Introduction to the OSI Layer-7 Model

Introduction to the OSI Layer-7 Model
OSI is an open reference model for System Interconnection. It is a well-defined protocol specification. The OSI model has a layer-7 structure, and each layer can have several child layers. Next I will briefly introduce these 7 layers and their functions.

Layer 7 of OSI is

7 Application Layer telnet, HTTP, FTP, WWW, NFS, SMTP
6 presentation layer encryption, asii
5 session-layer RPC and SQL
4 Transport Layer TCP, UDP, SPX
3 network layer IP, IPX
2. data link layer ATM, FDDI
1 physical layer RJ45, 802.3

Among them, Layer 7, layer 6, layer 5, and Layer 4 define the functions of the application. Layer 3, Layer 3, and Layer 1 are mainly oriented to end-to-end data streams through the network. Next I will introduce the features of these 7 layers:

(1) Application Layer: an application that communicates with other computers. It corresponds to the communication service of applications. For example, a word processing program without communication functions cannot execute communication code, and programmers engaged in word processing do not care about OSI Layer 7th. However, if a file transfer option is added, the programmer of the word processor needs to implement the OSI Layer 7th. Example: Telnet, HTTP, FTP, WWW, NFS, SMTP, etc.

(2) Presentation Layer: The main function of this layer is to define the data format and encryption. For example, FTP allows you to transfer data in binary or asii format. If binary is selected, the sender and receiver do not change the file content. If the asii format is selected, the sender converts the text from the sender's character set to the standard asii and then sends the data. The receiver converts the standard asii to the character set of the receiver's computer. Example: encryption, asii, etc.

(3) Session Layer: it defines how to start, control, and end a session, including control and management of multiple two-way hours, so that the application can be notified when only part of the continuous message is completed, so that the data seen in the presentation layer is continuous. In some cases, if the presentation layer receives all the data, data is used to represent the presentation layer. Example: RPC and SQL.

(4) Transport Layer: This layer features whether to select the error recovery protocol or error-free recovery protocol, and reuse the input of data streams of different applications on the same host, it also includes the re-sorting function for packets that are not in the correct order. Example: TCP, UDP, and SPX.

(5) network layer: This layer defines the end-to-end packet transmission. It defines the logical addresses that can identify all nodes, and also defines the routing implementation and learning methods. To adapt to the transmission medium whose maximum transmission unit length is smaller than the packet length, the network layer also defines how to break a packet into smaller packets. Example: IP, IPX, etc.

(6) data link layer: it defines how data is transmitted on a single link. These protocols are related to the song media discussed. Example: ATM, FDDI, etc.

(7) Physical Layer: OSI physical layer specifications are characteristic standards for transmission media. These specifications are generally referred to standards developed by other organizations. Connector, needle, needle usage, current, current, encoding and optical modulation all belong to the content of various physical layer specifications. Multiple specifications are commonly used in the physical layer to define all details. Example: RJ45, 802.3, etc.