: OSI (Open Systems Interconnection) refers to open systems interconnection. Commonly known as OSI reference model, it is a network interconnection model studied by ISO (International Organization for Standardization) in 1985. The architecture standard defines the seven-layer framework of network interconnection (physical layer, data link layer, network layer, transport layer, session layer, presentation layer and application layer), that is, ISO open system interconnection.
Under this framework, the functions of each layer are further specified in detail to realize interconnection, interoperability and application transplantation in the open system environment.
Layer 7 Application Layer:
The highest level in OSI. It provides a way for specific types of network applications to access the OSI environment. The application layer determines the nature of inter-process communication to meet the needs of users. The application layer should not only provide the information exchange and remote operation required by the application process, but also serve as the user agent of the application process to complete some functions necessary for information exchange. It includes FTAM, VT, TP, RDA, MMS, DS and other protocols. The application layer can communicate with the application program interface to achieve the purpose of showing it to users. Common protocols here are: HTTP, HTTPS, FTP, TELNET, SSH, SMTP, POP3, etc.
Layer 6 presentation layer:
Mainly used to deal with the representation of information exchanged in two communication systems. Solve the syntax problem of user information for upper users. It includes data format exchange, data encryption and decryption, data compression and terminal type conversion.
Session layer 5:
Establish an end connection between two nodes. It provides a dialogue control mechanism between applications in the terminal system. This service includes setting whether the connection is established in full-duplex or half-duplex mode, although the duplex mode can be handled at Layer 4; The session layer manages the login and logout process. It specifically manages the dialogue between two users and processes. If only one user is allowed to perform certain operations at a time, the session layer protocol will manage these operations, such as preventing two users from updating the same set of data in the database at the same time.
Layer 4 transport layer:
-General data transfer-Connection-oriented or connectionless. Provide end-to-end reliable, transparent and optimized data transmission service mechanism for session layer users. Including full-duplex or half-duplex, flow control and error recovery services; The transport layer divides the message into several data packets and reassembles them at the receiving end. Different data packets can be transmitted to the host through different connections. In this way, higher bandwidth can be obtained without affecting the session layer. When establishing a connection, the transport layer can request the quality of service, which specifies acceptable parameters such as bit error rate, delay and security, and can also realize the end-to-end flow control function.
Layer 3 Network Layer:
This layer establishes the connection between two nodes by addressing, selects the appropriate routing and switching nodes for the data packets sent by the source transport layer, and correctly transmits them to the destination transport layer according to the address. It includes routing and forwarding data through the Internet; In addition to routing, the network layer is also responsible for establishing and maintaining connections, controlling network congestion, and generating billing information when necessary.
Layer 2 Data Link Layer:
Organize data and handle flow control at this level. Shield the physical layer, provide data link connection for the network layer, and carry out almost error-free data transmission (error control) on a physical connection that may go wrong. This layer specifies the topology and provides hardware addressing. Commonly used devices include bridges and switches;
Layer 65438 +0 Physical Layer:
At the bottom of the OSI reference model. The main function of the physical layer is to provide physical connection for the data link layer by using the physical transmission medium, so as to transmit the bit stream transparently. Common devices include (various physical devices) network cards, hubs, repeaters, modems, network cables, twisted pair cables and coaxial cables.
References:
Baidu Encyclopedia -OSI Reference Model