Series: The OSI Model

The OSI (Open System Interconnection) model is the model most commonly used when describing networking.  It was created by a lot of groups, most credited by ISO (International Organization for Standardization.)  In order to truly understand how computer networks work, knowing the OSI model is a must.  Most people believe that the OSI model was invented for educational purposes only.  The reason why the OSI model was created is to provide standards complete with protocols that are to be used to connect computers together.  Unfortunately for OSI, TCP/IP (Transport Control Protocol with Internet Protocol) won the race for protocols.  While ISO was fixed on an organized model, companies using Arpanet (the grandfather of the Internet) were interested on just getting everything to work.  TCP/IP was convenient and easy to use.

The OSI model is divided into seven layers.  Using a layered model for networking has many great benefits:

-Software or equipment changed in one level does not effect the other layers

-Using a layered model is easier to manage

-A layered model is easier to learn

-Following specifications allow compatibility with different vendors

-The OSI model is universal in any county

The seven layers listed from top to bottom are: Application, Presentation, Session, Transport, Network, Datalink and Physical.  A great way to memorize the layers from top to bottom is to memorize the saying: All people seems to need digital processors.  If you want to memorize the layers from bottom to top, use this saying: Please do not throw sausage pizza away.

Interestingly enough, IBM was working on its own network computer system called SNA (Systems Network Architecture.)  Originally, it consisted of six layers.  Recently it was updated to include a seven layer model.  TCP/IP has its own four layer model.  The OSI model concept was basically developed in response to IBM’s SNA model.  However, the OSI is commonly used as a baseline to develop other models.  Here is a list of the different models side by side so that you can see the difference:

Notice how the TCP/IP model has less layers?  That is because TCP/IP’s Application layer combines the OSI’s Application, Presentation and Session into one layer.  Also, the Network Access layer in the TCP/IP model combines the Data Link and Physical layer into one.  All three models have the same layer 4.  Layer 3 also does pretty much the same thing in all the models.

All three models are still being used today.  OSI is mainly used for education purposes.  Any of the OSI’s protocols are not being used.  The SNA is mainly used to network banks and governmental networks together.  These networks use very high-end security.  TCP/IP is the most widely used suite of protocols.  It is used to network devices at your home, in most businesses, as well as the required protocol suite to connect to the Internet.

This post is a beginning of a series of posts that I will write.  After the first few posts, I will also create a ‘classroom’ page which will have all the information in the OSI Model series for convenience.