• Deirra Footman

The OSI Model

No matter where you are in the IT world, understanding the OSI Model is key. The OSI (Open System Interconnection) model is a framework used to implement standards for network communication.  Created in 1984, the goal of the OSI model was to provide a vendor-neutral set of standards for Technology vendors to ensure interoperability. It is a hierarchical architecture that logically partitions the functions required to support system-to-system communication.

The OSI model has seven (7) layers that define specific tasks. The layers are:



  • Layer 1: Physical

  • Layer 2: Data Link

  • Layer 3: Network Layer

  • Layer 4: Transport Layer

  • Layer 5: Session Layer

  • Layer 6: Presentation Layer

  • Layer 7: Application Layer

The OSI model was created to provide software developers and hardware manufacturers a standard interface allowing interoperability. Other key benefits include:

  • Reduces complexity by breaking concepts down into smaller, more digestible parts and simplifying troubleshooting.

  • Ensures Vendor Interoperability by providing a standard for vendors to meet when creating computers, network devices, and software.

  • Provides modular engineering allowing a vendor to write code for a specific layer. For example, a web browser for the application layer.

  • Simplifies Learning and Development by breaking the concepts down into digestible pieces.

In figure 1.1 we will take a look at each layer and see some protocol and device examples:

Fig. 1.1

Device Type by Layer

Let's take a look at where our network devices fall on the OSI Model.

  • Layers 7 - 5 - Hosts and firewalls

  • Layer 3 – Routers and Layer 3 Switches (Switches that perform routing functions)

  • Layer 2 – Switches (No routing functionality, Switches at Layer 2 forward traffic based on MAC Address), Network bridges (similar function to switches but with fewer features and intelligence.)

  • Layer 1 – Hub, Repeaters, ethernet cables

Layer 4 In Depth - Transmission Protocols

There are two types of transmission protocols available at this layer,  TCP, and UDP.