International Organization for Standardization (ISO) proposed most important and widely used layered protocol architecture known as Open Systems Interconnection (OSI). The model is known as the OSI 7 layer reference model as it comprises 7 layers each of which corresponds to a group of functions of related functionalities.
Open Systems are the systems that are capable of interconnection by each system having implemented a common set of protocols regardless of their underlying architecture.
Principle of OSI Layers:
As communication through a network requires a large number of tasks to be done on the data, a variety of functions were required to be developed for each purpose. Groups of functions formed modules, formally known as layers. These layers interact with each other through the proper interface. Some principles were developed for the creation of these layers:
1. Don’t create many layers such that system engineer’s task of describing and integrating the layers remain simpler.
2. Create a boundary at a point where the description of service can be small and information flow across the boundary can be minimized.
3. Create a separate layer to perform separate functions.
4. Collect similar functions into the same layer.
5. Select boundaries at a point where experience has demonstrated to be successful.
6. Create a layer of easily localized functions so that the layer could be easily redesigned and changed without affecting the services provided.
7. Create a boundary where it may be useful at some point of time to have corresponding interface standardized.
8. Create a layer where there is a need for different levels of abstraction in handling data.
9. Allow changes of functions or protocols to be made within a layer without affecting the other layers.
10. For each layer create boundaries with its upper and lower layers only.
11. Create sub-grouping by organizing functions to form a sub-layer within a layer.
12. Create two or more sub-layers, if required, with a common and minimal functionality to allow interface operation with adjacent layers.