Introduction to the OSI Model

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Before you start

Objectives: learn what OSI model is, which layers does it contain and what are the advantages of using OSI model.

Prerequisites: no prerequisites.

Key terms: OSI layers, communication, modularity, features, advantages


OSI Model

The Open Systems Interconnection (OSI) model is a theoretical way of classifying and talking about the complex process of sending data on a network. The goal of the OSI model is to design a common networking communications model and to make this communications model modular. This modularity fixed the problem of proprietary products. The modularity allows us to purchase various networking components from various vendors and have them work without problems. Without the OSI model we would still be back in the proprietary world. OSI model divides the communication process between two hosts into layers. These layers break down the communication process into general tasks. Let’s take a look at what this model looks like.

OSI Layers

Image 175.1 – OSI Model Layers

The OSI model provides seven different layers of communication. Let’s imagine that on the left side we have the sender in the communication process, and on the right side we have the receiver. For the sender to create a message that will be transmitted to the receiver, the sender processes this message down through the layers of the model before it actually hits the network medium.

Sender

Image 175.2 – Sender

Understand that those layers are conceptual, it’s a model for how the hardware and software is implemented on the computer. This concept is really valuable because you we can have an application written in the Application layer, and that application does not need to understand what type of physical medium is used, it doesn’t need to understand the network, how information gets from one network to another, it doesn’t need to understand how sessions are open between hosts and so on. All it needs to know is how to format its information according to the particular layer and send it on down. The next layer then processes it and sends it to the next layer. That next layer knows what to do with that information to prepare it, goes down to the next layer and so on until it reaches down to the Physical layer where it is actually transmitted on the network medium. When it arrives at the receiver, it goes through the reverse process.

Receiver

Image 175.3 – Receiver

Each layer on the Receiver processes the information, stripping off the information that was added on the Sender, until it reaches the receiving application layer.

Advantages

OSI model divides networking tasks into logical layers for easier comprehension so it aids in troubleshooting. It provides a common language or reference point. It also allows specialization of features at different levels, promotes standards of interoperability between networks and devices and provides modularity in networking features. Modularity means that developers can change features without changing the entire approach.

Although OSI has all those advantages, OSI layers are theoretical and do not actually perform real functions. Industry implementations rarely have a layer-to-layer correspondence with the OSI layers. In the real world different protocols within the stack perform different functions that help send or receive the overall message. A particular protocol implementation may not represent every OSI layer or may spread across multiple layers.

Remember

The goal of the OSI model is to design a common networking communications model and to make this communications model modular. It provides seven different layers of communication. OSI layers are theoretical and do not actually perform real functions.