Basic functions: The application layer not only provides the information exchange and remote operation required by the application process, but also serves as the user agent of the application process to complete some functions required for information exchange.
2. Layer 6 presentation layer: the presentation layer mainly used to process the information exchanged between two communication systems.
Basic function: to solve the grammatical problems of user information for upper users. It includes data format exchange, data encryption and decryption, data compression and terminal type conversion.
3. Session Layer 5: Establish an end-to-end connection between two nodes. It provides a dialogue control mechanism between terminal system applications. This service includes establishing a connection 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.
Basic function: It 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.
4. Layer 4 transport layer: the transport layer is the interface layer between the upper layer and the lower layer in the network architecture. The transport layer is not only a single structural layer, but also the core of the whole analysis architecture protocol. The transport layer provides end-to-end reliable, transparent and optimized data transmission service mechanism for session layer users.
Basic functions: including full-duplex or half-duplex, flow control and error recovery services; The transport layer divides messages into several groups and reorganizes them at the receiving end. Different packets can be sent 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, security, etc. It can also realize end-to-end flow control function.
5. Layer 3 network layer: This layer establishes the connection between two nodes through addressing, and selects the appropriate routing and switching nodes for the data packets sent by the source transport layer. And correctly transmits it to the destination transport layer according to the address.
Basic functions: including routing and relaying 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.
6. Layer 2 Data Link Layer: In this layer, data is framed and flow control is handled. Shield the physical layer, provide data link connection for the network layer, and carry out almost error-free data transmission (error control) for the physical connection that may be wrong.
Basic function: This layer specifies the topology and provides hardware addressing. Common devices include bridges and switches.
7. Physical layer 1: at the bottom of the OSI reference model. Common devices include network cards, hubs, repeaters, modems, network cables, twisted pair cables and coaxial cables.
Basic function: The main function of the physical layer is to provide physical connection for the data link layer by using the physical transmission medium, and realize the transparent transmission of the bit stream.
Extended data:
History of OSI reference model:
The two major international organizations that play an important role in formulating computer network standards are: the Telecommunication Standardization Department of the International Telecommunication Union and the International Organization for Standardization (iso). Although their work fields are different, with the development of science and technology, the boundary between communication and information processing has become blurred, which has also become a common concern of the telecom standardization departments of the International Telecommunication Union (itu) and ISO.
1984, ISO issued the famous ISO/IEC 7498 standard, which defined the seven-layer framework of network interconnection, namely open system interconnection.
Baidu encyclopedia -OSI model
Baidu Encyclopedia -OSI Reference Model