計(jì)算機(jī)網(wǎng)絡(luò)使得計(jì)算機(jī)組及其用戶之間可以進(jìn)行廣泛的信

1、12I.IntroductionII. Modems and Computer BureauxIII. Local Area NetworksIV. Routers and BridgesV.Wide Area NetworksVI. Distributed ComputingVII.Security and Management3 Computer networks, the widespread sharing of information among groups of computers and their users, are a central part of the inform

2、ation age. The popular adoption of the personal computer (PC) and the local area network (LAN) during the 1980s had led to the capacity to access information on a distant database; download an application from overseas; send a message to a friend in a different country; and share files with a collea

3、gue all from a personal computer. 計(jì)算機(jī)網(wǎng)絡(luò)使得計(jì)算機(jī)組及其用戶之間可以進(jìn)行廣泛的信計(jì)算機(jī)網(wǎng)絡(luò)使得計(jì)算機(jī)組及其用戶之間可以進(jìn)行廣泛的信息共享，構(gòu)成了信息時(shí)代的核心部分。個(gè)人計(jì)算機(jī)和局域息共享，構(gòu)成了信息時(shí)代的核心部分。個(gè)人計(jì)算機(jī)和局域網(wǎng)在網(wǎng)在20世紀(jì)世紀(jì)80年代的普遍采用，使人們得以訪問遠(yuǎn)端數(shù)據(jù)年代的普遍采用，使人們得以訪問遠(yuǎn)端數(shù)據(jù)庫中的信息；從海外下載應(yīng)用程序；給另外一個(gè)國家的朋庫中的信息；從海外下載應(yīng)用程序；給另外一個(gè)國家的朋友發(fā)送消息；以及與同事共享文件友發(fā)送消息；以及與同事共享文件這一切只需在個(gè)人這一切只需在個(gè)人計(jì)算機(jī)上就可以實(shí)現(xiàn)。計(jì)算機(jī)上就可以實(shí)現(xiàn)。

4、 4 The networks that allow all this to be done so easily are sophisticated and complex entities. They rely for their effectiveness on many cooperating components. The design and deployment of the worldwide computer network can be viewed as one of the great technological wonders of recent decades. 使這

5、一切如此容易地得以實(shí)現(xiàn)的網(wǎng)絡(luò)，是精密而使這一切如此容易地得以實(shí)現(xiàn)的網(wǎng)絡(luò)，是精密而復(fù)雜的實(shí)體。它們之所以能夠發(fā)揮作用，依靠的復(fù)雜的實(shí)體。它們之所以能夠發(fā)揮作用，依靠的是許多協(xié)作的組件。世界范圍的計(jì)算機(jī)網(wǎng)絡(luò)的設(shè)是許多協(xié)作的組件。世界范圍的計(jì)算機(jī)網(wǎng)絡(luò)的設(shè)計(jì)與部署，堪稱近幾十年來的一項(xiàng)偉大技術(shù)奇跡。計(jì)與部署，堪稱近幾十年來的一項(xiàng)偉大技術(shù)奇跡。5 As recently as the 1970s, computers were expensive, fragile machines that had to be looked after by specialists and kept in a cont

6、rolled environment. They could be used either by plugging in a terminal directly or by using a phone line and modem to gain access from a distance. Because of their high cost, they tended to be centralized resources to which a user had to arrange their own access. During this time, organizations tha

7、t offered access time on a mainframe computer computer bureaux flourished. 直到直到20世紀(jì)世紀(jì)70年代，計(jì)算機(jī)還是一種嬌貴的機(jī)器，得由專年代，計(jì)算機(jī)還是一種嬌貴的機(jī)器，得由專家維護(hù)并放置在受控的環(huán)境中。其工作方式可以是直接插家維護(hù)并放置在受控的環(huán)境中。其工作方式可以是直接插接終端，也可以是通過電話線和調(diào)制解調(diào)器遠(yuǎn)程接入。由接終端，也可以是通過電話線和調(diào)制解調(diào)器遠(yuǎn)程接入。由于其造價(jià)高，它們往往是集中式資源，用戶需要安排才能于其造價(jià)高，它們往往是集中式資源，用戶需要安排才能訪問。這個(gè)時(shí)期，提供大型機(jī)訪問時(shí)間的機(jī)構(gòu)訪問。這個(gè)

