Before I show you how to setup and configure your Cisco Router and Switch I thought that I would go over the OSI Model with you. The OSI Model is probably the topic that shows up most frequently on the ICND1 Exam. Make sure you take the extra time to Memorize the OSI Model.
The OSI Model was created because when the internet was in its infancy there were many companies producing different types of networking technologies. Networks during this time were increasing very rapidly and many problems began to arise when networks from different vendors could not talk to one another.
By following the OSI Model networks created by different vendors can now communicate with one another.
Layers of The OSI Model:
- (7) Application
- (6) Presentation
- (5) Session
- (4) Transport
- (3) Network
- (2) Data Link
- (1) Physical
The OSI Model is broken up into many different layers which provides many advantages:
- Increases growth in the network technologies because one layer can be improved upon without effecting the other layers or having to completely rewrite the entire model.
- By breaking the networking communication into smaller pieces it reduces complexity.
- Allows many different types of hardware and software to work together.
YOU WILL HAVE TO MEMORIZE THE OSI MODEL FOR THE EXAM!
The way that I memorized the OSI Model was by remembering the phrase, “All People Seem To Need Diet Pepsi.” That is what worked for me, you can either memorize that one or come up with one on your own. My networking teach Tom Rosen taught me that while I was in high school and I still remember it to this day.
The OSI Model was created in such a way that each layer talks to the layer directly above it and directly below it. This is a really important design because it allows changes or improvements to take place at one layer without affecting the other layers at all. It also makes it easier for software developers to write applications that only need to talk to the presentation layer and not have to worry about any of the other layers.
OSI Layer Summaries
Below I will summarize what happens at each of the 7 layers of the OSI Model.
Application Layer - Pretty much any application that you use on the computer that will interact with the network will operate at this layer.
Presentation Layer - This layer is called the presentation layer because it presents data to the application layer. It provides encryption and decryption of data. For example a .jpg file is a type of encryption to compress photos that will be used by the application layer.
Session Layer - The session layer keeps track of the many different sessions that a program might be running so that it doesn’t get things mixed up. A good example of why we need a session layer is Instant Messaging. If you are Instant Messaging several different people at on time, the computer needs to know which messages belong to which session. If the session layer did not exist then maybe your messages would be sent to the wrong person.
Transport Layer - Combines every application that is sending data over the network into one ordered stream. I like to think of your network cable coming out of your computer as a pipe. Each application has its own little pipe and the transport layer is in charge of putting the data from each applications pipe into the big pipe leaving your computer.
Network Layer - Is in charge of routing the data packets across the network and choosing the best path. Routers live in this layer.
Data-Link Layer - Transports the data using frames and provides error detection. An example device that operates at the data-link layer is a switch.
Physical Layer - The Physical Layer consists of all the networking interfaces and cabling. At this layer only zeros and ones get transferred.
Learning the OSI Model is probably one of the most important things that you will need to learn in order to be prepared for the ICND1 exam and to be a good network administrator. By knowing the layers of the OSI Model and how they work together it will help you greatly when troubleshooting network problems. And just as a reminder Routers are in Layer 3 and Switches are in Layer 2.
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