater distanceperhaps in neighboring cities or on opposite sides of the world.spanspnvt.跨越;遍及communitykmju:ntn.社區(qū);社會團體;群落城域網屬于中型網絡，如一個覆蓋某一社區(qū)的網絡。廣域網連接地理范圍更廣的計算機，這些計算機或許在相鄰的城市，或許在地球相反的兩面。Unit7 Com

9、puter Network9 Another means of classifying networks is based on whether the networks internal operation is based on designs that are in the public domain or on innovations owned and controlled by a particular entity such as an individual or a corporation. domaindmenn.范圍，領域；innovationnvenn. 革新，創(chuàng)新 網絡

10、分類的另一種方式是根據網絡的內部運行是基于無專利權保護的設計還是基于特定實體（如個人或公司）所擁有和控制的革新。賓語從句賓語從句Unit7 Computer Network10A network of the former type is called an open network; a network of the latter type is called a closed, or sometimes a proprietary prietaryprpratradj.專有的，專利的前一種類型的網絡稱為開放式網絡，后一種類型的網絡稱為封閉式網絡，有時也稱為專有網絡。U

11、nit7 Computer Network11 The Internet is an open system. In particular, communication throughout the Internet is governed by an open collection of standards known as the TCP/IP protocol suite. Anyone is free to use these standards without paying fees or signing license agreements. suiteswi:tn.計（軟件的）套

12、件 因特網 屬于開放式系統(tǒng)。尤其是，整個因特網的通信是由一組稱為TCP/IP協(xié)議組的開放標準來控制的。任何人都可以自由地使用這些標準，而不需要付費或簽署許可協(xié)議。 Unit7 Computer Network12In contrast, a company such as Novell Inc. might develop systems for which it chooses to maintain ownership rights, allowing the company to draw income from selling or leasing these products. In

13、c. (縮寫)incorporated (責任有限)公司leaseli:svt.出租; 租借incomenkm n.收入，進款；相反，像Novell股份有限公司這樣的公司可能選擇為其開發(fā)的系統(tǒng)保留所有權，通過出售或出租這些產品獲得收入。Unit7 Computer Network13Networks based on such systems are examples of closed networks.基于這類系統(tǒng)的網絡屬于封閉式網絡的例子。Unit7 Computer Network14 Still another way of classifying networks is based

14、 on the topology of the network, which refers to the pattern in which the machines are connected. The bus topology, ring topology, and star topology are three popular topologies. topologytpldn.地質學; 拓撲結構 還有一種網絡分類方法，它基于網絡的拓撲結構，即計算機相互連接的模式?？偩€拓撲結構、環(huán)形拓撲結構和星形拓撲結構是3種流行的拓撲結構。Unit7 Computer Network15Of these

15、, the star network is perhaps the oldest, having evolved from the paradigm of a large central computer serving many users. paradigmprdam n.范例，樣式，模范；其中，星形網絡或許是最古老的，這種網絡由一臺大型中央計算機服務許多用戶的范式演變而來。Unit7 Computer Network16As the simple terminals employed by these users grew into small computers themselves,

16、 a star network emerged. 隨著這些用戶所使用的簡單終端本身發(fā)展成小型計算機，星形網絡也就出現了。Unit7 Computer Network17A point to emphasize is that the connections between machines in a network do not need to be physical. Wireless networks, using radio broadcast technology, are becoming quite common.emphasizeemfsazvt.強調，著重需要強調的一點是，在一

17、個網絡中，計算機之間的連接并不一定是物理連接。使用無線廣播技術的無線網絡正在變得相當常見。Unit7 Computer Network18Protocols協(xié)議 For a network to function reliably, it is important to establish rules by which network activities are conducted. Such rules are called protocols. functionfknvi.工作;運轉;運行;發(fā)揮功能;reliablyrlabladv.可靠地，確實地conductkndktvt.&

18、 vi.引導; 帶領; 控制 為了網絡可靠運行，確立進行網絡活動所遵循的規(guī)則很重要。這類規(guī)則稱為協(xié)議。Unit7 Computer Network19By developing and adopting protocol standards, venders are able to build products for network applications that are compatible with products from other venders.vendervendn.賣主, 售賣者, 供應商compatiblekmptbladj.兼容的，相容的通過開發(fā)和采用協(xié)議標準，不同

