640-407 | Cisco Certified Network Associate |
The 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.
The OSI model is composed of 7 interconnected layers, as detailed in the following table:
OSI Reference Model | ||
Layer Number | Layer Name | Layer Description |
7 | Application | Provides network services, such as file transfer and network management to user applications. |
6 | Presentation | Provides formatting services for data. This would include data encryption and decryption and data translation. |
5 | Session | Establishes a session between two hosts and provides maintenance of the session as well as session breakdown. |
4 | Transport | 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. |
3 | Network | Provides a mechanism for addressing and routing of data through an internetwork. |
2 | Data Link | Specifies topoloy such as Ethernet or Token Ring and frames data according to that topology. Hardware (MAC) addresses are invovled at this layer. |
1 | Physical | 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 benfits to having an industry accepted layered model. Some of these benefits, as listed in ICRC, include:
There are several mnemonics that have been devised to help remember this model. The most popular is a top to bottom approach:
All People Seem To Need Data Processing
Another popular choice, for learning the order from bottom to top is:
People Don't Need This Stuff Presented Anyway
But wait, there's more ...Please Do Not Tthrow Sausage Pizza Away
Princess Di Never Tried Screwing Prince Andrew
Philosophers Deign Not To Solipstically Ponder Anachronisms
Philys Did Networking Till She Passed Away
From the top again ...All People Sstanding Totally Nude Don't Perspire