Network communication model -- Network OSI tcp/ip layering

1.osi

1. The physical layer ： The main definition of physical equipment standards , Such as the interface type of network cable 、 Interface type of optical fiber 、 Transmission rate of various transmission media, etc . Its main function is to transmit bitstreams （ Is the 1、0 It's converted into current to transmit , After arriving at the destination, it will be transformed into 1、0, That is to say, we often talk about analog-to-digital conversion and analog-to-digital conversion ）. The data in this layer is called bit .

2. Data link layer ： Defines how formatted data is transmitted in frames , And how to control access to physical media . This layer also usually provides error detection and correction , To ensure the reliable transmission of data . Such as ： Used in serial communication 115200、8、N、1
3. The network layer ： Provides connectivity and path selection between two host systems in a network in different geographic locations .Internet With the development of Internet, the number of users accessing information from all sites in the world has increased greatly , And the network layer is the layer that manages this kind of connection .
4. Transport layer ： Some protocols and port numbers for data transmission are defined （WWW port 80 etc. ）, Such as ：TCP（ Transmission control protocol , Low transmission efficiency , High reliability , High requirements for transmission reliability , A large amount of data ）,UDP（ User datagram protocol , And TCP The opposite is true , The requirements for transmission reliability are not high , A small amount of data , Such as QQ Chat data is transmitted in this way ）. It is mainly to segment and transmit the data received from the lower layer , Arrive at the destination address and recombine . This layer of data is often called segment .
5. The session layer ： Through transport layer ( Port number ： Transmission port and receiving port ) To establish a path for data transmission . It is mainly used to initiate or accept session requests between your systems （ Equipment needs to know each other can be IP It can also be MAC Or the host name ）.
6. The presentation layer ： It can ensure that the information sent by the application layer of one system can be read by the application layer of another system . for example ,PC Program communicates with another computer , One of the computers uses the extended binary decimal interchange code (EBCDIC), The other uses the American Standard Code for information exchange （ASCII） To represent the same characters . If necessary, , The presentation layer will use a common format to achieve the conversion between multiple data formats .
7. application layer ： Is the closest to the user OSI layer . This layer is the user's Application （ Like E-mail 、 File transfer and terminal emulation ） Provide network services .


Hair ： From the top down
closed ： From bottom to top