Data Flow of the OSI Model

The purpose of the OSI model and its layers is data flow.  The data is the most important component of the OSI model, the computers, and all the equipment in between.  The process starts when the user requests to send data at layer 7.  Layer 7 passes the data down to layer 6.  The data gets passed down from layer to layer until layer 1.  Layer 1 transmits the data to the destination computer.  The destination receives the data at layer 1, passes it to layer 2, then layer 3… all the way back up to layer 7.  Below is a diagram that shows how this works:

OSI data flow

It is that simple.  This is how the data flows in the OSI model.  However, there is a lot more information that flows in the OSI model than just the data.  Remember the session layer?  It starts and terminates the session of the transmission.  The transport layer may sometimes provide a way to ensure the data was received.  The network layer makes decisions on how the data gets to the destination.  The data link layer makes sure that the data physically gets to the destination.

Seeing the data flow from top to bottom, then bottom to top does not explain how the services at each layer work.  It is important to know that each layer from the source computer communicates to the same layer of the destination computer.  What this means is that layer 7 on the source computer communicates to layer 7 at the destination computer.  Layer 6 on the source computer communicates to layer 6 on the destination computer.  Layer 5 on the source computer communicates with layer 5 on the destination computer…you get the idea.

Sometimes, the information that is being sent is not the data itself but information for one of the lower layers to establish communication between the devices.  For example, the transport layer may provide reliability of the transmission to ensure delivery of all the data.  In order for this to happen, layer 4 from both computers must first negotiate rules to use.  So, the source computer will send rules to the destination computer to follow, and wait for the destination computer to send back whether or not it accepts these rules.  This information will start at layer 4, move to layer 3, then layer 2, then layer 1, and start from layer 1 on the destination computer.  This information will be passed up the layers of the OSI model until it reaches layer 4.  Since the information is attended for layer 4 protocols, this data will not be passed up to layer 5.  EVEN THOUGH INFORMATION BEING SENT IS NOT THE DATA ITSELF AND IS FROM A LOWER LAYER, THAT INFORMATION NEEDS TO PASS DOWN ALL LAYERS TO LAYER 1, THEN PASS UP ALL LAYERS UNTIL IT REACHES THE ATTENDED LAYER.

When the data is sent, it remains in tact until it reaches layer 4, the transport layer.  This layer divides the data into smaller chunks and numbers them so that they can be placed back in order on the destination computer.  In order for this to happen, layer 4 encapsulates the data, surrounding the data with layer 4 information that only makes sense to the destination’s layer 4.  Layer 3 then encapsulates the data with layer 3 information.  Layer 2 encapsulates the data with layer 2 information.  Layer 1 is just bits.  Just look at the diagram below.  It will make sense.

Encapsulation diagram

When this information is sent to the destination computer, that computer receives the information at layer 1.  Layer 2 at the destination computer removes the layer 2 information and sends it to layer 3.  Layer 3 will remove its own information.  Same with layer 4.  This is called de-encapsulation.

The information that is exchanged at each of the layers are called PDUs (Protocol Data Units.)  Data at layer 4 is called segments.  Layer 3 data is called packets.  Layer 2 data is called frames.  Data at layer 1 is called bits.

Finally, we will look at how information flows between the source and destination computers.  Do you have one cable that you physically plugged in your computer at one end, drove miles upon miles to my house, and plugged the other end in my web server so you can see this page?  Of course not.  You plugged your cable in your router, which is plugged in your modem, which goes in your phone or cable outlet, which goes to your ISP provider.  There is a good chance that there are at least 10 layer 3 devices between your computer and my web server.  This does not include layer 2 devices or layer 1 devices.

When a device refers to a specific layer on the OSI model, that device make  its decisions (or lack there of) at that layer.  For example, routers are layer 3 devices.  That is the network layer.  The purpose of layer 3 is to route the information to the destination computer.  So, the router receives the information at layer 1 on one of its ports.  The data gets passed to layer 2.  Layer 2 removes its information (its PDU,) and passes it to layer 3.

The information of layer 3 consists of a logical address.  It will read this information, make a decision on what port to pass the data to and update the information on layer 3.  It will then pass the data back down to layer 2.  Layer 2 will encapsulate it again and forward it down to layer 1 at the port it routed the data to.

Based on this post alone, you should start to see how useful the OSI model is when trying to understand computer networks.  However, we have a long way to go.  I will be discussing each of the layers in later posts.