3 concepts are central to OSI model:
1.Services-performs some services for the layer above it,tells what layer does.It defines the layer's semantics.
2.Interfaces-tells processes above it how to access it.
3.Protocols-layer's own business.
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.
Data Link Layer originally dealt with point-to-point networks. The committee originally expected that each country would have one network, run by govt. and using OSI protocols, so no thought was given to internetworking.
With TCP/IP the reverse was true:the protocols came first and the model was just a description of existing protocols. The only trouble was that the model did not fit any other protocol sticks.
An obvious difference between the 2 models is the number of layers:the OSI model has 7 layers and the TCP/IP has 4 layers.Both have (inter)network, transport and application layers,but other layers are different.
The OSI model supports both connectionless and connection oriented communication in the network layer.The TCP/IP model has only one mode in the network layer (connectionless)but supports both modes in the transport layer,giving the users a choice.