Linux Learning notes: OSI seven layer model

OSI Seven layer model:

The OSI (open System Interconnection) seven-layer network model, known as the Open System Interconnect Reference Model, is a logical definition, a specification that divides the network logically into 7 layers. Each layer has relevant, corresponding physical devices, such as routers, switches. OSI Seven layer model is a framework design method, the main purpose of building seven-layer model is to solve the compatibility problem of heterogeneous network interconnection, and its main function is to help different types of host to realize data transmission. Its greatest advantage is to distinguish between the three concepts of service, interface, and protocol, and to enable reliable communication between different networks with different systems through seven hierarchical structural models.

The OSI seven layer structure is as follows:

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1. Physical layer: The main definition of physical equipment standards, such as the interface type of network cable, the interface type of optical fiber, the transmission rate of various transmission media. Its main role is to transmit the bitstream (that is, 1, 0 is converted to the current strength to carry out the transmission, after reaching the destination in the conversion to 1, 0, that is, we often say the digital-to-analog conversion and A/D conversion). This layer of data is called a bit.

2. Data Link layer: Defines how to make data formatted for transmission and how to control access to physical media. This layer also often provides error detection and correction to ensure reliable data transmission.

3. Network layer: Provides connectivity and path selection between two host systems located in geographically diverse networks. The development of the Internet has greatly increased the number of users accessing information from various sites around the world, and the network layer is the layer that manages this connection.

4. Transport Layer: Defines some transfer data protocol and port number (WWW port 80, etc.), such as: TCP (Transmission Control Protocol, transmission efficiency is low, reliable, for transmission reliability, high data volume data), UDP (User Datagram Protocol, the opposite of TCP characteristics, For transmission reliability requirements are not high, data volume of small data, such as QQ chat data is transmitted in this way. The main point is to segment and transmit the data received from the lower layer, and then reorganize after reaching the destination address. This layer of data is often called a segment.

5. Session Layer: The path of data transmission is established through the Transport Layer (port number: Transport port and receive port). Primarily initiates a session between your systems or accepts a session request (the device needs to know each other either IP or Mac or hostname)

6. Presentation layer: Ensures that the information sent by the application layer of one system can be read by the application layer of the other system. For example, a PC program communicates with another computer, where one computer uses an extended 210 binary Interchange Code (EBCDIC) and the other uses the U.S. Information Interchange Standard Code (ASCII) to represent the same characters. If necessary, the presentation layer enables conversion between multiple data formats by using a pass-through format.

7. Application layer: The OSI layer closest to the user. This layer provides network services for user applications such as e-mail, file transfer, and terminal emulation.

Advantages of the OSI seven layer model:

The main purpose of building a seven-tier model is to solve the compatibility problems encountered in heterogeneous network interconnection. Its greatest advantage is a clear distinction between the three concepts of service, interface, and protocol: The service describes what functions a layer provides for the previous layer, how the previous layer uses the underlying service, and how the service is implemented in the layer, so that the layers are highly independent, There is no limit to what protocols are used by entities in the interconnection network, as long as the same service is provided upwards and the interface of the adjacent layer is not changed.

The Network Seven Layer division is also in order to make the network of different functional modules (different levels) to share different responsibilities, thus bringing the following benefits:

1, reduce the complexity of the problem, once the network failure, can quickly locate the level of fault, easy to find and error correction;

2, in each layer to define the standard interface, so that the same peer layer of different network equipment can be interoperable, each layer is relatively independent, a high-level protocol can be placed in a variety of low layer protocol operation;

3, can effectively stimulate the network technological innovation, because each update can be carried out in a small scope, do not need to the whole network of major surgery;

4, easy to study and teaching.

Linux Learning notes: OSI seven layer model