19、廠商制造的網絡應用產品能夠相互兼容。Unit7 Computer Network20Thus, the development of protocol standards is an indispensable process in the development of networking technologies.indispensable,ndspensbladj.不可缺少的； 絕對必要的；因此，在聯(lián)網技術的開發(fā)中，協(xié)議標準的開發(fā)是一個必不可少的過程。Unit7 Computer Network21 As an introduction to the protocol concept,

20、let us consider the problem of coordinating the transmission of messages among computers in a network. coordinatek:dnetvt.&vi.協(xié)調, 協(xié)同message報文 作為對協(xié)議概念的介紹，讓我們考慮在一個網絡的計算機之間協(xié)調報文傳輸的問題。Unit7 Computer Network22Without rules governing this communication, all the computers might insist on transmitting me

21、ssages at the same time or might fail to relay messages when that assistance is required.relayri:levt.轉播，傳達assistancesstnsn.幫助，援助如果沒有控制這種通信的規(guī)則，所有的計算機就可能堅持同時傳輸報文，或者在需要傳遞報文時而未能傳遞。Unit7 Computer Network23 One approach to solving this problem is the token ring protocol, which was developed by IBM in the

22、 1970s and continues to be a popular protocol in networks based on the ring topology. tokentknn.象征; 令牌; 標記 解決這個問題的一種方法是令牌環(huán)協(xié)議，該協(xié)議由IBM公司20世紀70年代開發(fā)，今天在基于環(huán)形拓撲結構的網絡中依然是一種很流行的協(xié)議。Unit7 Computer Network24In this protocol, all the machines in the network transmit messages in only one common direction, meanin

23、g that all messages sent over the network move around the ring in the same direction by being forwarded from computer to computer.forwardf:wdvt.促進, 助長; 轉寄, 發(fā)送在這個協(xié)議里，網絡中的所有計算機都只沿一個共同的方向傳輸報文。也就是說，通過網絡發(fā)送的所有報文都沿一個相同的方向繞環(huán)形網絡移動，依次經由各個計算機轉發(fā)。Unit7 Computer Network25When a message reaches its destination, th

24、e destination machine keeps a copy of it and forwards a copy on around the ring. destination,destnenn.目的，目標； 目的地，終點；一份報文到達目的地后，目的地的計算機保留一個副本并繞環(huán)形網絡轉發(fā)一個副本。Unit7 Computer Network26When the forwarded copy reaches the originating computer, that machine knows that the message must have reached its destina

25、tion and removes the message from the ring. originaterdnetvt.&vi.發(fā)源; 來自; 產生; 創(chuàng)始; 開創(chuàng)當轉發(fā)的副本到達始發(fā)計算機時，該計算機知道報文一定到達了目的地，并將其移出環(huán)形網絡。Unit7 Computer Network27Of course, this system depends on inter-machine cooperation. If a machine insists on constantly transmitting messages of its own rather than forwar

26、ding those of the other machines, nothing will be accomplished.constantlyknstntliadv.不斷地，時常地當然，這種系統(tǒng)的運行依靠計算機之間的合作。如果一臺計算機堅持不斷地傳輸自己的報文，而不轉發(fā)其他計算機的報文，那么這個系統(tǒng)就什么也執(zhí)行不了。Unit7 Computer Network28 To solve this problem, a unique bit pattern, called a token, is passed around the ring. 為了解決這個問題，在環(huán)形網絡中傳遞一個稱為令牌的獨特

27、位模式。Unit7 Computer Network29Possession of this token gives a machine the authority to transmit its own message; without the token, a machine is only allowed to forward messages. authority:rtin.權威; 權力; 權限擁有令牌的計算機得到權利傳輸自己的報文，而沒有令牌的計算機則只允許轉發(fā)報文。Unit7 Computer Network30Normally, each machine merely relay

28、s the token in the same manner in which it relays messages.merelymili adv.僅僅，只不過； 只是； 純粹； 全然一般來說，每臺計算機只是像傳遞報文那樣傳遞令牌。Unit7 Computer Network31Unit7 Computer Network32If, however, the machine receiving the token has messages of its own to introduce to the network, it transmits one message while holding

29、 the token.然而，如果收到令牌的計算機有自己的報文需要通過網絡傳輸，它就會保存令牌，同時傳輸一份報文。Unit7 Computer Network33When this message has completed its cycle around the ring, the machine forwards the token to the next machine in the ring. 這份報文在環(huán)形網絡中環(huán)繞一周后，該計算機就將令牌傳遞給環(huán)形網絡中的下一臺計算機。Unit7 Computer Network34Likewise, when the next machine r

