OSI seven-layer model

       Layer 7th Application layer: The highest layer in the OSI. Provides a means to access the OSI environment for specific types of network applications. The application layer determines the nature of the communication between processes to meet the needs of the user. The application layer not only provides the information exchange and remote operation required by the application process, but also acts as a user agent for the application process to accomplish some functions necessary for the exchange of information. It includes: File transfer access and management Ftam, virtual terminal VT, transaction TP, remote database access RDA, manufacturing Message Specification MMS, directory service DS and other protocols;
Layer 6th: Mainly used to deal with the representation of exchanging information in two communication systems. The syntax of user information is resolved for the upper user. It includes data format Exchange, data encryption and decryption, data compression and recovery functions;
Level 5th Session Layer:-Establishes an end connection between two nodes. A dialog control mechanism is provided between applications for the end system. This service includes establishing whether the connection is set in full or half duplex mode, although the duplex mode can be handled in layer 4;
Layer 4th Transport:-General Data Delivery-connection-oriented or no connection. Provides an end-to-end, reliable, transparent, and optimized data transfer service mechanism for session-level users. Includes full-duplex or half-duplex, flow control, and error recovery services;
Layer 3rd Network layer:-This layer through addressing to establish a connection between two nodes, for the transport layer from the source side of the packet sent, select the appropriate routing and switching nodes, correctly and accurately transmitted to the destination end of the transport layer. It includes routing and relaying data through an interconnected network;
Layer 2nd Data Link layer:-This layer will frame the data and process flow control. Shields the physical layer, providing a data link connection to the network layer, and making almost error-free data transfer on a physical connection that is likely to make a mistake. This layer specifies the topology and provides hardware addressing;
Layer 1th Physical layer: At the very bottom of the OSI reference Model. The primary function of the physical layer is to use physical transport media to provide a physical connection to the data link layer for transparent transfer of bitstream.
When data is sent, it is transmitted from the seventh layer to the first layer, while receiving data is reversed.
The upper three layer is the general application layer, used to control software aspects. The next four layers are collectively the data flow layer used to manage hardware.
Data is split when it is sent to the data flow layer.
The data at the transport layer is called a segment,
       The network layer is called a packet,
       The data link layer is called the frame,
       The physical layer is called a bitstream, which is called a PDU (protocol data Unit)

OSI seven-layer model