Computer Network Technology and Application

Chapter 3 A Brief Introduction to Computer Networks (1)
Computer networks are not new to us through the use of the Internet, campus networks, and community networks, but what exactly is a computer network (referred to as a network), what it can do for us, and what types of networks are there... These are not all People can answer right away.

In this chapter, we will give concise answers to each of them.

【Main content of this chapter】

Definition of computer network.

protocol and architecture.

Classification of computer networks.

1.1 Definition of Computer Network
First we will give an example of a network to illustrate its functionality and define the concept of a network.

Teachers and students can use this campus network to share resources on the Internet, access the Internet, send emails, communicate with chat software, and interact on the learning platform.

Computer network is the product of the development and combination of computer technology and communication technology. It is "a system that uses communication lines to connect computers distributed in different locations, and uses communication protocol software to achieve resource sharing."

In this definition, the following three main points should be mastered.

First, computers in different locations, including computers that are not in the same location.For example, 3 computers in a home, or computers in a school laboratory, classroom, or dormitory.

Second, the communication protocol, which is a collection of certain conventions.Just like the contract that people sign before doing business, there are many clauses written in it, such as how to deliver the goods, how to pay, how to solve quality problems, etc. In the specific implementation process, it is enough to implement the terms of the contract.On a network with a wide variety of devices and complex service functions, there will be more necessary agreed terms, which are collectively called agreements.The agreement is like the rules of the game. Without the rules, the game cannot be played.

For the rules of the network, many organizations, manufacturers and even individuals have designed a large number of protocols, among which the most famous and commonly used protocol is the TCP/IP protocol (this protocol will be introduced in the following chapters).

Third, resource sharing.On the network, there are many data resources, software and hardware resources, and sharing the rich resources with others through the network is one of the main goals of computer network applications.

1.2 Function and classification of computer network

1.2.1 Function
The function of computer network is mainly reflected in the following three aspects, the most basic function of which is to realize resource sharing and data communication.

1. Resource sharing
Resource sharing is one of the main purposes of establishing a computer network.The so-called resources include hardware resources, software resources and data resources.Hardware resources include various models and types of computers and other equipment connected to the network. The sharing of hardware resources can improve the utilization rate of equipment and avoid duplication of investment.For example, using the network to set up a network printer in an office allows one printer to be shared by all computers.The sharing of software resources and data resources can make full use of existing software and data, reduce duplication of labor in the software development process, and avoid duplication of database settings.

2. Data communication
Data communication refers to the use of computer networks to realize the data exchange process between computers in different geographic locations.For example, people send and receive information by email (E-mail), and can also use IP phones for voice communication. This method is widely used by many e-commerce websites to provide services to customers.

3. Centralized/distributed processing
When a computer system in a computer network is overloaded, it can distribute its processing tasks to other computers in the network to improve the utilization of the entire system and balance the load.For large-scale, comprehensive scientific computing and information processing, it is a reasonable and effective method to disperse tasks to different computer systems in the network for distributed processing through appropriate algorithms.For example, it is possible to conduct global scientific research cooperation through computers connected to the Internet, analyze information from space, analyze the structure of viruses such as SARS and bird flu vaccines, and perform disaster forecasting.

In today's highly informationized and globalized society, all walks of life and all corners of the world are constantly generating a large amount of information that needs to be processed in a timely manner. Computer networks have played a very important and necessary role.

1.2.2 Classification
1. Divided according to the geographical range of node distribution
(1) Local Area Network (LAN)

The coverage of the local area network is within ten kilometers.For example, the campus network of a school, the network of a hospital, the network of a community, the internal network of a building, etc.

(2) Wide Area Network (WAN)

The wide area network has the largest geographical range and can span cities, countries, and continents.For example, China Netcom and China Mobile Network are owned by an organization, and the management right is within the organization.Compared with the local area network, the transmission rate is generally relatively low, and the network topology is complex, usually a mesh topology.

(3) Metropolitan Area Network (MAN)

A metropolitan area network usually covers a city, an oil field, or a mine.At present, many oil fields and mines in my country, as well as emerging cities, have established metropolitan area networks.There are also examples of multiple organizations in an industry building a MAN.For example, schools in Beijing, Nanjing, Shanghai and other places have established metropolitan area networks within their respective cities as the regional backbone network of the China Education and Research Network.

