附录1英文及其译文Computer NetworksNetwork GoalsSome 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 NetworksOne of the main areas of potential network use is access to remote data bases. It may someday be easy for people sitting at their terminalsat 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 StructureIn 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 channelsIn 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 itcan 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 HierarchiesTo 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 LayersOne 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-duplexcommunicative?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 ModelThe 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 LayerThe 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 networkconnector 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 LayerThe 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 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 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.The Transport LayerThe 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 LayerWith 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 LayerThe 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：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。