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Detailed explanation of OSI seven-layer model

In the network, the seven-layer framework model of the Internet (Open system Interconnection reference Model) constitutes the foundation of the whole network. The seven-layer framework is divided into physical layer, data link layer, network layer, transport layer, session layer, presentation layer and application layer.

1. Physical layer (network cable, hub, repeater, modem)

The physical layer is at the lowest level of the OSI reference model. The main function of the physical layer is to define the mechanical, electrical and functional characteristics between activating and maintaining and shutting down the communication power off. The cover layer provides a physical medium for the upper layer protocol to transmit data so that the bit stream can be transmitted transparently. The data transmitted at the physical layer is called a bitstream. The physical layer does not do any processing to the data, but only completes the transmission operation.

2. Data link layer (network card, bridge, switch)

The data link layer provides reliable transmission over unreliable physical media. The functions of this layer include physical address addressing, framing of data, flow control, error detection and retransmission of data, etc. in this layer, the data is framed and processed for flow control. This layer specifies the topology and provides hardware address addressing. The data transmitted at the data link layer is called a frame. The effective level of the data link layer can be understood as at the intranet level, the addressing based on the Mac address in the intranet, and the reception, verification and splicing of data frames are all completed in the data link layer.

3. Network layer (router)

The network layer is responsible for routing packets between subnets, and can also achieve congestion control, internetwork interconnection and other functions. Establish a connection between two nodes by addressing, including routing and relaying data over the Internet. The data transmitted at the network layer is called a packet. The network layer works in the external network, and the data transmission control in the external network is realized by it.

4. Transport layer

The transport layer is the first layer in the seven-layer model that shifts from network transmission to host. The transport layer is responsible for segmenting the upper layer data and providing end-to-end, reliable or unreliable transmission. In addition, the transport layer also has to deal with end-to-end error control and flow control. The data transmitted at the transport layer is called a segment. The transport layer determines the mode of end-to-end transmission.

5. Session layer

The session layer establishes an end-to-end connection between the two phases, manages the session processes between hosts, and is responsible for establishing, managing, and terminating sessions between processes, including whether the connection is established in full-duplex or half-duplex mode. The session layer is responsible for end-to-end links or scheduling between processes. (session management and process scheduling)

6. Presentation layer

It is mainly used to deal with the expression of information exchanged in two communication systems, and to exchange information on the application layer to ensure that one host can be understood by the application of another host. It includes data format exchange, data encryption and decryption, data compression and recovery and other functions. The presentation layer is responsible for translating what the program wants to express to another program.

7. Application layer

The highest layer in OSI that provides an interface for operating systems or network applications to access network services and determines the nature of communication between processes to meet the needs of users.

The network card belongs to the data link layer (some netizens say it is the physical layer).

Although the network card can realize the physical connection and electrical signal matching with the local area network transmission medium, but its main functions are frame sending and receiving, frame encapsulation and unpacking, media access control, data encoding and decoding and data cache.

1. Encapsulation and unencapsulation of data

When sending, the data handed over from the previous layer is added to the head and tail to become the frame of Ethernet. When receiving, the Ethernet frame is stripped of the head and tail, and then sent to the upper layer.

2. Link management

Mainly the implementation of CSMA/CD protocol

3. Coding and decoding

Manchester coding and decoding.

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