However, instead of sending a coffee mug to a friend, you send
information from an application from one device to another. The information sent on a
network is referred to as data or data packets.
Encapsulation wraps data with the necessary protocol information before network transit.
Therefore, as the data moves down through the layers of the OSI reference model, each OSI
layer adds a header (and a trailer, if applicable) to the data before passing it down to a lower
layer. The headers and trailers contain control information for the network devices and
receiver to ensure proper delivery of the data and to ensure that the receiver can correctly
interpret the data.
Figure 1-21 illustrates how encapsulation occurs. It shows the manner in which data travels
through the layers. These steps occur to encapsulate data:
Step 1 The user data is sent from an application to the application layer.
Step 2 The application layer adds the application layer header (Layer 7 header)
to the user data. The Layer 7 header and the original user data become
the data that is passed down to the presentation layer.
Step 3 The presentation layer adds the presentation layer header (Layer 6
header) to the data. This then becomes the data that is passed down to the
session layer.
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