Open System Interconnection Model (OSI Model)

Today we will learn about the popular Open System Interconnection Model for interconnection. It is a hypothetical model to visualize the network interconnection. This model is divided into seven layers.

These seven network layers are classified into two types namely Media Layers and Host Layers.

Media Layers further subdivided into three types : 

• Physical Layer (This layer is responsible for bit streams transmissions over the physical medium)

• Data link layer (Transfer of data frames takes place)

• Network Layer (this layer used for routing and traffic operations)

Host layers are further subdivided into four types :

• Transport Layer (data fragmentation takes place)

• Session Layer (Manages the session for uninterrupted connection)

• Presentation Layer (translation of data between network service and an application happens)

• Application Layer (layer in which resources and file sharing takes place).

Now, let us look about all of these layers in brief and try to understand their functioning.

Physical Layer

This Layer is responsible for the transmitting and receiving data between device and physical transmission medium (i.e. wires, optical fiber cables, etc.). Conversion of bits (binary digits) into electrical, optical and radio signals takes place here. This layers correspond to the connection with physical devices directly like resistances, Bluetooth, Ethernet, USB, etc.

Data Link Layer

Data link layer is responsible for node-to-node transfer of data.it also correct the error of data packet losses and repair the packets. According to IEEE standard this layer is classified into further two parts MAC (Medium access control) and LLC (Logical Layer control). P2P (peer to peer) data linking takes place in this layer. Encryption can be applied to this layer.

Network Layer

This layer is the medium for transferring the packets of data from different nodes with different networks. Data fragmentation also takes place by breaking data into small fragments and transfer them independently from several nodes and it also reassembles those data fragments back again at the destination node.

Transport layer

Transport layer takes care of reliability of packets retrieved from the previous layer and verifies the data if the data packet is good and error-free then it acknowledges the successful transfer.

There are classes defined in the layer according to the packet reliability. In the previous blog, studied about the SSL/ TLS Encryption, which is associated to this layer. TLS (Transport Layer Security) provides the security to the Transport Layer by verifying the reliability.

Session Layer

It is responsible for the graceful session handling. This layer manages the session categorization and control of session. It can terminate, and start a session which is handled over TCP (Transmission Control Protocol) and this layer is implemented explicitly in Desktop Environment using the RPCs (Remote Procedure calls).

Presentation Layer

It translates the data between application and network in that form that application require and also provide the independence from data representation. It is also known as syntax layer because semantics are being taken care by this layer. Data compression takes place in this layer.

Application Layer

This is the OSI layer, where direct interaction of user and software applications take place. Interaction with applications that fall outside the scope of OSI model. It determines identity and availability of communication partner where data needs to be transmitted. Application layer does not deal with the resource availability as host may be free to use the different resources for example a website can be hosted using the http resource and https as well both independently.

However, there are nowadays other new models been developed like TCP/IP (Transmission control protocol/ Internet Protocol) model to understand the computer networking and they are being used too but most of the institutions uses this basic OSI model in their regular classes.

Reference : Wikipedia.org

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