計算機英語專業(yè)外語第三版unit8

1、幻燈片1Unit 8 Computer NetworkSection A Network Fundamentals幻燈片2 The need to share information and resources amongdifferent computers has led to linked computer systems, callednetworks, in which computers are connected so that data can betransferred from machine to machine. In these networks,computer u

2、sers can exchange messages and share resourcessuch as printing capabilities, software packages, and data storagefacilities-that are scattered throughout the system. The underlyingsoftware required to support such applications has grown fromsimple utility packages into an expanding system of networks

3、oftware that provides a sophisticated network wide infrastructure.In a sense, network software is evolving into a network wideoperating system. 在不同計算機之間共享信息和資源的需要，導(dǎo)致了相互連接的計算機系統(tǒng)的產(chǎn)生。這種相互連接的計算機系統(tǒng)被稱為網(wǎng)絡(luò)。在網(wǎng)絡(luò)中，計算機連接在一起，從而數(shù)據(jù)可以從一臺計算機傳輸?shù)搅硪慌_計算機。在網(wǎng)絡(luò)中，計算機用戶可以交換信息，并共享分散在整個網(wǎng)絡(luò)系統(tǒng)的資源，如打印能力、軟件包以及數(shù)據(jù)存儲設(shè)備。支持這類應(yīng)用所需的基本軟件，已

4、經(jīng)從簡單的實用軟件包發(fā)展成一個不斷擴展的網(wǎng)絡(luò)軟件系統(tǒng)，該系統(tǒng)提供了一個復(fù)雜的網(wǎng)絡(luò)范圍的基礎(chǔ)結(jié)構(gòu)。從某種意義上說，網(wǎng)絡(luò)軟件正在演變成一個網(wǎng)絡(luò)范圍的操作系統(tǒng)。 幻燈片3. 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). A LAN normally consists of a collection of

5、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. A MAN is a network of intermediate size, such as one spanning a local community. A WAN links machines over a greater distance-perhaps i

6、n neighboring cities or on opposite sides of the world. 一、網(wǎng)絡(luò)分類計算機網(wǎng)絡(luò)常常劃分為局域網(wǎng)、城域網(wǎng)和廣域網(wǎng)。局域網(wǎng)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(xué)校園里的計算機或制造工廠中的計算機可以用局域網(wǎng)連接。城域網(wǎng)屬于中型網(wǎng)絡(luò)，如一個覆蓋某一社區(qū)的網(wǎng)絡(luò)。廣域網(wǎng)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。 complex n 綜合體，集合體幻燈片4. Network Classifications Another means of classifying networks is based

7、 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. A network of the former type is called an open network; a network of the latter type is called a closed,

8、 or sometimes a proprietary network. 網(wǎng)絡(luò)分類的另一種方式是根據(jù)網(wǎng)絡(luò)的內(nèi)部運行是基于無專利權(quán)保護的設(shè)計還是基于特定實體（如個人或公司）所擁有和控制的革新。前一種類型的網(wǎng)絡(luò)稱為開放式網(wǎng)絡(luò)，后一種類型的網(wǎng)絡(luò)稱為封閉式網(wǎng)絡(luò)，有時也稱為專有網(wǎng)絡(luò)。 幻燈片5. Network Classificationson the contrary The Internet is an open system. In particular, communication throughout the Internet is governed by an open collectio

9、n of standards known as the TCP/IP protocol suite. Anyone is free to use these standards without paying fees or signing license agreements. In 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 sel

10、ling or leasing these products. Networks based on such systems are examples of closed networks. 因特網(wǎng)屬于開放式系統(tǒng)。尤其是，整個因特網(wǎng)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。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(quán)，通過出售或出租這些產(chǎn)品獲得收入。基于這類系統(tǒng)的網(wǎng)絡(luò)屬于封閉式網(wǎng)絡(luò)的例子。 幻燈片6. Network Classifications Still another way of

11、classifying networks is based 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. Of these, the star network is perhaps the oldest, having evolved from the paradigm of a larg

12、e central computer serving many users. As the simple terminals employed by these users grew into small computers themselves, a star network emerged. A point to emphasize is that the connections between machines in a network do not need to be physical. Wireless networks, using radio broadcast technol

