The communication principle of each layer in the OSI seven-layer model

(1) Physical layer

In the OSI reference model, the Physical Layer is the lowest layer of the reference model and the first layer of the OSI model. The main function of the physical layer is to use the transmission medium to provide a physical connection for the data link layer to realize transparent transmission of the bit stream. The transparent transmission of the bitstream between adjacent computer nodes is realized, and the difference between the specific transmission medium and the physical device is shielded as much as possible. The data link layer above it does not have to consider what the specific transmission medium of the network is. The "transparent transmission bit stream" indicates that the bit stream transmitted after the actual circuit has not changed. The task of the physical layer is to provide a physical connection to its upper layer, as well as their mechanical, electrical, functional, and protocol characteristics. For example, the type of cable and connector, the voltage of the transmitted signal, etc. are specified. At this level, the data has not yet been organized, only as the original bit stream or electrical voltage processing, the unit is biting. It is really difficult to understand, but in one sentence, it is to put the initial current and provide the physical connection to the upper layer. Ensure the transparency of the current, etc.

(2) Data link layer

Datalink Layer, the second layer of the OSI reference model, which controls the communication between the network and the physical, is a bridge whose main function is how to reliably transfer data over unreliable physical lines. To put it bluntly, it is to ensure the reliability of transmission. To ensure transmission, the data received from the network layer is split into specific frames that can be transmitted by the physical layer. A frame is a structural package used to move data. It includes not only raw data but also the physical address of the sender and receiver, as well as error detection and control information. The address determines where the frame will be sent, and the error correction and control information ensures that the frame arrives without errors. If the receiving point detects an error in the transmitted data while transmitting the data, the sender is notified to resend the frame.

The functionality of the data link layer is independent of the network and its nodes and the type of physical layer it is in, and it is not concerned with whether Word, Excel or the Internet is being used. There are some connected devices, such as switches, that work at the data link layer because they decode frames and use frame information to send data to the correct receiver. The data link layer establishes a data link between adjacent nodes based on the bitstream service provided by the physical layer, provides error-free transmission of data frames on the channel through error control, and performs an action series on each circuit. The data link layer provides reliable transmission over unreliable physical media. The role of this layer includes physical address addressing, data framing, flow control, data error detection, retransmission, and so on. It feels very difficult to understand, simple summary to ensure the reliability of data transmission, as well as error correction and retransmission mechanism.

(3) Network layer

Network Layer, the third layer of the OSI reference model. Its main function is to translate the network address into the corresponding physical address and decide how to route the 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 considering the cost of sending priority, the degree of network congestion, the quality of service, and the cost of optional routes. Because the network layer processes and intelligently guides data transmission, the router connects to various segments of the network, so the router belongs to the network layer. The network is responsible for establishing the routes they use between the source machine and the target machine. This layer itself does not have any error detection and correction mechanisms, so the network layer must rely on the reliable transport service provided by the DLL between the end terminals.

Simply put, a path is found in the network, transmitted one by one, because the data link layer ensures that the data between the two points is correct, so the source-to-destination data is also correct so that on a machine The information can be passed to another one. But the end user of the computer network is not the host, but an application process on the host. This process is implemented by the transport layer.

(4) Transport layer

Transport Layer, the fourth layer of the OSI reference model. The transport protocol performs flow control at the same time or specifies an appropriate transmission rate based on how fast the receiver can receive data. In addition to this, the transport layer forcibly splits longer packets according to the maximum size that the network can handle. For example, Ethernet cannot receive packets larger than 1500 bytes. The transport layer of the sender node splits the data into smaller pieces of data and arranges a sequence number for each piece of data so that when the data arrives at the transport layer of the receiver node, it can be reassembled in the correct order. This process is called sorting. One service that works at the transport layer is TCP (Transmission Control Protocol) in the TCP/IP suite, and the other transport layer service is SPX (Sequenced Packet Exchange) of the IPX/SPX protocol suite.

After the network layer is handed over to the transport layer, the transport layer must identify which process the service is requesting and who to hand over. When I hand you something, I want to inform you that it is the work of the conversation layer.

