Comparison between OSI and TCP/IP Reference Models

OSI and TCP/IP models are based on the concept of a stack of independent protocols. Functions of the layers are more or less similar.

Three concepts are central to OSI Model: Services, Interfaces, and Protocols.

• Services: This definition tells what the layer does. It defines the layers semantics.
• Interface: It tells the process above it how to access it.
• Peer Protocols: Protocols used in a layer are the layer’s own business.

TCP/IP model did not clearly distinguish between service, interface and protocol. The protocols in the OSI model are better hidden than in the TCP/IP model and can be replaced relatively easily as the technology changes.

It is learnt that the OSI  model was devised before the protocols were invented. This ordering means that the model was not biased toward one particular set of protocols. Downside of this ordering is that the designers did not have much experience with the subject and did not have good idea of which functionality to put in which layer. The committee originally expected that each country would have one network ,run by the government and using the OSI Protocols, so no thought was given to internetworking.

With TCP/IP, the protocols came first and the model was really just a description of existing protocols. Protocols fit the model perfectly. The only trouble was that the model did not fit any other protocols stacks.

The difference between two models is the number of layers: the OSI model has seven layers and the TCP/IP has four layers .Both have (inter)network, transport and Application layers, but the other layers are different.

Another difference is in the area of connectionless and connection oriented communication. OSI Model supports connection oriented communication in transport layer, whereas in network layer it supports both connectionless and connection oriented. The TCP/IP model has only one mode in the network layer but supports both modes in transport layer.