Networks Have Layers? | The OSI Reference Model - Pt1

Many people don't like the sound of their voice, but if you're on a podcast or a radio show, the person you talk with and the microphone you speak into capture your speech on different levels. If the microphone was a person, it would hear something different from what the guy sitting opposite you is hearing.

When you want to speak, you articulate your speech in your head and your brain sends signals to your mouth to speak. Ultimately, your mouth ends up creating changes in the air pressure around it, known as sound waves. Let's take that as the first layer of your speech. Those sound waves travel outwards and are picked up by the ears of the person you're talking to.

The person's ears convert those pressure variations into electrical signals (or changes in voltage). Let's call that layer 2.?

These signals are sent to the listener's brain to be processed. The brain deciphers the sounds, finds out what language you're speaking and decodes the words according to the detected language. Let's call those layers 3, 4 and 5.

From the words, the brain is able to decode the message you're trying to pass and even perform error correction. If you say something that sounds like the wrong word in a context, the listener can sometimes, round that off to the nearest correct word. Let's call that layer 6.

In that situation, the guy is listening to you at layer 6. At layer 6, comprehension of what you're saying occurs, at layer 1, only the sound waves conveying your message are understood. The microphone is listening at layer 1. It's too dumb to understand what you're saying. It can only see you creating changes in air pressure, but the other guy is listening at layer 6. He can process everything you're saying.

Apply this analogy to computer networks and you have what we call the OSI reference model (also known as the OSI model). The Open Systems Interconnection (OSI) model is a reference model that describes how data moves from one computer, through a network, to another computer. It makes it easier to organize network protocols into layers based on what they do, and know where to troubleshoot if there's a problem. It also makes it easier to communicate what's happening in a network. It basically gives names to things. It was developed by the International Organization for Standardization (ISO) in 1984.

The OSI model consists of seven layers. The physical layer, data link layer, network layer, transport layer, session layer, presentation layer and application layer. By the time we're done, you should be able to understand how they work and what they do so much that you can't just forget them.

When you send data from a host to another, the data moves from layer 7 of the sender through layers 6, 5, 4, 3, 2, 1, 2, 3, 4, 5, 6, and to the?layer 7 of the receiver. Just like you process your speech first and articulate them before converting them to electrical signals and sending them to your mouth and speech muscles which create sound waves which get converted back to electrical signals in the listener's ears and is processed to understand the meaning.

The data sent through the network is call a Protocol Data Unit (PDU). The PDU, however, has different names at different levels.?

At the physical layer (layer 1), it is called a bit. At the data link layer (layer 2), it is called a frame. At the network layer (layer 3), it is called a packet. At the transport layer (layer 4), it is called a segment. At layers 5, 6 and 7, it is simply called PDU or data. No fancy names there.

It's like a guy whose name is Dave Ndubuisi Gregory. He puts on casual clothes and hangs out with his friends. He's like the average Nigerian that is ashamed of his native name, so his friends know him as Dave. He dresses up in a suit and goes to work where he's known officially as Mr. Gregory. Then he puts on traditional attire and goes to his ancestral home where they know him by his native name, Ndubuisi. The same man.

The PDU actually has clothes on. It's called encapsulation, and it has different clothes at the different layers. Data encapsulation simply means wrapping the data in another piece of information. When you write a letter, put it in an envelope with an address, you've just practiced data encapsulation (or would that be letter encapsulation?). Anyways, I'm not sure you've done that before.

At layer 4, a layer 4 header is added to the PDU and it becomes a datagram. At layer 3, an IP header is added and it becomes a packet. At layer 2, the data is encapsulated in a frame header and trailer, and becomes a frame.?

To avoid writing something too long, I decided to split this article into two parts. In the next article, I'll explain the layers in more detail. You'd see what they actually do and learn mnemonics you could use to remember them faster. I'll also show you where some protocols like HTTP and SSL lay in this model.

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UPDATE: I'm done with part two The OSI Reference Model Pt 2