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Application layer

An application that communicates with other computers that corresponds to the communication service of the application. For example, a word processor without a communication function cannot execute communication code, and programmers engaged in word processing do not care about layer 7 of OSI. However, if you add an option to transfer files, then the word processor programmer needs to implement layer 7 of OSI. Example: TELNET,HTTP,FTP,NFS,SMTP and so on.

Presentation layer

The main function of this layer is to define data format and encryption. For example, FTP allows you to choose to transmit in binary or ASCII format. If binary is selected, the sender and receiver do not change the contents of the file. If you choose ASCII format, the sender will convert the text from the sender's character set to standard ASCII and send the data. Convert the standard ASCII to the character set of the receiver computer at the receiver. Example: encryption, ASCII, etc.

Session layer

It defines how to start, control, and end a session, including the control and management of multiple two-way messages, so that the application can be notified when only part of the continuous message is completed, so that the data seen by the presentation layer is continuous. In some cases, if all the data is received by the presentation layer, the data is represented by the presentation layer. Example: RPC,SQL and so on.

Transport layer

The functions of this layer include whether to choose an error recovery protocol or an error-free recovery protocol, and to reuse the input of data streams from different applications on the same host, as well as the function of reordering packets received in the wrong order. Example: TCP,UDP,SPX.

Network layer

This layer defines end-to-end packet transmission, which defines logical addresses that can identify all nodes, as well as how routing is implemented and how to learn. In order to adapt to the transmission medium where the maximum transmission unit length is less than the packet length, the network layer also defines a segmentation method of how to decompose a packet into smaller packets. Example: IP,IPX and so on.

Data link layer

It defines how data is transmitted on a single link. These protocols are related to the various media under discussion. Example: ATM,FDDI and so on.

Physical layer

The physical layer specifications of OSI are specific to transmission media. These specifications usually refer to standards developed by other organizations. Connectors, frames, the use of frames, current, coding and optical modulation all belong to various physical layer specifications. Multiple specifications are commonly used in the physical layer to define all the details. Example: Rj45802.3 and so on.

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