Discover the OSI Model
Remember, the purpose of a network is to allow devices to communicate with each other. However, in any network, a range of different devices will be present.
Standardization rules are necessary to enable communication between these different device types.
There are two categories of rule in computer networks:
Rules concerning hardware, which are known as standards.
Rules concerning software, or protocols.
We’ve already come across some of these standardization rules already, such as:
The use of RJ45 cables as a communication medium.
The requirement to assign an IP address to devices in a network.
And there are many more!
The OSI model classifies each of these rules into a total of seven different levels, known as layers. Each of these layers has a name that corresponds to its function.
Let’s go back to our mail analogy.
As you’re a caring grandchild, you decide to write a letter to wish your dear grandmother a happy birthday. You follow a set of rules from the moment you write the letter to when Grandma receives it.
There are some rules which we’ve already mentioned, such as writing the name of the intended recipient on the envelope. But if we look even closer, we can see there are many more rules. Let me give you a few examples:
When writing the letter, you’ll need to follow the grammatical rules of the language you are writing in.
You’ll need to put the letter in an envelope and write your grandmother’s name and address on a specific part of it.
You’ll then need to place the letter in a mailbox corresponding to the mail shipping service you want to use.
Once the mail shipping service picks up the letter, it will be sorted by region and grouped with others according to its destination before being loaded into vehicles (which also have to meet certain manufacturing specifications) in order to be distributed.
I did say we were going to look closer!
These rules, when followed, all contribute in some way to the goal of wishing your grandmother a happy birthday, but they don’t all apply to every party involved in the process.
Postal workers don’t care whether your letter is written in French, Hebrew, or Latin—it doesn’t affect their work at all. However, they do need the address to know where the envelope is going.
The same applies with the OSI model. Each party, or “component”, involved in the network follows the rules on their specific layer. The rules on the other layers don’t interest it in the slightest, and it often won’t even understand them.
We can link each component to a layer:
This detailed model helps us understand the stages involved in creating and sending a packet of data. However, there is also a simpler model, with fewer layers: the TCP/IP model.
The TCP/IP model
The TCP/IP model only has four layers. It’s much less complex and more applicable to real-life situations than the OSI model.
This model’s name is composed of the two most important protocols within it.
The Internet Protocol
This is located in the Internet layer, the equivalent of layer three in the OSI model: the Network layer. You’ve already heard of IP in the context of its addressing system.
The Transmission Control Protocol (TCP)
This is located in the Transport layer, the equivalent of layer four of the OSI model, of the same name. Its role is to establish the rules for sending a message from the source to the destination, ensuring that nothing is lost along the way.
One of these rules states that each packet must be numbered. Therefore, when a message is too long to be sent in one packet, it is split into several parts and placed into different packets, which have to individually be given a number.
All of the rules or instructions for sending a message through a network must be sent in the packet containing the message. The packet is therefore made up of the message, plus all of the necessary instructions, split into layers. This is what we call the encapsulation mechanism.
This is what an encapsulated message looks like, following the TCP/IP model, in diagram format:
Each part of the network—the router, the switch, the application to which the message is being sent, etc.—will look for the instructions intended for it in the packet.
Use This in Practice
The TCP/IP model better reflects the reality of communications on IP networks. However, it is less detailed than the OSI model.
Therefore, in practice, the historical OSI model is still used to define which layer network devices are on.
Over to You!
Now you know the different layers of the OSI model, try to identify which layer of the OSI model each device in your network architecture corresponds to. Write down your answers for each different device before checking the answer sheet below.
Answers
You can find the answer sheet here to check your work.
Out of the many level seven protocols, HTTP is one of the most well-known level seven protocols. It allows you to switch between and open web pages—the internet owes its success to it!
Let’s Recap!
The OSI model allows the rules and protocols that enable communication on a network to be classified into different layers.
This model is composed of seven layers with a name indicating their function: Physical, Data Link, Network, Transport, Session, Presentation, Application. A popular acronym for this is Please Do Not Throw Sausage Pizza Away.
Each protocol and part of the network are linked to a specific layer. The IP protocol, and routers that handle and understand IP addresses, are linked to layer three, the Network layer.
The TCP/IP model is a similar model with four layers.
When a message is generated, it is assembled with all of the rules to allow it to reach its destination. This assembly process is called encapsulation.
Now you know how to create and configure a functional network that a technician can use. In the next part, you’ll see how to optimize this network for the end user. But first, a quiz!
