One of the first subjects approached by almost any documentation I’ve read in relation to the CCNA, or just networking in general, is the Open Systems Interconnection(OSI) Model. The OSI model is a conceptual model for the subdivision of communications system into smaller concentrations called “layers”. I know, that sentence doesn’t exactly make a lot of sense now, but continue reading and I think you’ll pick it up.
Each layer of the OSI model groups together certain functions that can be performed on the network. There are 7 layers to the actual OSI model as follows : Application, Presentation, Session, Transport, Network, Data-Link and Physical. Below is a model that is commonly used as diagram for how to view the OSI model.
One thing that I have learned in the last couple months from all of the studying I’ve done is that the ONLY layers of the OSI model that you’ll be dealing with in general “Routing and Switching” networking are the bottom three layers of the OSI model. As we proceed a little further into the networking field we’ll start to incorporate some features of the TRANSPORT layer which handles security protocols and other interesting features on the network.
When I first learned the OSI model, I was also taught a phrase to help remember all of the layers : All People Seem To Neet Data Processing
It definitely helps in remembering all 7 layers. It’s more of a formality to know all 7 layers like the back of your hand, but it will be asked in certifications if you plan on taking that route.
As stated previously the OSI model is a conceptual model for grouping similar network functions together into theoretical “layers” to help us understand the way data is transferred from end to end in the networking world. We can now take a deeper look into the functions of each layer and see what protocols and standards are handled by each.
Application – The Application layer of the OSI model is layer that sits closest to the end user while using the computer. When an application – (not to be confused with the Application layer) – is being used on the computer by and end user the Application Layer is at the same time processing data that is being fed to the application by the end user and packing it and preparing it to be passed down to stack.
Presentation – The Presentation layer of the OSI model is responsible for mapping the data between the application on the computer and the way the information is “presented” on the network. Encryptions are usually pre-processed on this level before they are passed down the stack to be sent over the network.
Session – The Session layer of the OSI model controls the connections between the computers on the network. It is establishes, manages and terminates the sessions(connections) between the local and remote applications. It also controls the full-duplex, half-duplex, or simplex operation, and establishes checkpointing, adjournment, termination, and restart procedures(not to be confused with restarting the computer, but restarting the session between the local and remote application if it times out or is terminated).
Transport – The Transport layer provides a transparent means of data transfers between users, while also providing a reliable transfer service to the upper layers of the model. The way the Transport layer guarantees the reliability of the transfer of data is through the use of flow control, segmentation and desegmentation, and error control. Protocols that traverse networks are sometimes connection oriented and the Transport layer can track the transmission of segments and re-transmit those that fail. One of the most popular connection oriented protocols that exists in the networks of today is the Tranmission Control Protocol (TCP).
I’m going to limit this post to the top 4 layers of the OSI model and leave the last 3 to the next post. The last 3 layers are the layers that Network Engineers or anyone that works directly on networking equipment deals with. There is a lot of in depth information that I will end up breaking up over the next couple of posts to make sure I can cover it all as thoroughly as possible.
Feel free to post any comments on any inaccurate data, or if you have any input, and also post comments if you have any questions.