The OSI reference model provides a framework for both designing network systems and for explaining how they work. The OSI model was originally created as a basis to design universal sets of protocols called the OSI protocol suite. The suite was never used but the model became a tool for education and development.

Understanding the purpose of the model

The OSI reference model is a framework that breaks down the internetworks into easier to understand components. Each component does a specific function. This component approach, allows us to design and develop functionality to combine components as needed. For these reasons, it is helpful to understand the OSI reference model as it describing real-world network operations. The OSI reference model is therefor a tool to better understand networking technologies. However, not all technologies, protocols and components fit into the layered model and trying to make it fit can add more confusion than anything else.

Beside the OSI reference model, there are networking architectures and protocol suites that also describe networking technologies. The DOD model or TCP/IP model has similarities to the OSI model but is often used when discussing the suit.

The OSI Layers

The OSI model divides functions into layers. The lower layers are concrete hardware specific functions while upper layers are more abstract. The four lower layers have clear boundaries and functionality while the three higher layers are sometimes even treated as one - TCP/IP.

OSI Layer	Layer Name	Network Layer
7	Application	N + 4
6	Presentation	N + 3
5	Session	N + 2
4	Transport	N + 1
3	Network	N
2	Data Link	N - 1
1	Physical	N - 2

In order to communicate between layers, defined interfaces have to be in place. Interfaces allow other layers to use functionality without understanding the specifics of each layer. Interfaces thus allow for vertical communication. Same layers can communicate with each other through protocols. Protocols pass data through lower layers until layer 1 to transmit it across a network and finally back up to the same layer on the receiving network. This gives the impression of direct communication.

Each Protocol communicates information through protocol data units (PDU). The PDU is passed down to the next lower layer becoming a service data unit (SDU) since the lower layer provides the service of handling the PDU. The lower layer encapsulates the SDU into its own PDU in turn sends it down to the next lower layer until it reaches the physical layer. The process is reversed on the recipient.

L4 (PDU) -> L3 (SDU) -> L3 (PDU) -> L2 (SDU)

In the OSI model routing occurs when data is sent through an intermediate system like a router. That receives data and passes it upt to layer 4 (Network) where it is repackaged and send out via the layer 2 (Data Link) interface.