計算機專業(yè)外文翻譯---網(wǎng)絡(luò)目標

1、<p><b>　　附錄1英文及其譯文</b></p><p>　　Computer Networks</p><p>　　Network Goals</p><p>　　Some reasons are causing centralized computer systems to give way to networks.<

2、/p><p>　　The first one is that many organizations already have a substantial number of computers in operation, often located far apart .Initially, each of these computers may have worked in isolation from the o

3、ther ones, but at a certain time, and management may have decided to connect them to be able to correlate information about the entire organization. Generally speaking, the goal is to make all programs, data, and other r

4、esources available to anyone on the network without regard to the physical loca</p><p>　　The second one is to provider high reliability by having alternative sources of supply. With a network, the temporary

5、loss of a single computer is much less serious, because its users can often be accommodated elsewhere until the service is restored.</p><p>　　Another important reason for distributing computing power has to

6、do with the relative price of computing versus communication. Now the cost of a small computer is negligible, so it becomes attractive to analyze the data at where it is captured, and only to send occasional summaries ba

7、ck to the computer center, to reduce the communication cost, which now represents a larger percentage of the total cost than it used to.</p><p>　　Yet another reason of setting up a computer network is that a

8、 computer network can provider a powerful communication medium among widely separated people.</p><p>　　Application of Networks</p><p>　　One of the main areas of potential network use is access t

9、o remote data bases. It may someday be easy for people sitting at their terminals at home to make reservations for airplanes, trains, bused, boats, restaurants, theaters, hotels, and so on, at anywhere in the world with

10、instant confirmation. Home banking, automated newspaper and fully automated library also fall in this category.</p><p>　　Computer aided education is another possible field for using network, with many differ

11、ent coursed being offered.</p><p>　　Teleconferencing is a whole new form of communication. With it, widely separated people can conduct a meeting by typing messages at their terminals. Attendees may leave at

12、 will, and find out what they missed when they come back. International contacts by human being may be greatly enhanced by network based communication facilities.</p><p>　　Network Structure</p><p&

13、gt;　　In any network exists a collection of machines intended to running user (i.e., application) program. These machines are called hosts. The hosts are connected by the communication subnet. The job of the subnet is to

14、carry messages from host to host.</p><p>　　The subnet consists of two basic components: nodes and channels. Nodes are specialized computers. All traffic to or from the host goes via its node. Channels are tr

15、ansmission lines. </p><p>　　Broadly speaking, there are two general types of designs for the communication subnet:</p><p>　　(1)Point-to-point channels</p><p>　　(2Broadcast channels&

16、lt;/p><p>　　In the first one, the network contains numerous cables or leased telephone lines, each one connecting a pair of nodes. If two nodes that do not share a cable wish to communicate, they must do this i

17、ndirectly via other nodes. When a message is sent from one node to another via one or more inter mediate nodes, each intermediate node will receive the message and store it until the required output line is free so that

18、it can transmit the message forward. The subnet using this principle is called a p</p><p>　　When a point-to-point subset is used, the important problem is how to design the interconnected topology between th

19、e nodes.</p><p>　　The second kind of communication architecture uses broadcasting. In this design there is a single communication channel shared by all nodes. The inherence in broadcast system is that messag

20、es sent by any node are received by all other nodes.</p><p>　　Protocol Hierarchies</p><p>　　To reduce their design complexity, most networks are organized as a series of layers or levels, each o

21、ne built upon its predecessor. Layer n on one machine carries on a conversation with layer n on another machine. The rules and conventions used in this conversation are collectively called the layer n protocol. In realit

22、y, no data are directly transferred from layer n on one machine to layer n on another machine (except in the lowest layer).Instead, each layer passes data and control information</p><p>　　Between each pair o

23、f adjacent layers there is an interface. The interface defines which primitive operations and services the lower offers to the upper one.</p><p>　　The set of layers and protocols is called network architectu

24、re.</p><p>　　Design Issues for the Layers</p><p>　　One set of design decisions are the rules for data transfer. Do data only travel in one direction, called simplex communication, or travel in e

25、ither direction, but not simultaneously, called half-duplex communication, or travel in both directions at once, call full-duplex communicative?</p><p>　　Error control is an important issue when the physical

26、 communication circuits are not perfect. Many error-detecting and error-correcting codes are known, but both ends of the connection must agree on what kind of code is being used. In addition, the receiver must have some

27、way of telling the sender which messages have been correctly received and which has not.</p><p>　　When there are multiple paths between source and destination, at some points in the hierarchy, a routing deci

28、sion must be made. </p><p>　　The ISO Reference Model</p><p>　　The Reference Model of Open System Interconnection (OSI), as ISO calls it, has seven layers. The major ones of the principles, from