13、ogy, are becoming quite common. 還有一種網(wǎng)絡(luò)分類方法，它基于網(wǎng)絡(luò)的拓撲結(jié)構(gòu)，即計算機相互連接的模式。總線拓撲結(jié)構(gòu)、環(huán)形拓撲結(jié)構(gòu)和星形拓撲結(jié)構(gòu)是3種流行的拓撲結(jié)構(gòu)。其中，星形網(wǎng)絡(luò)或許是最古老的，這種網(wǎng)絡(luò)由一臺大型中央計算機服務(wù)許多用戶的范式演變而來。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(wǎng)絡(luò)也就出現(xiàn)了。需要強調(diào)的一點是，在一個網(wǎng)絡(luò)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(shù)的無線網(wǎng)絡(luò)正在變得相當常見?；脽羝?. Protocols For a network to function reliably, it is important t

14、o establish rules by which network activities are conducted. Such rules are called protocols. By developing and adopting protocol standards, venders are able to build products for network applications that are compatible with products from other venders. Thus, the development of protocol standards i

15、s an indispensable process in the development of networking technologies. 為了網(wǎng)絡(luò)可靠運行，確立進行網(wǎng)絡(luò)活動所遵循的規(guī)則很重要。這類規(guī)則稱為協(xié)議。通過開發(fā)和采用協(xié)議標準，不同廠商制造的網(wǎng)絡(luò)應(yīng)用產(chǎn)品能夠相互兼容。因此，在聯(lián)網(wǎng)技術(shù)的開發(fā)中，協(xié)議標準的開發(fā)是一個必不可少的過程。 幻燈片8. Protocols As an introduction to the protocol concept, let us consider the problem of coordinating the transmission of me

16、ssages among computers in a network. Without rules governing this communication, all the computers might insist on transmitting messages at the same time or might fail to relay messages when that assistance is required. One approach to solving this problem is the token ring protocol, which was devel

17、oped by IBM in the 1970s and continues to be a popular protocol in networks based on the ring topology. 作為對協(xié)議概念的介紹，讓我們考慮在一個網(wǎng)絡(luò)的計算機之間協(xié)調(diào)報文傳輸?shù)膯栴}。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。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(jié)構(gòu)的網(wǎng)絡(luò)中依然是一種很流行的協(xié)議。 幻燈片9. Protocols In this protocol, all the mac

18、hines in the network transmit messages in only one common direction, meaning that all messages sent over the network move around the ring in the same direction by being forwarded from computer to computer. When a message reaches its destination, the destination machine keeps a copy of it and forward

19、s a copy on around the ring. When the forwarded copy reaches the originating computer, that machine knows that the message must have reached its destination and removes the message from the ring. Of course, this system depends on inter-machine cooperation. If a machine insists on constantly transmit

20、ting messages of its own rather than forwarding those of the other machines, nothing will be accomplished. 在這個協(xié)議里，網(wǎng)絡(luò)中的所有計算機都只沿一個共同的方向傳輸報文（圖8A-1）。也就是說，通過網(wǎng)絡(luò)發(fā)送的所有報文都沿一個相同的方向繞環(huán)形網(wǎng)絡(luò)移動，依次經(jīng)由各個計算機轉(zhuǎn)發(fā)。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(wǎng)絡(luò)轉(zhuǎn)發(fā)一個副本。當轉(zhuǎn)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(wǎng)絡(luò)。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

21、斷地傳輸自己的報文，而不轉(zhuǎn)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。 幻燈片10. Protocols To solve this problem, a unique bit pattern, called a token, is passed around the ring. Possession of this token gives a machine the authority to transmit its own message; without the token, a machine is only allowed to forward messages. Normal

22、ly, each machine merely relays the token in the same manner in which it relays messages. If, however, the machine receiving the token has messages of its own to introduce to the network, it transmits one message while holding the token. When this message has completed its cycle around the ring, the

23、machine forwards the token to the next machine in the ring. Likewise, when the next machine receives the token, it can either forward the token immediately or transmit its own new message before sending the token on to next machine. In this manner, each machine in the network has equal opportunity t

