The full name of the OSI Reference Model is the Open System Interconnection Reference Model, which was developed by the International Organization for Standardization ISO in the 1980s. It was first proposed. Since ISO was established in 1946, it has put forward a number of standards, and ISO/IEC 7498, the standard for network architecture, defines the basic reference model for network interconnection. At that time, several network architectures represented by IBM's SNA appeared in the network industry. The focus of these architectures is often the internal network connections of various companies. There is no unified standard, so it is difficult to interconnect them. Under this circumstance, ISO proposed the OSI reference model, and its biggest feature is openness. Network products from different manufacturers can achieve interconnection, interoperability, and portability as long as they follow this reference model. In other words, any system that follows the OSI standard, as long as they are physically connected, they can communicate with each other.
The OSI reference model defines the hierarchical structure of the open system and the services provided by each layer. One of the successes of the OSI reference model is that it clearly separates the three confusing concepts of service, interface, and protocol. The service describes the function of each layer, the interface defines how the service provided by a certain layer is accessed by the upper layer, and the protocol is the realization method of each layer function. By distinguishing these abstract concepts, the OSI reference model distinguishes the function definition from the implementation details, with high generality, making it universally adaptable.
The OSI reference model is a framework with 7 levels. The 7 levels from the bottom up are the physical layer, the data link layer, the network layer, the transport layer, the session layer, the presentation layer, and the application layer.
Features of the Open System Interconnection Reference Model:
①Corresponding entities at each layer communicate through their own protocols.
②Each computer system has the same hierarchical structure.
③The corresponding levels of different systems have the same functions.
④All levels of the same system are connected through interfaces.
⑤ Between two adjacent layers, the lower layer provides services for the upper layer, and the upper layer uses the services provided by the lower layer.
Reference model level division principle
OSI divides the entire communication function into 7 levels, the division principle is as follows:
(1) All nodes in the network are Divided into the same hierarchical structure, each same level has the same function.
(2) The interface protocol can be used to communicate between adjacent levels in the same node.
(3) Each layer uses the services provided by the next layer and provides services to its upper layer.
(4) The peer layer of different nodes implements the communication between peer layers according to the protocol.
Functions at various levels
The physical layer is not the physical media itself, it is only the description and execution of the functions of the physical media to realize the physical connection in the open system Connection procedures. The physical layer provides mechanical, electrical, functional, and procedural conditions for establishing, maintaining, and disconnecting physical connections. In short, the physical layer provides a means for synchronization and bitstream transmission on physical media. Typical protocols include EIA-232-D, etc.
Data link layer
The data link layer is used to establish, maintain, and tear down link connections to achieve error-free transmission. On point-to-point or point-to-multipoint links, reliable information transmission is guaranteed. This layer performs error control, data framing, synchronization and other controls on connecting adjacent paths. Generally, cyclic redundancy check (CRC) is used to detect errors, and techniques such as timer recovery and automatic retransmission (ARQ) are used to correct errors. Its typical protocol is the high-level data link control protocol HDLC in the OSI standard protocol set.
The network layer specifies the protocol for establishing, maintaining, and tearing down network connections. Its main function is to use the error-free data transmission function between adjacent nodes provided by the data link layer, and realize the connection between the two systems through routing and relay functions. In the computer network system, the network layer also has the function of multiplexing.
The transport layer completes data transmission control between open systems. The main function is to confirm the sending and receiving of data between open systems. At the same time, it is also used to make up for the quality differences of various communication networks, to recover transmission errors that still exist after the lower three layers, and to further improve reliability. In addition, technical measures such as multiplexing, segmentation and combination, connection and separation, splitting and merging have also been adopted to improve throughput and service quality.
The session layer relies on the communication function below the transport layer to enable the data transmission function to be carried out effectively between open systems. Its main function is to send and receive data in the correct order according to the agreement between the application processes, and conduct various forms of dialogue. The control methods can be summarized into the following two categories: One is to easily realize the successive alternate conversion of receiving processing and sending processing in conversational applications, and set only one end to send data at a certain time. Therefore, it is necessary to alternately change the transmission control of the sender. Second, in the case of transferring large amounts of data in one direction such as file transfer, in order to prevent accidents in application processing, it is necessary to mark the data in the process of transferring the data. When an accident occurs, it can be retransmitted from the mark. For example, you can send a long file in pages, and after receiving the acceptance confirmation of the previous page, the content of the next page can be sent.
The main function of the presentation layer is to transform the information provided by the application layer into a form that can be understood together, providing character codes, data formats, control information formats, encryption, etc. Unified representation. The presentation layer only transforms the form of the information content of the application layer without changing the content itself.
The application layer is the highest layer of the OSI reference model. Its function is to realize the information exchange between application processes (such as user programs, terminal operators, etc.). At the same time, it also has a series of service functions required for business processing.
OSI model has the following advantages.
(1) Division of labor and cooperation, clear responsibilities. Jobs of similar nature are divided into the same level, and jobs of different nature are divided into different levels. In this way, the function of each level is clear, and each level has its responsibility. Once a problem occurs, it is easy to find the problem. Only improve this layer.
(2) Peer-to-peer conversation. When computers communicate through the network, they follow the principle of peer-to-peer conversation, that is, the same layer is found at the same layer, and the communication is carried out through the protocol of each peer layer. For example, two peer-to-peer network layers communicate using network protocols.
(3) Process layer by layer, and layer by layer is responsible. In OSI, the communication between two entities must involve the next layer. Only adjacent layers can communicate. The lower layer provides services to the upper layer. The upper layer calls the lower layer services through the interface. There can be no leapfrog call relationship between the layers. The function of each layer is realized. They are all done on the basis of the services provided by the lower layer. That is, each layer uses the services provided by the lower layer to complete the functions of the layer, and on this basis, provides further services for the upper layer.