Do You really Need to Know the OSI Model?

Published on September 6, 2021 at 6:32 am by LEW


Do you really need to know about the Open Systems Interconnection (OSI) Model? And what is it anyway?

The first question is easy. Unless you are applying for work in the Information Technologies (IT) field, you don’t need detailed knowledge of the OSI model. And even then it is questionable how much you actually need to know, or will ever use.

As to what it is, the OSI model is a theoretical model of how network communication between computers work, that was never implemented in its entirety anywhere outside of a lab environment. It is mainly used as a reference for system design, for teaching networking concepts, or as a source for questions during IT job interviews.

I should note that there are some OSI terms that have seen common adoption that you will want to be aware of.

Lack of Acceptance

There are reasons why the OSI model has never seen full scale implementation.

The TCP/IP four layer model was implemented and well on the way to becoming the de-facto standard before the OSI seven layer model was introduced.

There is also a political/religious component that affected the adoption of the OSI model. Many did not see the need for another standard, and felt the OSI model was being pushed for political, rather than technical, reasons.

And finely, in the world of computers and networking, like anything else, you have the true believers who are never going to be persuaded to adopt anything outside of their belief system (on any side of the argument).

Origins and Standard

The OSI model is defined in ISO/IEC 7498. Development of the OSI Model started in 1977 from two sources. The International Organization of Standards (ISO), and the International Telegraph and Telephone Consultative Committee (CCITT). The basic model was first defined in 1978, then initially published in draft form in 1980.

Review of OSI Model

What follows is my understanding of the OSI model. I have not made a detailed study of the actual standard, as like any ISO publications it is a guaranteed cure for insomnia (try reading any ISO publication some time and see if you can stay awake).

The OSI model proposes seven layers within any network device. Communications starts at layer seven in one computer and ends at layer seven in another computer. Even though most descriptions and diagrams map all seven layers within a single computer, conceptually layer one is distributed between all network equipment on a segment and not confined to any one computer.

Also, conceptually, you do not need all seven layers in a device. For example a simple network switch would never require anything above Data Link (layer 2). A simple router may never require anything above Network (layer 3).

Layer 7: Application

Starting at the top and working down, the application layer is where the user interacts with various network enabled applications. Some of the more common network enabled applications you will use include web browsers, email clients, multiplayer games, hosted productivity applications, and cloud storage clients. However any application the user interacts with, that is network enabled fits into this layer.

Layer 6: Presentation

In this layer data conversion occurs. An image file, for example, will be converted into a stream of data for transmission, or a received data stream will be converted into a copy of the original file. Depending on the system this will generally be done by the operating system, rather than the application.

Layer 5: Session

This layer is where communications are both established and terminated. In the image example, a communication channel would be established, then after the data for the image was transmitted the channel would be terminated.

Layer 4: Transport

This layer handles the actual transmission and verification of data onto the network. In the image example, this is where the actual data transmission occur. The data is broken down into segments small enough to be reliably transmitted across the network, or received segments will be assembled into a data stream. Acknowledgment of receipt, and multiplexing (multiple streams of data being segmented and transmitted/received) also occur at this level.

Layer 3: Network

This is where the data segments are routed. Routing is the process of sending data to other devices not directly connected to the local network segment. IP address live at this level. Addressing, routing, and traffic control exist at this level.

Layer 2: Data Link

This layer moves data between directly connected devices on the same network segment. This is where the MAC address resides.

Layer 1: Physical

The actual physical real world network, consisting of cables, switches, routers, bridges, etc.

Common Usage Terms

Some OSI terms that have come into common usage are related to network maps or diagrams. Often referred to as layer diagrams, for example a network layer three map or diagram would show IP addresses and logical routing. A layer 2 map would have MAC addresses and physical layout.

Final Thoughts

It is important to remember that the OSI model is for particle purposes, theoretical. I find it interesting that many books have been published on a non commercial theoretical model. So while it is a nice reference for network requirements and structure, and possibly a useful trouble shooting tool. I would not spend to much time getting into the details of the model. Rather, what I did, was become familiar with the basic functions of each layer. Other than being able to discuss it during job interview, it is not something I would typically use.

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