How It Works
As we explained before, each layer only talks to the layer above or below it. When your computer is transmitting data, the flow of information is from the program to the network (i.e., the data path is from top to bottom), so the program talks to the seventh layer, which in turns talks to the sixth layer and so on. When your computer is receiving data, the flow of information is from the network to the program (i.e., the data path is from bottom to the top), so the network talks to the first layer, which in turn talks to the second layer and so on.
When transmitting data, each layer adds some control information to the data it got from the layer above, and when receiving data the opposite process occurs: each layer removes control information from the data it got from the layer below.
So when sending data to the network, the seventh layer gets the data send by the program and adds its own control information, and sends this new packet formed by the original data plus its own control data to the layer below. The sixth layer will add its own control data to the packet it received from above and sends the new packet down to the fifth layer, now containing the original data, control data added by the seventh layer plus control data added by the sixth layer. And so on. When receiving data the opposite process occurs: each layer will remove the control data it is in charge of.
Each layer only understands control data it is in charge of. When a layer receives data from the layer above it doesn’t understand the control data added by the above layer, so it treats the set data plus control data as if everything was a single data packet.
In Figure 2 we illustrate this idea, where you can see a computer sending data to the network. Each number added to the original data represents the control data added by each layer. Each layer treats the packet it is receiving from the layer above it as a single packet, not differentiating what is the original data from what is control data added by the upper layers.
We can also say that each layer on the transmitting computer talks directly to the same layer on the receiving computer. For example, the fourth layer on the transmitting computer is talking directly to the fourth layer on the receiving computer. We can say that because the control data added by each layer can only be interpreted by the same layer on the receiving computer.