The ISO/OSI 7-Layer Reference Model

The OSI (Open System Interconnection) reference model, in accordance with the international standard ISO 7498, is often used as a reference framework to aid the comprehension and division of fieldbus system requirements. The model describes seven layers of functionality available to a networked device and these are outlined in the table below.
Layer Name Purpose Operation
7 Application Device Application Compatibility This is the code written by the user that governs the operation of the device. 
6 Presentation Data Interpretation Deals with the common application orientated functions for instance dealing with known data structures such as ASCII strings or conversion between codes. (SNVTs)
5 Session Remote Access Takes the communication from the transport layer below and supports application orientated functions (BINDING) 
4 Transport End to End Reliability Accepts data from the above and passes it down to the network layer in convenient packets for transmission
3 Network Destination Addressing Controls the operation of the subnet to ensure that packets of information are presented to the correct devices
2 Datalink Media Access and Framing Takes the raw transmission from below and presents it above as error free data frames. Deals also with acknowledgements and other error checking.
1 Physical Electrical Inter-connection Concerned with transmitting the raw bits over the physical medium of the network, e.g. twisted pair, RF, IR, optical fibre, power line etc.