The OSI Model

The Open System Interconnection Model, shortened as the OSI Model is an important topic in computer networking. It is a conceptual model that covers all the processes that are involved for data to be transferred between any two types of endpoints.

In simpler definition, it is a standard way computers must follow when communicating with themselves.

Developed by the International Organization for Standardization to allow computer manufacturers follow a universal pattern of computer communication.

Why Understand the OSI Model?

Though it is a networking concept, the OSI Model cuts across different domains of computing, from hardware engineers to software developers and even network administrators. Cybersecurity professionals? They can’t do without networking, so it is equally important to them. In fact, anyone serious about computers and how they communicate should be familiar with this concept.

Its use is not only restrained for the sake of manufacturing, it is also important for troubleshooting, due to the fact that each of its seven layers involves protocols and endpoints that are constantly referred to when troubleshooting network related issues.

Layers of the OSI Model

Each of its seven layers concerns itself with a different aspect of networking that flows logically onto the next, flowing from the seventh to the first.

The last layer at the top is the Application Layer and that’s standard, but some professionals often have an eight layer, known as the User. I won’t be surprised that much and that’s because even the classification of computers based on software and hardware, there is also an invisible third–the human-ware.

The users are always present, officially or not, so that isn’t a serious topic to debate on.

Now, let’s get on with our seven layers!

The Application Layer

The Application layer is the part of the OSI Model that deals with the data and protocols utilized by softwares in interacting with different computers.

A web browser for instance uses the http protocol to send a request to a web server, requesting for specific data.

It is at the application layer that the data makes sense to a user at an endpoint , once it leaves the application layer, end users would require special skills to make usage of whatever that data is, i.e data at the physical layer is in the form of electronic signals and can’t be understood in that state.

Another application layer protocol is the SMTP or Simple Mail Transfer Protocol, used to enable email applications to convey email messages.

The Presentation Layer

Just like the name suggests, the presentation layer is concerned with how data is presented. It is at this layer that data is encrypted/decrypted and compressed/decompressed depending on what direction is it taking (up or down the layer)

It also provides translation services too. If for instance, an email server is sending a client an email using its unique encoding format, before the email gets to the application layer which presents it to the client, at the presentation layer, the email would have to be translated into a format that the client’s application layer understands, ensuring that the client gets a readable message.

Its process is usually like this:

Up the layer: Decryption → Decompression → Decoding/Translation

Down the layer Encryption → Compression → Encoding

The Session Layer

Have you ever logged into a site and after a while, you were automatically logged out? That’s the role of the session layer. The session layer is the layer that’s used to maintain sessions when a connection has been established in any networked communication.

Another area where the session layer plays an important role is in downloads. When you download a file and pause along the line, probably due to a network interruption, depending on the configuration, you may have to restart the download or continue from where you stopped.

In simpler terms, the session layer is used to open and close communication. Once the communication has been closed, to connect on the network, a new session would have to be established.

The Transport Layer

The transport layer is like the road that vehicles carrying data utilize in network communication. It is at this layer that the data that is to be transmitted actually goes on its voyage, utilizing frameworks like the TCP & UDP protocols.

The TCP protocol is used when there is a focus on reliable data transfer, making it ideal for transmitting stuff like web pages and all that’s in it.

When you X feed loads up, or you are watching a youtube video, or even reading this blog, you are able to see all the information contained because the data was transmitted using TCP or the Transmission Control Protocol.

UDP or the User Datagram Protocol is used when the focus shifts from reliability to speed. In scenarios like watching a YouTube Live feed, or listening on an X space, UDP is preferred, this is because unlike TCP that would go back to pick up lost packets in order to ensure that all packets are transmitted in order, UDP just keeps transmitting, leaving dropped packets so as to focus on delivering on time.

This is why when your internet fluctuates during a live broadcast or WhatsApp call, you lose what the other person was saying and you’d have to ask that they repeat themselves.

There is a solution though, but that will be in a later blog. Sign up to the newsletter to get reminded when I upload that.

The Network Layer

Before you embark on a journey, it is essential you decide the best route for you to take, right? Especially if you are on a journey to where you are not too familiar with.

This essentially is the network layer. It helps in finding the best routes that data, known as network packets, should take when moving across different networks. Though it is unnecessary when the communication is to take place between the same networks, just as how you don’t need to plan your route when walking from your sitting room to the kitchen.

The Internet protocol is an essential protocol used at this layer. It helps computers identify themselves on a network. For instance, if I am sending a web page request from my phone’s web browser to a web server hosting a particular website, for such communication to be effective, my web browser would utilize my phone’s IP address in communicating with routers, ISPs and finally the web server that has the web page I am looking for. But how is the web server identified on the network? Through its own IP address. When I type in my URL, a DNS resolver translates that URL into its IP address because computers understand that better.

The Data Link Layer

The younger brother of the network layer because of their similarity in functions.

It has two sub layers: the logical link control and the Media Access Control (MAC)

The logical link control helps to determine what type of IP is being used at the moment, helps in controlling data flow.

The media access control is used to identify network interface cards and regulates how data flows from one NIC to another.

The Physical Layer

This layer interacts directly with the medium used in transferring data. The use of binary bits, cables carrying electrical signals, etc. is the base of the OSI model, and on the originating device, the actual 0s and 1s are worked up until they get to the user readable format of the Application layer.

An Example

Let's make sense of all of these with a pragmatic scenario. A Google Meet Call

Suppose I have an interview with a client and we are scheduled to use Google Meet.

Now, my Google Meet is using an application layer protocol called WebRTC, this protocol would transfer my data to the presentation layer, which would check for the encoding and compression and pass on to the session layer. The session layer using the same WebRTC helps in managing the session with my interviewer on the other end, a session that would close once I leave the call.

Once my session is up and running, the transport layer kicks in, using UDP to transmit my data to the network layer, which uses the Internet Protocol to route my call packets to the right server, in this case, the Google server in charge of the google meet call.

After that, it hits the data link layer, and assuming my interviewer is using his mobile phone, the MAC forwards the further broken down frames to the physical layer which interacts with the physical medium used in data transfer and the entire process is rebuilt again, from layer one to layer seven.

Conclusion

The role of the OSI Model can’t be overemphasized in networking. It helps you and I easily create communication channels that would have been impossible.

It should be noted that each layer isn’t entirely independent. They often overlap in functions due to the broad nature of computer networks.

Understanding how it works is indeed a valuable knowledge if you are in IT.

All of them are useful, but if I were to choose which I love the most, it would be the 4th and 3rd. What’s yours?