The OSI model is a theoretical stack of seven layers that can be used as a reference to better understand how networks operate.
The model was introduced to standardize networks in a way that allowed multi-vendor systems before this you will only be able to have one vendor network because the device from one vendor couldn’t communicate with others.
It’s worth noting at this point that we don’t use the OSI model we use something called the TCP/IP model the concepts are the same the layers are slightly different.
So if we don’t use the OSI model why would we bother learning it well that’s because it is still referenced a lot when troubleshooting or describing network operations.
Now, look at the layers starting from the bottom up layer.
Layer 1 = is the physical layer is the lowest layer of the OSI model its key responsibility is to carry the data across physical hardware such as Ethernet cables.
Layer 2 = is the data link layer at this layer the physical addresses are added to the data this is the source and destination MAC addresses switches are located at this layer
The network layer handles IP addressing and routing at this stage of the OSI model the source also destination IP addresses are added Reuters operate on this.
Layer 3,4 = The transport layer of the OSI model adds the transport protocols such as TCP and UDP, TCP for example is used for error handling and sequencing to ensure on data is lost. That layer also adds the source and destination port numbers.
Layer 5 = Session this layer is responsible for establishing and terminating the connection between devices.
Layer 6 = Presentations this layer formats the data in a way the receiving application can understand it this layer also encrypts and decrypt data if needed.
Layer 7 = Application this layer is where the application and user communicate application-specific protocols are used here such as SMTP if you’re sending an email for example.
A really good way to remember these layers is simply all people seem to need data processing A, P, S, T, and D, P.
So to fully how this model works you need to see a real-world example let’s say you send an email the data travel through the OSI model adding and processing data on every layer and this process is called encapsulation.
Step 1 The application layer our load creates the data the email you wrote the email addresses etc-etc and ready to send it using SMTP which is the simple mail transfer protocol.
Step 2 presentation there the data is formatted in a way the receiving device will understand in this example probably an SCIII this layer could also encrypt the data if needed.
Step 3 session layer a session with receiving mail server is started.
Step 4 transport this is where it decides to use TCP or UDP in this case we’ll use TCP to make sure every packet gets delivered also the source and destination port are added to the data.
Step 5 Network the IP address of the mail server is added as the destination and the source IP address is also added through the data.
Step 6 data link the MAC address of the router and the source MAC address of the host is added to the data.
Step 7 physical the data is sent out on the network using eth Ethernet so when the data reaches the other side the receiving device will process the data in that same way but in reverse starting from that bottom-up.
How can we use this to Troubleshoot
Okay so that’s the OSI model but how can we use this to troubleshoot you may have heard the term or that’s a layer 2 problem or sounds like a layer 3 issue when you learn that that’s people referring to this model.
Let’s say there‘s a problem with the network if we go through this model checking every layer we can soon diagnose the problem for example.
Layer 1 or the cables plugged in is the network card functioning could it be a faulty cable.
Layer 2 may be the switch has gone bad.
Layer 3 is the route to function do I have the right IP address and the process goes on and on from there.