1、第 1 页 共 20 页英文资料Computer Networks and Database1. NetworksSome reasons are causing centralized computer systems to give way to networks.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
2、 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 speaking ,this goal is to make all programs ,data ,and other resources available to anyone on the network wit
3、hout regard to the physical location of the resource and the user .The second one is to provide high reliability by having alternative 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 serv
4、ice is restored .Yet another reason of setting up a computer network is computer network can provide a powerful communication medium among widely separated people .Application of Networks One of the main areas of potential network sue is access to remote database .It may someday be easy for people s
5、itting at their terminals at home to make reservations for airplanes trains , buses , boats , restaurants ,theaters ,hotels ,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 educati
6、on is another possible field for using network ,with many different courses being offered.Teleconferencing is a whole new form communication. With it widely separated people can conduct a meeting by typing messages at their terminals .Attendees may leave at will and find out what they missed when th
7、ey come back .International contacts by human begin may be greatly enhanced by network based communication facilities .Network Structure .Broadly speaking,there are two general types of designs for the communication subnet:(1) Point to point channels第 2 页 共 20 页(2) Broadcast channelsIn the first one
8、 ,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 other nodes .When a message is sent from node to another via one or more inter mediate modes ,each inte
9、rmediate node will receive the message and store it until the required output line is free so that it can transmit the message forward .The subnet using this principle is called a point to piont or store and forward subnet .When a point to point subnet is used ,the important problem is how to design
10、 the connected topology between the nodes .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 are received by all other nodes .The ISO Referen
11、ce ModelThe Reference Model of Open System Interconnection (OSI),as OSI calls it ,has seven layers .The major ones of the principles ,from which OSI applied to get the seven layers ,are as follows:(1)A layer should be created where a different level of abstraction is needed.(2)Each layer should perf
12、orm a well defined function .(3) The function of each layer should be chosen with an eye toward defining internationally standardized protocols.(4) The layer boundaries should be chosen to minimize the information flow across the interfaces .(5) The number of layers should be large enough so that di
13、stinct need not be put together in the same layer without necessity ,and small enough so that the architecture will not become out control .The Physical LayerThe physical layer is concerned with transmitting raw bits over a communication channel .Typical questions here are how many volts shoule be u
14、sed 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 connection and what kind of function each pin has .The design issues here largely deal with mechanical ,electrical and proce
15、dural interfacing to the subnet .第 3 页 共 20 页The data link layer 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 data up into data f
16、rames ,transmitting the frames sequentially ,and processing the acknowledgment frames sent back the receiver .Since the physical layer merely 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 .Thi
17、s can be accomplished by attaching special bits patterns to the beginning and the end of the frame .But 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 mechani
18、sm must be employed to let the transmitter know how much buffer space the receiver has at the moment .The network layerThe network layer controls the operation of subnet .It determines the chief characteristics of the node-host interface ,and how packets ,the units of information exchanged in this l
19、ayer ,are routed within the subnet .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 destination .The key design issue is how the route is determined .It could not only base on static table ,either ar
20、e “wired into”the network and rarely changed ,by also adopt highly dynamic manner ,which can determine packet again to reflect the current network load .The transport layer The basic function of transport layer is to accept data from the session layer ,split it up into smaller units ,if necessary ,p
21、ass these to the network layer ,and ensure that the pieces all arrive correctly at the other end .This layer is a true end-to-end layer .In other words ,a program on the source machine carries on a convene station with as similar program on the destination machine , using the message header and cont
22、rol messages .The session layer 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 user to log into a 第 4 页 共 20 页remote time-sharing system or to transfer a file
23、 between two machines .The operation of setting up a session between two processes is often called binding .Another function of the session layer is to manage the session once it has been setup .The presentation layer The presentation layer could be designed to accept ASCstrings as input and produce
24、 compressed bit patterns as output .This function of the presentation layer is called text compression .In addition ,this layer can also perform other trans formations .Encryption provide security is one possibility .Conversion between character codes ,such as ASCto EBCDIC,might often be useful .Mor
25、e generally ,different computers usually have incompatible file formats ,so a file conversion option might be useful at times .The application layer Many issues occur here .For example ,all the issues of network transparency ,hiding the physical distribution of resources from user .Another issue is
