1、毕业设计（论文）文献翻译 英文资料 ： Computer Networks and Database 1. Networks Some 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 comput
2、ers 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 speaking ,this goal is to make all programs ,data ,and other resources available to anyone on the netw
3、ork without 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
4、the service 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 fo
5、r people sitting 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 aid
6、ed education 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 mis
7、sed when they 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 (2) Broadcast channels In the first o
8、ne ,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 in
9、termediate 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 des
10、ign 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 Re
11、ference Model The 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 sho
12、uld perform 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
13、 so that distinct 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 Layer The physical layer is concerned with transmitting raw bits over a communication channel .Typical questions here are how many volt
14、s 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 connection and what kind of function each pin has .The design issues here largely deal with mechanical ,electri
15、cal and procedural interfacing to the subnet . 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
16、 frames ,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 .
17、This 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 mech
18、anism must be employed to let the transmitter know how much buffer space the receiver has at the moment . The network layer 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 exchanged in t
19、his layer ,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 ,eit
20、her are “ 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 nec
21、essary ,pass 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 heade
22、r and control 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 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 ASC strings as input and produ
24、ce 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 ASC to EBCDIC,might often be useful
25、.More 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 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 .