24、o introduce messages of its own as the token circles around the ring. 為了解決這個問題，在環(huán)形網(wǎng)絡(luò)中傳遞一個稱為令牌的獨特位模式。擁有令牌的計算機得到權(quán)利傳輸自己的報文，而沒有令牌的計算機則只允許轉(zhuǎn)發(fā)報文。一般來說，每臺計算機只是像傳遞報文那樣傳遞令牌。然而，如果收到令牌的計算機有自己的報文需要通過網(wǎng)絡(luò)傳輸，它就會保存令牌，同時傳輸一份報文。這份報文在環(huán)形網(wǎng)絡(luò)中環(huán)繞一周后，該計算機就將令牌傳遞給環(huán)形網(wǎng)絡(luò)中的下一臺計算機。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

25、機。這樣，隨著令牌繞著環(huán)形網(wǎng)絡(luò)傳遞，網(wǎng)絡(luò)中的每臺計算機都有同等的機會傳輸自己的報文。幻燈片11. Protocols Another protocol for coordinating message transmission is used in bus topology networks that are based on the Ethernet protocol collection. In an Ethernet system, the right to transmit messages is controlled by the protocol known as Carrier

26、Sense Multiple Access with Collision Detection (CSMACD). This 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 itselfTo transmit a message, a machine waits until the bus is silent，and at this tim

27、e it begins transmitting while continuing to monitor the bus. 協(xié)調(diào)報文傳輸?shù)牧硗庖环N協(xié)議用于基于以太網(wǎng)協(xié)議集的總線拓撲網(wǎng)絡(luò)。在以太網(wǎng)系統(tǒng)中，傳輸報文的權(quán)利由稱為帶有沖突檢測的載波偵聽多路訪問（CSMA/CD）的協(xié)議控制。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（圖8A-2）。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。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。 幻燈片12. Protocols If another machine also begins transmit

28、ting, both machines detect the clash and pause for a brief random period of time before trying to transmit again. The result is a system similar to that used by a small group of people in a conversation. If two people start to talk at oncethey both stop. The difference is that people might go throug

29、h a series such as，"I'm sorry, what were you going to say?", "No, no. You go first." whereas under the CSMACD protocol each machine merely tries 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?jīng)_突，并暫停短暫而隨機的一段時間，然后再嘗試傳輸。結(jié)果是形成一種體制，這種體制類似于一小群人對話時所采用的。如果兩個人同時開始說話，他們都會停下來。不同的是，人們隨后可能進行這樣的對話：“抱歉，

30、您想說什么？”，“不，不，您先說?！倍鶕?jù)CSMA/CD協(xié)議，每臺計算機只是重新進行嘗試。 幻燈片13. Interprocess Communication The various activities (or processes) executing on thedifferent computers within a network (or even executingon the same machine via time sharing) must oftencommunicate with each other to coordinate their actionsand to p

31、erform their designated tasks. Suchcommunication between processes is calledinterprocess communication.三、進程間通信在一個網(wǎng)絡(luò)內(nèi)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(jīng)常互相通信，以便協(xié)調(diào)行動，并完成指定的任務(wù)。這種進程之間的通信稱為進程間通信。幻燈片14. Interprocess Communication A popular convention used for interprocess communication is the client/s

32、erver model. This model defines the basic roles played by the processes as either a client，which makes requests of other processes, or a server, which satisfies the requests made by clients. 進程間通信使用的一種流行規(guī)約是客戶機/服務(wù)器模型。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(wù)器。 幻燈片15. Interprocess Communication

33、 An early application of the client/server model appeared in networks connecting all the computers in a cluster of offices. In this situation a single, high-quality printer was attached to the network where it was available to all the machines in the network. In this case the printer played the role

34、 of a server (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. 客戶機/服務(wù)器模型的一種早期應(yīng)用，出現(xiàn)在將一組辦公室里的所有計算機都連接起來的網(wǎng)絡(luò)中。在這種應(yīng)用中，一臺高質(zhì)量的打印機被連接到網(wǎng)絡(luò)上，供其中的所有計算機使用。在這種情況下，打印機扮演了服務(wù)器（常稱為打印服務(wù)器）的角色，而其他計算機則通過程序設(shè)計而扮演了向打印服務(wù)器發(fā)送打印請求的客戶機角