2. Classified according to the topology of the network
The "topology" method is a method of designing on paper, no matter where the network devices used by the nodes are, and no matter how far the distance between the devices is, the devices and connections are represented by dots and lines.

Usually we refer to the point and line structure of a network as topology, and its quality directly affects the performance of the network.

(1) Star structure

The star structure means that each workstation is connected into a network in a star manner.

(2) Ring structure

The ring structure consists of several nodes in the network connected end to end through a point-to-point link to form a closed ring. This structure makes the public transmission cable form a ring connection, and data travels between nodes along one direction in the ring. Transmission, the transfer of data from one node to another.

(3) Bus structure

The bus-type structure means that all devices (computers and network devices) are hung on a bus-type transmission line, and the data on the public bus is mostly transmitted in the form of baseband ([-]) serial ([-]), and its transmission direction always starts from the node that sends the information Diffusion to both ends, just like the broadcast broadcast by a radio station, so it is also called a broadcast computer network.

(4) Tree structure

The tree structure refers to a hierarchical centralized control network, similar to the administrative organization of an enterprise.

The above types are suitable for local area networks.

(5) Network structure

In the mesh topology, each device in the network is connected by a point-to-point link. This connection is not economical. This method is only used when each site must send data frequently, so it is usually used in large and complex networks.

3. Classification by Ownership of Networks
By ownership, networks can be divided into public and private networks.The former includes communication networks provided by China Netcom, China Telecom and other service providers; the latter includes networks of organizations such as schools, hospitals, and enterprises.

In general, it is common to classify network types by their geographic scope.

51.3 History and development trend of computer network development
1.3.1 History of Network Development

The emergence of any new technology must meet two conditions: the strong social demand and the maturity of advanced technology.

The formation and development of computer network technology also confirmed this rule.Generally speaking, the development of computer network can be divided into four stages.

The first stage: the combination of computer technology and communication technology forms the prototype of computer network.

The second stage: Based on the computer communication network, complete the research on the network architecture and protocol, and form a computer network.

The third stage: In the context of solving computer networking and network interconnection standardization issues, the open system interconnection reference model and protocol are proposed, which promotes the development of computer network technology that meets international standards.

The fourth stage: the computer network develops towards the direction of interconnection, high speed and intelligence, and is widely used.

In the early days of the computer age, the computer world was ruled by large systems called time-sharing systems.The time-sharing system allowed users to use the mainframe through "dumb" terminals that consisted only of a monitor and keyboard, much like a PC but without a CPU, memory, or hard disk.Through such a terminal, hundreds or even thousands of users can access the mainframe at the same time.Since the time-sharing system divides the time of the host into slices and distributes it to users, the time slice is very short, which will give the user the illusion that the host is completely serving him/her.

In December 1969, the predecessor of the Internet, the ARPA network in the United States, was put into operation, which marked the rise of computer networks.

Then, in the 20s, the large time-sharing systems were replaced by microcomputer systems.The microcomputer system uses a time-sharing system on a small scale.On the basis of the time-sharing computer system, the remote terminal computer system provides shared resource services to remote terminal users with different geographical distributions through the modem (Modem) and the public telephone network (PSTN).Although this is not a real computer network system, it is the first attempt to combine a computer with a communication system.Remote terminal users have experienced the service of "computer network".On the basis of the remote terminal computer system, people began to study the interconnection of computers through PSTN and other existing communication systems.In order to make the communication connection between computers reliable, a layered communication system and corresponding network communication protocols have been established, so a computer network with the main purpose of resource sharing was born.Due to the ability of data exchange between computers in the network, it provides the ability to work collaboratively between computers in a larger range, realize distributed processing or even parallel processing, and the communication ability of networked users to exchange information directly through the computer network is also greatly improved. improve.

In the early 20s, with the promotion of PC applications, the demand for PC networking also increased, and various microcomputer LANs based on PC interconnection appeared one after another.This period is a period of rapid development of microcomputer LAN, which is a typical client/server model.The typical structure is to share the file server on the shared transmission line communication network platform, and the network file server provides shareable file resource services for all networked PCs.Each PC user accesses the file server through the network only when he needs to access the shared file resource, which reflects the cooperative work among the computers in the computer network.