30、eceives the token, it can either forward the token immediately or transmit its own new message before sending the token on to the next machine. likewiselakwazadv.同樣地同樣，當下一臺計算機收到令牌時，它既可以立即傳遞令牌，也可以先傳輸自己的新報文，然后將令牌傳遞給下一臺計算機。Unit7 Computer Network35In this manner, each machine in the network has equal op

31、portunity to introduce messages of its own as the token circles around the ring.circles:klvt.繞運轉, 環(huán)繞, 盤旋這樣，隨著令牌繞著環(huán)形網絡傳遞，網絡中的每臺計算機都有同等的機會傳輸自己的報文。Unit7 Computer Network36Unit7 Computer Network37 Another protocol for coordinating message transmission is used in bus topology networks that are based on t

32、he Ethernet protocol collection. Etherneti:netn.以太網 協(xié)調報文傳輸的另外一種協(xié)議用于基于以太網協(xié)議集的總線拓撲網絡。Unit7 Computer Network38In an Ethernet system, the right to transmit messages is controlled by the protocol known as Carrier Sense Multiple Access with Collision Detection (CSMA/CD). 在以太網系統(tǒng)中，傳輸報文的權利由稱為帶有沖突檢測的載波偵聽多路訪問(

33、CSMA/CD)的協(xié)議控制。Unit7 Computer Network39This protocol dictates that each message be broadcast to all the machines on the bus. Each machine monitors all the messages but keeps only those addressed to itself. addressed to寄往;寄給該協(xié)議規(guī)定，每一份報文必須向總線上的所有計算機廣播。每臺計算機監(jiān)控所有的報文，但只保留發(fā)送給自己的那些報文。Unit7 Computer Network40

34、To transmit a message, a machine waits until the bus is silent, and at this time it begins transmitting while continuing to monitor the bus.一臺計算機要等到總線靜默時才可以傳輸報文。這時，它開始傳輸報文，同時繼續(xù)監(jiān)控總線。Unit7 Computer Network41If another machine also begins transmitting, both machines detect the clash and pause for a bri

35、ef random period of time before trying to transmit again.如果另一臺計算機也開始傳輸報文，這兩臺計算機都會檢測到沖突，并暫停短暫而隨機的一段時間，然后再嘗試傳輸。Unit7 Computer Network42The result is a system similar to that used by a small group of people in a conversation. If two people start to talk at once, they both stop.結果是形成一種體制，這種體制類似于一小群人對話時所

36、采用的。如果兩個人同時開始說話，他們都會停下來。Unit7 Computer Network43The difference is that people might go through a series such as, “Im sorry, what were you going to say?”, “ No, no. You go first,” whereas under the CSMA/CD protocol each machine merely tries again.whereas,werzconj.鑒于; 然而; 反之不同的是，人們隨后可能進行這樣的對話：“抱歉，您想說什

37、么？”，“不，不，您先說?！倍鶕﨏SMA/CD協(xié)議，每臺計算機只是重新進行嘗試。Unit7 Computer Network44Interprocess Communication進程間通信 The various activities (or processes ) executing on the different computers within a network (or even executing on the same machine via time sharing) must often communicate with each other to coordinate

38、their actions and to perform their designated tasks.designatedeznetvt.指明，指出； 指派 在一個網絡內不同計算機上（甚至使用分時方法在同一臺計算機上）執(zhí)行的各種活動（或進程）必須經?；ハ嗤ㄐ?，以便協(xié)調行動，并完成指定的任務。Unit7 Computer Network45Such communication between processes is called interprocess communication.這種進程之間的通信稱為進程間通信。Unit7 Computer Network46 A popular con

39、vention used for interprocess communication is the client/server model. conventionknvennn.習俗;慣例;常規(guī);公約;協(xié)定;協(xié)約 進程間通信使用的一種流行規(guī)約是客戶機/服務器模型。Unit7 Computer Network47This model defines the basic roles played by the processes as either a client, which makes requests of other processes, or a server, which sati

40、sfies the requests made by clients.這種模型將進程扮演的基本角色或定義為向其他進程提出請求的客戶機，或定義為滿足客戶機所提請求的服務器。Unit7 Computer Network48 An early application of the client/server model appeared in networks connecting all the computers in a cluster of offices. clusterklst(r)n.叢； 簇，串； 群 客戶機/服務器模型的一種早期應用，出現在將一組辦公室里的所有計算機都連接起來的網絡