29、which ISO applied to get the seven layers, are as follows:</p><p>　　(1) A layer should be created where a different level of abstractions is needed.</p><p>　　(2) Each layer should perform a well

30、 defined function.</p><p>　　(3) The function of each layer should be chosen with an eye toward defining internationally standardized protocols.</p><p>　　(4) The layer boundaries should be chosen

31、 to minimize the information flow across the interfaces.</p><p>　　(5) The number of layers should be large enough so that distinct functions need not be put together in the same layer without necessity, and

32、small enough so that the architecture will not become out of control.</p><p>　　The Physical Layer</p><p>　　The physical layer is concerned with transmitting raw bits over a communication channel

33、. Typical questions here are how many volts should be used to represent an 1 and how many a 0, how many microseconds a bit occupies, whether transmission may proceed simultaneously in both directions, how to establish th

34、e initial connection and how to tear town the connection when both sides are finished, how many pins the network connector has and what kind of function each pin has. The design issues here lar</p><p>　　The

35、Data Link Layer</p><p>　　The task of the data link layer is to obtain a raw transmission facility and to transform it into a line that appears free of transmission errors to the network layer. It accomplishe

36、s this task by breaking the input data up into data frames, transmitting the frames sequentially, and processing the acknowledgment frames sent back by the receiver.</p><p>　　Since the physical layer merely

37、accepts and transmits a stream of bits without any regard to meaning or structure, it can create and recognize frame boundaries until the data link layer. This can be accomplished by attaching special bits patterns to th

38、e beginning and the end of the frame. But it may produce two problems: one is a noise burst on the line can destroy a frame completely. In this case, the software on the source machine must retransmit the frame. The othe

39、r is that some mechanism mus</p><p>　　The Network Layer</p><p>　　The network layer controls the operation of subnet. It determines the chief characteristics of the node-host interface, and how p

40、ackets, the units of information exchanged in this layer, are routed within the subnet.</p><p>　　What this layer of software does, basically, is to accept messages from the source host, convert them to packe

41、ts, and observe the packets to get to the destination. The key design issue is how the route is determined. It could not only base on static table, which are "wired into" the network and rarely changed, but els

42、e adopt highly dynamic manner, which can determine each packet again to reflect the current network load.</p><p>　　The Transport Layer</p><p>　　The basic function of transport layer is to accept

43、 data from the session layer, split it up into smaller units, if necessary, pass these to the network layer, and ensure that the pieces all arrive correctly at the other end.</p><p>　　This layer is a true en

44、d-to-end layer. In other words, a program on the source machine carries on a conversation with a similar program on the destination machine, using the message headers and control messages.</p><p>　　The Sessi

45、on Layer</p><p>　　With the session layer, the user must negotiate to establish a connection with a process on another machine. The connection is usually called a session. A session might be used to allow a u

46、ser to log into a remote time-sharing system or to transfer a file between two machines. The operation of setting up a session between two processes is often called binding.</p><p>　　Another function of the

47、session layer is to manage the session once it has been set up.</p><p>　　The Presentation Layer </p><p>　　The presentation layer could be designed to accept ASCII strings as input and produce co

48、mpressed bit patterns as output. This function of the presentation layer is called text compression.</p><p>　　In addition, this layer can also perform other transformations. Encryption to provide security is

49、 one possibility. Conversion between character codes, such as ASCII to EBCDIC, might often be useful. More generally, different computer usually have incompatible file formats, so a file conversion option might be useful

50、 at times.</p><p><b>　　網(wǎng)絡(luò)目標：</b></p><p>　　第一是許多機構(gòu)已擁有大量正在工作的計算機，這些計算機通常相距較遠。起先，每臺機器可能獨立工作，但是在某些時候使整個機構(gòu)的信息能夠相互關(guān)聯(lián)。一般來說，這上目標是使所有程序、數(shù)據(jù)和其它資源能夠被網(wǎng)絡(luò)上的任何人所獲取，而不必考慮資源和用戶的物理位置。</p><p&

51、gt;　　第二個原因是通過可選資源提供更高的可靠性。在網(wǎng)絡(luò)中，單個計算機臨時的損失無關(guān)緊要，因為它的用戶可以從別處獲取信息，直到該計算機恢復功能。</p><p>　　將計算能力分散開的另一個重要原因和計算與通訊的價格比有關(guān)?，F(xiàn)在， 一臺小計算機的價格是微不足道的，因此，在獲取數(shù)據(jù)的地方分析數(shù)據(jù)，并且僅把臨時的概要傳回計算機中心，這種做法是有吸引力的，這樣做可以降低通訊費用，通訊費用占費用的比例比以往要高。<