The computer models and other equipment in the computer network system are relatively complicated, and the software used is also different. The communication between computers involves many complicated technical issues. In order to realize the "communication" and "communication" and " In 1984, the International Organization for Standardization ISO officially promulgated the "Basic Reference Model for Open Systems Interconnection" OSI international standard. The original intention was to use this set of standards to promote the standardization process of computer network architecture.

In the 20s, computer technology, communication technology and computer network technology developed rapidly.Especially in 90, after the United States announced the establishment of the National Information Infrastructure (NII), many countries formulated and established their own NII, which greatly promoted the development of computer network technology and brought the computer network into a new era. stage of development.

网络互联和高速计算机网络正成为最新一代的计算机网络的发展方向。全球以美国为核心的高速计算机互联网络Internet已经成为人类最重要的、最丰富的知识宝库。美国政府分别于1996年和1997年开始研究更加快速可靠的互联网2（Internet2）和下一代互联网（Next Generation Internet）。我国目前也在实验下一代互联网，已经开通IPv6实验网。

1.3.2 Development Trend
In recent years, the network has made significant progress in basic research such as wireless communication and next-generation Internet, dual-core or multi-core semiconductor device technology, artificial intelligence, high-performance computers, and network security.In the development trend of cutting-edge information technology, the development and research of the following network technologies have become hot spots.

(1) Bluetooth technology

Bluetooth technology is a short-range wireless communication technology that has been used for the longest time.The focus of Bluetooth technology development will be on performance, security and power consumption to complete its functions.

(2) Wi Fi (Wireless Compatible Certification) technology

Wi Fi (Wireless Compatibility Certification) technology has been deployed in large numbers in airports, transportation hubs and business districts in Europe, America and Asia-Pacific regions, and its development momentum is strong.

(3) ZigBee technology

ZigBee technology has the characteristics of short distance, low power consumption, low cost, etc., but the data transmission rate is low, and it will have a wide range of applications in the wireless connection of fixed or mobile devices.

(4) Ultra-wideband wireless UWB technology

Ultra-wideband wireless UWB technology is expected to complement and replace Bluetooth technology and WiFi technology due to its wide range of applications.Therefore, many manufacturers flock to carry out corresponding product development.

(5) W1Msx Technology

W1Msx technology is the most noteworthy wireless broadband Internet access technology. It has many advantages and can provide users with real wireless broadband network services and even mobile communication services. Its standards have been formulated.

(6) Broadband communication technology

DSL technology, which relies on existing telephone lines, is currently the most rapidly developing broadband communication technology.The fiber-coaxial hybrid network HFC technology vigorously developed by the radio and television sector has entered the stage of commercial application.Optical fiber combined with LAN access mode is becoming a new broadband access mode.Electric broadband (BRL) is conducive to market competition.The combination of Orthogonal Frequency Division Multiplexing OFDM technology and multiple-input multiple-output (MIMO) technology in the field of wireless communication, applied to the next generation wireless broadband network, will be a long-term hot spot in the field of wireless communication technology.The software defined radio is expected to realize the compatibility of multiple standards and realize the integration of wired network and wireless network.

(7) [-] Gigabit Copper Ethernet Transmission Technology

Ten Gigabit copper Ethernet transmission technology has been used, but there are still some problems to be solved.

(8) Post-3G technology

Although 3G communication has not yet been popularized in the world, research on post-3G technology has been vigorously carried out, and some companies have released samples.

The biggest impact on the post-3G era is the upgrade to 3.5G and the integration of mobile Internet (WLAN) into the wide area network.Among them, the most noteworthy is the high-speed downlink packet data service access HSDPA technology (some people call it 3.5G technology), which may expand the capability of 3G into an all-IP network and integrate wireless communication and wired communication.

(9) Video communication technology

Video communication technology has been widely used, and many products have come out. IP telephone (VoIP) has been promoted rapidly in developed countries, and the triple play of computer network, cable TV network and Internet based on Internet protocol (full IP) has gained great importance. progress.

In addition, network technology will further improve and improve broadband technology, the application of mobile network and wireless communication technology, the combination of multimedia network and traditional network, the close combination of network technology and computer technology, and network security environment. Greater room for development.

1.4 Composition of computer network
Below we introduce several basic concepts of the network.

1. Host
In a resource subnet, there are two types of hosts.One type is a server that provides network resources, such as computers that provide news browsing, forums, and download services; the other type is a desktop computer that provides users with access to Internet resources through these PCs, which are also what we often call PCs.

(End of this chapter)