The OSI Model

The Open Systems Interconnection (OSI) model provides standards to hardware that allows it to only focus on the type of communication that is relevant to that specific type of hardware(Ex. routers). The OSI model is broken up into the following seven layers:

1. Physical Layer - This layer communicates between two points through a physical connection. It communicates using raw bit streams and is responsible for activating, maintaining, and deactivating these communications. 
2. Data-link Layer - This layer actually sends the data between the two points and provides high-level functions, like error correction and flow control. 
3. Network Layer - The network layer passes information between the lower and higher layers. It provides the addressing and routing that is necessary to pass the information.
4. Transport Layer - This layer is responsible for transferring the data between the different systems. 
5. Session Layer - The session layer establishes and maintains connections between the different network applications. 
6. Presentation Layer - This layer makes it possible for applications to understand the incoming data by presenting it in a language that the application can interpret. 
7. Application Layer - This layer keeps track of the requirements for a particular application. 

Data is transported by packets through the different layers. The data is wrapped by each layer, starting with the application layer and ending with the physical layer, which is called encapsulation. Each layer has a header, which contains the protocol information for that layer; and a body, which contains the data for that particular layer.

Original image from: http://compnetworking.about.com/library/graphics/basics_osimodel.jpg