Terminology Tuesday: 7 Layers of the OSI Model


Over the weekend I was enjoying a delicious 7-Layer salad while contemplating what term I should blog about this week. It’s funny how the mind connects different thoughts, but needless to say, I found myself trying to remember the 7 layers of the Open Systems Interconnection (OSI) model. I admit, I needed to look some of this up, but thought that I would share them as I can never seem to remember them all off the top of my head.

The layers were defined to separate “how things happen” to network data to prepare it to move from one computer to the next whether half-way around the world, or in the next cubicle. Note, I will start at the TOP layer and work down.

Layer 7: The Application layer – When you choose to read an email, or transfer a file on the network (or do other network related activities), this is the layer that interacts with the operating system (or application – hence, the name).

Layer 6: The Presentation layer – This layer converts the data that you send from your application (layer 7) into a “standard” format that the other layers will understand.

Layer 5: The Session layer – This layer communicates with the receiving device in has 3 simple purposes:

  • Establish communication
  • Maintain communication
  • End communication

Layer 4: The Transport layer – This layer ensures that data transfer is complete. It provides error-checking and flow control.

Layer 3: The Network layer – Routing and forwarding take place here. It makes sure your data goes to the correct recipient.

Layer 2: The Data Link layer – The physical protocol is assigned to the data here. It defines the packet sequencing and the type of network. (Here is a link to a simple explanation of Packet Sequencing that I really like)I

Layer 1: The Physical layer – I like to call this layer the “stuff” – it defines the physical characteristics of the network, stuff like media (type of cable being used) and connections (hardware and connectors). This also includes electrical and mechanical requirements like voltage levels.

These are pretty simplistic descriptions, if you have more details to add, please share them in the comments!

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