第 1 页 共 20 页英文资料ComputerComputer NetworksNetworks andand DatabaseDatabase 1.1. NetworksNetworks 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 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 speaking ,this goal is to make all programs ,data ,and other resources available to anyone on the network 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 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 . ApplicationApplication ofof NetworksNetworks One of the 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 ,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 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 missed when they come back .International contacts by human begin may be greatly enhanced by network based communication facilities . NetworkNetwork StructureStructure . . 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 channels 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 other 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 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 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 . TheThe ISOISO ReferenceReference ModelModel 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 should 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 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 . TheThe PhysicalPhysical LayerLayer The physical layer is concerned with transmitting raw bits over a communication channel .Typical questions 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 connection and what kind of function each pin has .The design issues here largely deal with mechanical ,electrical and procedural 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 frames ,transmitting the frames sequentially ,and processing the acknowledgment frames sent