外文翻譯---計算機網(wǎng)絡(luò)和數(shù)據(jù)庫

1、<p>　　畢業(yè)設(shè)計（論文）文獻翻譯</p><p><b>　　英文資料：</b></p><p>　　Computer Networks and Database</p><p><b>　　Networks</b></p><p>　　Some reasons are causing

2、 centralized computer systems to give way to networks.</p><p>　　The first one is that many organizations already have a substantial number of computers in operation ,often located far apart .Initially ,each

3、of these computers may have worked in isolation from the other ones ,but at a certain time ,management may have decided to connect them to be able to correlate information about the entire organization .Generally speaki

4、ng ,this goal is to make all programs ,data ,and other resources available to anyone on the network without regard to the physical locati</p><p>　　The second one is to provide high reliability by having alte

5、rnative sources of supply .With a network ,the temporary loss of a single computer is much less serious ,because its users can often be accommodated elsewhere until the service is restored .</p><p>　　Yet ano

6、ther reason of setting up a computer network is computer network can provide a powerful communication medium among widely separated people .</p><p>　　Application of Networks </p><p>　　One of the

7、 main areas of potential network sue is access to remote database .It may someday be easy for people sitting at their terminals at home to make reservations for airplanes trains , buses , boats , restaurants ,theaters ,h

8、otels ,and so on ,at anywhere in the world with instant confirmation .Home banking ,automated newspaper and fully automated library also fall in this category .Computer aided education is another possible field for using

9、 network ,with many different courses being offere</p><p>　　Teleconferencing is a whole new form communication. With it widely separated people can conduct a meeting by typing messages at their terminals .At

10、tendees may leave at will and find out what they missed when they come back .International contacts by human begin may be greatly enhanced by network based communication facilities .</p><p>　　Network Structu

11、re </p><p>　　Broadly speaking,there are two general types of designs for the communication subnet:</p><p>　　Point –to –point channels</p><p>　　Broadcast channels</p><p>

12、;　　In the first one ,the network contains numerous cables or lesased 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 indirectly via othe

13、r nodes .When a message is sent from node to another via one or more inter mediate modes ,each intermediate node will receive the message and store it until the required output line is free so that it can transmit the me

14、ssage forward .The subnet using this principle is called a poin</p><p>　　When a point –to –point subnet is used ,the important problem is how to design the connected topology between the nodes .</p>&

15、lt;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 systems is that messages sent by any node a

16、re received by all other nodes .</p><p>　　The ISO Reference Model</p><p>　　The Reference Model of Open System Interconnection (OSI),as OSI calls it ,has seven layers .The major ones of the princ

17、iples ,from which OSI applied to get the seven layers ,are as follows:</p><p>　　(1)A layer should be created where a different level of abstraction is needed.</p><p>　　(2)Each layer should perfo

18、rm a well defined function .</p><p>　　The function of each layer should be chosen with an eye toward defining internationally standardized protocols.</p><p>　　The layer boundaries should be chos

19、en to minimize the information flow across the interfaces .</p><p>　　The number of layers should be large enough so that distinct need not be put together in the same layer without necessity ,and small enoug

20、h so that the architecture will not become out control .</p><p>　　The Physical Layer</p><p>　　The physical layer is concerned with transmitting raw bits over a communication channel .Typical que

21、stions here are how many volts shoule be used to represent an 1 and how many a 0,how many microseconds a bit occupies ,whether transmission may proceed simultaneously in both are finished ,how to establish the initial co

22、nnection and what kind of function each pin has .The design issues here largely deal with mechanical ,electrical and procedural interfacing to the subnet .</p><p>　　The data link layer </p><p>　

23、　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 accomplishes this task by breading the input dat

24、a up into data frames ,transmitting the frames sequentially ,and processing the acknowledgment frames sent back the receiver .</p><p>　　Since the physical layer merely accepts and transmits a stream of bits

25、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 the beginning and the end of the frame .B

26、ut it produce two problems :one is a noise burst on the line can destroy a frame completely .In this case ,the software in the source machine must retransmit the frame .The other is that some mechanism must be</p>

