Understand OSI Model

What is OSI model

As you begin your quest to learn computer networking one of the first tasks you have before you is a basic understanding of the OSI model.
For many folks understanding the OSI model is like trying to understand some mystical formula that controls the way computer networks operate. 
As we help you to begin your journey to understanding computer networking   We will tackle explaining the complex subject of the computer networking OSI model simple terms in hopes that you will gain an understanding of the reasons behind the definitions
You can find a lot of resources that define the components of the OSI model, but an understanding of the reasons behind the definitions will go a lot way to fully understanding this complex technology model.

The acronym and the organization behind it can get confusing. The formal name for the OSI model is the Open Systems Interconnection model.  Open Systems refers to a cooperative effort to have development of hardware and software among many vendors that could be used together.  The model is a product of the International Organization for Standardization (2) which is often abbreviated ISO.

The logic behind ISO

Before we delve into the OSI model, let us take a moment to understand the organization behind it.  You may have seen the term ISO certified in various technology areas. ISO, International Organization for Standardization,  (1) is the world's largest developer and publisher of International Standards. ISO helps to manage and create many international standards in many technical areas to insure the same quality of a product or process regardless of location or company.

The OSI (Open Systems Interconnection) model provides a set of general design guidelines for data communications systems and gives a standard way to describe how various layers of data communication systems interact. Applying the logic of the ISO standards to computer networking, a computer component, or computer software needs to comply to set of standards so that the product or process will work no matter where in the world we are, and no matter who is the world is producing it.

Putting the OSI model into perspective

Strive for a good understanding of the intent of the model and a few of the core principles, that will go a long way in an overall understanding of computer networking.  Do not focus on the intricate details of the OSI model at first, as the more you read the more confused you may get. The model was created in the 1970s and the technology is ever changing. Many text books will contradict each other on some aspects of the upper layers.  Some of the reasoning behind the upper layers are for processes that are not nearly as useful today as they were many years ago, and for that reason many other network models will blend together the upper three layers into a single layer.

Basic definitions of the OSI Model

The seven layers of the OSI Model  can be remembered by using the following memory aide: All People Seem To Need Data Processing. As you say the phrase, write down the first letter of each word, and that will help you to remember the seven layers in order from highest to lowest: Application, Presentation, Session, Transport, Network, Data Link, and Physical.  We will briefly discuss the lower four layers from the bottom up.

Layer one, the Physical layer provides the path through which data moves among devices on the network.

Layer two, the Data Link layer provides a system through which network devices can share the communication channel.

Layer three, the Network layer's main purpose is to decide which physical path the information should follow from its source to its destination.

Layer four, the Transport layer provides the upper layers with a communication channel to the network.

An analogy to understand the model

Some of reasons behind the OSI model are, to break network communication into smaller, simpler parts that are easier to develop and to facilitate standardization of network components to allow multiple vendor development and support.

Let's take the reasons behind the OSI model and apply them to something totally different to illustrate how they are used. If we wanted to start a railroad and build a new type of train from scratch, and we wanted this train to be able to use existing train tracks, and existing train stations so our new system could get up and running quickly, we would need to understand what existing standards are currently in place.

Even if we never had to build a set of train tracks we would need understand the standards by which train tracks were build and designed so we could assure our train could operate on them, and how the track is shared.  Likewise, in order for components to operate, manufactures must understand the track, layer one, and how the track is shared, layer two.

If we are building trains, not train stations, we need to know the size and shape of other vehicles using the tracks so our trains could use the same track as all the other trains. Layer one of the OSI model gives us the path, or the track we use for communication.  Layer one, referred to as the media, is the wire, or anything that takes the place of the wire, such as fiber optic, infrared, or radio spectrum technology.

Once you have more than one train on the track, you need to find a way to share the track.  Layer two provides a system through which network devices can share the communication channel, or in the case of our analogy, share the track. One of the functions of layer two is called media access control (MAC). If you think about the term media access control you can break it down into the two parts it represents, the media or the track, and access control, or the sharing of the track.

In the OSI model layers one and two represent the the media, or the physical components.  Layers three through seven represent the logical, or the software components.

In layer three of the OSI model, the Network later, the logical decision is made to decide which physical path the information should follow from its source to its destination.

In order to continue our analogy to understand this complex set of rules, think of the track system that has already been built as layers one and two. Once this track system is in place we need a system to control the routing of the train system that runs on the tracks. Think of layers three through seven as processes which affect the train itself, which would represent the actual package of information being transported along the tracks.  The main purpose of layer three is switching and routing.

Layer four of the OSI model, the transport layer ensures the reliability of data delivery by detecting and attempting to correct problems that occurred. In terms of our analogy, think of this as a set of standards and procedures that allows our train to arrive safely at its destination in a timely manner.

Learning and understanding the OSI model can be confusing.. The goal of this article was not meant to define the layers of the OSI model from purely a technical nature, but to offer an analogy to understand why it is needed and how it used to establish standards for data communications.  In our next article we will go over the basic definitions of all the layers of the OSI model.



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