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OSI reference model

1- The OSI reference model introduction:

The International Organization for Standardization ISO has introduced the Open Systems Interconnection Reference Model (Open Systems Interconnection Reference Model, ISO-OSI RM). This model defines the standards for different computer interconnections and is the basic framework for designing and describing computer network communications.

The system structure of the open system interconnection reference model is divided into 7 layers. Peer-to-peer communication is performed between the layers in the model, and the communication is only logical. The real communication is implemented at the lowest level – the physical layer. The layer needs to complete the corresponding functions, and the next layer provides services for the upper layer, thus dividing the complex communication process into multiple independent sub-problems that are easier to solve.

2- OSI reference model Layering:


Data Format

Function and Connection Mode

Typical Device



An interface between web services and consumer applications

Terminal equipment (PC, mobile phone, tablet, etc.)



Data representation, data security, data compression

Terminal equipment (PC, mobile phone, tablet, etc.)



Session layer connection to transport layer mapping; session connection flow control; data transfer; session connection recovery and release; session connection management, error control

Terminal equipment (PC, mobile phone, tablet, etc.)



Use an addressing mechanism to identify a specific application (port number)

Terminal equipment (PC, mobile phone, tablet, etc.)



Path selection between different network systems based on network layer address (IP address)

Gateway, router

Data Link

Bit information turned into Frame data

Functions such as establishing, revoking, identifying logical links and link multiplexing, and error checking on the physical layer. Addressed by using the hardware address or physical address of the receiving system

Bridge, switch


Bit stream

Establish, maintain, and cancel physical connections

Optical fiber, coaxial cable,

Twisted pair, network card, repeater, hub

3- OSI reference model layering Characteristics:

People can easily discuss and learn the specification details of the agreement.

The standard interface between the layers facilitates the modularization of the project.

Create a better interconnect environment.

Reduced complexity, making the program easier to modify, and product development faster.

Each layer utilizes the immediately adjacent service, making it easier to remember the functions of each layer.

OSI is a well-defined set of protocol specifications and has many optional parts to accomplish similar tasks.

It defines the hierarchy of open systems, the interrelationships between levels, and the possible tasks involved in each layer. It serves as a framework to coordinate and organize the services provided by each layer.

The OSI reference model does not provide an achievable method, but rather describes concepts for coordinating the development of inter-process communication standards. That is, the OSI reference model is not a standard, but a conceptual framework used in the development of standards.

4- TCP/IP Comparison

Application layer, corresponding to OSI application layer, presentation layer, session layer

Transport layer, corresponding to the transport layer of OSI

Network layer, corresponding to the network layer of OSI

Network interface layer, corresponding to the data link layer and physical layer of OSI

The network layer of the OSI model supports both connection-oriented and connectionless communication, but the transport layer only supports connection-oriented communication; the network layer of the TCP/IP model only provides connectionless services, but two communication modes are provided simultaneously on the transport layer.

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