The Seven Layer of OSI Model are generally divided into two groups:

Layer 1-4 are called Media Layers or Lower Layers

Layer 5-7 are called Upper Layers or Host Layers

Lower/Media Layers are responsible for Networking & they handle data transport issues. The physical layer and the data link layer are implemented in hardware and software. The physical layer is responsible for actually placing information on the medium.

Upper/Host layers are about Applications. Upper layers deal with application issues and generally are implemented only in software.

PDU’s in OSI Model

During Data communication, user data is passed from layer to layer & Control information comprising Header & Trailer is updated at each layer. So, each layer has different ‘units of information’. This combination of Header, Trailer & Data is called PDU (or protocol data unit). So, every Layer has a different name for these PDU’s.

They are called Bits on Physical Layer, Frames on Data Link Layer, Packets on Network Layer, Segments on Transport Layer & Data on all Application Layer. Below are the PDU’s at each OSI Layer & their names:

Modular Nature of OSI

OSI Model is based on modular methodology. Different vendors, people & teams can work on different parts & then combine them together to form a fully functional Communications System. It is an easy to understand & easy to implement model.

With this approach, one vendor can write software that implements higher layers …. for example, a web browser like Google Chrome or Firefox…. and another vendor can write software that implements the lower layers to support it …. for example, TCP/IP software by Microsoft in its Windows OS. In this way, Chrome can work on Windows OS which was not possible without OSI. Because without OSI, Google would have built Chrome following its own standards while Microsoft would have built WindowsOS on its own standards … & both of these proprietary standards would never work together if they were not standardised under OSI.

OSI Working/Operation

Each Layer in OSI serves the layer above it. Data flow occurs b/w layers through Encapsulation & Decapsulation.

Encapsulation: “Preparing & passing the data by any Upper layer to the layer below it, is called Encapsulation”. Means, going from the application layer all the way down to the physical layer.

Decapsulation: Going Upwards from the physical layer till application layer is called decapsulation.

Whenever moving the data b/w computers/hosts, we can’t skip any layers in the OSI model. We always need to go through all the layers to send data over the network. We have to follow the whole Encapsulation Decapsulation Process.

Sending a message from one computer to another computer working under OSI Model is similar to real world example of posting a letter. If we want to send a letter to your friend through Post Mail,

1. First, we will write a letter
2. Then we will put the letter in an envelope
3. We will write the Sender/Receiver detail
4. Envelope is put into the nearest Letterbox
5. Postman will bring all the letters from Letterbox to the central processing Office
6. Our envelope will be delivered to the city of receiver
7. The letter will be delivered to our friend
8. He will open the envelope and read its contents

Although, we are interested in only the last step i.e. our friend should read the letter but we have to follow all the steps. If we put our letter directly in the mailbox without encapsulation into envelop, it won’t be delivered. Unless someone at the postal office is friendly enough to deliver it anyway … but it never happens in Network World.

Let us take a Network World example of Opening a Website from our Laptop. We open our Web Browser (Google Chrome, Firefox or Internet Explorer) and type in the URL of a website like ATech:

1. First, our computer will send a data request to the web server hosting ATech website using HTTP at application layer
2. The message moves to presentation layer which will structure the information of the application in a certain format which is readable by receiver i.e. ASCII or any other format
3. The session layer will make sure to separate all the different sessions
4. Transport Layer will transmit this data using TCP or UDP
5. Network Layer will complete the Logical Addressing & Routing
6. Physical Layer will complete the Physical Addressing & will put the IP Packets into Ethernet Frames
7. Frames will be converted into bits and will be sent down the cable using electric signals

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