Discovering the purpose of each layer, the protocols, and the PDUs

In this section, we'll take a closer look at each of the seven layers of the OSI model and describe the following:

• The layers of the OSI model, from layer seven to layer one: Application, Presentation, Session, Transport, Network, Data Link, and Physical.
• The three layers that use source addressing and destination addressing during encapsulation: the Transport, Network, and Data Link layers.
• The PDU for each layer, which defines what shape the data is in as it is passed to the layer above or the layer below.

In the following diagram, we can see a summary of the OSI model:

Figure 8.1 – The OSI model

Before diving into the layers, it is helpful to use a mnemonic device to remember the first letter of each layer. With the OSI, we have two, as shown in the following diagram:

Figure 8.2 – OSI mnemonics

Once you know the names of...