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I believe everyone is familiar with the "Internet", because we are in contact with the Internet almost every day, and it has been fully integrated into our daily life. But you know what? In fact, all the applications of the Internet are layered, which is the OSI seven-layer model in the mouth of many network experts. Next, we will teach you an in-depth understanding of the iOS seven-tier model.

OSI layering thought:

Because there are many architectures in the computer network, such as IBM's SNA (system Network Architecture, layer 7) and DEC's DNA (Digital Network Architecture, layer 3) and so on. Due to the differentiation of architecture, there are serious compatibility problems in network products, which affects the rapid development of the network. In order to solve this problem, ISO formally promulgated OSI RM in 1984. This model divides the work of network communication into seven layers. Layers 1 to 4 are considered lower layers, and these layers are closely related to data movement. Layers 5 to 7 are high-level and contain application-level data. Each layer is responsible for a specific task, and then transfers the data to the next layer. From low to high, it is divided into physical layer, data link layer, network layer, transport layer, session layer, presentation layer and application layer.

In-depth understanding of the OSI seven-tier reference model:

Physical layer (introduced from the bottom): the physical layer includes physical networking media, which are actually cabling, optical fiber, network cards, and other things used to connect two network communication devices together. It defines the mechanical, electrical, functional and process characteristics between activating, maintaining and closing communication endpoints. Although the physical layer does not provide error correction service, it can set the data transmission rate and monitor the data error rate.

The key to data movement in layer 2 to layer 4 of OSI

The second layer of the OSI model: the data link layer the main function of the data link layer is to control the communication between the network layer and the physical layer. It ensures the reliable transmission of data on unreliable physical lines. It divides the data received from the network layer into specific frames that can be transmitted by the physical layer, ensuring the reliability of transmission. Its main functions include: physical address addressing, data framing, flow control, data error detection, retransmission and so on. It is independent of the network layer and the physical layer, and does not need to care whether the computer is running software or other operations.

Layer 3 of the OSI model: many users in the network layer often confuse layer 2 and layer 3 related problems. To put it simply, if you are talking about a problem related to IP addresses, routing protocols, or address resolution protocols (ARP), then this is the layer 3 problem. The network layer is responsible for routing packets between subnetworks. it determines the best path from two nodes in a network by comprehensively considering transmission priority, network congestion, quality of service and the cost of optional routes. In addition, it can also achieve congestion control, Internet interconnection and other functions.

The fourth layer of the OSI model: the transport layer is the most important layer in the OSI model. It is the first end-to-end layer that serves as a buffer when two computers communicate through the network. When the quality of service of the network layer can not meet the requirements, it will improve the service to meet the requirements of the high level; but when the quality of service of the network layer is good, it only needs to do little work. In addition, it also deals with end-to-end error control and flow control, and ultimately provides reliable and error-free data transmission for the session.

The fifth layer of the OSI model: the session layer is responsible for establishing and maintaining communication between the two nodes in the network, and keeping the session synchronized. It also determines whether the communication is interrupted and where to resend when the communication is interrupted.

The sixth layer of the OSI model: the function of the presentation layer is to manage the decryption and encryption of the data, such as the common system password processing, when your account data is encrypted before sending, at the other end of the network, the presentation layer will decrypt the received data. In addition, the presentation layer needs to decode and encode the picture and file format information.

The seventh layer of the OSI model: simply speaking, the application layer provides an interface for the operating system or network applications to access network services, including file transfer, file management and e-mail information processing.

The typical devices for each layer are as follows:

Physical layer... Network: repeater, hub, network cable, HUB

Data link layer. .. Network: network card, bridge, switch

Network layer... Network: routers, firewalls, multilayer switches

Transport layer... . Computers: processes and ports

Session layer... . Computer: establish session, SESSION authentication, breakpoint retransmission

Presentation layer... . Computer: coding mode, image coding and decoding, URL field transmission coding

Application layer … . Computers: applications, such as FTP,SMTP,HTTP

Advantages of OSI layering

1) people can easily discuss and learn the specification details of the protocol.

(2) the standard interface between layers facilitates engineering modularization.

(3) A better interconnection environment is created.

(4) reduce the complexity, make the program easier to modify, and the speed of product development is faster.

(5) each layer makes use of the adjacent lower-level services to make it easier to remember the functions of each layer.

Summary:

The OSI seven-layer model effectively solves the compatibility problems encountered in the interconnection of different network systems, and its appearance reduces the complexity of the network. Once a network failure occurs, it can quickly locate the fault level, making it easy to find and correct errors; by defining standard interfaces on each layer, it enables interoperability between different network devices belonging to the same layer; it also ensures the relative independence of each layer. On the other hand, the high-level protocol can run on a variety of low-level protocols, which improves the efficiency of the network; because each update only needs to be carried out at one level and is not restricted by the whole network, so its emergence effectively stimulates the innovation of network technology. it is the source of the development of network technology.

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