35、色。 幻燈片16. Interprocess Communication Another early application of the client/server model was used to reduce the cost of disk storage while also removing the need for duplicate copies of records. Here one machine in a network was equipped with a high-capacity mass storage system (usually a magnetic

36、disk) that contained all of an organization's records. Other machines on the network then requested access to the records as they needed them. Thus the machine that actually contained the records played the role of a server (called a file server)，and the other machines played the role of clients

37、 that requested access to the files that were stored at the file server客戶機/服務(wù)器模型的另外一種早期應(yīng)用是為了降低磁盤存儲費用，同時消除復(fù)制記錄的需要。在這種情況下，網(wǎng)絡(luò)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(gòu)的所有記錄。這樣，網(wǎng)絡(luò)中的其他計算機可根據(jù)需要請求訪問這些記錄。因此，實際含有記錄的計算機扮演了服務(wù)器（稱為文件服務(wù)器）的角色，而其他計算機則扮演了請求訪問文件服務(wù)器上存儲的文件的客戶機角色。 幻燈片17. Interprocess Communication Today the clien

38、t/server model is used extensively in network applications. However, the client/server model is not the only means of interprocess communication. Another model is the peer-to-peer (often abbreviated P2P) model, whose properties provide insightful contrast to the client/server model. Whereas the clie

39、nt/server model involves one process (the server) communicating with numerous others (clients)，the peer-to-peer model involves two processes communicating as equals .今天，客戶機/服務(wù)器模型在網(wǎng)絡(luò)中廣泛應(yīng)用。不過，客戶機/服務(wù)器模型并非進程間通信的唯一方式。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(wù)器模型的特性形成鮮明對照。客戶機/服務(wù)器模型涉及一個進程（服務(wù)器）與許多其他進程（客戶機

40、）通信，而對等模型則涉及兩個進程進行對等通信。treating as equals 平等待人幻燈片18. Interprocess Communication Moreover, whereas a server must execute continuously so that it is prepared to serve its clients at any time，the peer-to-peer model usually involves two processes that execute on a temporary basis. For example, applicati

41、ons 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 people play games such as chess or checkers. 而且，服務(wù)器必須持續(xù)運行，以準備好隨時服務(wù)于客戶機，而對等模型則通常涉及兩個進程臨時運行。例如，對等模型的應(yīng)用包括兩個人通過因特網(wǎng)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

42、況。 continuously 連續(xù)不斷的；continually 不斷的（中間有小間斷）幻燈片19. Interprocess Communication 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). In this case individuals who are seeking particular ite

43、ms 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. 對等模型還是通過因特網(wǎng)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）。在這種情況下，尋求特定項目的個人在因特網(wǎng)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

44、。幻燈片20完全倒裝. Interprocess Communication This is in contrast to earlier approaches that applied the client/server model by establishing a central “distribution center” (the server) from which clients downloaded music recordings (or at least found sources for those items). The central server, however,

45、proved to be a focal point at which the music industry could enforce copyright laws, leading to the eventual dismantling of these music distribution centers. In contrast, the lack of a centralized base of operation obtained via the peer-to-peer model makes legal efforts to enforce copyright laws mor

46、e difficult.這與應(yīng)用客戶機/服務(wù)器模型的較早期方法截然不同，這些方法要建立一個“分發(fā)中心”（服務(wù)器），供客戶機下載音樂錄音（或者至少找到可以提供這些項目的地方）。然而，中心服務(wù)器證明是音樂行業(yè)可以運用版權(quán)法的一個焦點對象，結(jié)果最終導(dǎo)致這些音樂分發(fā)中心的解體。與此形成對照的是，對等模型缺乏一個操作中心，這使得運用版權(quán)法的努力難度增加。 幻燈片21. Interprocess Communication You might often read or hear the term peer-to-peer network, which is an example of how misus

47、e of terminology can evolve when technical terms are adopted by the nontechnical community. The term peer-to-peer refers to a system by which two processes communicate over a network (or internet). It is not a property of the network (or internet). A process might use the peer-to-peer model to communicate with another process and later use the client/server model to communicate with another process over the same network. Thus, it would be more accurate to speak of communicating by means of the peer-to-peer model rather than communicating over a peer-to-peer network. 你或許