What the heck am I talking about when I say THE OSI model? OSI Reference model is the model that was developed to show the different independent sections of a protocol. The tcp/ip protocol pre-dates the OSI model, but it lends itself very well to using this model to describe it, because it is hardware independent. TCP/IP consists of only 4 layers, where the OSI model consists of 7 layers. But, each of the layers of tcp/ip corresponds to one or more layers of the OSI model. Below is a diagram of the layers of the OSI model and the corresponding tcp/ip layers.
Table 1-1. Layer Model
| OSI REFERENCE MODEL | REAL WORLD EQUIVALENT TASKS | TCP/IP PROTOCOL |
|---|---|---|
| Application | Program to program transfer of information | Application |
| Presentation | Text formatting and display code conversion | |
| Session | Establishing, maintaining, and coordinating communication | |
| Transport | Accurate delivery, service quality | Transport |
| Network | Transport routes, message handling and transfer | Internet |
| Data Link | Coding, addressing, and transmitting information | Network Interface |
| Physical | The hardware connections (the NIC and the cables) |
**** All People Seem To Need Data Processing ****
**** Please Do Not Throw Sausage Pizza Away ****
These two somewhat silly sayings will help you remember the layers of the OSI model and what the first letters and order is. If you can get the letters in the right order, you can figure out the names of the layers. This is a common question on certification tests for all networking type subjects. Know it, live it, love it. Or not, just remember it. You will see it again!
Why is this silly diagram so important? Because tcp/ip is NOT one single protocol, but it is actually a suite of protocols, and each one of the parts that make up tcp/ip function at a specific layer of the model. This is important to understand what is actually happening when you are setting up a network using tcp/ip, or sending files to the Internet. Without a good understanding of the layers and what they do, it is more difficult to understand where to look when things go wrong with your network. Below is a table that explains in detail what each layer does for the network.
Table 1-2. Layer Model
| OSI REFERENCE MODEL | Layer Definitions |
|---|---|
| Application - Layer 7 | This is the layer where you perform your tasks, writing the email message, running your browser, opening an ftp session, receive and read your email, etc. |
| Presentation - Layer 6 | The presentation layer works with the file system and the operating system. This is the layer that files get transferred from one format to another. This allows computers of different platforms to swap data. Without this layer, you could only network same platform computers. And if you remember, that would defeat one of the main purposes of developing the tcp/ip protocol suite. |
| Session - Layer 5 | The other protocols of the tcp/ip suite work at layer 5 and above. This layer establishes and coordinates a session, which is simply a connection between two computers. You cannot transfer data between two computers without a session being established. |
| Transport - Layer 4 | This is the layer of the model that the tcp protocol works. While layer 3 sends your data to it's destination, Layer 4 is the one that maintains the order of the packets. This layer makes sure the packets are received without error and that the packets are reassembled in the proper order. Without this layer your network would not be trustworthy for valuable data. |
| Network - Layer 3 | This is the layer that the IP portion of the tcp/ip protocol fits into the model. This layer gets the packets from the data link layer and sends them to the correct network address. If there are multiple paths, this is the layer that determines what is the best possible route. Without this layer your data would never make it to the right destination. |
| Data Link - Layer 2 | This is the layer that splits your data into packets readying them to be sent across the wires to their destination. The data link works to assure accurate data transmission. This is also the layer where the token or ethernet packets are handled. |
| Physical - Layer 1 | The physical layer is pure hardware, i.e. Cables, network cards, hubs, etc. All the parts that the electrical signals are transported between the machines. |