27、<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 packets ,the units of information exchan

28、ged in this layer ,are routed within the subnet .</p><p>　　What this layer if software does ,basically ,is to accept messages from the source host ,convert them to packets ,and observe the packets to get the

29、 destination .The key design issue is how the route is determined .It could not only base on static table ,either are “wired into”the network and rarely changed ,by also adopt highly dynamic manner ,which can determine p

30、acket again to reflect the current network load .</p><p>　　The transport layer </p><p>　　The basic function of transport layer is to accept data from the session layer ,split it up into smaller

31、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 end-to-end layer .In other words ,a program on the sourc

32、e machine carries on a convene station with as similar program on the destination machine , using the message header and control messages .</p><p>　　The session layer </p><p>　　With the session

33、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 user to log into a remote time-sharing system or

34、 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 session layer is to manage the session once it

35、 has been setup .</p><p>　　The presentation layer </p><p>　　The presentation layer could be designed to accept ASCⅡstrings as input and produce compressed bit patterns as output .This function o

36、f the presentation layer is called text compression .</p><p>　　In addition ,this layer can also perform other trans formations .Encryption provide security is one possibility .Conversion between character co

37、des ,such as ASCⅡto EBCDIC,might often be useful .More generally ,different computers usually have incompatible file formats ,so a file conversion option might be useful at times .</p><p>　　The application l

38、ayer </p><p>　　Many issues occur here .For example ,all the issues of network transparency ,hiding the physical distribution of resources from user .Another issue is problem partitioning :how to divide the p

39、roblem among the various machine in order to take maximum advantage of the network .</p><p>　　Database system </p><p>　　The conception used for describing files and databases has varied substant

40、ially in the same organization .</p><p>　　A database may be defined as a collection of interrelated data stored together with as little redundancy as possible to serve one or more applications in an optimal

41、fashion ;the data are stored so that they are independent of programs which use the data ;a common and retrieving existing data within the databases if they are entirely separate in structure .</p><p>　　A da

42、tabase may be designed for batch processing ,real-time processing ,or in-line processing .A database system involve application program ,DBMS ,and database. </p><p>　　One of the most important characteristic

43、s of most databases is that they will constantly need to change and grow .Easy restructuring of the database must be possible as new data types and new applications are added .The restructuring should be possible without

44、 having to rewrite the ap0plication program and in general should cause as little upheaval as possible .The ease with which a database can be changed will have a major effect on the rate at which data-processing can be d

45、eveloped in a corpor</p><p>　　The tem data independence is often quoted as being one of the main attributes of a data base .It implies that the data and the application programs which use them are independen

46、t so that either may be changed without changing the other .When a single set of data items serves a variety of applications ,different application programs perceive different relationships between the data items .</p

47、><p>　　To a large extent ,data-base organization is concerned with the representation between the data item about which we store information referred to as entities .An entity may be a tangible object or nont

48、angible .It has various properties which we may wish to record .It can describes the real world .The data item represents an attribute ,and the attribute must be associated with the relevant entity .We design values to t

49、he attributes ,one attribute has a special significance in that it identifies</p><p>　　An attribute or set of attributes which the computer uses to identify a record or tuple is referred to as a key .The pri

50、mary key is defined as that key used to uniquely identify one record or tuple .The entity identifier consisting of one or more attributes .The primary key is of great importance because it is used by the computer in loca

51、ting the record or tuple by means of an index or addressing algorithm .</p><p>　　If the function of a data base were merely to store data ,its organization would be simple .Most of the complexities arise fro

52、m the fact that is must also show the relationships between the various items of data that are stored .It is different to describe the data in logical or physical .</p><p>　　The logical data base description

53、 is referred to as a schema .</p><p>　　A schema is a chart of the types of data that one used .It gives the entities and attributes ,and specifics the relations between them .It is formwork into which the va

54、lues of the data-items can be fitted .</p><p>　　We must distinguish between a record type and a instance of the record .When we talk about a “personnel record”,this is really a record typed .There are no dat

