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:

- Clarification of general function rather than specific implementation.
- Reduced complexity of networking into manageable layers.
- Improve interoperability by implementing stadardized interfaces.
- Modular modification. Changes to one layer don't affect the others.
- Increased speed of development through development specialization.
- Ease protocol comparizons through OSI model comparisons
- Orders network troubleshooting, facilitating systematic approach.

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