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(1) physical layer (Physical Layer)

The physical layer is the lowest layer of the OSI reference model, which uses transport media to provide physical connectivity to the data link layer. It is mainly concerned with the transmission of bit streams from one node to another through physical links, which may be copper wires, satellites, microwaves or other communication media. Its concerns are as follows: how many volts represent 1? How many volts represent zero? What's the clock rate? Full-duplex or half-duplex transmission? Generally speaking, the physical layer is concerned with the mechanical, electrical, functional, and procedural characteristics of the link.

(2) data Link layer (Data Link Layer)

The data link layer provides services for the network layer and solves the communication problem between two adjacent nodes. The protocol data unit transmitted is called data frame.

The data frame contains information such as physical address (also known as MAC address), control code, data and check code. The main function of this layer is to convert unreliable physical links into error-free data links for the network layer by means of verification, confirmation and feedback retransmission.

In addition, the data link layer also needs to coordinate the data transmission rate between the transceiver and the sender, that is, to control the flow, so as to prevent the receiver from overflowing the buffer and blocking the line due to the delay in processing the high-speed data from the sender.

(3) Network layer (Network Layer)

The network layer provides services to the transport layer, and the protocol data units transmitted are called packets or packets. The main function of this layer is to solve the problem of how to send the packet through each node, that is, to send the packet to the destination through the path selection algorithm (routing). In addition, in order to avoid network congestion caused by too many packets in the communication subnet, it is necessary to control the number of incoming packets (congestion control). When the packet has to cross multiple communication subnets to reach its destination, it is necessary to solve the problem of interconnection.

(4) Transport layer (Transport Layer)

The role of the transport layer is to provide end-to-end reliable and transparent data transmission services for the upper layer protocols, including dealing with error control and flow control. This layer shields the details of the lower layer data communication to the upper layer, so that the high-level users can only see a host-to-host reliable data path that can be controlled and set by the user between the two transmission entities.

The protocol data unit transmitted by the transport layer is called a segment or message.

(5) session layer (Session Layer)

The main function of the session layer is to manage and coordinate the communication (dialogue) between various processes on different hosts, that is, it is responsible for establishing, managing and terminating sessions between applications. The conversation layer gets its name because it is very similar to the concept of conversation between two entities. For example, an interactive user session begins with a login to the computer and ends with a logout.

(6) presentation layer (Presentation Layer)

The presentation layer deals with the representation of the data coding that flows through the nodes to ensure that the information sent by the application layer of one system can be read by the application layer of another system. If necessary, this layer can provide a standard representation for converting a variety of data representation formats within the computer into standard representations used in network communications. Data compression and encryption is also one of the conversion functions provided by the presentation layer.

(7) Application layer (Application Layer)

The application layer is the highest layer of the OSI reference model and is the interface between the user and the network. This layer completes the application needs of network users through applications, such as file transfer, sending and receiving e-mail, and so on.

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