What is OSI Reference Model ??

							

The Open System Interconnection reference model was created by the International Organizaiton for Standardization (ISO) to provide a model that network vendors could follow when developing network products. The concept for the model was established to further support for interconnectivity between systems and to modularize components used in network communications.
OSI Seven Layer
Layer-7  Application Layer:   Provides network services, such as file transfer and network management to user applications.
Layer-6  Presentation Layer: Provides formatting services for data. This would include data encryption and decryption and data translation.
Layer-5  Session Layer:






Establishes a session between two hosts and provides maintenance of the session as well as session breakdown.
Layer-4  Transport Layer






Provides for the transport of data from upper layers (Layers 5- 7). This layer abstracts the actual network data transfer from applications. Protocols at this layer are dealing with issues of connection reliability, flow control, and error detection.
Layer-3  Network Layer






Provides a mechanism for addressing and routing of data through an internetwork.
Layer-2 DataLink Layer Specifies topology such as Ethernet or Token Ring and frames data according to that topology. Hardware (MAC) addresses are involved at this layer.
Layer-1  Physical Layer This layer is concerned with the actual interpretation of the bit stream into an electrical signal that can be carried across a physical medium. This layer would also specify physical medium properties.

A good portion of the exam focuses on a thorough understanding of these layers and how various protocols and services may map to these layers. Spend some time considering a variety of applications and protocols and think about where on the model they may fall.


There are many benefits to having an industry accepted layered model. Some of these benefits, as listed in ICRC, include: 

•    Clarification of general function rather than specific implementation. 

•    Reduced complexity of networking into manageable layers. 

•    Improve interoperability by implementing standardized interfaces. 

•    Modular modification. Changes to one layer don't affect the others. 

•    Increased speed of development through development specialization. 

•    Ease protocol comparisons through OSI model comparisons 

•    Orders network troubleshooting, facilitating systematic approach.