8、時(shí)期，提供大型機(jī)訪問時(shí)間的機(jī)構(gòu)計(jì)算機(jī)計(jì)算機(jī)管理局管理局涌現(xiàn)出來。涌現(xiàn)出來。 6 Computer networks during this period were not commercially available. Even so, one of the most significant developments to shape the modern world of technology was initiated at this time: experimentation by the US Defence Department in distributing computer re

9、sources to provide resilience against failure. This work is now known as the Internet. 這期間的計(jì)算機(jī)網(wǎng)絡(luò)還沒有進(jìn)入商業(yè)使用階段。即便如此，這期間的計(jì)算機(jī)網(wǎng)絡(luò)還沒有進(jìn)入商業(yè)使用階段。即便如此，對(duì)現(xiàn)代技術(shù)領(lǐng)域起到塑造作用的最重要發(fā)展之一這時(shí)開始對(duì)現(xiàn)代技術(shù)領(lǐng)域起到塑造作用的最重要發(fā)展之一這時(shí)開始啟動(dòng)：美國國防部為提供抗故障彈性而在分布計(jì)算機(jī)資源啟動(dòng)：美國國防部為提供抗故障彈性而在分布計(jì)算機(jī)資源方面進(jìn)行的實(shí)驗(yàn)。這項(xiàng)工作的成果現(xiàn)在稱為因特網(wǎng)。方面進(jìn)行的實(shí)驗(yàn)。這項(xiàng)工作的成果現(xiàn)在稱為因特網(wǎng)。 7 One of the m

10、ost dramatic events in computer networking has been the introduction and rapid growth of the local area network (LAN) as a way to standardize the system of linking computers used in office systems. As the name suggests, this is a means of connecting a number of computing elements together. 在計(jì)算機(jī)聯(lián)網(wǎng)方面最

11、引人注目的事件之一，是局域網(wǎng)作為規(guī)在計(jì)算機(jī)聯(lián)網(wǎng)方面最引人注目的事件之一，是局域網(wǎng)作為規(guī)范辦公系統(tǒng)計(jì)算機(jī)互聯(lián)體制的方法，而被采用及其快速發(fā)范辦公系統(tǒng)計(jì)算機(jī)互聯(lián)體制的方法，而被采用及其快速發(fā)展。顧名思義，它是連接若干計(jì)算單元的手段。展。顧名思義，它是連接若干計(jì)算單元的手段。8 At the simplest level, a LAN provides no more than a shared medium (such as a coaxial cable to which all computers and printers are connected) along with a set of

12、rules that govern the access to that medium. The most widely used LAN, Ethernet, uses a mechanism called Carrier Sense Multiple Access-Collision Detect (CSMA-CD). 在最簡單的層面上，局域網(wǎng)只是提供一種共享的介質(zhì)（如在最簡單的層面上，局域網(wǎng)只是提供一種共享的介質(zhì)（如所有計(jì)算機(jī)和打印機(jī)都跟其相連接的同軸電纜），以及一所有計(jì)算機(jī)和打印機(jī)都跟其相連接的同軸電纜），以及一套用來管理對(duì)該介質(zhì)的訪問的規(guī)則。以太網(wǎng)這種使用最普套用來管理對(duì)該介質(zhì)的訪問

13、的規(guī)則。以太網(wǎng)這種使用最普遍的局域網(wǎng)，使用一種被稱為遍的局域網(wǎng)，使用一種被稱為“帶有沖突檢測的載波偵聽帶有沖突檢測的載波偵聽多路訪問多路訪問”（CSMA-CD）的機(jī)制。）的機(jī)制。 9 This means that each connected device can only use the cable when it has established that no other device is using it. If there is contention, the device looking for a connection backs off and tries again lat

