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12.6 During transmission:

• The packet passes from the user program to the upper layer in the model (the
application layer) and then to the lower layer (the transport layer) until it is sent
over the data transmission medium.

• At each layer, user data is wrapped with additional assisting data that helps in
addressing, routing, and receiving data correctly.

Upon reception:
• The data comes from the transmission medium and is delivered to the lower layer

(the link layer), then sent to the upper layer, and so on until it reaches the
recipient's program.
• When passing a packet from the lower layer to the upper layer, the wrapping
done at the sender is undone, and its data is used to correctly route and receive
the data.
An illustrative example of how data is sent between two devices connected through
routers is shown below.

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