osi model


OSI stands for Open System Interaction. It has been developed by International Organization for Standardization (ISO) in the  year 1984. It has 7 layers models with each layer having specific function to perform. All these layers work cordinally to transmit data from one source to another destination across the globe.

osi model


  1. Physical layer
  2. Data link layer
  3. Network layer
  4. Transport layer
  5. Session layer
  6. Presentation layer
  7. Application layer


This layer of OSI model represent the physical connection between the systems. It contains the information in the form of bits, which is simply a series of 0s and 1s, while taking care of bit rate control. Media, signal and binary transmission happens in the layer. In this layer, sniffing  attack works.

Hub, Repeater, Modem are Physical Layer devices. 

The data link layer establishes the connection and breaks the packets into fragments and send them from one connection to another. The main function of this layer is that the data transferred  from source to destination is free from error. When a packet arrives in a network, it is the responsibility of the Data Link Layer to transmit it to the destination  using its MAC address. Data link layer also saves the MAC address  of senders and receivers in its headers. Physical addressing. MAC, fibre cables comes under this layer.


The network layer perform two main function first is selection of path to transmit the packets from the number of routes available. Also break the fragments into network packets and again join them at the other end. The sender’s and receiver’s IP address are written in the headers by  the   network layer. 

Routing and logical addressing works here.  IP, MAC, ARP etc.


The transport layer provides services to the application layer and also receives services from the network layer. The transport layer also admit  the successful data transmission and re-transmits the data if an error is found. It’s responsible for reassembling the parts on the entering end, turning it back into data that can be used by the session layer. The transport layer carries out flow control,  segmentation and reassembly etc.  It confirms that data is received completely and if not  data is requested again.

In this layer TCP UDP works. TCP is the connection-oriented protocol and UDP is connection-less protocol.


The function of the session layer is that it build connection  or session between the systems. It is responsible for the opening session when then communication starts and closing when the  data is fully transferred, if in case  of any reason is the data transmitted is interrupted,  the session build up.


The presentation layer performs the function to encode, encrypt  and compress the data so it is transmitted  correctly from one end to another end. The presentation layer receives any data from the application layer and prepares it for lower layer.


At the top of the OSI layer application layer presents the end to end user connection through web browsers. It provides protocols that allow software to send and receive information and present meaningful data to destination users. A few examples of application layer protocols are the (HTTP), (FTP), Simple Mail Transfer Protocol (SMTP). It also serves as a opening to the services access to the network and also for displaying entered  information from the user.

In the TCP / IP model, the application layer, session layer and presentation  consists of single application layer.


  1. Network Access layer
  2. Network layer
  3. Transport layer
  4. Application layer

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