What is OSI Model ?

In this article, we will discuss the OSI (Open System Interconnection), Model. It is a reference model which is design to understand large and complex network architecture in a simple layered approach. OSI model itself does not have any direct relationship with the network. Actually, it defines a networking framework in which various networking protocols are work in different layers.

OSI Model in Networking
OSI Model

History of Open System Interconnection (OSI) Model

In 1970, we had several network communication protocols but they designed for the same types of machines. For example, IBM machines could communicate only with IBM machines.

In 1978, ISO (International Standard Organisation) starts working to develop a network reference module. In 1984, finally, a reference model has been developed and this is known as Open System Interconnection.

OSI model has 7 different layers. Each layer has different functions. Different types of protocols are works on different layers.

7 Different layers of the Open System Interconnection (OSI) Model

As we already discussed, the OSI Model has 7 different layers. In this session, we will cover all these layers.

Application Layer

This is the 7th protocol of the OSI Model. In this layer, different protocols have direct interacted with the user application. On this layer, various protocols works. They encapsulate their header on this layer. After the encapsulation, they handover this data to the presentation layer.

Examples : HTTP, HTTPS, FTP, RIP, DNS, DHCP, BGP etc.

Presentation Layer

This layer is the 6th layer of the OSI Model. In this layer, various operations are performed on original data from the application layer. Here, mainly Data Encryption, Data Decryption, Data conversion, Compression, and Serialization operations are performed on original data. After performing the various operation, it handovers this data to the Session Layer.

Session Layer

Session Layer is the 5th layer of the OSI model.  This layer maintains sessions between different computers. The session layer is responsible for session creation, session managing, and session termination between different computers. This layer is also important for session checkpointing and recovery. After the creation or termination of data, it handover the data to the transport layer (described below).

Transport Layer

The transport layer is the 4th layer of the OSI model. In this layer, two major protocol TCP and UDP is works. This layer chucks the actual data into segments and then encapsulate their head on this. After the encapsulation process, it handover this segment to the network layer on which Internet Protocol (IP) protocol is working.

Ports: Port is a virtual path from source to destination. Both protocols TCP and UDP, having two 16 bit fields which are known as Source Port and Destination Port. Protocols at the application layer having their pre-defined port numbers. These port number is also known as “Well define ports“. During transmission of data, a client puts a random port number in Source Port filed and a well-defined port in the Destination field.

Protocols: TCP, UDP.

Network Layer

The network layer is the 3rd layer of the OSI model. In this layer, Internet Protocol (IP) works. This layer receives data from the Transport layer. Here, IP protocol encapsulates their header on received data and thus forms a data packet. IP protocol, here mainly adds source and destination IP address. This data packet further sends to the Data Link Layer of the OSI model.

Protocols: IPv4, IPv6, EIGRP, OSPF.

Data Link Layer

Data Link layer is the 2nd layer of the OSI model. This layer is responsible for node to node data transfer. This layer has two sub-layer.

Medium Access Control (MAC) layer: Responsible for how devices gain access to transmit data over a medium.

Logical Link Control (LLC) layer: It is responsible for identifying and encapsulating Network Layer protocols. It is also used for error checking and error correction.

Example: On the data link layer, IEEE standards 802.3 (Ethernet) and 802.11 (Wi-Fi) works.

Physical Layer

This is the first protocol of the OSI model. This layer is responsible for sending and receiving data bits from one end to another end.

Example: Here, Transmission media like Wire, RJ45, etc works.


OSI (Open System Interconnection), is a reference model developed by ISO (International Standard Organisation) in 1984. This model has 7 different layers. Each layer has different protocols. It is very helpful for understanding data flow between computers.

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