Today I'm going to get back to basics and discuss the 7 layer OSI reference model and how it applies to networking.  Most experienced network engineers know the OSI model all to well and most if not all certifications will expect you to know it. So lets unwrap this 7 layer burrito and see whats inside.

The first thing we need to understand is what is the OSI reference model anyway?  Well,  Wikipedia defines it as - The 'Open Systems Interconnection Basic Reference Model' (OSI Reference Model or OSI Model) is an abstract description for layered communications and computer network protocol design.

Basically it is structure of guide lines for developers and equipment manufacturers to be able to follow when designing new network cards, create new network drivers, new network software, etc and feel confident that the widget will work once it hits the wire.

So the 7 layers of the OSI Model is broken down as follows:  Application, Presentation, Session, Transport, Network, Data Link, and Physical.  Lets look at each one to get a better idea of how each one works and what each layer is responsible for.

Application - This is the layer that is closest to the user. In a web application this would be the browser.

Presentation - This layer would define how the application is presented to the user. Again using the web example. The html of a web page would fall into the presentation layer.

Session - This layer established the communication between computers. From client to server. In our web application the http protocol would fit nicely in the preentations layer.

Transport - This layer provides reliable transport services to the upper layers between users. At this point the TCP session of our web application is functioning here. The TCP syn-synack-ack occurs at the transport layer.

Network - This is the layer that ensures a functional means of having variable data lengths transfer from source to destination between different networks.  Here is where IP is defined and routing occurs.

Data Link - This layer provides a functional means to transfer data between network entities. This is where the MAC address lives and switching and bridging occurs.

Physical - This layers defines the physical and electrical specifications for devices. This is where the rubber meets the road.  Here is where the one's and zero's get put out onto the wire. This is also where cable specs are maintained, network drivers and the transition from physical to electrical occurs.

Now not everyone may agree with my definitions and there has been volumes written about exactly what occurs within the OSI Model. In my example I am merely trying to give a brief overview and understanding of what the OSI reference model is.

In order to help you memorize the OSI model its often helpful to use an analogy.  There are a couple common ones you can use.  One popular one is - All People Seem To Need Data Processing.  Or you can make one up.  Either way the, more you understand the  OSI Model the more it will help you in your networking career.

Until next time

Get your Freak On!