The OSI model will help you understand how computers communicate.

This is just a model that helps us understand how things work. Not everything has to fit exactly into one of these layers and maybe you might design something that doesn’t fit perfectly. That’s okay.

Here is a summary of each of the layers. Make sure to listen to episodes 158 through 161 as well as this episode for more details.

  1. Physical layer is responsible for making sure that individual binary data can be transmitted and received.
  2. Data link layer is responsible for making sure that computers and devices can recognize each other and send and receive multiple bits of data.
  3. Network layer helps by routing information through multiple computers to reach the final destination.
  4. Transport layer adds reliability and flow control.
  5. Session layer adds longer communications through sessions.
  6. Presentation layer translates information between formats that an application expects to and from whatever format is needed for the communication.
  7. Application layer covers everything in an application.

Listen to the full episode for more or you can read the full transcript below.

Transcript

I’ve already been explaining how to communicate with other computers and introduced the OSI model in the previous episode. You’ll get the most benefit from this episode if you first make sure to listen to episodes 158, 159, 160, and 161.

As a quick review, you’ve learned how the OSI model describes communication between computers or other devices by focusing on seven layers that each build on each other. In other words, the lowest layer provides very basic functionality that is used by the next layer which adds new abilities.

This is just a model that helps us understand how things work. Not everything has to fit exactly into one of these layers and maybe you might design something that doesn’t fit perfectly. That’s okay.

Layer one is the physical layer and is responsible for making sure that individual binary data can be transmitted and received.

Layer two is the data link layer and is responsible for making sure that computers and devices can recognize each other and send and receive multiple bits of data.

Layer three is the network layer and helps by routing information through multiple computers to reach the final destination.

Layer four is the transport layer and adds reliability and flow control.

This episode continues explaining the remaining layers 5, 6, and 7.

Layer five is the session layer and adds longer communications. Think of it like this. When you walk up to a friend and start talking, what’s the first thing you say? You should probably start with a polite greeting, right? Your friend also responds. Maybe you then comment about something your friend is doing or just mention something that seems remarkable. In other words, the two of you start conversing. Back and forth. Even when the conversation turns to more important issues and then back again, it’s still an exchange of thoughts and ideas. When you’re done, you both say goodbyes. That whole conversation is a session. And that’s what this layer provides.

You need to have some way to group multiple messages together. Imagine the trouble it would cause if halfway through your conversation with your friend, you remembered something important and just stopped talking in the middle of a sentence and walked away. Your friend wouldn’t be too happy with you.

Layer six is the presentation layer. What kind of presentation is needed for communications? Of all the layers, I always felt this one has the strangest name. In order to understand it, you have to first have some experience building well-designed user interfaces. Let’s say that you have an application that displays some dates. It doesn’t matter for this example where the dates appear or what they mean. The important part is that different users want to view dates in different formats. Some want the month first and some want the day first. And some want completely different formats. The question is how do you store the actual date that appears? A well designed user interface will be based on a lower layer that exposes the dates in a single standard format. It’s the job of the user interface application to do the formatting. A user interface is often called a presentation.

The presentation layer in the OSI model is also responsible for translating information between formats that an application expects to and from whatever format is needed for the communication. Maybe you need to send and receive information through computer systems that are outside of your control. It would be better to encrypt the information before you send it. This encryption is an example of what might be done in the presentation layer.

And the final layer seven is the application layer. This covers everything in an application. If the application is a web browser, then it has to allow communications to be configured properly, and then issue the requests for web pages and respond to the replies from the servers. And if you’ve ever tried to connect your computer to a wireless access point, then the various windows and dialog boxes are part of the application layer.