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CCNA Training Course: OSI Model Part 1

Posted: October 21st, 2016


This next chapter will describe two very important concepts in networking known as protocol stacks. There are two very important protocol stacks which as a network engineer you must know the OSI model and the TCP/IP model. Knowledge of both these models will help you understand the role of various devices that go to make up a network also help you design, configure and fault find a network more efficiently.

The OSI model
One of the very first concepts I learnt when I entered this networking arena back in the 90’s was that of the “Protocol Stack”, I still recall how I was a little confused by this term, what was this protocol stack?, where does it live, can I see it?, can I hold it?.

What I would like to do over the course of this section is to explain the purpose of the “Protocol Stack” and in particular the OSI protocol stack long with the IP/TCP protocol stack.

Note: It is a good idea to note terms down. I.T is awash with weird and wonderful TLA (Three letter acronyms) even if right now the terms do not mean anything, just note them down.

Here we go, so what is the OSI model, well to put it in its simplest terms, the OSI model is a conceptual model which determines what should happen to your data when it moves from your “Screen” to the “Wire”. The emphasis is in the “what should happen” since the OSI model is a conceptual model or rather a reference model and not an actual working model

Another way to explain it is like this, imagine you wanted to build a car, how would you do it?, you would learn all the necessary skills required to build the car, from welding the body panels, to stitching the fabric of the seat, building the engine and wiring the electrics, this is going to take a little while as you have to learn all the skills needed, so instead of doing all the work yourself you decide to hire seven (7) people to build the car for you, but rather than teach each person all the skills needed to build a complete car you we teach each person an individual skill, so one person does one job only.

Your car is a huge success so you decide you are going to give up the day job and build cars for a living, for this you need a bigger space like a factory, as you have no experience in setting up a car factory you bring in a “Car factory consultant”, who reports that a car factory will need the following:
1. Goods in
2. Storage and inventory
3. Conveyor system to transport the cars down the production line
4. Robots to do the heavy lifting
5. Paint shop
6. Interior station
7. Goods out

Note I have never built a car let alone built a car factory so these are just guesses

The consultant has not and will not specify where to purchase any of these items but will lay out a design how the factory ought to be laid out, the specifics are up to you, where you get the robots is your decision, the paint shop is your decision, all you have from the consultant is the basic frame work or a reference of what should happen in the car factory and what at which stages certain actions and process ought to occur.

The OSI model is the same as the car factory production line. Your car factory you will have a production line, the basic car frame will be placed at the top of the of the production line and will move down the line, as it moves down the line each of your 7 faithful workers will each perform their individual tasks.

1. Body shop to press the panels
2. Panels are welded together to form the chassis
3. Doors, hood and boot lid are fitted
4. Paint shop
5. Assembly were the electrics and mechanicals are fitted
6. Assembly were the interior is fitted
7. Driven out of the factory into the showroom

Why have a production line? Simple, since we are able to change how any of the individual processes is performed without affecting the process before or the process to follow, therefore if you decided the paint shop process is to be changed what will not affect step 3 or step 5 provided whatever happens are step 3 presents the car to step 4 in a manner that step 4 can perform it’s task. Whatever happens at step 4 is of no concern to step 5 provided step 5 is presented a car with which is recognises and can work with.

Now going back to the OSI model, just like our production line, the OSI model is a layer model which will take our data say for instance an email and move the “1’s and 0’s” that make up our email down through 7 layers of its production line, but unlike our production line the OSI layered model starts at layer 7, layer 7 is the top layer and the lowest layer is referred to as layer 1.

As our email goes down through the layers each layer will perform certain actions on the email, from encoding using ASCII to encryption, placing addresses, sequence numbers, error detection and so on and so forth. When the e-mail now wrapped up in layers and layers of communications information to deliver the email reaches layer 1 it is then transmitted onto the wire.

Having a layered approach like our car production line makes it much easier to fault find as we can narrow problems down to specific layers, it also means that any layer can be changed without affecting the next layer above or the next layer below.

The diagram above shows the 7 layers of the OSI model and their respective names