52、;/p><p>　　建立計算機網(wǎng)絡(luò)的另一個原因是計算機網(wǎng)絡(luò)能在廣泛分布的人們之間提供強有力的通訊介質(zhì)。</p><p><b>　　網(wǎng)絡(luò)的應(yīng)用</b></p><p>　　潛在的網(wǎng)絡(luò)應(yīng)用的主要領(lǐng)域之一是訪問遠程數(shù)據(jù)庫?？赡苡幸惶烊藗兛梢宰诩抑械慕K端旁，方便地預(yù)定飛機、火車、汽車、般、飯店、劇院、旅館等的位置或床位，無論是在世界上的什么地方都可以立

53、即得到回復。家庭銀行、自動報紙和全自動圖書館也在此列。</p><p>　　計算機輔助教育是另一個可能利用網(wǎng)絡(luò)的地方，其中提供了許多不同的教程。</p><p>　　電話會議是一個全新的通訊形式，通過這種形式距離很遠的人們可以通過在他的終端上鍵入信息召開一次會議。參加者可以隨意離開，并可以在回來的時候發(fā)現(xiàn)剛才漏掉的信息?；诰W(wǎng)絡(luò)的通訊設(shè)備將大幅度增強人類之間的國際接觸。</p>

54、<p><b>　　網(wǎng)絡(luò)結(jié)構(gòu)</b></p><p>　　在任何網(wǎng)絡(luò)中都存在著一批用以運行用戶（即應(yīng)用）等程序的機器，這些機器被稱</p><p>　　為主機。主機通過通信子網(wǎng)相互聯(lián)接，子網(wǎng)的任務(wù)是把信息從一個主機傳輸?shù)搅硪粋€主機。</p><p>　　子網(wǎng)由兩個基本部分組成：節(jié)點和通道。節(jié)點是特殊的計算機。所有進出主機制信息均要

55、通過它的節(jié)點，通道是傳輸線。</p><p>　　一般而言，有兩種通用的子網(wǎng)的設(shè)計方案：</p><p><b>　　點對點通道</b></p><p><b>　　廣播式通道</b></p><p>　　在第一種子網(wǎng)中，網(wǎng)絡(luò)包含大量電纜或是租用的電話線，每條線連接一對節(jié)點。如果兩個節(jié)點之間沒有電纜

56、卻相互通信，它們必須通過其它節(jié)點間接地達到目的。當信息從一個節(jié)點，通過一個多個節(jié)點，發(fā)向另一個節(jié)點的時候，每個中間節(jié)點都接收信息，存儲信息，直到所需的輸出線空閑，再把信息向前傳送。遵從這一原則的子網(wǎng)稱為點對點或存儲傳發(fā)子網(wǎng)。</p><p>　　當采用點對點子網(wǎng)時，一個重要的問題是怎樣設(shè)計節(jié)點相互聯(lián)接的拓撲結(jié)構(gòu)。</p><p>　　第二種通訊結(jié)構(gòu)使用廣播方式。在這種設(shè)計中所有節(jié)點共享單一

57、通信通道。廣播系統(tǒng)的基本是由任一節(jié)點發(fā)送的信息均能被其它所有節(jié)點接收。</p><p><b>　　協(xié)議層次</b></p><p>　　為了減少設(shè)計的復雜性，大多數(shù)網(wǎng)絡(luò)被組織成一系列層次或等級，每一層次都建筑在前一層次的基礎(chǔ)之上。 一臺機器上的第n層與另一臺機器上的第n 層協(xié)議。實際中，數(shù)據(jù)并非直接從一臺機器的第n層轉(zhuǎn)換到另一臺機器的第n層（除了最低層以外），而是每

58、層都把數(shù)據(jù)和控制信息傳到下面一層，直至最低層。在最低層存在與其它機器的物理通訊，這是和高層中的虛擬通訊相對而言的。</p><p>　　在每對相鄰層次之間有一個接口，接口定義了低層提供給高層的原始操作和服務(wù)。</p><p>　　層次與協(xié)議的集合被稱為網(wǎng)結(jié)構(gòu)。</p><p><b>　　層次的設(shè)計問題</b></p><p

59、>　　設(shè)計中需要決定的一部分內(nèi)容是數(shù)據(jù)傳輸?shù)囊?guī)則。數(shù)據(jù)沿一個方向傳播（稱單工通信），還是可以在不同時刻不同方向傳播（稱半雙工通信），還是可以同時沿兩個方向傳播（稱全雙工通信）？</p><p>　　當物理通信線路不很完善的時候，錯誤控制是一個重要的問題。許多錯誤檢測和錯誤糾正碼是人們知道的，但連線的兩端必須商定當前使用哪一個代碼。另外，接收者必須有辦法告訴發(fā)送者哪些信息已被正確接收，哪些信息未被正確接收。&

