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:
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.