The OSI Model Explained and Simplified

(Last Updated On: January 22, 2020)

The OSI Model Simplified with Explanation.

The OSI model (Open System Interconnect) is a theoretical stack of 7 layers that can be used as a reference to understand how network data flows using the TCP IP protocol. It is also known as the TCP IP model.

In this article, I have tried to explain all the aspects of the OSI model and its layers with some examples. So do check out below.

What is The OSI Model?

                         The OSI Model is incredibly a useful tool if you can learn it, it will make life easier for you if you are inter IT field. The OSI Model is actually a logical model for how network systems are supposed to communicate with each other.

It has been developed by ISO – ‘International Organization of Standardization‘, in the year 1974 and maintained by the identification of ISO/IEC 7498-1.

So all this model does is, it breaks down the different components of network communication and divide them into some layers. It was introduced to standardize communication to allow different vendors to share the same network.

So the 7 layer of The OSI Model allows you to think that how the networking system does work. If you understand all these layers perfectly, it will help you to figure out the source of any networking problems that occur and you can think the solution accordingly.

What are the Layers that The OSI Model contains?

OSI  model has seven layers:

The Osi Model

I have already mentioned that the Open System Interconnection (OSI) model defines a networking framework to implement protocols in seven layers. So check this out below.

Application Layer (Layer 7)

The Application Layer is the seventh layer of the Open Systems Interconnection Model.  Application layer provides the platform to send and receive data over the network.  There are various applications available which facilitate different types of communication over a network. Typical examples are web browsers, email clients, and remote file access, etc.

In the OSI model, the definition of the application layer is very narrow in scope, because this is the layer which interacts with the user directly and displays all the results for the query that user has made.

Ex:  Web Browsers(Chrome/Firefox/Safari), Outlook Client. etc are the examples of Application  Layer.

Presentation Layer (Layer 6)

The Presentation Layer is the sixth layer of the Open Systems Interconnection Model. This layer is responsible for the delivery and formatting of information to the application layer for further processing.

A web browser supports various text formats (UNICODE and ASCII), image file formats (JPG, GIF, PNG, BMP) and audio file formats (WAV, MP3, AIFF) as well as HTML and more scripting languages (JavaScript, JScript, VBScript).

In the PRESENTATION layer, the web browser converts these file formats into local formats that are used on the local computer for displaying images, playing sounds and displaying text, etc.

Ex: Examples of the Presentation Layer are  ASCII, EBCDIC, TIFF, GIF, PICT, JPEG, MPEG.etc.

 

Session Layer (Layer 5)

The Session Layer is the Fifth layer of the Open Systems Interconnection Model that provides the mechanism for opening, closing and managing a session between end-user application processes,

It provides the ability to have multiple devices use a single application from multiple locations. It deals with session and connection coordination. Protocols like NFS, NetBIOS names, RPC, SQL work at this layer.

Ex:  examples of the session layer are   OSI protocol suite and Zone Information Protocol (ZIP).

Transport Layer (Layer 4)

The Transport layer is the fourth layer of the Open Systems Interconnection Model that provides host-to-host communication services for applications. Some of the services are connection-oriented communication, reliability, flow control, and multiplexing, etc.

The most commonly used protocols of the transport layer include the Transport Control Protocol (TCP) and User Datagram Protocol (UDP). The connection-oriented method uses the TCP protocol and connectionless method uses UDP protocol.

Connection-oriented transmission is said to be reliable and the other is considered as unreliable. Reliability means guaranteed data delivery. To ensure delivery of every single segment, a connection-oriented method is used. In this approach before sending any segments three-way handshake process is done.

Five main functions of Transport Layer:
1. Segmentation.
2. Connection management.
3. Reliable and unreliable data delivery.
4. Flow control.
5. Connection multiplexing.

Ex: Two great examples of this layer are TCP and UDP.

Network Layer (Layer 3)

The network layer is the third layer of the Open Systems Interconnection Model and it provides data routing paths for network communication.

The main function of the 3rd layer of the OSI model is to permit different networks to connect with each other. It does this by forwarding packets to the routers, which rely on algorithms to determine the best paths for the data to travel. These paths are known as the virtual circuit.

This is why The network layer is considered the backbone of the OSI Model.

Main functions of this layer are:-

1. Define IP address
2. Find routes based on IP address to reach its destination.
3. Connect different data link type together like Token Ring, Serial, Ethernet etc.

Ex: The IP address is a 32-bit long software address, made from two basic components: Network and Host component.

A segment of the device is defined by the Network component and the Host component Defines a specific device on that particular network segment. Where subnet mask distinguishes between the network and host component.

IP addresses are divided into five classes:

  • Class A:  1-126.
  • Class B:  128-191.
  • Class C:  192-223.
  • Class D:  224-239.
  • Class E:  240-254.
  • APIPA (Automatic IP Adress):  It is a class B IP address from 169.254.0.0 to 169.254.255.255
  • Loopback Address / Localhost address : 127.0.0.1

Data Link Layer (Layer 2)

This is the second layer of the Open Systems Interconnection Model. Data Link layer transfers data and it is called as the protocol layer that transfers data.

Data bits are encoded, decoded and organized in this layer before they are transported as frames between network nodes in a wide area network (WAN) or between nodes on the same local area network (LAN)

At data link layer, it extracts the message from the sender and provides it to the receiver by providing sender’s and receiver’s address.

The data link layer consists of two sublayers in its framework, the first one is called  logical link control (LLC) sublayer and the other one named as media access control (MAC) sublayer

Ex: Ethernet, MAC, Token Ring

Physical Layer (Layer 1)

The lowest layer of the OSI reference model is the physical layer. This layer is all about the physical stuff that connects computer together. The physical layer contains information in the form of bits.

The physical layer is where all the hardware related stuff is working out in a network. It is the fundamental layer underlying the higher level functions in a network.

Ex: Hub, Repeater, Modem, Cables are Physical Layer devices.

Conclusion

So, guys, I have tried my best to simplify things so that you can have a better understanding and user experience. Now it’s your turn to pay me back.

Now I am expecting your responses. If you like this content or even dislike,  just leave a comment. Your responses will motivate me to do better and deliver better. I will try my best to enrich my skill.

So thank you guys, Have a good day.

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