The OSI Model

  Beginning in the early eighties, the ISO (Internationa Standards Organization) put together a specification, to define a network communications model.
A theoretical representation of the protocols and methodology required for network nodes to communicate with each other on a network
This model has proved to be very helpful in the undestanding and development of network computer communications.
The model divides network communication into seven layers:- Data follows the "Realisation of the communication" path indicated in the diagram, opposite.
Data is loaded into PDU's (Protocol Data Units) in the Application Layer and passed down through the layers, across the Network data transmission medium (wire, radio, fibre-optic, etcetera), and up through similar layers, to the receving node's Application Layer.
Logically, each layer communicates with the corresponding layer, between communicationg nodes.
OSI Model
        {From: The Industrial Ethernet Book}