14、er. The Ethernet transfers data at 10M bits/sec, which is fast enough to make the distance between devices insignificant. They appear to be connected directly to their destination. 這就意味著，連接在電纜上的每件設(shè)備，只有在確定沒有其這就意味著，連接在電纜上的每件設(shè)備，只有在確定沒有其他設(shè)備使用電纜的情況下，才能使用電纜。如果發(fā)生沖突，他設(shè)備使用電纜的情況下，才能使用電纜。如果發(fā)生沖突，意欲連接的設(shè)備就會(huì)退出，以后再

15、進(jìn)行嘗試。以太網(wǎng)的傳意欲連接的設(shè)備就會(huì)退出，以后再進(jìn)行嘗試。以太網(wǎng)的傳輸速率可達(dá)輸速率可達(dá)10兆位秒，這個(gè)速度足以使設(shè)備之間的距離兆位秒，這個(gè)速度足以使設(shè)備之間的距離無關(guān)緊要。它們感覺起來就好像是與目的地直接相連。無關(guān)緊要。它們感覺起來就好像是與目的地直接相連。 10 There are many different layouts (such as bus, star, ring, see Figure 9A-1) and a number of different access protocols for LANs. Despite this variety, all LANs share

16、 the feature that they are limited in range (typically they cover one building) and are fast enough to make the connecting network invisible to the devices that use it. 局域網(wǎng)有許多不同的布局（如總線式、星形、環(huán)形）和許局域網(wǎng)有許多不同的布局（如總線式、星形、環(huán)形）和許多不同的訪問協(xié)議。盡管種類繁多，所有的局域網(wǎng)都具有多不同的訪問協(xié)議。盡管種類繁多，所有的局域網(wǎng)都具有如下的共同特征：它們都范圍有限（通常覆蓋一幢建筑如下的共同特征

17、：它們都范圍有限（通常覆蓋一幢建筑物），而且都有足夠快的速度，使得使用連接網(wǎng)絡(luò)的設(shè)備物），而且都有足夠快的速度，使得使用連接網(wǎng)絡(luò)的設(shè)備感覺不到網(wǎng)絡(luò)的存在。感覺不到網(wǎng)絡(luò)的存在。 11 In addition to providing shared access, modern LANs can also give users a wide range of sophisticated facilities. Management software packages are available to control the way in which devices are configured o

18、n the LAN, how users are administered, and how network resources are controlled. 除了提供訪問共享，現(xiàn)代局域網(wǎng)還能為用戶提供各種高級(jí)功除了提供訪問共享，現(xiàn)代局域網(wǎng)還能為用戶提供各種高級(jí)功能。管理軟件包可以用來控制設(shè)備在局域網(wǎng)上的配置方式、能。管理軟件包可以用來控制設(shè)備在局域網(wǎng)上的配置方式、用戶的管理方式和網(wǎng)絡(luò)資源的控制方式。用戶的管理方式和網(wǎng)絡(luò)資源的控制方式。 12 A widely adopted structure on local networks is to have a number of servers

19、 that are available to a (usually much greater) number of clients. The former, usually powerful computers, provide services such as print control, file sharing, and mail to the latter, which are usually personal computers. 局域網(wǎng)廣泛采用的結(jié)構(gòu)是設(shè)立若干個(gè)服務(wù)器，供若干個(gè)（數(shù)局域網(wǎng)廣泛采用的結(jié)構(gòu)是設(shè)立若干個(gè)服務(wù)器，供若干個(gè)（數(shù)量通常比服務(wù)器多得多的）客戶機(jī)訪問。前者通常是高性

20、量通常比服務(wù)器多得多的）客戶機(jī)訪問。前者通常是高性能的計(jì)算機(jī)，為后者提供打印控制、文件共享以及郵件發(fā)能的計(jì)算機(jī)，為后者提供打印控制、文件共享以及郵件發(fā)送等服務(wù)，而后者通常是個(gè)人計(jì)算機(jī)。送等服務(wù)，而后者通常是個(gè)人計(jì)算機(jī)。 13 The facilities on most LANs are very powerful. Most organizations do not wish to have small isolated islands of computing facilities. They usually want to be extend facilities over a wide

