Layer-7 model of OSI Network Structure

Source: Network

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The OSI Open System Interconnection model is a reference model proposed by the International Organization for Standardization (ISO) in 1984. As a conceptual standard framework for network communication, this model makes it possible to conduct communication on networks formed by devices and application software of different manufacturers. Now, this model has become a major structural model for communication between computers and networks. Most of the network communication protocols currently used are based on the structure of the OSI model. OSI defines it as layer-7, which divides tasks related to activity information in network computers into seven smaller and easier-to-Process Task Groups. A task or task group is assigned to an OSI layer. Each layer exists independently. Therefore, tasks assigned to each layer can be executed independently. In this way, the solution provided by a layer can be updated without affecting other layers.

Each layer of the OSI Layer-7 model has clear features. Basically, Layer 7 to Layer 4 processes the communication between the data source and destination address at the beginning and end, while Layer 3 to Layer 3 processes the communication between network devices ,. On the other hand, the seven layers of the OSI model can be divided into two groups: the upper layer (Layer 7, layer 6 and layer 5) and the lower layer (Layer 4, Layer 3, Layer 2 and Layer 1 ). The upper layer of the OSI model processes application information and is executed only on the software. The top layer, that is, the application layer is the closest to the end user. The lower layer of the OSI model processes data transmission. Information on the physical layer and data link layer is executed on hardware and software. The bottom layer, that is, the physical layer is closest to the physical network media (such as wires) and is responsible for replacing data on the media.

The specific description of each layer is as follows:

Layer 7: Application Layer

• Define User Interface programs for network communication and data transmission;
• Provides standard services, such as virtual terminals, documents, and task transmission and operations.

Layer 6: Presentation Layer

• Define different data formats between different systems;
• Specific description of the data transmission format of the independent structure;
• Data encoding and decoding; data encryption and decryption; data compression and decompression

Layer 5: Session Layer

• Manage sessions and conversations between users;
• Controls connections and hangs up connections between users;
• Report upper-level errors

Layer 4: Transport Layer

• Manage Information Transmission of the first and last connections in the network;
• Provides reliable and ordered packets transmitted through error recovery and flow control devices;
• Provides connectionless packet-oriented transmission

Layer 3: Network Layer

• Determines how data is transmitted between network devices;
• Select a package based on the unique network device address;
• Provides flow and congestion control to prevent network resource loss.

Layer 2: Data Link Layer

• Define the program for controlling communication connections;
• Packets;
• Monitor and correct packet transmission errors

Level 1: Physical Layer

• Define the physical method for sending data through a network device;
• Interfaces between network media and devices;
• Define optical, mechanical, and mechanical features

Through the OSI Layer, information can be transmitted from a computer's software application to another application. For example, if computer A wants to send information from its application to computer B's application, the application in computer A needs to first send the information to its application layer (Layer 7 ), this layer then sends the information to the presentation layer (layer 6), which transfers the data to the Session Layer (layer 5) until the physical layer (Layer 1 ). On the physical layer, data is replaced by physical media and sent to computer B. The physical layer of computer B receives data from the physical media, sends the information to the data link layer (Layer 2), and then forwards the information to the network layer until the information reaches the application layer of computer B. Finally, the application layer of computer B transmits information to the application receiver to complete the communication process. The following figure shows the process:

 

Layer 7 of OSI uses various control information frameworks to communicate with the corresponding layers of other computer systems. Control Information consists of special requests and descriptions that are exchanged between the corresponding OSI layers. The protocol header and end of each layer of data are two basic frameworks carrying control information.

For data transmitted from the upper layer to the lower layer, the protocol header must be added, and the end of the Protocol must be added at the same time. However, adding protocol headers and Protocol tails to data from the above layer is not necessary for an OSI layer.

Each protocol layer may add protocol headers and Protocol tails to the data. During transmission, each layer of data is composed of the Upper-layer protocol header, tail, and data. The protocol header contains layer-to-layer communication information. Protocol headers, protocol tails, and data are only relative concepts. They depend on the protocol layer of the analysis information unit. For example, the transport layer protocol header contains information that can only be seen by the transport layer. Other layers under the transport layer only transmit this protocol header as part of the data to the following layer. For the network layer, an information unit consists of a layer-3 protocol header and data. For the data link layer, all information passed down by the network layer (layer-3 protocol header and data) is processed. In other words, at a specific OSI layer, the data part of an information unit may contain the protocol header, the end of the Protocol, and the data, which is called encapsulation.

 

For example, if computer A wants to send data from an application to computer B, computer A wants to send the data to its application layer. the application layer of computer A transmits the control information required by the application layer of any computer B by adding a protocol header to the data in advance. The final message unit, which contains the protocol header and data, may end with the Protocol and be sent to the presentation layer. The presentation layer then adds the protocol header that contains the presentation layer of computer B and understands the control information. The size of the message unit increases with the addition of each layer of the protocol header and Protocol tail. These protocol headers and Protocol tails contain the control information to be used by the corresponding layer of computer B. On the physical layer, the entire Information Unit is transmitted through the network media.

The physical layer in computer B receives information units and sends them to the data link layer. Then, the data link layer in B reads the control information, including the protocol header pre-added to the data link layer of computer. The protocol header and Protocol tail are removed, and the rest of the Information Unit is transferred to the network layer. Each layer performs the same action. The protocol layer reads and removes the protocol header and end from the corresponding layer, and then sends the remaining information to the upper layer. After the application layer is executed, the data is transmitted to the application receiver of computer B, and the data generated by the application of computer A is received at the end.

The communication between one OSI Layer and the other layer is completed using the services provided by the second layer. The services provided by the adjacent layers help the specific OSI layer to communicate with the corresponding layers of another network system. A specific layer of the OSI model usually communicates with three other OSI layers: directly adjacent to the upper and lower layers, and the corresponding layers of the object network system. For example, the data link layer of computer A should communicate with its network layer, physical layer, and data link layer of computer B. The following chart can explain this example.