Introduction#
There are a lot of “standards” when it comes to networking. We need those standards to communicate. One organization that makes some of those standards is the International Organization for Standardization (ISO, and yes, the acronym doesn’t line up with the name). ISO created a standard called the Open Systems Interconnection model (OSI model).
ISO created OSI. The acronyms are just reversed.
OSI Model#
OSI divides up the concept of networking into seven layers:
Physical
Data-Link
Networking
Transport
Session
Presentation
Application
This book is organized around these layers. We are going to start at the Physical layer and work our way up to the Presentation Layer. As for the Application layer? We could write another whole other book on that.
OSI is a conceptual model. There isn’t a specific “implementation” of any layer in this model. What the OSI model helps with is talking to other people about networking. It also helps when you want to break apart a complex networking problem into smaller logical parts. You know when you talk about the “east-side” of your town? Exactly what fits into that definition is a little fuzzy and can change a bit over time, but the term gives most people an idea of what part of town you are talking about. It works the same way with OSI layers.
It is worth remembering the name of each layer and their order. The physical layer is considered “layer 1.” If you talk to a networking expert and mention that some process happens at the “networking layer”, or “layer 3” they will know what you are talking about. It is such a popular way to talk about networking that there are even companies named after “layer 3.”
If you have trouble remembering the layer names, use the mnemonic “Please Do Not Touch Steve’s Pet Alligator.” Take the first letter from each word of that sentence, and you have the first letter of each layer.
What does each layer do?#
At the application layer, the user interacts with our application. For example, we can give a thumbs-up to a video we like. Our application needs to send that data to the website’s server. Each OSI layer below the application layer works to process that “thumbs-up”, passing it down until our request is finally turned into a signal on a wire, as shown in Fig. 1 and Fig. 2.
When the computer that receives the signal, when it is picked up at the physical layer, we pass it up all the layers until we finally hit the application layer where we store that information in a database.
Fig. 1 The seven OSI Layers#
Incoming Data#
In detail, what might it look like when we receive a web page?
Physical: Translates pulses of electricity, radio waves, or light into binary 1’s and 0’s. The physical layer then takes that binary data and passes it up to the data-link layer.
Data-Link: Decodes the binary data from the physical layer into chunks of data called “frames.” A frame has a data “payload” and if the frame is addressed for this computer and uncorrupted, it is passed up to the networking layer. Otherwise it is ignored.
Networking: Each chunk of data at this level is called a “packet.” At the networking level, we look to see if this the final destination for the packet. If it is, pass it up a level. If not, figure out the next “hop” to pass the packet to. Pass the packet back down to the data-link level to go to that hop. The networking layer routes packets.
Transport: Takes the data contained in the packets, and reassembles them into the original file, image, or larger data stream. If we are missing data, we’ll ask for it again. We will also pass it to the correct program on the computer that is expecting the data.
Session: At this level we track a whole “conversation.” So if multiple files are being sent, we’ll know to send them to the right program. We’ll also decrypt and uncompress any zipped up data at this level.
Presentation: This is where we display information. In our example we combine images, web HTML files, and CSS style documents together to render a web page.
Application: At this level, we interact with the user and wait for a “thumbs up” or “thumbs down” on our post.
Outgoing Data#
What if the user clicks a “thumbs-up” on the video? The layers might work like this:
Application: Realize the user pressed a mouse button on the document. Pass that down to the presentation layer.
Presentation: Receive that the user pressed the “thumbs up” icon. Turn that into a small file that holds info on which thumbs-up was clicked.
Session: Add information about who the user was. Encrypt the message. Compress it.
Transport: Take the file and break it into smaller packet-sized parts.
Networking: Add an address with our final destination. We probably aren’t going to get there in one hop, this layer routes to the destination computer.
Data-link: Add an address for our next hop. This layer only worries about point-to-point communication.
Physical: Turn the data into electrical pulses on a wire.
Fig. 2 The seven OSI Layers#
OSI Model vs. Reality#
That’s the theory behind the OSI model. But reality? The technologies we use don’t always fit neatly into those layers. Here are some of the more common technologies and approximately where they fit into the model.
You may have heard of Ethernet. Ethernet is a set of protocols for networking we use when we network with cables or wirelessly. It covers both the physical layer (layer 1) and the data-link layer (layer 2).
You may have heard of TCP/IP. It is the one of the main protocols that gets Internet traffic to its destination. TCP/IP stands for Transmission Control Protocol/Internet Protocol. IP covers the networking layer (layer 3), TCP covers the transport layer (layer 4). TCP/IP (layers 3 and 4) can run on top of Ethernet (layers 1 and 2), but it doesn’t have to.
Have you seen those letters, “http” as part of an address for a web page? They stand for HyperText Transport Protocol. That protocol covers parts of transport and session layers (layers 4 and 5). HTTP can run on top of TCP/IP that runs on top of Ethernet.
The display of web pages, PDFs, images, even 3D graphics fall under presentation (layer 6), while the management of menus, buttons and other are covered by the application (layer 7).
Just remember, many of the boundaries between these layers are fuzzy and not clearly defined. Often technologies do tasks outside of what their “layer”, and software may lump multiple layers together.