21、r area so that groups can work without having to be located. 大多數(shù)局域網(wǎng)上的設(shè)施都具有十分強(qiáng)大的功能。大多數(shù)機(jī)構(gòu)大多數(shù)局域網(wǎng)上的設(shè)施都具有十分強(qiáng)大的功能。大多數(shù)機(jī)構(gòu)都不希望計(jì)算設(shè)施成為一個(gè)個(gè)小小的孤島。它們通常都希都不希望計(jì)算設(shè)施成為一個(gè)個(gè)小小的孤島。它們通常都希望將設(shè)施擴(kuò)展到更廣的范圍，這樣各個(gè)計(jì)算機(jī)組不必定位望將設(shè)施擴(kuò)展到更廣的范圍，這樣各個(gè)計(jì)算機(jī)組不必定位就可以工作。就可以工作。14 Routers and bridges are specialized devices that allow two or more LANs

22、to be connected. The bridge is the more basic device and can only connect LANs of the same type. The router is a more intelligent component that can interconnect many different types of computer network. 路由器和橋接器就是使兩個(gè)或更多的局域網(wǎng)能夠連接起來路由器和橋接器就是使兩個(gè)或更多的局域網(wǎng)能夠連接起來的專用設(shè)備。橋接器相對(duì)而言屬于更基礎(chǔ)的設(shè)備，只能連的專用設(shè)備。橋接器相對(duì)而言屬于更基礎(chǔ)的設(shè)備

23、，只能連接同類型的局域網(wǎng)。路由器則是智能化程度更高一些的元接同類型的局域網(wǎng)。路由器則是智能化程度更高一些的元件，可以連接多種不同類型的計(jì)算機(jī)網(wǎng)絡(luò)。件，可以連接多種不同類型的計(jì)算機(jī)網(wǎng)絡(luò)。 15 Many large companies have corporate data networks that are founded on a collection of LANs and routers. From the users point of view, this arrangement provides them with a physically diverse network that

24、looks like one coherent resource. 許多大公司擁有建立在一批局域網(wǎng)與路由器基礎(chǔ)上的公司許多大公司擁有建立在一批局域網(wǎng)與路由器基礎(chǔ)上的公司數(shù)據(jù)網(wǎng)絡(luò)。從用戶的角度來看，這種布局提供了一個(gè)似乎數(shù)據(jù)網(wǎng)絡(luò)。從用戶的角度來看，這種布局提供了一個(gè)似乎是相干網(wǎng)絡(luò)而實(shí)際上具有物理多樣性的網(wǎng)絡(luò)。是相干網(wǎng)絡(luò)而實(shí)際上具有物理多樣性的網(wǎng)絡(luò)。 16 At some point, it becomes impractical to extend a LAN any further. Physical limitation sometimes drives this, but more oft

25、en than not there are more convenient or cheaper ways to extend a computer networks. Two major components in most real computer networks are the public telephone and data networks. These provide long distance links that extend a LAN into a wide area network. 局域網(wǎng)擴(kuò)展到某個(gè)程度之后，再進(jìn)一步擴(kuò)展就變得不現(xiàn)實(shí)了。局域網(wǎng)擴(kuò)展到某個(gè)程度之后，再