41、中。Unit7 Computer Network49In this situation, a single, high-quality printer was attached to the network where it was available to all the machines in the network. attached to附著; 連接到; 附加到; 隸書于在這種應用中，一臺高質量的打印機被連接到網絡上，供其中的所有計算機使用。Unit7 Computer Network50In this case the printer played the role of a ser

42、ver (often called a print server), and the other machines were programmed to play the role of clients that sent print requests to the print server.在這種情況下，打印機扮演了服務器（常稱為打印服務器）的角色，而其他計算機則通過程序設計而扮演了向打印服務器發(fā)送打印請求的客戶機角色。Unit7 Computer Network51 Another early application of the client / server model was use

43、d to reduce the cost of disk storage while also removing the need for duplicate copies of records. duplicatedju:plikitadj.復制的,副本的,重復的 客戶機/服務器模型的另外一種早期應用是為了降低磁盤存儲費用，同時消除復制記錄的需要。Unit7 Computer Network52Here one machine in a network was equipped with a high-capacity mass storage system (usually a magne

44、tic disk) that contained all of an organizations records. Other machines on the network then requested access to the records as they needed them. 在這種情況下，網絡中的某一臺計算機配備了大容量存儲系統(tǒng)（通常是磁盤），存儲一個機構的所有記錄。這樣，網絡中的其他計算機可根據需要請求訪問這些記錄。Unit7 Computer Network53Thus the machine that actually contained the records play

45、ed the role of a server (called a file server), and the other machines played the role of clients that requested access to the files that were stored at the file server.因此，實際含有記錄的計算機扮演了服務器（稱為文件服務器）的角色，而其他計算機則扮演了請求訪問文件服務器上存儲的文件的客戶機角色。Unit7 Computer Network54 Today the client/server model is used exte

46、nsively in network applications. However, the client/server model is not the only means of interprocess communication. extensivelyikstensivliadv.廣大地，廣泛地 今天，客戶機/服務器模型在網絡中廣泛應用。不過，客戶機/服務器模型并非進程間通信的唯一方式。Unit7 Computer Network55Another model is the peer-to-peer (often abbreviated P2P) model, whose proper

47、ties provide striking contrasts to the client/server model.strikingstraikiadj.顯著的,引人注目的contrasts to 與形成對比，和相對照另外一種模型是對等（peer-to-peer，常縮寫為P2P）模型，其特性與客戶機/服務器模型的特性形成鮮明對照。Unit7 Computer Network56Whereas the client/server model involves one process (the server) communicating with numerous others (clients

48、), the peer-to-peer model involves two processes communicating as equals.反之，客戶機/服務器模型涉及一個進程（服務器）與許多其他進程（客戶機）通信，對等模型則涉及兩個進程進行對等通信。Unit7 Computer Network57Moreover, whereas a server must execute continuously so that it is prepared to serve its clients at any time, the peer-to-peer model usually involv

49、es two processes that execute on a temporary basis. 而且，服務器必須持續(xù)運行，以準備好隨時服務于客戶機，而對等模型則通常涉及兩個進程臨時運行。Unit7 Computer Network58For example, application of the peer-to-peer model include instant messaging in which two people carry on a written conversation over the Internet as well as situations in which p

50、eople play games such as chess or checkers.例如，對等模型的應用包括兩個人通過因特網進行書面對話的即時通信，以及人們參與下國際象棋或西洋跳棋等游戲的情況。Unit7 Computer Network59 The peer-to-peer model is also a popular means of sharing files such as music recordings and motion pictures via the Internet (sometimes with questionable legality). motion pict

51、ure電影； 影片questionablekwestnbladj.可疑的，有疑問的legalityli:gltin.合法性 對等模型還是通過因特網共享音樂錄音、影片等文件的一種流行方法（其合法性有時值得懷疑）。Unit7 Computer Network60In this case, individuals who are seeking particular items broadcast their desire over the Internet and are contacted by those who own the items of interest. Then, the items are transferred between the two parties using the peer-to-peer model. 在這種情況下，尋求特定項目的個人在因特網上發(fā)布其需求，而擁有他所感興趣項目的人則會與他取得聯(lián)系。然后，雙方之間就可以使用對等模型傳輸這些項目。Unit7 Computer Network61This is in contrast to earlier approaches that applied the client/server model by establishing a central “distribution