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The concept of OSI

The    Open System Interconnect Open Systems Interconnection reference model is defined by ISO (International Organization for Standardization). It is a flexible, robust and interoperable model.

The purpose of OSI model

   standardizes the interconnection standards of different systems, making it easier for two different systems to communicate without changing the underlying hardware or software logic.

OSI seven-layer model

   from bottom to top are: physical layer, data link layer, network layer, transport layer, session layer, presentation layer, application layer.

OSI layer 7 function

Application layer

   provides an interface for software, so that applications can use network services.

Presentation layer

Decoding and encoding, encryption and decryption, compression and decompression of    data.

Session layer

   establishes, manages, and terminates session connections between entities.

Transport layer

Segment and reorganize      data to provide end-to-end data services (reliable or unreliable: TCP, UDP)

Network layer

   transmits packets from source to destination, logical addressing and routing.

Data link layer

   encapsulates the packet data into frames to realize the communication between two adjacent nodes and error detection.

Physical layer

   transmits bits over the medium, providing mechanical and electrical protocols.

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