55、a vales associated with it .</p><p>　　The term schema is used to mean an overall chart of all of the data-types and record types stored in a data base .The term subschema refers to an application programmer’

56、s view of the data he uses .Many different sub schemas can be derived from one schema .</p><p>　　The schema and the subschema are both used by the data-base management system ,the primary function of which i

57、s to serve the application programs by executing their data operations .</p><p>　　A DBMS will usually be handing multiple data calls concurrently .It must organize its system buffers so that different data

58、operations can be in process together .It provides a data definition language to specify the conceptual schema and most likely ,some of the details regarding the implementation of the conceptual schema by the physical sc

59、hema .The data definition language is a high-level language ,enabling one to describe the conceptual schema in terms of a “data model”.</p><p>　　The choice of a data model is a difficult one ,since it must b

60、e rich enough in structure to describe significant aspects of the real world ,yet it must be possible to determine fairly automatically an efficient implementation of the conceptual schema by a physical schema .It should

61、 be emphasized that while a DBMS might be used to build small data bases ,many data bases involve millions of bytes ,and an inefficient implementation can be disastrous .</p><p>　　We will discuss the data mo

62、del in the following </p><p>　　ASP.NET and the .NET Framework</p><p>　　ASP.NET is part of Microsoft's overall .NET framework, which contains a vast set of programming classes designed to sat

63、isfy any conceivable programming need. In the following two sections, you learn how ASP.NET fits within the .NET framework, and you learn about the languages you can use in your ASP.NET pages.</p><p>　　The .

64、NET Framework Class Library</p><p>　　Imagine that you are Microsoft. Imagine that you have to support multiple programming languages—such as Visual Basic, JScript, and C++. A great deal of the functionality

65、of these programming languages overlaps. For example, for each language, you would have to include methods for accessing the file system, working with databases, and manipulating strings.</p><p>　　Furthermor

66、e, these languages contain similar programming constructs. Every language, for example, can represent loops and conditionals. Even though the syntax of a conditional written in Visual Basic differs from the syntax of a c

67、onditional written in C++, the programming function is the same.</p><p>　　Finally, most programming languages have similar variable data types. In most languages, you have some means of representing strings

68、and integers, for example. The maximum and minimum size of an integer might depend on the language, but the basic data type is the same.</p><p>　　Maintaining all this functionality for multiple languages req

69、uires a lot of work. Why keep reinventing the wheel? Wouldn't it be easier to create all this functionality once and use it for every language?</p><p>　　The .NET Framework Class Library does exactly that

70、. It consists of a vast set of classes designed to satisfy any conceivable programming need. For example, the .NET framework contains classes for handling database access, working with the file system, manipulating text,

71、 and generating graphics. In addition, it contains more specialized classes for performing tasks such as working with regular expressions and handling network protocols.</p><p>　　The .NET framework, furtherm

72、ore, contains classes that represent all the basic variable data types such as strings, integers, bytes, characters, and arrays.</p><p>　　Most importantly, for purposes of this book, the .NET Framework Class

73、 Library contains classes for building ASP.NET pages. You need to understand, however, that you can access any of the .NET framework classes when you are building your ASP.NET pages.</p><p>　　Understanding N

74、amespaces</p><p>　　As you might guess, the .NET framework is huge. It contains thousands of classes (over 3,400). Fortunately, the classes are not simply jumbled together. The classes of the .NET framework a

75、re organized into a hierarchy of namespaces.</p><p>　　ASP Classic Note</p><p>　　In previous versions of Active Server Pages, you had access to only five standard classes (the Response, Request,

76、Session, Application, and Server objects). ASP.NET, in contrast, provides you with access to over 3,400 classes!</p><p>　　A namespace is a logical grouping of classes. For example, all the classes that relat

77、e to working with the file system are gathered together into the System.IO namespace.</p><p>　　The namespaces are organized into a hierarchy (a logical tree). At the root of the tree is the System namespace.

