Back to Basics: The OSI Model

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When I taught high school we always went over the OSI Model in the student’s junior year. Students originally hated the concept until their senior year when it was their turn to manage the student network in the IT lab.  Then, without knowing it they were talking like professional troubleshooters when it came to determining what layer the problem was in so they could resolve the problem.


With that in mind, let’s revisit the OSI Model in today’s Back to Basics post.


The OSI Model (Open Systems Interconnection model) explains the different internal functions of a communication system by partitioning it into various layers. 


Below is one of the best images showing how this model works 

Source: bmc blogs: https://www.bmc.com/blogs/osi-model-7-layers/ as viewed on 3/21/19


Here is a brief explanation of what each layer does.


(7) Application Layer- This layer is where the end-user (computer user) begins the communication process. At this layer user authentication is identified and protocols like FTP, TELNET, E-MAIL, and other file transfer protocols begin with the user starting the process.


(6) Presentation Layer - This layer is where encryption occurs. This layer converts the file and/or information into a language (format) that the remaining layers can read and handle without any compatibility problems.


(5) Session Layer - This layer is where the communication is coordinated. Applications are managed in this layer. This layer manages the exchange of information by creating and terminating all communication between the application layer at both ends.


(4) Transport Layer - This layer is responsible for ensuring the data completes its transmission to the other user. It's also responsible for flow control and error recovery.


(3) Network Layer - This layer is where the routing and switching takes place. Packet sequencing, addressing, congestion control, and internetworking happen at this layer. Logical paths are created at this level to allow users to communicate to each other. This layer utilizes IP addresses.


(2) Data Link Layer - This layer is where packets (information) is broken into bits through encoding and decoding. At this layer you have two sections; Media Access Control (MAC), and Logical Link Control (LLC). The MAC section is where packets (information) is gathered and gets permission to transmit the data. The MAC section is where your computer's MAC address is located to allow proper delivery to a specific system (computer). The LLC section controls synchronization, flow control, and error checking.


(1) Physical Layer - This layer is where your hardware such as ethernet cable, Network Interface Cards, etc., provide the transmission through electrical impulse, light, and radio waves. This layer is the hardware means of sending and receiving data (information).


Some acronyms to remember the OSI model include:


  • Please Do Not Throw Sausage Pizza Away (Physical, Data Link, Network, Transport, Session, Presentation and Application)
  • All People Seem To Need Data Processing (Application, Presentation, Session, Transport, Network, Data Link and Physical


Please go to bmc blogs for a more comprehensive explanation of the OSI model. 


For a quick overview of the OSI model watch this 10 minute video by Deep Ramanayake as he demonstrates how an e-mail moves through the OSI model https://www.youtube.com/watch?v=O_rsqVtaloI


For a more in depth presentation of the OSI model watch this 49 minute video by Nicholas Andre https://www.youtube.com/watch?v=0Rb8AkTEASw.


If you are studying for the CCNA Routing and Switching exam and need more help to prepare beyond understanding the OSI Model Click Here for more information on our CCNA Route Switch Boot Camp.

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