Ten-Minute Series: OSI Reference Model

Hi, welcome to another 10 Minute Series. My name is Kelly Handerhan. You can reach me at KellyH@CyberTrain.it. We’re here to bring some clarity to some of the concepts that you’ll find can be a little bit difficult when you’re studying for certification exams. We’re going to release these 10 minute series every couple of weeks, or more frequently if we’re able to do so, and try to help those of you that are working on certifications find a way to make some of these ideas easier. I’ve been a technical instructor for going on about 20 years now. I teach a wide range of classes from PMP, I teach cloud security, ethical hacking, the CompTIA courses. Been doing this for quite a while, and I want to take those things that I’ve learned, and I want to make them easier for you.
Today we’re going to talk about the OSI Model. I think complicated topics, if you can find something that they’re similar to, if you can find an analogy, or say, “Hey this is like that,” makes it easier to understand. What I want to talk to you about is first of all, why the OSI Model, and then what it means, okay?

It’s not as boring as it seems

When you’re new to networking and telecom, you may find that the OSI Model is just one of those boring things you have to memorize for a test. I understand that. To be honest with you, 20 years ago I learned the OSI Model as something boring I needed to know to take a test, but the longer that I worked with it, the more I understood its meaning, and how it helped me take networking to a different level, to a deeper step. Right now, if you’re just memorizing, that’s okay. The longer you’re in this field, the more meaningful it will become. Understand the big reasons for the OSI, first of all is that so much has to happen in networking to get information from computer one to computer two. I mean, just so much has to happen even if the two PCs are sitting right beside each other. That there’s no one device that can do everything, right? You can’t take a switch that does logical addressing, and error detection, and packet filtering, and reassembly, and inspect for ports, and look at non repudiation, you just can’t have a single device that does everything.

The OSI Model allows us to break up the functions of networking into seven distinct categories, or seven distinct layers, so that when a vendor, this the other part, when a vendor builds a device that performs a specific function at a specific layer, if he builds to that standard, then that device is going to be standardized, and is going to be interoperable with the other devices. OSI Model really is helpful to us. Hopefully in the next 10 minutes, or so, I’m going to help make this make a little bit more sense, what happens along the way.

I think the best way to think about this is think about an executive. We have Antoinette, she’s our CEO of OSI Incorporated. She’s up on the seventh floor of a building and she wants to send a letter to her peer at the branch office, wherever that may be located, okay? What she has to do is she has to ensure that that letter is going to be able to get from her seventh floor, where she creates the content, all the way over to the branch office, and all the way up to her peer at the branch office, okay? Antoinette is responsible up on the seventh floor, she’s the creator of the content. Antoinette is going to decide what’s in that letter, she’s going to determine what information needs to be exchanged, we’re strictly dealing with the content up here at layer seven, okay? She’s going to put the date and time on it, so we’ve got a little bit of time notification. She is going to sign the letter that she writes, so that they know that it comes from Antoinette. Then of course, the content.

Now, Antoinette has associates on each of the remaining six floors that are going to perform a function in order to get this letter and make it so that it’s deliverable. Antoinette is going to walk down to Pasqual, who is on the sixth floor. She’s going to ask him, and she’s going to say, “Hey, can you make sure this letter is formatted? Make sure I have good grammar, make sure that spelling is okay. Make sure that I’ve written it in such a way that they can understand it at the branch office.” Okay? Pasqual does that and then he walks down to the fifth floor. He’s going to hand the letter off to Sophia. He’s going to say, “Sophia, I need you to make sure that Albert knows,” Albert’s company, he’s our CEO at the branch office. “Make sure that his organization knows that we’ve got a message that’s sending and I want to make sure that they can contact me if they need to, that we’ve got that established pathway for communication.”

Sophia takes care of that and walks down to the fourth floor. She says, “Tamir, can you attach a read receipt to this letter? We need to know, we need an acknowledgement, that the letter was received or that it wasn’t, we need to know one way or the other.” Tamir does that. He also jots down on an envelope the specific type of message that it is. For instance, he says, “This is a letter for … ” If it were something else, you know, he would jot down, so that ultimately when it gets up to Albert he’ll have an idea of what type of message it is. Okay? Like in that regarding field with a letter, Tamir might write, “Regarding software services,” or, “Regarding scheduling for next month,” whatever. Okay?

Now, Tamir has done that, walks it down to Naheem on the third floor. Naheem looks up the street address for Albert’s office, the branch office. He’s also going to look up online what’s the best path to get there. He’s going to scribble those notes on the envelope, all right? Then he walks down to Daryl on the second floor and he says, “Daryl, what is the floor and suite number in which Albert works? Add that to the envelope as well.” Okay? Daryl does it and he walks down to Pauline on the first floor. He says, “Pauline, take the instructions on this envelope and drive this message over to our branch office,” and she does.