26、進(jìn)一步擴(kuò)展就變得不現(xiàn)實(shí)了。有時(shí)是物理限制使然，但更常見的是因?yàn)閾碛袛U(kuò)展計(jì)算機(jī)網(wǎng)有時(shí)是物理限制使然，但更常見的是因?yàn)閾碛袛U(kuò)展計(jì)算機(jī)網(wǎng)絡(luò)的更方便或更廉價(jià)的方法。在大多數(shù)現(xiàn)實(shí)的計(jì)算機(jī)網(wǎng)絡(luò)中，絡(luò)的更方便或更廉價(jià)的方法。在大多數(shù)現(xiàn)實(shí)的計(jì)算機(jī)網(wǎng)絡(luò)中，兩個(gè)主要組成部分是公用電話網(wǎng)和數(shù)據(jù)網(wǎng)。它們提供了遠(yuǎn)程兩個(gè)主要組成部分是公用電話網(wǎng)和數(shù)據(jù)網(wǎng)。它們提供了遠(yuǎn)程鏈接，使局域網(wǎng)延伸到廣域網(wǎng)之中去。鏈接，使局域網(wǎng)延伸到廣域網(wǎng)之中去。 17 Nearly all of the national network operations offer services for the interconnection of com

27、puter networks. These services range from simple, low speed data links that work over the public telephone network to sophisticated high-speed data services that are ideally suited to the interconnection of LANs. These high-speed data services are usually referred to as broadband connections. It is

28、anticipated that they will provide the necessary links between LANs that make what is called the information superhighway a reality. 幾乎所有的國家網(wǎng)絡(luò)運(yùn)營商都提供計(jì)算機(jī)網(wǎng)絡(luò)的互聯(lián)服務(wù)。幾乎所有的國家網(wǎng)絡(luò)運(yùn)營商都提供計(jì)算機(jī)網(wǎng)絡(luò)的互聯(lián)服務(wù)。這些服務(wù)從基于公用電話網(wǎng)的簡單、低速數(shù)據(jù)鏈路到非常適這些服務(wù)從基于公用電話網(wǎng)的簡單、低速數(shù)據(jù)鏈路到非常適合局域網(wǎng)互聯(lián)的復(fù)雜、高速的數(shù)據(jù)服務(wù)。這些高速數(shù)據(jù)服務(wù)合局域網(wǎng)互聯(lián)的復(fù)雜、高速的數(shù)據(jù)服務(wù)。這些高速數(shù)據(jù)服務(wù)通常稱為寬帶連接。據(jù)預(yù)

29、測，這些服務(wù)將在局域網(wǎng)之間提供通常稱為寬帶連接。據(jù)預(yù)測，這些服務(wù)將在局域網(wǎng)之間提供必要的鏈接，從而使所謂的信息高速公路成為現(xiàn)實(shí)。必要的鏈接，從而使所謂的信息高速公路成為現(xiàn)實(shí)。 18 It would be easy to assume that computers will all be able to work together once they have broadband connection. But how do you get computers made by different manufactures in different countries to work toget

30、her across the world? Until recently, most computers were built with their own interfaces and were structured their own unique way. 人們很容易認(rèn)為：計(jì)算機(jī)一旦有了寬帶連接，就能全部具備人們很容易認(rèn)為：計(jì)算機(jī)一旦有了寬帶連接，就能全部具備協(xié)同工作的能力。但是，怎樣才能使不同國家不同制造商協(xié)同工作的能力。但是，怎樣才能使不同國家不同制造商生產(chǎn)的計(jì)算機(jī)在世界范圍內(nèi)協(xié)同工作呢？直到最近，大多生產(chǎn)的計(jì)算機(jī)在世界范圍內(nèi)協(xié)同工作呢？直到最近，大多數(shù)計(jì)算機(jī)還是用各自的接口生產(chǎn)的，

31、并且采用了各自獨(dú)特?cái)?shù)計(jì)算機(jī)還是用各自的接口生產(chǎn)的，并且采用了各自獨(dú)特的結(jié)構(gòu)。的結(jié)構(gòu)。 19 A computer could talk to one of its own kind but would have difficulty communicating with a “foreigner”. There were only a privileged few with the time, knowledge, and equipment to extract what they wanted from a variety of computing resources. 一臺(tái)計(jì)算機(jī)可以與同

