OSI AND TCP/IP MODEL

• OSI stands for Open System Interconnection is a reference model that describes how information from a software application in one computer moves through a physical medium to the software application in another computer.
• OSI consists of seven layers, and each layer performs a particular network function.
• OSI model was developed by the International Organization for Standardization (ISO) in 1984, and it is now considered as an architectural model for the inter-computer communications.
• OSI model divides the whole task into seven smaller and manageable tasks. Each layer is assigned a particular task.
• Each layer is self-contained, so that task assigned to each layer can be performed independently.

The International Standards Organization (ISO) developed a model for the connection of open systems (systems that are open for communication with other systems) known as the Open Systems Interconnection (OSI) reference model. The OSI model is not network architecture in that it does not specify the actual protocols to be used in each layer. Rather, the model clearly defines the functions to be performed by each layer and the boundaries between each layer. The OSI model is built of seven ordered layers:

1. Layer-1 : Physical Layer
2. Layer-2 :Data-Link Layer
3. Layer-3 :Network Layer
4. Layer-4 :Transport Layer
5. Layer-5 :Session Layer
6. Layer-6 :Presentation Layer
7. Layer-7 :Application Layer

The seven layers can be thought of as belonging to three sub groups:

• Network Support Layers (Layers 1-3): Deal with the physical aspects of moving data from one device to another.
• User Support Layers (Layers 5-7): Allow interoperability among unrelated software systems.
• Layer-4 ensures end to end reliable data transmission.

Prior to, and concurrently with the development of the ISO‘s OSI model, the US Department of Defense Advanced Research Projects Agency (DARPA) was heavily involved in internet working activity. It was such activity that resulted in the creation of the Internet. As a result of this activity, DARPA created a four layered model of data communication known as the TCP/IP suite, which
has become the de facto standard for interconnection, despite the  official status of the OSI model. The TCP/IP and OSI standards are the two major open system, vendor-independent, standards in use.

Figure: OSI Layers in Detail