Trouble Remembering OSI?

One of the key topics covered in all of the major professional certifications in IT is the Open Systems Interconnection (OSI) Model. You will find this requirement in leading certifications including Certified Internet Webmaster (CIW) from Prosofttraining.com, Novell?s CNA, Microsoft?s MCSE and CompTIA?s Network+ certifications. Understanding the OSI model is one of the more difficult topics to grasp for certification candidates. To help relieve some of the confusion for students here?s an easy way to remember the components of the OSI model that will get you through the initial frustration of learning this important model.

The OSI model is a seven-layered model that describes how systems communicate over a network. The description applies to all systems and is a basic model in computing. The model is mapped out in seven distinct layers that describe each piece of the data transmission process. Going from top to bottom, the layers are labeled: Application, Presentation, Session, Transport, Network, Data Link, and Physical. You can easily remember the model by a simple pneumonic that puts the layers together going from the bottom of the model back up to the top. It goes like this: Please (Physical) Do (Data Link) Not (Network) Throw (Transport) Sausage (Session) Pizza (Presentation) Away (Application). or alternatively: Please (Physical) Do (Data Link) Not (Network) Thrust (Transport) Sales? (Session) People (Presentation) Advice (Application). :-)) You can continue with the pizza metaphor to remember how the model describes data transmission. The OSI Model can easily be explained as a reference to something familiar, such as pizza. We?re not actually concerned with the pizza itself when describing how the OSI model operates, but with the way in which a pizzeria transfers food to its customers. We will start at the top of the model, with the application layer, and work our way down as if to describe a typical transmission transaction on a computer. The application layer does not refer to a computer?s applications, but to the processes that are being used to send data. Some common processes are file and print services. They are similar to determining what type of food the customer is ordering ?did he or she want a pizza, or the lasagna?

After the Application layer adds a header to the packet of information (noting the type of transaction or, in our analogy, the type of food the customer will receive), it sends the packet down to the next layer in the model, the Presentation layer. The Presentation layer determines the format of the data; whether it is using compression or encryption, and what character set is being used. Another important part of this layer is the redirector, which determines where to send (or redirect) the data.
This task is similar to determining what side dishes go with the meal that was ordered to properly format the entire order, and also determining where to send the meal (was it going to a table, or was it a delivery?).

Once the destination is determined for the packet, it is passed down to the Session layer, which deals with the packet as a whole and determines what parts have been prepared to send.
In this layer, the model makes sure that all the food ordered has been cooked or fully prepared. If something is not finished, the order waits here until it is complete. When all is complete and ready, the packet (the whole packet) is sent down to the Transport layer.

The Transport layer deals with packaging and sequencing. It divides the packet down into manageable chunks called "datagrams".
Basically, this is where the staff places the lasagnas together in one box, the salads in plastic bags, and makes sure the pizza is in a warmer.

Now it is time to pass it to the Network layer. This layer deals directly with addressing and routing functions.
In other words, we are making sure we have the customer?s address on the box, and putting the datagrams into the car to actually route from the pizzeria (point A) to customer?s house (point B).

We then pass the datagrams down to the Data Link layer, which is responsible for error correction/detection, and link management.
The error correction/detection feature is like making sure we have all the meals that were ordered in the appropriate boxes.

When the information is passed from the Data Link layer to the Physical layer, it must be modified to the appropriate electrical or optical signals for the type of physical medium (cable) being used.
The Physical layer is essentially the road between the pizzeria and the customer?s house.

This analogy, simple as it may be, describes data transmission between multiple systems. Hopefully you will find some value in remembering the OSI model in these simple terms. If not, at least you will know how to order dinner next time you forget to thaw the chicken.

Get to know the OSI model well; you will find it on many certification exams as well as in your design, engineering and systems administration roles. Good luck!

For more information on the OSI Model, visit any of these great Web sites: