Figure 16-1 OSI Model Layers
Applications that need to transmit data over the network
start at the application layer, gathering the data that
needs to be transmitted. This data makes its way down
the transmission model and gets processed at each layer
by being split into smaller chunks, with header and
footer information added, and then being encoded on the
physical medium. As the bits arrive at the destination,
the reverse process takes place: As the data moves up the
layers at the receiver, headers and footers are removed,
and data is recombined into bigger chunks and made
available to the receiving application. This is similar in a
way to how postal mail works. When sending out
heartwarming holiday wishes via snail mail, first you
need to put your message on a piece of paper. This is the
equivalent of the application layer at the sender. Then
you put the paper with the message in an envelope, seal
the envelope, write down the sender and receiver
information, and drop the envelope in a mailbox or at the
post office. The postal service plays the role of the
physical layer in this case, as it sorts through the mail
and delivers each piece to its destination. The person
receiving the mail makes sure that the envelope is meant