What happens is the content starts up at the seventh floor with Antoinette and goes all the way down to the first floor where it’s ready to get on the road. All right? Pauline takes it over to the branch office and hands it off to Peter. Peter works on the first floor. He takes it out of her car and he walks up to the second floor. Dae on the second floor highlights the suite number from the envelope so that they’ll know where to go with it. She walks it up to Nadia. Nadia removes the envelope and passes the letter and the read receipt up to the Tom on the fourth floor. Tom completes the read receipt to acknowledge the letter was received, so they now know at the first location we got the message. He circles the regarding field so that when it gets up to Albert he’ll know what mindset to be in to process this message.
He goes on up to the fifth floor. Shelecia calls Sophia from the main office and says, “Hey, let’s stay on the phone while Albert reads this letter and we’re going to manage it to ensure that there’s a pathway of communication.” Takes it up to the sixth floor where Paul is waiting. He looks at the letter contents and makes sure it’s formatted properly so that his boss, Albert, up on the seventh layer, can get the message, can view the content, and respond accordingly. Okay?

The point here is on the sending, from the sending office, the main branch where Antoinette works, each floor is responsible for some sort of activity. Then on the branch office, the corresponding floor uses the information that was placed on it from the branch office. You have this idea of what’s put on at the sixth floor at the main office is needed by the sixth floor at the branch office, and so on. Sometimes we talk about this is as a peer to peer model. All right? Now, if you want to take that, and so you see that data coming in to the branch office goes up the OSI Model.

Now how does that work with data? It’s the exact same idea. When we’re up at layer seven it’s about content. It’s about those protocols driving the applications that we need in order to process data, right? Up at layer seven we’re looking at HTTP, FTP, SMTP, you know, all of those application protocols, and they’re about the content and the communication. At upper level it’s really about the data. I might click on a link that says, “Show me an HTTP site. Give me that webpage, I want to view it.” All right, well down at layer six it’s about formatting, and compression, it’s about making sure the message is in universal format so that if you’re on a Mac and I’m on PC, we can still read messages you and I send to each other. It’s about file level encryption, if that’s necessary, or if that’s part of the message before getting it ready to go, it’s basically packaged up in a universal format.

Okay, we go down to the session layer. The session layer is about establishing a session from one application to another. When your client connects to a server. Session layer makes sure that that session exists from app to app. Layer four is that pony express layer that’s ultimately responsible for a couple of things. It’s responsible for read receipt and acknowledgement, it’s about end to end delivery, and port numbers are added here. Port numbers are kind of like that regarding field, because a port number indicates to the receiving system what type of message this is. Are you going to need a mail application to open it, or are you going to need a web server, are you going to need DNS, what is the message about so to speak.

All right, down at layer three you get logical addressing and best path determination. That’s added to your packet. What’s happening with your data is it’s growing, and growing, and growing. The data is staying the same, but at each layer of the OSI Model from the sending computer heathers are being added that are going to help on the receiving end to process the packet. Okay? Now, down at data link layer, we learn Mac addresses, that’s added to the packet, as well as specific framing, which essentially is it’s about making sure that the right sort of communication is working, and being able to detect collisions, that kind of stuff. Then down at the very bottom layer, the physical layer, this is where information is converted to signal and it’s put on the wire, just across the line.
It comes in across the line at the receiving computer. Each step along the way, like for instance next up on the second layer. Layer one on the receiving end passes it up to layer two where that frame information is examined, Mac information examined and removed. Then it goes up to layer three where the network information is removed. Up at layer four, acknowledgements are sent, segments are put back together if it’s been chopped up into segments. Session layer makes sure that we got that communication presentation, is the letter formatted correctly, how is that interpreted. Up at layer seven, the corresponding application layer protocol is able to display that HTP page accordingly.

Here’s the tip, get that concept, get that concept of data in each of the OSI Model adds something to the packet to help it find its way to the destination. On the receiving end at each layer of the OSI, those packets are stripped away, and information as necessary is used to further the delivery of the packet up the OSI layer, so that the application layer protocol can respond accordingly. If it’s something that you just have to memorize for now, I’ve given you just kind of a short and sweet couple of things that happen, and major protocols, and so on. This is fine for most of the certification exams, but the longer you work, you want to get that little bit of a deeper understanding, this isn’t meant to be a doctorate degree in the OSI Model, but I’m hoping it will help you make a little bit of sense of this. For now, if it’s something you have memorize, go out and pass your test so that you can get the job where this does become relevant to you.

Like always, I wish you the best of luck in your certification studies. If I can help in anyway, KellyH@CyberTrain.IT, would love to hear from you.