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China public computer Internet （ChinaNet）
On 1994 year 2 In June, the former Department of Posts and telecommunications and the United States Sprint The company signed , Provide... For the whole society Internet All kinds of services .

Science and technology network of Chinese Academy of Sciences （NCFC）/ China Science and technology network （CSTNET）

The science and technology network of the Chinese Academy of Sciences is also known as the Zhongguancun education and scientific research demonstration network （NCFC）. Hosted by the Chinese Academy of Sciences , Jointly implemented with Peking University and Tsinghua University .1989 year 8 month , The Chinese Academy of Sciences has undertaken the project of the State Planning Commission “ Zhongguancun education and scientific research demonstration network ”（NCFC）——— China Science and technology network （CSTNET） The construction of the predecessor .1994 year 4 month ,NCFC Take the lead with the United States NSFNET Directly connected , China and nternet Full function network connection , It marks the birth of China's earliest international Internet .1995 year 12 month , The networking project of 100 Institutes of the Chinese Academy of Sciences has been completed ,1996 year 2 month , The Chinese Academy of Sciences has decided to officially NCFC The network of Chinese Academy of Sciences developed on the basis of （CASNET） Name it “ China Science and technology network ”（CSTNET）

3-2 Basic concepts of computer network

3-2-1 Computer network definition

Computer network refers to multiple computers with independent functions and their external devices with different geographical locations , To connect by means of a communication line , In the network operating system , Network management software and network communication protocol management and coordination , Realization Resource sharing and information transmission The computer system of

3-2-2 Classification of computer networks

According to geographical scope

According to the topological structure

Star structure ： It means that all computers are connected into a network in a star mode . The network has a central node , Other nodes （ The workstation 、 The server ） They are all directly connected to the central node , This structure takes the central node as the center , So it's also called centralized network . It has a simple structure 、 Easy to manage 、 Simple control 、 The network delay time is small 、 Low transmission error . But the disadvantages are also obvious ： Low reliability 、 Poor resource sharing ability 、 Low line utilization .

Bus structure ： It means that all workstations and servers are hung on one bus , All workstations are equal , No central node control , It has a simple structure 、 Good scalability 、 Easy to install , But maintenance is difficult .

Ring structure ： Several nodes in the network are connected end to end through communication links to form a closed ring , This structure makes the public transmission cable form a ring , Data is transmitted between nodes in one direction in the loop , Information from one node to another . It has strong real-time performance 、 Transmission control is easy , But maintenance is difficult , Low reliability .

Network structure ： It refers to the point-to-point link connection between each device , This connection is not economical , Use this method only if every site has to send information frequently . Its installation is also complicated , But the system has high reliability , Strong fault tolerance . Sometimes referred to as distributed architecture .

Tree structure ： This is a hierarchical centralized control network , Compared with star shape , Its total length of communication line is short , Lower cost 、 Nodes are easy to expand 、 Finding a path is convenient , But apart from the leaf nodes and their connected lines , Any node or its connected line failure will affect the system .

3-2-3 Main functions and applications of computer network

3-3 The composition of the computer network

3-3-1 The composition of the computer network

1、 So-called Communication subnet It is the part in charge of data communication in computer network .

2、 Resource subnet It is the user oriented part of computer network , Responsible for application-oriented data processing of the whole network .

3、 The rules and agreements that both sides of the communication must abide by are called Communication protocol , Whether it exists or not is the fundamental difference between computer network and general computer interconnection system .

3-3-2 The edge and core of the network

Fringe part It's connected to the Internet All the hosts , The host can also be called “ End system ”. It is directly used by users , For communication ( Transmit data 、 Audio or video ) And resource sharing .

The core part of the By a large number of The Internet And connecting these networks Router form . This part is the edge part Provide services Of .

In conclusion , We, as service users , What you can touch is the host , That is, the edge part ; Accordingly, we are provided with services , Is the core part .

3-3-3 The transmission medium of the network

3-3-4 Packet switching technology

In the process of communication , The communication parties are grouped 、 Using storage - Forwarding mechanism realizes the communication mode of data interaction , go by the name of Packet switching （PS:packet switching）

Packet switching is also known as Packet exchange , It divides the user communication data into multiple smaller equal length data segments , The necessary control information is added to the front of each data segment as the header of the data segment , Each data segment with a header constitutes a packet . The header indicates the address where the packet is sent , When the switch receives the packet , The packet will be forwarded to the destination according to the address information in the header , This process is packet switching . A communication network capable of packet switching is called Packet switching network .

Packet switched The essence Namely Store and forward , It temporarily stores the received packets , Queue on destination route , When it can send messages , Then send the information to the corresponding route , Complete forwarding . The process of storing and forwarding is the process of packet switching .

Packet switched characteristic

1. The smallest unit of information transmission is packet

2. There are connection oriented （ Logical connection ） And no connection

3. Statistical time division multiplexing （ Dynamically allocate bandwidth ）

The basic principle of statistical time division multiplexing is to divide time into unequal time slices , Time slices of different lengths are the time required to transmit packets of different lengths , There is no fixed time slice allocated to each communication , It's about using it on demand . This means how long it takes to transmit packets using this multiplex line , It can be seen that statistical time division multiplexing is the key to dynamically allocate bandwidth .

