First of all, we need to understand the functions of each layer of the OSI seven-layer model.
The seventh layer: application layer data user interface, which provides user program "interface".
The sixth layer: the representation of data in presentation layer and the realization of specific functions, such as data encryption.
The fifth layer: session layer data allows users on different machines to establish session relationships, such as WINDOWS.
The fourth layer: the transport layer realizes the data communication between user processes on different hosts of the network, which is reliable.
As well as unreliable transmission, error detection at the transport layer, flow control, etc.
Layer 3: Network layer packets provide logical addresses (IP) and routes, and data goes from source to destination.
transmit
Layer 2: Data Link Layer Frames encapsulate the upper layer data into frames, use MAC addresses to access the media, and detect errors.
And correction.
The first layer: the transmission, physical interface and electrical characteristics of bit stream between bit stream devices in the physical layer.
The following is a detailed description of the functions of each layer of the OSI seven-layer model:
OSI seven-layer model OSI seven-layer model is called open system interconnect, which is a framework design method.
The OSI seven-layer model realizes reliable communication between different systems and different networks through seven hierarchical models, so it is the most important.
The important function is to help different types of hosts realize data transmission.
Physical layer: the lowest or first layer of the O S I model, including physical network media, such as cable connectors. The protocols of the physical layer generate and detect voltages in order to send and receive signals carrying data. Plug in a network card on your desktop, and you have established the foundation of a computer network. In other words, you provide a physical layer. Although the physical layer does not provide error correction service, it can set the data transmission rate and monitor the data error rate. Network physical problems, such as disconnection, will affect the physical layer.
Data link layer: the second layer of the O S I model, which controls the communication between the network layer and the physical layer. Its main function is how to transmit data reliably on unreliable physical lines. In order to ensure transmission, data received from the network layer is divided into specific frames that can be transmitted by the physical layer. Frame is a structured data packet for mobile data, which includes not only the original data, but also the network addresses of the sender and receiver, as well as error correction and control information. The address determines where the frame will be sent, while the error correction and control information ensure that the frame arrives error-free.
The function of the data link layer has nothing to do with the network, its nodes and the type of physical layer adopted, and it doesn't care whether it is running Wo r d, E x c e l or I n t e r n e t. Some connected devices, such as switches, work at the data link layer because they decode frames and use frame information to send data to the correct recipients.
Network layer: the third layer of the O S I model, whose main function is to translate network addresses into corresponding physical addresses and decide how to route data from the sender to the receiver.
The network layer determines the best path from node A in one network to node B in another network by comprehensively considering transmission priority, network congestion, quality of service and alternative routing cost. Because the network layer handles routing, the router belongs to the network layer, because it connects all segments of the network and intelligently guides data transmission. In the network, "routing" is based on addressing scheme, usage mode and reachability to guide data transmission.
Transport layer: the most important layer in the O S I model. The transmission protocol controls the traffic at the same time, or specifies an appropriate transmission rate according to the speed at which the receiver receives data. In addition, the transport layer will force the division of longer packets according to the maximum size that the network can handle. For example, Ethernet cannot receive packets larger than 1 5 0 0 bytes. The transport layer of the sender node divides the data into smaller segments and arranges a sequence number for each segment, so that when the data reaches the transport layer of the receiver node, the data can be reassembled in the correct order. This process is called sorting.
One service working in the transport layer is T C P (Transmission Control Protocol) in T C P/I P protocol family, and another service in the transport layer is S P X (Sequential Packet Switching) in I P X/S P X protocol family.
Session layer: responsible for establishing and maintaining communication between two nodes in the network. The functions of the session layer include: establishing a communication link, keeping the communication link open during the session, synchronizing the session between two nodes, deciding whether the communication is interrupted, and deciding where to retransmit when the communication is interrupted.
You may often hear someone call the conversation layer the "traffic police" of network communication. When you ask your I. S. P. (Internet Service Provider) to connect to the Internet through dial-up, the session layer on the I.S.P server will negotiate your connection with the session layer on the P C client. If your telephone line accidentally falls off the wall jack, the session layer on the terminal will detect the disconnection and start the connection again. Session layer sets communication deadline by determining the priority of node communication and the length of communication time.
Presentation layer: the converter between application and network. At the presentation layer, the data will be formatted according to a scheme that the network can understand. This format also varies depending on the type of network used.
The presentation layer manages the decryption and encryption of data, such as the handling of system passwords. For example, when you look up your bank account on the Internet, you use a secure connection. Your account data is encrypted before sending, and at the other end of the network, the presentation layer will decrypt the received data. In addition, the presentation layer protocol also decodes and encodes picture and file format information.
Application layer: responsible for providing interfaces for software so that programs can use network services. The term "application layer" does not refer to a special application running on the network. The services provided by the application layer include file transfer, file management and e-mail information processing.