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CCNA Thursday's Discussion: The OSI Model

Welcome to Cisco Certified Network Associate (CCNA) Thursday's! Each week I will cover different technologies that are covered in the CCNA routing and switching (R&S) exam. If you don't know what CCNA is click here to read about the exam objectives. Currently You can earn your CCNA by choosing one of two routes.

200-125 CCNA Interconnecting Cisco Networking Devices: Accelerated (CCNAX) or 100-105 ICND1 & 200-105 ICND2 Interconnecting Cisco Networking Devices Part 1 (ICND1) & Interconnecting Cisco Networking Devices Part 2 (ICND2). Whichever route you decide to take you will need to understand and demonstrate the objectives of the exam. I currently hold both CCNA R&S and CCNA Security. One of the foundational topics that helped me pass the exam is fully understanding the Open System Interconnect (OSI) Model.

The OSI model is a framework for defining the conventions and tasks required for network systems to communicate with one another (Siris, 2002). The OSI Model is made up of 7 layers. Starting from the top down the first three layers is where the applications communicated with users and end devices. The three layers are Application, Presentation, and Session. An application is how the user interfaces with the information that moves up and down the OSI model. When sending an e-mail, the application could be Microsoft Outlook or Google Chrome depending on the setup.

The presentation is where the data is prepared to present to the various applications. Presentation layer has been referred to as the translator, providing the translation of information to and from the session layer. The session layer is similar to an air traffic controller. The session layer is responsible for ensuring that all traffic between different hosts and applications maintain integrity.

Three methods are used at the session layer, simplex, half-duplex and full-duplex. One easy way to remember how each mode works is to connect them to something you use daily. Full-duplex is like talking on a cell phone. You can talk and listen at the same time. Half-duplex is similar to a walkie-talkie. You can receive and transmit, but it must be done one at a time. Simplex is identical to the announcement you heard in school. Simplex is a one-way broadcast that does not get a response. That was a mouth full. So let's review....

Cisco provides two methods for earning certification 200-125 or 100-105 plus 200-105. The OSI model consisted of 7 layers and defined as an industry standard for devices to communicate on a network. The first three layers of the OSI model are Application, Presentation, and Session. The Application Layer is the how the user interacts with data. The Presentation Layer formats the data and presents it to the application. The Session Layer ensures the data stays segmented and maintains integrity.

Next Week we will cover the following four layers of the OSI model which are Transport, Network, Data Link and Physical. Until next time.


Siris, V. A. (2002). The OSI Model and Switching. In Enterprise Networking: Multilayer Switching and Applications(pp. 1-14). IGI Global.