外文资料(3)

through the cellular telephone system. Present in many cities abroad have been able to provide cellular digital packet data (Cellular Digital Packet Data, CDPD) business, which can be directly set up a wireless LAN CDPD system. Wireless network is currently a research focus at home and abroad, the research of wireless networks is a huge market demand-driven. Characteristics of the wireless network is to enable users at any time, anywhere access to computer networks, and this feature make it a powerful

application. At present there have been many products based on wireless networks, such as personal communications system (Personal CommunicationS ystem, PCS) phones, wireless data terminals,

portable video phone, personal digital assistants (PDA), etc. The development of wireless networks rely on wireless communication technology support. Current wireless communication systems are: low-power cordless telephone system, analog cellular systems, digital cellular systems, mobile satellite systems, wireless LAN and wireless WA N so.

Network architecture computer network

To get two computers to communicate, to make them use the same rules for information

exchange. We provided in the computer information network for the format and how to send and receive information set of rules known as Internet Protocol (network protocol) or communications protocol (communication protocol).

To reduce the complexity of network protocol design, network designers do not design a single, large of an agreement to all forms of communication for the whole of the provisions of the details, instead of using the communication problem into many small problems, then to each small question design method of a separate agreement. This allows each protocol design, analysis, coding and testing are relatively easy. Hierarchical model (layering model) is a network protocol for the development of the design method. In essence, the hierarchical model describes the communication problem is divided into several small problems (called level) approach, each corresponding to a layer of small problems.

1, protocol layer

To reduce the complexity of network design, most network stratified design. The so-called hierarchical design method, that is the flow of information process in accordance with the network's overall function is decomposed into functional layers one by one, the same function on different

machines using the same protocol layer, the same machine by adjacent functional layers messaging interface. In order to facilitate understanding of the concept of interfaces and protocols, we first postal system as an example to explain. When ordinary people to write, have a contract, that is, the format and content of the letter. First, we must use both knew how to write the language and style, starts with the other titles, the last is dated and so on. In this way, the other received the letter, we can understand the contents of the letter, know who wrote it, when so written. Of course, there may be some other special terms, such as letters of the code, the secret spying and writing. Written the letter, the letter must be packaged and sent by post, so the return and the post office should also have agreed that this is the wording of the provisions of the envelope and stamp. Send a letter must be written in the Chinese

recipient address, name, and then write the return address and name. Post office received the letter, the first for letters of sorting and classification, and delivery of the transport department for transport, such as air mail delivery of civil aviation, surface mail by rail or road transport departments, such as when written. Of course, there may be some other special terms, such as letters of the code, the secret spying and writing. Written the letter, the letter must be packaged and sent by post, so the return and the post office should also have agreed that this is the wording of the provisions of the envelope and stamp. Send a letter must be written in the Chinese recipient address, name, and then write the return address and name. Then, post offices and transport sectors have agreed, such as arrival location, time, format, etc. parcels. Correspondence delivered to the destination for the opposite process, the final letter sent to the recipients, the recipient can read in accordance with the format of the letter agreement. . Throughout the process, mainly related to the three subsystems, namely, the user subsystem, postal and transport subsystem subsystem. That agreements are to reach the letter sent from a source point to a certain

purpose designed with this objective, which means that they are due to the flow of information generated. These conventions can be divided into equal inter-agency agreement, as agreed between users, the agreement between the Post Office and the agreement between the transport sector, and the agreement between different agencies, such as the agreement between the user and post offices, post offices and agreement between the transport sector. Although the two users, two post offices, two transport sector branches A and B in two places, but they all correspond to the same institutions, belong to one

subsystem; the same office location of different institutions are not in a subsystem, and the relationship between them is the relationship between service and being served. Obviously, these two conventions are different, the former agreement within the department, which is the agreement between the different departments.

In the computer network environment, the two computers to communicate between two processes in the process and the process is very similar to postal communications. User process corresponds to the user, the computer to communicate with the process (can also be a special

communication processor〕 corresponding post office, communication facilities corresponding to the transport sector. In order to reduce the complexity of computer network design, often divided by function, the computer network layer for several different functions. Network communication between the rules of the same layer is the layer protocol used, such as the first N level set of communication rules, the

agreement is the first N layers. The same computer for communication between different functional layer called the interface, rules (interface), in the first N layers and (N + 1) layer as the interface between the N

/ (N + 1) layer interfaces. Overall, the agreement is between the different machines the same level of communication agreement, the interface is the same machine communication between adjacent layers agreement. Different networks, hierarchical number, name and function of each layer and the protocols are different. However, all the networks, the purpose of each layer to its upper layer is to provide a service. Agreement is different from the procedures hierarchical modular design concept. In program design, the module can be independent of each other, any assembly or in parallel, while the level is necessarily divided from top to bottom, it is according to the flow of data streams generated. Composed of different computers on the same level as other physical processes (peer process). Not necessarily on the other process is the same procedure, but its function must be exactly the same, and use the same protocol. Hierarchical design method to the entire network communication functions into a collection of vertical levels, the lower layer in the communication process will lower the upper hidden implementation details. But the level of division should first determine the level of collection and the task should be completed on each floor. Logic combination function should be separated and have sufficient levels to enable small-to easily handle each floor. Same level can not be too difficult to avoid the burden of processing overhead. Computer network architecture is hierarchical network model and the precise

definition of functions of each. Description of the network architecture must include enough information to make realization of those functions for each layer of hardware design or programming, and consistent with relevant agreements. However, we should note that the details of network protocols are not the contents of network architecture, because they are hidden inside the machine to the outside that is not visible.