The OSI reference model is known as a layered architectural model. It defines the 7 layers that data passes through as it is transmitted from one device to another. These 7 layers will be described very shortly.
Each layer consists of different networking protocols that perform specific functions in order to enable network communication. Apart from the obvious advantage of providing standardization and therefore interoperability between network devices, advantages of using this layered approach include:
- The simplification of troubleshooting, design and development by breaking down the communication process into smaller components or stages.
- Provides compartmentalization, that is, because the network functions that occur at each layer are standardized, protocols for each layer can be developed almost independently.
You should remember that the OSI Reference Model is only a logical model, not a physical one. It is really only a framework used by developers of network applications , protocols and devices.
The 7 layers of the OSI Reference Model are:
- 7 Application: Interfaces with the user.
- 6 Presentation: Presents the data and may handle functions like encryption and compression.
- 5 Session: Ensures that different application's data is kept separate from each other.
- 4 Transport: Offers reliable or unreliable delivery of data and performs error-correcting functions before retransmission.
- 3 Network: Provide logical addressing, this is used for path determination.
- 2 Data Link: Provides access to the media, uses hardware (MAC) addressing, performs error detection.
- 1 Physical: Responsible for moving bits between devices.
These 7 layers can be furthered categorized into 2 groups. The top three layers pertain to the way that network applications on the actual devices (i.e. routers, switches, computers) communicate with each other and with their users. The bottom four layers on the other hand pertain to the way that data is transmitted from one device to another.
In the next hour we will describe each of these 7 layers individually in a lot more detail, looking in particular at their role within the overall end-to-end transmission and examining the various protocols that exist at each of these layers.