OSI Referance Model

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OSI Model and TCP/IP Model

Communication in a network has different layers and these different layers are mentioned in two different models. In other words,there are two common models in computer networking. These two common models are OSI Model and TCP/IP Model.

As a beginner, TCP/IP Model and the OSI Model is very important to understand the network world. The two common terms has also been a classical question of a network engineer technical interviews. What are the layers of OSI Model? How many Layers does TCP/IP Model has? Etc..

Let’s start with OSI Model and then continue with TCP/IP Model.


OSI Referance Model

OSI Referecen Model is the first term that a network engineer learn. It is an important concept and generally always encountered especially at the technical job interviews of junior network engineers.

So, what is OSI Referance Model?


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OSI Referance Model is the standard model of how devices communicate each other over a network . Beginning with Physical Layer, throught the Application Layer, OSI Referance Model shows the 7 Layers of these Network Communication. These layers are orderly;

• Physical Layer (Layer 1),
• Data-link Layer (Layer 2) ,
• Network Layer (Layer 3),
• Transport Layer (Layer 4),
• Session Layer (Layer 5),
• Presentation Layer (Layer 6) and
• Application Layer (Layer 7)

The first four Layers are the Lower Layers and the last three Layers are Upper Layers. Upper Layers are also the Application Service Layer of TCP/IP Model. OSI Model Lower Layers are a little different than TCP/Model Lower Layers.

So, Whats are the roles of these Specific OSI Referance Model Layers? Lets talk about one by one.


Physical Layer (Layer 1)

The Physical Layer (Layer 1) is the actual connectivity between the source and the destination. It is the Layer in which the physical characteristic of the network is defined and bit flow tranmission is done.

Voltage levels and the meaningful 0 and 1 bits are transfered here. Basically it is the layer in which bit streams are transfered.


Data-link Layer (Layer 2)

Data-link Layer (Layer 2) is the medium provision Layer. It encodes and decodes the data in Frames. Encoding means adding a header at the source and decoding means removing this header at the destination. A Layer 2 Header is added in this layer and this header contains source and destination MAC Addresses. So it is responsable to find the next destination in the network.


Lesson tags: tcp/ip, osi model
Back to: CCNA > Network Fundamentals

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