4. Information transmission is error control

Packet switching is a switching mode specially designed for data communication network , Data service is characterized by high reliability requirements , The requirement for real-time is not as high as that of telephone communication , Therefore, in packet switching, in order to ensure the reliability of data information , Equipped with CRC check 、 Error control mechanism such as retransmission , To meet the needs of data business characteristics .

5. Information transmission is not transparent

Packet switching processes the transmitted data information , Such as splitting 、 Reorganization information, etc .

6. Flow control based on call delay system

In packet switching , When the data flow is large , Waiting for packet processing , Instead of calling loss immediately like circuit switching , Therefore, its flow control is based on call delay .

3-4 Network architecture and network protocol

3-4-1 Network architecture

Computer network architecture It refers to the hierarchical structure model of computer network , It is a collection of protocols and ports between layers .

3-4-2 OSI/ISO Reference model

application layer （Application Layer）： yes OSI The highest level of the reference model , It's the user 、 Interface between application and network , It provides services directly to users , Complete all kinds of work for users on the network .

The presentation layer （Presentation Layer）： yes OSI The sixth layer of the reference model , It interprets instructions and data from the application layer , Give meaning to all kinds of grammar , Its main function is to deal with the representation of user information , For example, data coding 、 Data format conversion, encryption and decryption, etc .

The session layer （Session Layer）： yes OSI The fifth layer of the reference model , Its main task is to provide the method of establishing and using connection for the presentation layer of two entities （ The connection of presentation layer between different entities is called session ）, Organize and coordinate communication between two session processes , And manage data exchange .

Transport layer （Transport Layer）： yes OSI The fourth layer of the reference model . The main task of this layer is to provide users with reliable end-to-end error and flow control , Ensure the correct transmission of messages , At the same time, the details of lower data communication are shielded to the upper layer .

The network layer （Network Layer）： yes OSI The third layer of the reference model , It is the most complex layer , It is also the highest layer of communication subnet . It provides services to the upper layer on the basis of the lower two layers . Its main task is through the routing algorithm , Select the most appropriate path for the message or packet . This layer controls the information forwarding between the data link layer and the transmission layer , establish 、 Maintain and terminate network connections .

Data link layer （Data Link Layer）： yes OSI Reference the second layer of the model , It is responsible for establishing and managing links between nodes . At the same time, through various control protocols , Change the physical channel with errors into error free 、 A data link that can reliably transmit data frames .

The physical layer （Physical Layer）： yes OSI The first layer of the reference model , And the bottom . Its main function is to use the transmission medium to provide physical connection for the data link layer , Transparent transmission of bitstream . It makes it unnecessary for the data link layer to consider the specific transmission medium of the network .“ Transparent transmission of bitstreams ” Indicates that the bit stream transmitted by the actual circuit has not changed , For the transmitted bitstream , This circuit seems to be invisible .

Protocols used by each layer

3-4-3TCP/IP Reference model

　　1、 Host to network layer 　　
　　 actually TCP/IP The reference model does not really describe the implementation of this layer , Just ask to be able to provide it to the top - The network interconnection layer is an access interface , In order to pass on IP grouping . Because this level is not defined , So its specific implementation method will vary with the network type .　　
　　2、 Network interconnection layer 　　
　　 The network interconnection layer is the whole TCP/IP The core of the protocol stack . Its function is to send packets to the target network or host . meanwhile , In order to send packets as soon as possible , It may be necessary to carry out group passing along different paths at the same time . therefore , The order in which packets arrive and are sent may be different , This requires the upper layer to sort the groups .　　
　　 The network interconnection layer defines the packet format and protocol , namely IP agreement （Internet Protocol）.　　
　　 In addition to the function of routing, the network interconnection layer , You can also complete different types of networks （ Heterogeneous network ） The task of Interconnection . besides , The network interconnection layer also needs to complete the congestion control function .　　
　　3、 Transport layer 　　
　　 stay TCP/IP In the model , The function of the transport layer is to enable peer entities on the source host and the target host to have sessions . Two protocols with different QoS are defined in the transport layer . namely ： Transmission control protocol TCP（transmission control protocol） and User datagram protocol UDP（user datagram protocol）.　　
　　TCP The agreement is a Connection oriented 、 A reliable agreement . It sends the byte stream sent by one host to other hosts on the Internet without error . At the sending end , It is responsible for dividing the byte stream transmitted from the upper layer into segments and delivering them to the lower layer . At the receiving end , It is responsible for reorganizing the received messages and delivering them to the upper level .TCP The protocol also deals with end-to-end traffic control , To avoid slow receivers not having enough buffers to receive the large amount of data sent by the sender .　　
　　UDP The agreement is a unreliable 、 No connection agreement , It is mainly applicable to the situation where there is no need to sequence and control the flow of messages .　　
　　4、 application layer 　　
　　TCP/IP The model will OSI The functions of the session layer and presentation layer in the reference model are integrated into the application layer implementation .　　