How much do you know about OSI and TCP/IP? Let us take you into the world of communications

When I first came into contact with computers, I was very curious about how data was transmitted. Through later learning, I gradually understood the principles of computer communication. I believe that when many people learn computers, the first course they take is the principles of computer communication, and the principles of communication are inseparable from two reference models: the OSI seven-layer reference model and the TCP/IP four-layer model.

1. Physical layer: mainly defines physical equipment standards, such as the interface type of network cables, the interface type of optical fibers, the transmission rate of various transmission media, etc. Its main function is to transmit bit streams (that is, converting 1 and 0 into current strength for transmission, and then converting them into 1 and 0 after reaching the destination, which is what we often call digital-to-analog conversion and analog-to-digital conversion). The data at this layer is called bits.

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

3. Network layer: Provides connections and path selection between two host systems in a network located in different geographical locations. The development of the Internet has greatly increased the number of users accessing information from sites around the world, and the network layer is the layer that manages this connection.

4. Transport layer: defines some protocols and port numbers for data transmission (such as WWW port 80), such as TCP (Transmission Control Protocol, low transmission efficiency, strong reliability, used to transmit data with high reliability requirements and large data volume), UDP (User Datagram Protocol, which is exactly the opposite of TCP characteristics and is used to transmit data with low reliability requirements and small data volume, such as QQ chat data is transmitted in this way). It mainly segments and transmits the data received from the lower layer, and reassembles it after reaching the destination address. This layer of data is often called a segment.

5. Session layer: Establish a data transmission channel through the transport layer (port number: transmission port and receiving port). Mainly initiate a session or accept a session request between your systems (devices need to know each other, which can be IP, MAC or host name).

6. Presentation layer: It 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 Decimal Interchange (EBCDIC) and the other uses American Standard Code for Information Interchange (ASCII) to represent the same characters. If necessary, the presentation layer converts between multiple data formats by using a common format.

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