REFERENCE MODEL

Reference Model





ISO-OSI Reference Model

· The international organization for standardization developed the open System Interconnection (OSI) reference model. OSI model is the most widely used model for networking.

· OSI model is a seven layer standard.
· The OSI model does not specify the communication standard or protocols to be used to perform networking tasks.

OSI model provides following services.

1) Provides peer-toper logical services with layer physical implementation.

2) Provides standards for communication between systems.

3) Defines point of interconnection for the exchange of information between systems.

4) Each layer should perform a well defined function.

5) Narrows the options in order to increase the ability to communicate without expansive conversations and translations between products.



Principles in defining OSI layers


· Following principles are used in defining the OSI layers.

1. Do not create so many layers as to make the system engineering task of describing and integrating the layers more difficult than necessary.

2. Create a boundary at a point where the description of services can be small and the number of interrelations across the boundary is minimized.

3. Create separate layers to handle function that are manifestly different in the process performed.

4. Collect similar functions in to the same layer.

5. Select the boundaries at a point which past experience has demonstrated to be successful.

6. Create a layer of easily localized functions so that the layer could be totally redesigned and its protocols changed in a major way to take advantage of new advances in architecture, hardware or software technology without changing the services expected from and provided to the adjacent layers.

7. Create a boundary where it may be useful at some points in time to have the corresponding interface standardized.

8. Create a layer where where there is a need for a different level of abstraction in the handling of data.

9. Allow changes of functions or protocols to be made within a layer without affecting other layers.

10. Create for each layer boundaries with its upper and lower layer only.