32、類型的計(jì)算機(jī)進(jìn)行對(duì)話，但是卻很難實(shí)一臺(tái)計(jì)算機(jī)可以與同類型的計(jì)算機(jī)進(jìn)行對(duì)話，但是卻很難實(shí)現(xiàn)與現(xiàn)與“異族異族”的通信。只有極少數(shù)特許的人，擁有必要的的通信。只有極少數(shù)特許的人，擁有必要的時(shí)間、知識(shí)和設(shè)備，從各種各樣的計(jì)算資源中提取他們所時(shí)間、知識(shí)和設(shè)備，從各種各樣的計(jì)算資源中提取他們所需要的東西。需要的東西。 20 By the 1990s, the level of commonality across different computers reached the stage where they could interwork effectively. This allows virtual

33、ly anyone to use a remote machine to good effect. The main contributors to this are: 到到20世紀(jì)世紀(jì)90年代，不同計(jì)算機(jī)的共性程度達(dá)到了彼此可以年代，不同計(jì)算機(jī)的共性程度達(dá)到了彼此可以有效地互工作的階段。這使得幾乎任何人都可以有效地使有效地互工作的階段。這使得幾乎任何人都可以有效地使用遠(yuǎn)程機(jī)器。使這一點(diǎn)得以實(shí)現(xiàn)的主要因素有：用遠(yuǎn)程機(jī)器。使這一點(diǎn)得以實(shí)現(xiàn)的主要因素有：21 1. Client Server Instead of building computer systems as monolithic sy

34、stems, there is now general agreement that they should be constructed as client/server systems. The client (a PC user) requests a service (such as printing) and the server (a LAN-connected processor) provides it. This consensus view on the structure of a computer system means that there is a separat

35、ion of functions previously bundled together. The implementation details that flow from a simple concept go a long way to enabling all computers to be treated uniformly. 現(xiàn)在的共識(shí)是，不應(yīng)將計(jì)算機(jī)系統(tǒng)建成單塊系統(tǒng)，而是應(yīng)建成客戶服現(xiàn)在的共識(shí)是，不應(yīng)將計(jì)算機(jī)系統(tǒng)建成單塊系統(tǒng)，而是應(yīng)建成客戶服務(wù)器系統(tǒng)?？蛻簦▊€(gè)人計(jì)算機(jī)用戶）提出服務(wù)（如打?。┱?qǐng)求，服務(wù)器務(wù)器系統(tǒng)。客戶（個(gè)人計(jì)算機(jī)用戶）提出服務(wù)（如打?。┱?qǐng)求，服務(wù)器（接在局域網(wǎng)上的

36、處理器）提供該服務(wù)。對(duì)計(jì)算機(jī)系統(tǒng)結(jié)構(gòu)的這種一致（接在局域網(wǎng)上的處理器）提供該服務(wù)。對(duì)計(jì)算機(jī)系統(tǒng)結(jié)構(gòu)的這種一致看法，意味著將先前捆綁在一起的功能分割開。產(chǎn)生自這個(gè)簡單概念的看法，意味著將先前捆綁在一起的功能分割開。產(chǎn)生自這個(gè)簡單概念的具體實(shí)現(xiàn)方案，對(duì)于使所有的計(jì)算機(jī)都能夠被一樣對(duì)待，有很大作用。具體實(shí)現(xiàn)方案，對(duì)于使所有的計(jì)算機(jī)都能夠被一樣對(duì)待，有很大作用。 22 2. Object Technology Another way to build computer systems works from the premise that they should be built from well-

37、defined parts objects which are encapsulated, defined, and implemented so that they can be independent agents. This adoption of objects as a means of building computer systems has helped to allow interchangeability of parts. 建立計(jì)算機(jī)系統(tǒng)的另外一種方法來自這樣一個(gè)假設(shè)：計(jì)算機(jī)建立計(jì)算機(jī)系統(tǒng)的另外一種方法來自這樣一個(gè)假設(shè)：計(jì)算機(jī)系統(tǒng)應(yīng)該用明確界定的部件系統(tǒng)應(yīng)該用明確界定的部