78、 This namespace contains all the classes for the base data types, such as strings and arrays. It also contains classes for working with random numbers and dates and times.</p><p>　　You can uniquely identify

79、any class in the .NET framework by using the full namespace of the class. For example, to uniquely refer to the class that represents a file system file (the File class), you would use the following:</p><p>

80、　　System.IO.File</p><p>　　System.IO refers to the namespace, and File refers to the particular class.</p><p><b>　　NOTE</b></p><p>　　You can view all the namespaces of th

81、e standard classes in the .NET Framework Class Library by viewing the Reference Documentation for the .NET Framework.</p><p>　　Standard ASP.NET Namespaces</p><p>　　The classes contained in a sel

82、ect number of namespaces are available in your ASP.NET pages by default. (You must explicitly import other namespaces.) These default namespaces contain classes that you use most often in your ASP.NET applications:</p

83、><p>　　?System— Contains all the base data types and other useful classes such as those related to generating random numbers and working with dates and times.</p><p>　　?System.Collections— Contai

84、ns classes for working with standard collection types such as hash tables, and array lists.</p><p>　　?System.Collections.Specialized— Contains classes that represent specialized collections such as linked l

85、ists and string collections.</p><p>　　?System.Configuration— Contains classes for working with configuration files (Web.config files).</p><p>　　?System.Text— Contains classes for encoding, dec

86、oding, and manipulating the contents of strings.</p><p>　　?System.Text.RegularExpressions— Contains classes for performing regular expression match and replace operations.</p><p>　　?System.Web

87、— Contains the basic classes for working with the World Wide Web, including classes for representing browser requests and server responses.</p><p>　　?System.Web.Caching— Contains classes used for caching th

88、e content of pages and classes for performing custom caching operations.</p><p>　　?System.Web.Security— Contains classes for implementing authentication and authorization such as Forms and Passport authenti

89、cation.</p><p>　　?System.Web.SessionState— Contains classes for implementing session state.</p><p>　　?System.Web.UI— Contains the basic classes used in building the user interface of ASP.NET p

90、ages.</p><p>　　?System.Web.UI.HTMLControls— Contains the classes for the HTML controls.</p><p>　　?System.Web.UI.WebControls— Contains the classes for the Web controls.</p><p>　　.N

91、ET Framework-Compatible Languages</p><p>　　For purposes of this book, you will write the application logic for your ASP.NET pages using Visual Basic as your programming language. It is the default language f

92、or ASP.NET pages (and the most popular programming language in the world). Although you stick to Visual Basic in this book, you also need to understand that you can create ASP.NET pages by using any language that support

93、s the .NET Common Language Runtime. Out of the box, this includes C# (pronounced See Sharp), JScript.NET (the .NET v</p><p><b>　　NOTE</b></p><p>　　The CD included with this book cont

94、ains C# versions of all the code samples.</p><p>　　Dozens of other languages created by companies other than Microsoft have been developed to work with the .NET framework. Some examples of these other langua

95、ges include Python, SmallTalk, Eiffel, and COBOL. This means that you could, if you really wanted to, write ASP.NET pages using COBOL.</p><p>　　Regardless of the language that you use to develop your ASP.NET

96、 pages, you need to understand that ASP.NET pages are compiled before they are executed. This means that ASP.NET pages can execute very quickly.</p><p>　　The first time you request an ASP.NET page, the page

97、is compiled into a .NET class, and the resulting class file is saved beneath a special directory on your server named Temporary ASP.NET Files. For each and every ASP.NET page, a corresponding class file appears in the Te

98、mporary ASP.NET Files directory. Whenever you request the same ASP.NET page in the future, the corresponding class file is executed.</p><p>　　When an ASP.NET page is compiled, it is not compiled directly int

99、o machine code. Instead, it is compiled into an intermediate-level language called Microsoft Intermediate Language (MSIL). All .NET-compatible languages are compiled into this intermediate language.</p><p>　

100、　An ASP.NET page isn't compiled into native machine code until it is actually requested by a browser. At that point, the class file contained in the Temporary ASP.NET Files directory is compiled with the .NET framewo