1. Background Overview of OSI Model (Why)
Reflection 1: the computer host needs CPU, motherboard, memory, graphics card, hard disk and other devices, and different devices are produced by many manufacturers. For example, the CPU includes Intel and AMD, the motherboard includes Gigabyte and ASUS, the memory includes Samsung and Kingston, and the hard disk includes Western Digital and Seagate. So, why do you use the combination of Gigabyte motherboard + Kingston + Seagate + Intel to work normally, and there is no problem?
Each manufacturer’s equipment, whether memory or hard disk, follows a standardization, which is formulated by the corresponding organization in advance.
Reflection 2: System Integration
After receiving a whole project, which involves networks, servers and applications, it is impossible for a manufacturer to complete it independently. It must be divided and coordinated.
When the network began to develop, at the beginning, each manufacturer operated and produced different equipment, and they were incompatible with each other. Therefore, the international standards organization (ISO) later developed a standard OSI, so that all manufacturers follow the standard, and the products produced can be compatible with each other
2. OSI Model
In 1983, ISO International released the famous OSI model.
OSI Full Name
Open system interconnection model
OSI model is an open architecture. It stipulates that the OSI model is divided into seven layers and specifies the functions of each layer.
3. Overview of OSI Model
Application layer: Application
Main functions: Provide user access interface
Software: QQ / Wechat, browser, Xunlei, various client software, office software, etc
Protocols: http, HTTPS, OICQ, SMTP, POP3, DNS, FTP, etc
Presentation Layer: Presentation
Main Functions: provide data display, such as data compression and data encryption in data formats (pictures: JPG, GIF, PNG, etc., videos: MP4, flv, avi, etc., office files: Doc, PPT, xls, etc.)
Session Layer: session
Main Functions: provide session management functions, such as establishing / distinguishing sessions, maintaining sessions and disconnecting sessions
Example: QQ, browser
Transport Layer: Transport
Main Function: provide reliable end-to-end connection
Related Protocols: TCP (transmission control protocol) UDP (user data protocol)
Case: Flying Pigeon Book transmission and online shopping
Network Layer: Network
Main Functions: it provides three-layer addressing and three-layer data forwarding functions
Related Equipment: router, layer 3 switch, firewall, etc
Data Link Layer: data link
Main Functions: provide layer-2 addressing and layer-2 data forwarding functions
Main Protocols: Ethernet (Ethernet protocol), token ring network
Related Equipment: network card, bridge, layer 2 switch
Main Functions: provide physical specifications (cable (medium) specifications – copper cable, twisted pair, optical fiber cable, interface standards: RJ45, RJ11, optical fiber interface)
Related Equipment: repeater, hub
Layered division of labor assistance
Theoretical framework (easy for problem analysis)
OSI Implementation Process
In the process of information transmission from the high-level to the low-level, each time we add information to the original data (hold the head), we become encapsulation. The essence of data encapsulation is to add user data to addressing information.
When the information is transmitted from the bottom layer to the top layer, the header and tail information of each layer are stripped.
Data Unpacking: the essence is the process of removing addressing information from encapsulated data packets.
The above is about OSI model of CCNA certification. To learn more about OSI model, you will have a better understanding of CCNA exam. In addition, CCNA also includes other directions, such as CCNA security, and CCNA routing and switching. Practice more and obtain CCNA certification as soon as possible.