(5) Session layer

Session Layer, the fifth layer of the OSI reference model. Responsible for establishing, maintaining, and terminating communications between two nodes in the network. The functions of the session layer include: establishing a communication link, keeping the communication link of the session process unblocked, synchronizing the dialogue between the two nodes, determining whether the communication is interrupted, and deciding where to resend when the communication is interrupted.

Notify someone to say hello to you to make a call. Just like now I am talking to you, your ears can understand and understand, but if the following is a foreigner, he heard my voice, did he understand? He can't understand. For the computer network, the client sent a request to the server, the server should be able to understand what the request is, so the next question is how do you understand? There are two levels of understanding. I speak Chinese and he can only understand English. Then there should be a translation. Translating Chinese into English, the work is done to the next layer of presentation.

(6) presentation layer

Presentation Layer, the sixth layer in the OSI reference model. The translator between the application and the network, at the presentation level, the data will be formatted according to the network's understandable scheme; this formatting will also vary depending on the type of network used. Indicates the decryption and encryption of layer management data, such as the processing of system passwords. For example, to check your bank account on the Internet, you are using a secure connection. Your account data is encrypted before it is sent, and at the other end of the network, it means that the received data will be decrypted. In addition to this, the presentation layer protocol also decodes and encodes picture and file format information.

(7) Application layer

Application Layer, the highest layer in the OSI reference model, the seventh layer. An application layer, also known as an application entity (AE), consists of several Application Specific Service Elements (SASEs) and one or more Common Application Service Elements (CASEs). Each SASE provides specific application services such as File Transport Access and Management (FTAM), Electronic Message Processing (MHS), Virtual Terminal Protocol (VAP), and the like. The CASE provides a common set of application services such as Contact Control Service Element (ACSE), Reliable Transport Service Element (RTSE), and Remote Operation Service Element (ROSE). Mainly responsible for providing an interface to the software to enable the program to use network services. The term "application layer" does not refer to a particular application running on the network. The services provided by the application layer include file transfer, file management, and information processing of email. Passing ccna r&s exam will help you find job easily.

To give an example to understand: I am speaking Mandarin in this case. I can only translate English into the face. The other party translates my words into Mandarin through the presentation layer, but it does not necessarily mean the meaning of certain special characters and phrases. Then you need professional knowledge and domain knowledge to understand after learning. The application layer is doing this. For computers, every application has its own specific expression.

To sum up:

1. Physical layer: mainly defines physical device standards, such as the interface type of the network cable, the interface type of the optical fiber, and the transmission rate of various transmission media. Its main function is to transmit the bitstream (that is, it is converted from 1,0 to current intensity for transmission, and then converted to 1,0 after reaching the destination, which is what we often call digital-to-analog conversion and analog-to-digital conversion). The data of this layer is called a bit.

2. Data Link Layer: Defines how to format data for transmission and how to control access to physical media. This layer usually also provides error detection and correction to ensure reliable transmission of data.

3. Network layer: Provides connectivity and path selection between two host systems in a network located in different geographic locations. 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 such connections.

4. Transport layer: defines some protocols and port numbers for transmitting data (WWW port 80, etc.), such as: TCP (Transmission Control Protocol, low transmission efficiency, high reliability, high reliability for transmission, large amount of data Data), UDP (User Datagram Protocol, contrary to TCP characteristics, is used for transmission reliability requirements, data with small data volume, such as QQ chat data is transmitted in this way). The main purpose 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: establishes a path for data transmission through the transport layer (port number: transport port and receive port). Mainly initiate a session or accept a session request between your systems (the devices need to know each other if they can be IP or MAC or hostname).

6. Presentation layer: ensures that information sent by the application layer of one system can be read by the application layer of another system. For example, a PC program communicates with another computer, one of which uses extended binary exchange (EBCDIC) and the other uses American Standard Code for Information Interchange (ASCII) to represent the same character. If necessary, the presentation layer implements conversion between multiple data formats by using a common format.

7. Application layer is the OSI layer closest to the user. This layer provides network services for user applications such as email, file transfer, and terminal emulation.