Cisco Certification: The Osi Model, Part I
To conquer the Introduction To Cisco Networking exam, and to begin the process of becoming an expert network troubleshooter, you have to master the OSI model and learn what happens at each of the seven layers.
In this three-part series, we'll examine each level of the OSI model, paying particular attention to the details that will help you pass the CCNA exams and give you the foundation you need to become a true networking professional.
We'll start at the top layer, the Application layer.
It won't surprise you to find that the Application layer is the OSI model layer where most end users have interaction with said applications. Passing the CCNA exam is all in the details, though, and you need to know what happens at the Application layer, as well as the common and not-so-common applications that run at this layer.
The Application layer's tasks include identifying the remote communication partner, ensuring that the needed resources for communicating with that partner exist, and user authentication. If you're prompted for authentication, you're most likely at the Application layer of the OSI model.
Keep that in mind if asked to identify Application layer protocols. I've noticed that CCNA candidates tend to identify Telnet as running at the Application layer. That's an understandable misconception, since the first thing you enter in Telnet is an IP address, and it's often used to communicate with a router. However, keep in mind that Telnet is an Application layer service, not a Network layer service. You've got to authenticate to Telnet to a Cisco router in the first place, remember!
Other common applications that run at Layer 7 are Simple Mail Transfer Protocol (SMTP, port 25) and File Transfer Protocol (FTP, port 21).
In short, if an end user is interacting with a program, especially if they're being prompted for authentication by a program such as Telnet or FTP, they're working at the Application layer of the OSI model.
Let's take a look at the Presentation layer.
Layer 6 of the OSI model is the Presentation Layer. While we don't have a great deal of interaction with this layer, you need to know what happens at this layer to pass your Intro and CCNA exams.
The main purpose of the Presentation Layer is making sure that the communication that will be seen at the Application Layer is presented in the appropriate format.
That's all well and good, but what does it mean? J Have you ever opened a document with MS Word and gotten screens and screen of garbage? That's a presentation layer problem - the program being used to open the document is unable to present the data in an appropriate format.
The three major tasks of the Presentation layer:
1. Compatibility with the operating system.
2. Proper encapsulation of data for network transmission.
3. Data Formatting (ASCII, binary)
Encryption and compression of data is also handled at the presentation layer.
Let's keep working our way down the OSI Model. Next up, the Session Layer!
The Session layer of the OSI model basically acts as the manager for the entire model. Some have called it the PHB (Pointy-Haired Boss) of the OSI model if that helps you remember its role, that's fine with me!
The Session layer establishes, manages, and tears down connections between applications. The Session layer uses port numbers to keep multiple conversations between two end points separate. You may have heard the term well-known port-numbers before. That term refers to port numbers that are often-used and static in that they use the same port numbers every time. You'll be expected to know common well-known port numbers to pass your Intro exam, such as 23 for Telnet and 21 for FTP.
That's about all there is to Layer 5, the Session Layer. From here on out, there's more you need to know about each layer, and that starts with Layer 4 - the Transport Layer. We'll take a look at that layer in Part II of this OSI tutorial.