38、件亦即可以作為獨(dú)立的主體亦即可以作為獨(dú)立的主體而封裝、定義和執(zhí)行的對(duì)象而封裝、定義和執(zhí)行的對(duì)象來構(gòu)建。將對(duì)象用作構(gòu)建來構(gòu)建。將對(duì)象用作構(gòu)建計(jì)算機(jī)系統(tǒng)的手段，促進(jìn)了部件的可換性計(jì)算機(jī)系統(tǒng)的手段，促進(jìn)了部件的可換性 。23 3. Open System This term covers the general aim of building computer systems so that they can readily be interconnected, and hence distributed. In practice, open systems are all about unbund

39、ling all the complexities of a computer system and using similar structures across different systems. And this entails a mixture of standards (which tell the manufactures what they should be doing) and consortia (groups of like-minded people who help them to do it). The overall effect is that they c

40、an talk to each other. 這個(gè)術(shù)語涵蓋了構(gòu)建計(jì)算機(jī)系統(tǒng)的總體目標(biāo)，即易于互聯(lián)并因此實(shí)現(xiàn)分布這個(gè)術(shù)語涵蓋了構(gòu)建計(jì)算機(jī)系統(tǒng)的總體目標(biāo)，即易于互聯(lián)并因此實(shí)現(xiàn)分布配置。實(shí)際上，開放式系統(tǒng)所要做的就是分解計(jì)算機(jī)系統(tǒng)的所有復(fù)雜性，配置。實(shí)際上，開放式系統(tǒng)所要做的就是分解計(jì)算機(jī)系統(tǒng)的所有復(fù)雜性，并在各種不同系統(tǒng)中使用類似的結(jié)構(gòu)。這就需要有（告訴制造商該做什并在各種不同系統(tǒng)中使用類似的結(jié)構(gòu)。這就需要有（告訴制造商該做什么的）各種標(biāo)準(zhǔn)和（幫助制造商做該做之事的志同道合之人組成的）各么的）各種標(biāo)準(zhǔn)和（幫助制造商做該做之事的志同道合之人組成的）各種協(xié)會(huì)的協(xié)同之力?？偟男Ч且共煌南到y(tǒng)能夠?qū)崿F(xiàn)

41、對(duì)話。種協(xié)會(huì)的協(xié)同之力。總的效果是要使不同的系統(tǒng)能夠?qū)崿F(xiàn)對(duì)話。24 The ultimate aim of all of the work in distributed systems is to allow anyone to buy computers from a number of different manufacturers, to site them whatever is convenient, to use broadband connections to link them, and to operate them as one cooperating machine th

42、at takes full advantage of the fast links. 在分布式系統(tǒng)領(lǐng)域所做的所有工作，其最終目標(biāo)就是使任何人在分布式系統(tǒng)領(lǐng)域所做的所有工作，其最終目標(biāo)就是使任何人可以從多個(gè)不同的制造商購買計(jì)算機(jī)，將其設(shè)置在任何方可以從多個(gè)不同的制造商購買計(jì)算機(jī)，將其設(shè)置在任何方便之處，使用寬帶鏈路來連接它們，并將它們作為充分利便之處，使用寬帶鏈路來連接它們，并將它們作為充分利用高速鏈路的一臺(tái)協(xié)同工作的機(jī)器來操作。用高速鏈路的一臺(tái)協(xié)同工作的機(jī)器來操作。 25 Having fast computer networks built of machines that can talk

43、 to each other is not the end of the story. The specters of the “information superhighwayman” and “information superroadworks” have yet to be dealt with. 擁有以彼此可以對(duì)話的機(jī)器構(gòu)建的快速計(jì)算機(jī)網(wǎng)絡(luò)，并不意味擁有以彼此可以對(duì)話的機(jī)器構(gòu)建的快速計(jì)算機(jī)網(wǎng)絡(luò)，并不意味著一切都萬事大吉了。還有著一切都萬事大吉了。還有“信息高速公路劫匪信息高速公路劫匪”和和“信信息高速公路故障息高速公路故障”的幽靈得對(duì)付。的幽靈得對(duì)付。 26 1. Security