60、lt;/p><p>　　當源節(jié)點與目的節(jié)點間存在多通道時，在層次中的一些點上必須進行路徑規(guī)劃。</p><p><b>　　ISO 參考模型</b></p><p>　　被ISO稱為開放系統(tǒng)互連（OSI）的參考模型有七個層次。ISO應(yīng)用了下面一些主要原則來得出這七個層次：</p><p>　　在需要一個不同的抽象等級時應(yīng)設(shè)立

61、一層次。</p><p>　　每一層應(yīng)執(zhí)行一種嚴格定義的功能。</p><p>　　每一層的功能應(yīng)參照國際標準化的協(xié)議來選擇。</p><p>　　選擇層次邊界時力圖使穿越接口的協(xié)議來選擇。</p><p>　　層次的數(shù)量應(yīng)中足夠多以便使不同的功能不致于毫無必要地放在同一層次中，層次的數(shù)量還應(yīng)中夠少以便結(jié)構(gòu)不會變得難以控制。</p>

62、;<p><b>　　物理層</b></p><p>　　物理層通過通信通道傳輸原始比特。此處典型的問題是用多少伏特表示1，用多少伏特表示0每一比特占多少微秒，千丈是否傳輸過程可以在兩個方向上同時進行，怎樣建立初始連接，當兩邊均已完成時怎樣拆卸連接，網(wǎng)絡(luò)連接器有多少管腳，每個管腳的功能如何。這里的大多數(shù)設(shè)計問題旨在處理子網(wǎng)中的機械、電子和過程接口。</p><

63、;p><b>　　數(shù)據(jù)鏈路層</b></p><p>　　數(shù)據(jù)鏈路層的任務(wù)是獲取原始的傳輸信息，并把它轉(zhuǎn)換到線路中。經(jīng)此轉(zhuǎn)換，到達網(wǎng)絡(luò)層時已消除了傳輸錯誤。該層是這樣來完成此項任務(wù)的：把輸入數(shù)據(jù)劃分成數(shù)據(jù)幀，按順序傳輸這些幀，處理從接收者返回的應(yīng)答幀。</p><p>　　由于物理層只接收和傳輸比特流而不考慮意義和結(jié)構(gòu)，所以直到數(shù)據(jù)鏈路層才產(chǎn)生和識別幀的邊界，這

64、可以通過在幀在開頭和末尾附加特殊的比特模式來完成。這樣可以產(chǎn)生兩個問題：第一個問題是線路上的噪聲段可能把一個幀全部毀壞。在這種情況下，源機器上的軟件必須重發(fā)該幀。另一個問題是必須應(yīng)用一些機制以使發(fā)送者能夠知道引刻接收者有多大的緩沖空間。</p><p><b>　　網(wǎng)絡(luò)層</b></p><p>　　網(wǎng)絡(luò)層控制子網(wǎng)的操作，它決定節(jié)點——主機接口的主要特性，以及包怎樣在

65、子網(wǎng)中按路徑送，包是指此層中信息換的單元。</p><p>　　這層軟件要做的基本上是從源主機接收信息，把它們轉(zhuǎn)換成包，并監(jiān)控著包傳向目的地。關(guān)鍵性設(shè)計問題是怎樣決定路徑。這可以基于插入網(wǎng)絡(luò)并很少修改的靜態(tài)表來實現(xiàn)。也可能采用高度動態(tài)的方式，重新確定每個包的傳輸路徑以反映當前網(wǎng)絡(luò)的負載情況。</p><p><b>　　傳輸層</b></p><p

66、>　　傳輸層的基本功能對話層接收數(shù)據(jù)，如果必要的話，將其切分成較小的單元，再傳輸?shù)骄W(wǎng)絡(luò)層，并確保這些數(shù)據(jù)片全部正確到達另一端。</p><p>　　這一層是真正的端對端層，換句話說，源機器上的程序和目標機器上相似的程序使用</p><p>　　信息頭和控制信息進行對話。</p><p><b>　　對話層</b></p>&

67、lt;p>　　在對話層用戶必須協(xié)商建立與另一機器上一個進程的連接，這一連接通常稱為對話。一個對話可能允許用戶登錄到一個遠程分進系統(tǒng)上，或者在兩個機器間傳輸文件。在兩個進程之間建立對話的操作通常稱為聯(lián)結(jié)。</p><p>　　對話層的加一個功能是在已建立對話之后對其進行管理。</p><p><b>　　表示層</b></p><p>　　

68、表示層可以接收ASCII碼字符串，并產(chǎn)生壓縮了比特模式作為輸出，表示層的這一功能稱為文本壓縮。</p><p>　　另外，這一層還可以執(zhí)行其它的變換。為提供安全而進行加密是一種可能性，字符間的轉(zhuǎn)換，例如ASCII碼到EBCDIC碼，經(jīng)常是有用的。更一般的，不同的計算機通常有不兼容的文件格式，因此文件轉(zhuǎn)換有時是有用的。</p><p><b>　　應(yīng)用層</b><