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Article Source: Network Code
Copyright: www.networkDictionary.com Network Current
The OSI open system interconnection model is a reference model proposed by the 1984 International Standardization Organization (ISO). This model is a conceptual standard framework for network communication, making communication on the network formed by different manufacturers and application software. Now this model has become a major structural model for communication between computers and between networks. Most network communication protocols currently used are based on the structure of the OSI model. OSI is defined as a seven floors, which is to divide the task of the activity information in the network computer into seven smaller and easier handling task groups. A task or task group is assigned to an OSI layer. Each layer exists alone, so the task assigned to each layer can be executed independently. This allows the solution provided by one layer to be updated without affecting other layers.
Each layer of the OSI seven layer model has a clear feature. Basically, the communication between the seventh to the fourth layer process the first-end data source and destination address, while the third to the first layer processes communication between network devices,. On the other hand, seven layers of the OSI model can be divided into two groups: the upper layer (layer 7, layers 6, and layer 5) and the lower layer (layer 4, layers 3, layers 2, and layer 1). The upper layer processing of the OSI model is applied and implemented on the software. The highest layer, that is, the application layer is closest to the end user. The lower layer of the OSI model is to process data transmission. The information on the physical layer and the data link layer is executed on hardware and software. The most underlying, that is, the physical layer is closest to physical network media (such as, wire) and is responsible for replacing data on the medium.
The specific description of each layer is as follows:
Seventh floor: Application layer
Define user interface programs for network communication and data transfer; provide standard services, such as virtual terminals, documents, and tasks transmission and operations
Layer 6: Representation
Define different data formats between different systems; specifically explain the data transmission format of the independent structure; encoding and decoding data; encryption and decryption data; compression and decompression data
Fifth layer: session layer
Manage the session and conversation between users; control the connection between the user and hang up the connection; report the upper layer error
Fourth floor: Transport layer
Manage information transfer of the first tail connection in the network; provide reliable and orderly packets transmitted by error recovery and stream control devices; providing communication without connection facing packets
Layer 3: Network Layer
Decide how to transfer data between network devices; select packets according to the unique network device address; provide stream and congestion control to prevent loss of network resources
Layer 2: Data Link Layer
Define programs that control communication connections; packets; monitoring and correcting packet transfer errors
First layer: physical layer
Define physical methods for sending data through network devices; network media and device interface; definition of optical, mechanics, and mechanical features
With the OSI layer, information can be transmitted from a computer software application to another application. For example, computer A is to send information from its application to a computer B application, and the application in computer A needs to send the information first to its application layer (seventh layer), then this layer sends information to Indicates the layer (sixth layer), indicating that the layer is transferred to the session layer (fifth layer), so that the physical layer (first layer) is continued. In the physical layer, the data is replaced by a physical network medium and is sent to the computer B. The physical layer of the computer B receives data from the physical medium, then transmits the information up to the data link layer (second layer), and then transferred to the network layer, and continues until the information reaches the application layer of the computer B. Finally, the application layer of the computer B will transfer the information to the application receiver to complete the communication process. The following illustration illustrates that the seven layers of the OSI uses a variety of control information frameworks to communicate with the corresponding layers of other computer systems. Control information consists of special requests and descriptions, and these requests and description exchange in the corresponding OSI layer. The protocol header and protocol tail of each layer of data are the basic framework of two carried control information.
For data transmitted from the upper layer to the next layer, an protocol head is to be added and the protocol end is added. However, adding protocol heads and protocols from the data from the upper layer, not necessarily necessary for an OSI layer.
Each protocol layer may increase the protocol head and protocol tail on the data, and each layer data consists of the upper protocol head, the tail, and data per layer data during transmission. The protocol header includes related layer-to-layer communication information. The protocol head and protocol tail and data are just relative concepts, depending on the protocol layer of the information unit. For example, the transport layer protocol header includes only information that the transport layer can be seen, and the other layers under the transport layer are only transmitted as part of the data to the following layers. For the network layer, one information unit consists of the protocol header and data of the third layer. For the data link layer, all the information transmitted downward through the network layer (Layer 3 protocol head and data) is processed as data processing. In other words, in a particular OSI layer, the data portion of the information unit may contain the protocol head and protocol tail and data from all upper layers, namely the package.
For example, if computer A is to send a data in the application to computer B, computer A should send data to its application layer. The application layer of computer A transmits any control information necessary for any computer B application layer, is passed Add protocol heads in the data. The final message unit contains the protocol head, data, may have a protocol tail, transmitted to the representation layer, indicating that the layer is added to the protocol header of the representation of the computer B and understand the control information. The size of the message unit increases with the addition of each layer protocol header and the protocol, and these protocol heades and protocols contain control information to be used by computer B. In the physical layer, the entire information unit is transmitted over a network media.
The physical layer in the computer B receives the information unit and transmits it to the data link layer. The data link layer in B is then read in the control information, in which the protocol header is added in advance of the data link layer of the computer A. Secondary protocol heads and protocols are removed, and the remainder of the information unit is transmitted to the network layer. Each layer performs the same action. The protocol layer reads the protocol head and protocol end from the corresponding layer, and removes the remaining information to the high layer. After the application layer is executed, the data is transmitted to the application receiving end in computer B, and finally received data generated by the application of computer A.
