Open System Interconnection Model

OSI Model

Open System Interconnection Model
Fig 1: Open System Interconnection Model

OSI Model stands for the Open System Interconnection Model. This model is the model that was published by the ISO "International organization for Standardization" in the 1984. This Model is published just for communication purposes of the different vendors. There were the different devices that manufacturing were different and their pattern of the communication mismatch to each other. So OSI model published for the communication of the different vendors because it is open source so that is the reason this model called the Open System Interconnection Model. This Model is known as the theoretical model. This model is classify into the seven layers just for the easy understanding of the networking how the data propagate from sender to the receiver. So following are the seven layers of Open System Interconnection Model:
  1. Application Layer  
  2. Presentation Layer 
  3. Session Layer
  4. Transport Layer 
  5. Network Layer
  6. Data Link Layer
  7. Physical layer

Application Layer  

Application layer is the 7th layer of the OSI Model and if we start the  top to bottom then then it will make the 1st layer. So the mainly sequence that mostly common is bottom to top. The application layer is used by user applications  that pass messages from one computer to another in a network. For example, Internet Explorer, Google  Chrome, Firefox, Microsoft Outlook, and so on. Note that the user applications do not reside on  the application layer, but they use application layer protocols to perform their activities. Application layer is the layer that provide the user interface for user to interact. There are many applications in the computer field and if we see applications are divided into the two types: 

  1. Computer Application 
  2. Network Application

We need the network application for the communication and communication is necessary for the resources sharing. There are the multiple protocols that operate on this layer that are the following:

  • Http that use the port number 80.
  • Https that   use the port number 443.
  • FTP that use the port number 20-21.
  • SSH that use the port number 22.
  • Telnet that use the port number 23.
  • SMTP that use the port number 25.
  • TFTP that use the port number 69.

Presentation Layer

Presentation layer is the layer that is the 6th layer of the OSI model and this layer concerns with the presentation of the data. presentation layer is the layer at which encoding of the data and decoding of the data  occur and that layers also concerns with the encryption and decryption of the data. Compression and decompression of the data also a part of the presentation layer .

Session Layer

Session layer is the 5th layer of the OSI model and that layer is play an important role for the checking the availability of the destination. That layer is very important layer which is use to establish  the session, maintain the session, and the terminating of  session. The other main role of this layer is data synchronization of the session streams that are the multiple. This layer work as the dialog controller. This layer is the layer that can directly communicate with presentation layer and the transport layer.

Transport Layer

Transport layer is the layer that is known as the 4th layer of the Open Systems Interconnection Model. This is the layer that concern with the identifying of the service. The main functionality of this layer is Multiplexing and Demultiplexing. The process of segmentation of data   also concerns with that layer. Sequencing and reassembling of the data happen on that layer. Flow Control and error correction are  also occurring on that layer. TCP transmission control protocol and UDP user datagram protocol work on this layer.

  • TCP {Reliable, Connection Oriented, Slower than UDP}
  • UDP{Unreliable, Connectionless, Faster}

Network Layer

Network layer is the 3rd layer of the OSI model and that layer is most common and important layer of the OSI model. Starting with the NETWORK LAYER The transport layer passes TCP segments or UDP datagrams to the Network Layer. The network layer adds logical addresses or IP addresses to the TCP segments or UDP datagrams to form IP packets and then uses routers to send the IP packets to other networks. The network layer also determines the best path for data delivery. So, the functions of the network layer are:

  • Logical Addressing  
  • Routing 
  • Path determination  

That layer is the layer on which our most important and effective device router works. Network layer is the layer that concern with the following :

  • Logical Addressing
  • Path Determination
  • Routing Protocols{RIP, RIPv2, OSPF, BGP, EIGRP}
  • Routed Protocols {IP, IPX, AppleTalk}

Datalink link Layer

Datalink link layer is the 2nd layer of the OSI model and here switch that is the networking device operate on this layer. Data link layer is the layer that concerns with the local delivery of the frames in between the devices. Here we see the most important protocols that are the following :


  • HDLC stands for the Higher Level Datalink Control Protocol.
  • Cisco Proprietary.
  • No support Authentication, compression, and error correction.


  • PPP stands for the point to point protocol.
  • Standard Protocol.
  • Support Authentication, compression, and error correction.

Physical layer

The physical layer is the place where actual communication takes place. We know that a sequence of 0s and 1s digitally represents the messages. The physical layer converts this binary sequence into signals and transmits them over local media. The signal can be electrical if the local media is Copper Cable or LAN cable, the Light signal in case of Optical Fiber and a Radio signal in case of Air/Vacuum. So, the signal generated by the Physical Layer depends on the type of media used to connect two devices. This layer is the first layer of the OSI model. Physical layer is the layer that concern with the bits transfer of the from one medium to other medium. Hybrid Universal Broadcast device is the device that work on that layer. This layer concerning with how the bit converted into the signals.

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