Wed. Jun 19th, 2024

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

Protocol: IP

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

Physical Layer:

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

OSI Function

Layered division of labor assistance

Standardization (compatibility)

Theoretical framework (easy for problem analysis)

OSI Implementation Process

Data encapsulation

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.

Data unpacking

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.

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