An Introduction to the OSI model: How the Internet was Born

Picture it. It was the dawn of the early internet. While computer networks had been invented, computers still could not communicate with each other. Even though the first “internet” a.k.a. ARPAnet had been invented, the implementation of such a network in the real world had not been fully realized. So, computer companies, such as IBM, began to build their own proprietary networks based off of the ARPAnet project. The problem with the concept of a proprietary network is that they do not play well with other networks. Thereby, they limit the ability of computers to communicate.

So, two models were invented to assist in creating a standardized communications system for all of the computers on the internet. One was the OSI model, and the other was the TCP/IP model. Both showed promise in the arena and the OSI model was expected to win. However, this did not turn out to be the case. When the dust settled, the TCP/IP model ended up being the clear winner. So, why are we talking about the OSI model, you ask? Well, since the OSI model was so popular during its battle for internet superiority, terminology used in the OSI model became common network engineer parlance. In fact, network engineers still use this terminology when describing network connectivity issues.

The OSI model has 7 conceptual “layers”. They are:

  • Layer 1: Physical
  • Layer 2: Data Link
  • Layer 3: Network
  • Layer 4: Transport
  • Layer 5: Session
  • Layer 6: Presentation
  • Layer 7: Application

You can remember the layers starting from 1 and ending with layer 7 by the simple pneumonic: Please Do Not Throw Sausage Pizza Away.

The physical layer consists of cabling like ethernet cables, fiber optic cables, and coaxial cables and wireless communication such as Wi-fi, 4G LTE, 5G, or Bluetooth.

The data link layer is designed to handle a type of address similar to the way your home has an address. To understand this type of address you need to know about two types of networks, Local Area Networks (LAN) and Wide Area Networks (WAN). Local Area Networks are small, typically similar to a home network. A Wide Area Network on the other hand is really big. A WAN can cross between cities, states, and even continents. Layer 2 addresses only work on LANs. Layer 2 addresses are known as MAC addresses. MAC addresses are often referred to a burned-in addresses because they are, in most cases, unchangeable. Every network enabled device has a MAC address.

Next, on layer 3, resides the IP address. This is the address you are probably most familiar with. IP address are routable between LANs where as MAC addresses are not. IP addresses form the basis of the internet as we know it. They function as a way for network devices to identify each other across various networks. This allows the end user to visit YouTube, use Facebook, order DoorDash, etc.

So far we’ve discussed the first 3 layers of the OSI model (Physical, Data Link, and Network). In the next article, we will discuss the last 4 layers: Transport, Session, Presentation, and Application.

Posted in