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毕业设计论文
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JSJ06-107@VB0门诊管理系统,毕业设计论文
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附录 1 英文及其译文 Computer Networks Network Goals 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, 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 resources available to anyone on the network without regard to the physical location of the resource and the user. The second one is to provider 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. Another important reason for distributing computing power has to 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 back to the computer center, to reduce the communication cost, which now represents a larger percentage of the total cost than it used to. Yet another reason of setting up a computer network is that a computer network can provider a powerful communication medium among widely separated people. Application of Networks One of the main areas of potential network use is access to remote data bases. It may someday be easy for people sitting at their terminals ntsat home to make reservations for airplanes, trains, bused, 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 coursed being offered. 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 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. Network Structure 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 carry messages from host to host. 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 transmission lines. Broadly speaking, there are two general types of designs for the communication subnet: (1)Point-to-point channels (2Broadcast channels 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 indirectly 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 it ntscan transmit the message forward. The subnet using this principle is called a point-to-point or store-and-forward subnet. When a point-to-point subset is used, the important problem is how to design the interconnected 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 system is that messages sent by any node are received by all other nodes. Protocol Hierarchies To reduce their design complexity, most networks are organized as a series of layers or levels, each one 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 reality, 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 to the following layer immediately, until the lowest layer is reached. At the lowest layer there is one physical communication with the other machine, as opposed to the virtual communication used by the higher layers. Between each pair of adjacent layers there is an interface. The interface defines which primitive operations and services the lower offers to the upper one. The set of layers and protocols is called network architecture. Design Issues for the Layers One set of design decisions are the rules for data transfer. Do data only travel in one direction, called simplex communication, or travel in either direction, but not simultaneously, called half-duplex communication, or travel in both directions at once, call full-duplex ntscommunicative? Error control is an important issue when the physical 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 way of telling the sender which messages have been correctly received and which has not. When there are multiple paths between source and destination, at some points in the hierarchy, a routing decision must be made. The ISO Reference Model The Reference Model of Open System Interconnection (OSI), as ISO calls it, has seven layers. The major ones of the principles, from which ISO applied to get the seven layers, are as follows: (1) A layer should be created where a different level of abstractions 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 functions need not be put together in the same layer without necessity, and small enough so that the architecture will not become out of control. The Physical Layer The physical layer is concerned with transmitting raw bits over a communication channel. 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 the initial connection and how to tear town the connection when both sides are finished, how many pins the network ntsconnector has and what kind of function each pin has. The design issues here largely deal with mechanical, electrical 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 breaking the input data up into data frames, transmitting the frames sequentially, and processing the acknowledgment frames sent back by 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. This can be accomplished by attaching special bits patterns to the 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 other is that some mechanism must be employed to let the transmitter know 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 this layer, are routed within the subnet. What this layer of software does, basically, is to accept messages from the source host, convert them to packets, 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 else adopt highly dynamic manner, which can determine each packet again to reflect the current network load. ntsThe Transport Layer The basic function of transport layer is to accept 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. This layer is a true end-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. 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 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 set up. The Presentation Layer The presentation layer could be designed to accept ASCII strings as input and produce compressed bit patterns as output. This function of the presentation layer is called text compression. In addition, this layer can also perform other transformations. Encryption to provide security is 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 at times. 网络目标： nts 第一是许多机构已拥有大量正在工作的计算机，这 些计算机通常相距较远。起先，每台机器可能独立工作，但是在某些时候使整个机构的信息能够相互关联。一般来说，这上目标是使所有程序、数据和其它资源能够被网络上的任何人所获取，而不必考虑资源和用户的物理位置。 第二个原因是通过可选资源提供更高的可靠性。在网络中，单个计算机临时的损失无关紧要，因为它的用户可以从别处获取信息，直到该计算机恢复功能。 将计算能力分散开的另一个重要原因和计算与通讯的价格比有关。现在， 一台小计算机的价格是微不足道的，因此，在获取数据的地方分析数据，并且仅把临时的概要传回计算机中 心，这种做法是有吸引力的，这样做可以降低通讯费用，通讯费用占费用的比例比以往要高。 建立计算机网络的另一个原因是计算机网络能在广泛分布的人们之间提供强有力的通讯介质。 网络的应用 潜在的网络应用的主要领域之一是访问远程数据库。可能有一天人们可以坐在家中的终端旁，方便地预定飞机、火车、汽车、般、饭店、剧院、旅馆等的位置或床位，无论是在世界上的什么地方都可以立即得到回复。家庭银行、自动报纸和全自动图书馆也在此列。 计算机辅助教育是另一个可能利用网络的地方，其中提供了许多不同的教程。 电话会议是一个全新的通讯形 式，通过这种形式距离很远的人们可以通过在他的终端上键入信息召开一次会议。参加者可以随意离开，并可以在回来的时候发现刚才漏掉的信息。基于网络的通讯设备将大幅度增强人类之间的国际接触。 网络结构 在任何网络中都存在着一批用以运行用户（即应用）等程序的机器，这些机器被称 为主机。主机通过通信子网相互联接，子网的任务是把信息从一个主机传输到另一个主机。 nts 子网由两个基本部分组成：节点和通道。节点是特殊的计算机。所有进出主机制信息均要通过它的节点，通道是传输线。 一般而言，有两种通用的子网的设计方案： （ 1） 点对点通道 （ 2） 广播式通道 在第一种子网中，网络包含大量电缆或是租用的电话线，每条线连接一对节点。如果两个节点之间没有电缆却相互通信，它们必须通过其它节点间接地达到目的。当信息从一个节点，通过一个多个节点，发向另一个节点的时候，每个中间节点都接收信息，存储信息，直到所需的输出线空闲，再把信息向前传送。遵从这一原则的子网称为点对点或存储传发子网。 当采用点对点子网时，一个重要的问题是怎样设计节点相互联接的拓扑结构。 第二种通讯结构使用广播方式。在这种设计中所有节点共享单一通信通道。广播系统的基本是由任一节点发送 的信息均能被其它所有节点接收。 协议层次 为了减少设计的复杂性，大多数网络被组织成一系列层次或等级，每一层次都建筑在前一层次的基础之上。 一台机器上的第 n 层与另一台机器上的第 n 层协议。实际中，数据并非直接从一台机器的第 n 层转换到另一台机器的第 n 层（除了最低层以外），而是每层都把数据和控制信息传到下面一层，直至最低层。在最低层存在与其它机器的物理通讯，这是和高层中的虚拟通讯相对而言的。 在每对相邻层次之间有一个接口，接口定义了低层提供给高层的原始操作和服务。 层次与协议的集合被称为网结构。 层次的设计问题 设计中需要决定的一部分内容是数据传输的规则。数据沿一个方向传播（称单工通信），还是可以在不同时刻不同方向传播（称半双工通信），还是可以同时沿两个方向传播（称全双工通信）？ 当物理通信线路不很完善的时候，错误控制是一个重要的问题。许多错误检测和错误纠正码是人们知道的，但连线的两端必须商定当前使用哪一个代码。另nts外，接收者必须有办法告诉发送者哪些信息已被正确接收，哪些信息未被正确接收。 当源节点与目的节点间存在多通道时，在层次中的一些点上必须进行路径规划。 ISO 参考模型 被 ISO 称为开放系统互连（ OS
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