44、With ever increasing amounts of important information being entrusted to ever more distributed computers, computer security becomes ever more important. In a highly distributed system it would be all too easy for an informed superhighwayman to access confidential information without being seen. The

45、Data Encryption System (DES) standard for protecting computer data, introduced in the late 1970s, has more recently been supplemented by “public key” systems that allow users to scramble and unscramble their messages easily without a third party intruding. 隨著越來越多的重要信息交托給越來越多的分布式計(jì)算機(jī)來處理，計(jì)算機(jī)隨著越來越多的重要信息

46、交托給越來越多的分布式計(jì)算機(jī)來處理，計(jì)算機(jī)安全變得越來越重要。在一個(gè)高度分布的系統(tǒng)中，一個(gè)諳熟情況的信息安全變得越來越重要。在一個(gè)高度分布的系統(tǒng)中，一個(gè)諳熟情況的信息高速公路劫匪不為人所知地訪問機(jī)密信息易如反掌。高速公路劫匪不為人所知地訪問機(jī)密信息易如反掌。20世紀(jì)世紀(jì)70年代末推年代末推出的用以保護(hù)計(jì)算機(jī)數(shù)據(jù)的數(shù)據(jù)加密系統(tǒng)標(biāo)準(zhǔn)，最近又得到出的用以保護(hù)計(jì)算機(jī)數(shù)據(jù)的數(shù)據(jù)加密系統(tǒng)標(biāo)準(zhǔn)，最近又得到“公用密鑰公用密鑰”系統(tǒng)的補(bǔ)充，而這種系統(tǒng)允許用戶在無第三方介入的情況下很容易地為系統(tǒng)的補(bǔ)充，而這種系統(tǒng)允許用戶在無第三方介入的情況下很容易地為其信息加密和解密。其信息加密和解密。27 2. Managem

47、ent It is a full-time job to keep a LAN operating as it should. Keeping a computer network that is distributed across the world running smoothly takes the challenge of network management a step further. The essential concepts for managing distributed and diverse networks have received a lot of atten

48、tion lately. There are now enough tools and standards for this important aspect of computer networks to allow global networks to be supervised effectively. 使局域網(wǎng)保持正常運(yùn)行，需要花費(fèi)大量時(shí)間。使分布在世界各地的計(jì)算使局域網(wǎng)保持正常運(yùn)行，需要花費(fèi)大量時(shí)間。使分布在世界各地的計(jì)算機(jī)網(wǎng)絡(luò)穩(wěn)定運(yùn)行，進(jìn)一步增加了網(wǎng)絡(luò)管理的挑戰(zhàn)。管理各種不同的分布機(jī)網(wǎng)絡(luò)穩(wěn)定運(yùn)行，進(jìn)一步增加了網(wǎng)絡(luò)管理的挑戰(zhàn)。管理各種不同的分布式網(wǎng)絡(luò)的基本概念，近來受到了大量的關(guān)注。

49、現(xiàn)在，計(jì)算機(jī)網(wǎng)絡(luò)的這個(gè)式網(wǎng)絡(luò)的基本概念，近來受到了大量的關(guān)注?，F(xiàn)在，計(jì)算機(jī)網(wǎng)絡(luò)的這個(gè)重要方面擁有足夠的工具和標(biāo)準(zhǔn)，使全球性網(wǎng)絡(luò)能夠得到有效的監(jiān)管。重要方面擁有足夠的工具和標(biāo)準(zhǔn)，使全球性網(wǎng)絡(luò)能夠得到有效的監(jiān)管。28 A local area network (LAN) is a group of computers and other devices dispersed (分散分散) over a relatively limited area and connected by a communications link that enables any device to interact w

50、ith any other on the network. LANs commonly include microcomputers and shared (often expensive) resources such as laser printers and large hard disks. Most (modern) LANs can support a wide variety of computers and other devices. Each device must use the proper physical and data-link protocols for the particular LAN, and all devices that want to communicate with each other on the LAN must use the same upper-level communications protocol. 2