What is the OSI Model?

The OSI model is an incredibly useful tool for understanding how different networks communicate to each other. The OSI Model breaks down communication into 7 logical layers that let you think about a network problem and how to fix that problem. It simplifies the troubleshooting process by allowing a technician to focus on which layer the problem appears to be in.

1.       The Application Layer- The Application layer is that layer that the user interacts with the data. For instance, you are reading this web document on a web browser. If you are using email, you might be viewing Outlook.

2.       Underneath the Application layer is the presentation layer. This is the layer that the operating system is on.

3.       The Session layer is the layer that deals with the communication creating a session between the two computers. If you go to a website, your computer has to create a session with the web server you are trying to get information from.

4.       The Transport layer is what decides how much information should be sent at one time. When you are communicating with a website, this layer is responsible for controlling how much data is sent back and forth. The transport level is where the process of windowing happens.

5.       The Network layer is the level that routers operate on. Your IP address is at the Network layer. This layer is related to connections between networks. The TCP/IP protocol suite resides on this layer.

6.       The Datalink layer is the level that switches work on.  This layer is related to connections within networks. Media Access Controll addresses reside on this layer. Every single device in the world has a MAC address.

7.       The Physical layer refers the physical components of a network. This includes wiring and cables. If your Ethernet cable has become disconnected, this is a Physical layer problem!

If you are new to networking, it is important to realise that the OSI model does not refer to actually layers hidden in your cables or the Internet! The OSI model refers to logically distinct aspects of a network and enables you to diagnose where your network issues lie. For example, if you notice you are not receiving an IP address from your router, this would be a Network layer problem.  If you can “ping” a domain but cannot connect to the domain in a web browser, this is likely an Application layer problem. 

Another benefit of the OSI model is that it is an exhaustive list of networking issues. If all layers are the OSI model are working, your networking with be working. Any problem that you encounter will be associated with a layer of the OSI model. Thus, once you identify where the problem is, you can take steps to fix it.

Nerd out.