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Network Layers in OSI

As you get deeper and deeper into networking, you eventually need a framework that can organize ideas and assist with development and implementation.

There are different frameworks that exist. In some cases, various models might directly compete with each other. In other cases, they can expand on the same ideas using different points of view, offering unique insights that can help with network design and development.

One of the most prominent models is the OSI model. By understanding this framework, you can see how many network protocols, devices, and systems are designed and why.

The OSI Model

The Open Systems Interconnection Model (OSI Model) is a framework that scientists and engineers use to classify and organize information and concepts related to network designs and functions.

It is one of the most recognized and utilized intellectual frameworks for network descriptions in the world.

The OSI model is largely built around an idea of splitting network functions into a seven-layer hierarchy. Each layer in the hierarchy is built on top of functions and systems in the lower layers, but it’s important to remember that this is an intellectual framework.

Since the bottom layer is the physical layer (which is just accounting for hardware), it’s clear that all of the other layers depend on hardware for their functions. In higher layers, virtual interactions and mechanisms take on specific tasks that keep the network functional, stable, and efficient.

By looking at each of the seven layers, you can see how different aspects of networking come together and where they fit into the overall design.

Physical

This is the easiest layer to understand. It describes the hardware in the network. Switches, routers, access points, etc. are all in the physical layer.

Data Link

At this point we start getting more into concepts, virtualizations, and the higher-level concepts.

The data link layer is where data is actually put into frames. The data link layer controls the timing of any transmissions within the network and it features fault tolerance mechanisms that help keep data intact.

Logical Link Control (LLC) and Media Access Control (MAC) are both managed in this layer.

Network

The network layer is where traffic is managed. The IP network protocol lives here — meaning that much of traffic routing is controlled in the network layer.

Pretty much everything in the network layer is geared around getting traffic to the right destination.

Transport

The transport layer is another streamlined concept within the OSI framework. This layer utilizes TCP and UDP. These resources sequence information in order to ensure data is free of errors.

Session

The session layer is where things get even deeper. This is where user interface mechanisms and resources are managed.

Remembering that a session is a period when two devices are in direct communication with each other, the session layer is aptly named. This features software controls that determine when a session begins or ends. It also controls how data is transmitted during any given session.

This is also where some security features live. Name lookup is what enables computers to find each other in a network, and this is a layer where you can institute security measures related to finding and sharing across a network.

Presentation

The presentation layer is where a lot of data-related computation happens. Using ASCII or EBCDEC, this layer is where data is translated to any computer-specific format. This can include things like byte ordering and compression.

Ultimately, the presentation layer prepares data. The nature of that preparation depends on the action at hand, but ultimately, various devices are able to communicate on the same network because of the work that happens in this layer.

Application

This is where applications live. The application layer has all of the software services that run through and in a network. Ultimately, this layer provides resources that users utilize to interact with the network.

Additional Learning Center Resources