26、problem partitioning :how to divide the problem among the various machine in order to take maximum advantage of the network .2. Database system The conception used for describing files and databases has varied substantially in the same organization .A database may be defined as a collection of inter
27、related data stored together with as little redundancy as possible to serve one or more applications in an optimal 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 str
28、ucture .A database may be designed for batch processing ,real-time processing ,or in-line processing .A database system involve application program ,DBMS ,and database. One of the most important characteristics of most databases is that they will constantly need to change and grow .Easy restructurin
29、g of the database must be possible as new data types and new applications are added .The restructuring should be possible without 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 第 5
30、 页 共 20 页effect on the rate at which data-processing can be developed in a corporation .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 independent so that either may be changed wit
31、hout 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 .To a large extent ,data-base organization is concerned with the representation between the data item about which we store
32、 information referred to as entities .An entity may be a tangible object or nontangible .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
33、he attributes ,one attribute has a special significance in that it identifies the entity .An attribute or set of attributes which the computer uses to identify a record or tuple is referred to as a key .The primary key is defined as that key used to uniquely identify one record or tuple .The entity
34、identifier consisting of one or more attributes .The primary key is of great importance because it is used by the computer in locating the record or tuple by means of an index or addressing algorithm .If the function of a data base were merely to store data ,its organization would be simple .Most of
35、 the complexities arise from 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 .The logical data base description is referred to as a schema .A schema is a chart of the types of data that o
36、ne used .It gives the entities and attributes ,and specifics the relations between them .It is formwork into which the values of the data-items can be fitted .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
37、 .There are no data vales associated with it .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 第 6 页 共 20 页an application programmers view of the data he uses .Many different sub schemas can be derived from
38、 one schema .The schema and the subschema are both used by the data-base management system ,the primary function of which is to serve the application programs by executing their data operations .A DBMS will usually be handing multiple data calls concurrently .It must organize its system buffers so t
39、hat different data 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 schema .The data definition language is a high-level language ,
40、enabling one to describe the conceptual schema in terms of a “data model”.The choice of a data model is a difficult one ,since it must be rich enough in structure to describe significant aspects of the real world ,yet it must be possible to determine fairly automatically an efficient implementation
41、of the conceptual schema by a physical schema .It should 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 .We will discuss the data model in the following ASP.NET and the .NET Frame
42、workASP.NET is part of Microsofts overall .NET framework, which contains a vast set of programming classes designed to satisfy 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
43、ASP.NET pages.The .NET Framework Class LibraryImagine that you are Microsoft. Imagine that you have to support multiple programming languagessuch as Visual Basic, JScript, and C+. A great deal of the functionality of these programming languages overlaps. For example, for each language, you would hav
44、e to include methods for 第 7 页 共 20 页accessing the file system, working with databases, and manipulating strings.Furthermore, these languages contain similar programming constructs. Every language, for example, can represent loops and conditionals. Even though the syntax of a conditional written in
45、Visual Basic differs from the syntax of a conditional written in C+, the programming function is the same.Finally, most programming languages have similar variable data types. In most languages, you have some means of representing strings and integers, for example. The maximum and minimum size of an
46、 integer might depend on the language, but the basic data type is the same.Maintaining all this functionality for multiple languages requires a lot of work. Why keep reinventing the wheel? Wouldnt it be easier to create all this functionality once and use it for every language?The .NET Framework Cla
47、ss Library does exactly that. 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, and generating graphics. In addition, it contains m
48、ore specialized classes for performing tasks such as working with regular expressions and handling network protocols.The .NET framework, furthermore, contains classes that represent all the basic variable data types such as strings, integers, bytes, characters, and arrays.Most importantly, for purpo
49、ses of this book, the .NET Framework Class Library contains classes for building ASP.NET pages. You need to 第 8 页 共 20 页understand, however, that you can access any of the .NET framework classes when you are building your ASP.NET pages.Understanding NamespacesAs you might guess, the .NET framework is huge. It contains
