The model is a theoretical stack of seven layers that can be used as a reference to help understand, “How networks operate?” The model was introduced to standardize the network in a way that allows multi-vendor systems. Prior to this you only be able to have one vendor network. Because Devices with one vendor could not communicate with other.
It is important to know that we don’t actually use OSI model. The practically implementation is TCP/IP model. The concepts are exactly the same, the leaves are slightly different.
Why to study OSI model?
The answer to this question is because it referenced a lot when troubleshooting or describing network operations.
Let’s describe the layer starting from the bottom up:
|Layer 7||Application layer||Layer seven is application layer. This is where the application and user communicate. Application specific Protocols are used such as SMTP.
|Layer 6||Presentation layer||This layer is presentation layer. This lead formats the data in a way, receiving application can understand it. This layer is also able to encrypt and decrypt data.
|Layer 5||Session layer||This layer is responsible for establishing and terminating connections between layer devices.
|Layer 4||Transport layer||The transport layer of the OSI model control the transport protocol such as TCP and UDP. TCP is used for error handling and sequencing to ensure no data is lost. This layer also add the source and destination port numbers.
|Layer 3||Network layer||The network layer handles IP addressing and routing. At this stage of the OSI model the source and destination IP addresses are added. Router operate on this layer.
|Layer 2||Data-link layer||It is data link layer. At this layer the physical addresses are added to the data. Source and destination Mac addresses and Switches are located at this layer.|
|Layer 1||Physical layer||It is the physical layer it is the lowest layer of the OSI model. Its key responsibility is to carry the data across the physical hardware such as Ethernet cables.
This was the brief description of an OSI model.
How can we use this to troubleshooting?
Sometimes you may have heard the term that’s a layer 2 problem or sounds like a layer 3 issue. When you hear that, that’s where people are referring to this model.
Let say there’s a problem with the network. If we go through this model checking every layer we can soon diagnose the problem.
Check whether the cables plug in or not? Is the network card functioning? Could it be a faulty cable?
May be the switch has gone bad?
The router is functioning? Do I have the right IP address?
And this process goes on and on till the last layer.