The design purpose of OSI model is to become an open network model that all manufacturers can realize, so as to overcome the difficulties and inefficiencies brought by using many private network models. OSI was completed with the participation of a respected international standards organization, namely ISO (International Organization for Standardization). What is OSI? OSI is the abbreviation of Open Systems Interconnection, which means open system interconnect. Before the appearance of OSI, there were many architectures of computer networks, among which the most famous ones were IBM's SNA (System Network Architecture) and DEC's DNA (Digital Network Architecture). In order to solve the interconnection problem of networks with different architectures, ISO (not to be confused with OSI) formulated Open System Interconnection (OSI/RM) in 198 1. The model divides the work of network communication into seven layers, which are physical layer, data link layer, network layer, transport layer, session layer, presentation layer and application layer from low to high. The first layer to the third layer belong to the lower three layers of the OSI reference model and are responsible for creating the link of network communication connection; The fifth to seventh layers are the upper three layers of the OSI reference model, which are responsible for end-to-end data communication. The fourth floor is responsible for connecting the high and low floors. Each layer performs certain functions, each layer directly provides services to its upper layer, and each layer supports each other, while network communication can be carried out from top to bottom (at the sender) or from bottom to top (at the receiver). Of course, not every communication needs to go through all seven layers of OSI, and some even need only one layer to correspond to both parties. The transmission between physical interfaces and the connection between repeaters only need to be carried out at the physical layer; The connection between routers only needs to go through three layers below the network layer. Generally speaking, the communication between the two sides is carried out on the level of reciprocity, not on the level of asymmetry.
The method used in the formulation of OSI standards is to divide the whole huge and complex problem into several small problems that are easy to deal with, which is the layered architecture method. In OSI, three levels of abstraction are adopted, namely architecture, service definition and protocol specification.
In order to facilitate memory, the seven layers from high to low can be regarded as: everyone seems to need data processing. Each capital letter corresponds to the first letter of the seven-layer name.