What is the OSI-7 model?

The Open-System Intercommunication model describes how the network communication layers are distinct and separate from one another. There are a few different network layer models, however this article only talks about the OSI-7 model.

The OSI-7 model breaks the communication layers as follows (The physical layer is called the bottom layer, while the application is the top):

1. Physical
2. Bit Link
3. Network
4. Transport
5. Session
6. Presentation
7. Application

If you have trouble remembering the order of the OSI 7 layers, check out this article I put together.

Why have separate layers on the OSI-7?

There are a few good reasons why the network communication stack is separate into discreet layers; troubleshooting, development and standards are but three.

With each layer segregated from the next, the protocols and data each layer needs relate only to its own layer. If data gets corrupted in a certain layer, the debugging and troubleshooting should be limited in scope to only that layer. As new technology are developed and new limitations found, the methods and actions within each layer will be changed. If there was no clear distinction between layer inputs and outputs, changes to one part would cascade through the entire system and cause severe development headaches. By forcing layers into the system, changes should now propagate outside the current layer.

Lastly we get to standards, one of the key reasons for the OSI-7 model. Without a rigid model for standards to be drawn up on, individual manufacturers and vendors would be free to develop things as they see fit. They would also be unable to rely on how systems outside of their control would operate. For example FTP programs work at the Application level, the developers of that program don’t know how the physical layer works, but without standards they would likely need to develop everything from the Application down to the Physical layer themselves. This would cause absolute chaos as one vendors technology failed to communicate with another.

What are the functions of the different OSI-7 layers?

7. The Application Layer

This layer, despite its name, doesn’t contain the applications that run on your computer. You wont find Microsoft Word in here. You will however get file-sharing utilities, (FTP), some email clients (SMTP) and web browsers (HTTP).

6. The Presentation Layer

No, not a slideshow presentation. This layer translates the data in various forms; think encryption, character encoding and compression etc. If you are aware of SSL (Secure Socket Layer), that is dealt with here at the Presentation Layer.

5. The Session Layer

Similar to how you might have a temporary session on a website, this layer is responsible for handling the back-and-forth communication exchanges between multiple nodes. This layer role is central to maintaining connections between two computers, also restarting and terminating them too.

4. The Transport Layer

Two common protocols you’ve probably heard of, TCP and UDP are handled here in the transport layer. This layer attempts to gaurantee the delivery of data to the destination.

3. The Network Layer

One role of the network layer is to split up your data (called datagrams) so that each part can be delivered separately. It also works out how to route the data through the network, while translating logical addresses into machine addresses

2. The Data Link Layer

This layer deals with how the network is physically structured. It also handles some error correction and flow control functionality

1. The Physical Layer

This handles the physical transmission of the data, converting it from digital bits to the corresponding form for whatever the network medium is. You get lots of different mediums these days, from WiFi (with different channels), fibre optics all the way down to standard copper cables. You’d find your home hub, your PC network adapter, cabling and network repeaters working at this level.