OSI is an open communication system interconnection reference model, which is a well-defined protocol specification. The OSI model has a seven-layer structure, and each layer can have several sub-layers. The 7 layers of OSI are from top to bottom: 7 application layer 6 presentation layer 5 session layer 4 transport layer 3 network layer 2 data link layer 1 physical layer; the upper layer (ie 7, 6, 5, 4 layers) defines the application The function of the program, the following three layers (ie, layers 3, 2, 1) are mainly oriented to the end-to-end data flow through the network.
An application that communicates with other computers that is the communication service for the application. For example, a word processor that does not have a communication function cannot execute the code for communication, and programmers engaged in word processing do not care about layer 7 of OSI. However, if you add an option to transfer files, the programmer of the word processor needs to implement layer 7 of OSI. Examples: TELNET, HTTP, FTP, NFS, SMTP, etc.
Presentation layerThe main function of this layer is to define the data format and encryption. For example, FTP allows you to choose to transfer in binary or ASCII format. If binary is selected, the sender and receiver do not change the contents of the file. If the ASCII format is selected, the sender will send the text after converting the text from the sender's character set to standard ASCII. The standard ASCII is converted to the character set of the receiving computer at the receiving end. Examples: encryption, ASCII, etc.
Session layerIt defines how to start, control, and end a session, including the control and management of multiple two-way messages so that the application can be notified when only a portion of the continuous message is completed, so that the data seen by the presentation layer is continuous, at some In some cases, if the presentation layer receives all the data, the data represents the presentation layer. Example: RPC, SQL, etc.
The functions of this layer include whether to select the error recovery protocol or the error-free recovery protocol, and to multiplex the input of the data streams of different applications on the same host, and also to reorder the received data packets with the wrong order. Examples: TCP, UDP, SPX.
Network layerThis layer defines the end-to-end packet transmission, which defines the logical address that identifies all nodes, and defines how the route is implemented and how it is learned. In order to accommodate transmission media with a maximum transmission unit length less than the packet length, the network layer also defines a segmentation method for how to decompose a packet into smaller packets. Examples: IP, IPX, etc.
data link layerIt defines how data is transmitted on a single link. These protocols are related to the various media in question. Examples: ATM, FDDI, etc.
Physical layerOSI's physical layer specifications are specific to transmission media. These specifications are also often referenced by standards established by other organizations. The use of connectors, frames, frames, current, coding, and optical modulation are among the various physical layer specifications. The physical layer often uses multiple specifications to define all the details. Examples: Rj45, 802.3, etc.
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