Learning by Doing: CISCO Certified Network ... - SCN Research

Encapsulation
As we move down the OSI model a process called encapsulation takes place. At
the session layer the information is called "data." At the transport layer the data is
converted into "segments." At the network layer the segments are encapsulated into
"packets." At the data link layer the packets are now encapsulated into "frames."
Finally, at the physical layer the frames are converted into "bits."
A good way to remember this is “Don’t Send People Free Beer.” Beer is on the
physical layer because its macho. If you want to remember it from the bottom up (which
might confuse you with the OSI model direction) you can remember “Been free people
since democracy.”
Pay close attention to when the information headers and footers are added. This
can be somewhat confusing. Let’s take a look at a make believe situation between two
users communicating over the Internet. Suppose Joe wants to send an email to Casey.
His message is 50,000 bytes in size at the application layer. This email is passed down to
the presentation layer where it is compressed, encrypted, and formatted down to a
message of 30,000 bytes in size (ok…so it really won’t be this neat but cut me a break it
is easier to explain this way). Then the 30,000 byte compressed, formatted, and
encrypted data is sent to the session layer. Here Joe’s computer establishes a session with
Casey’s computer…
Session layer communication:
Joe: Hey Casey…can I hook up with you (no pun intended)
Casey: I acknowledge that you are requesting a hook up
Joe: I received your acknowledgement of my request for a hookup.
Casey: I received your acknowledgement of my acknowledgment of your request
for a hookup.
Then the data is passed to the transport layer for numbering. Here the 30,000 byte data is
broken down into 6 segments and numbered: 1 of 6, 2 of 6, 3 of 6, 4 of 6, 5 of 6 and 6 of
6. Handshaking and windowing takes place to finish the establishment of the session.
Transport layer communication:
Joe: I want to send information so how quickly can I send it?
Casey: I acknowledge that you are requesting to send information.
Joe: I received your acknowledgement of my request to send information.
Casey: I received your acknowledgement of my acknowledgment of your request
to send information.
Casey: I am not busy so you can transmit at 22300 bps.
Joe: I acknowledge that you can transmit at 22300 bps.
Casey: I received your acknowledgement of my request to transmit at 22300 bps.
Joe: I received your acknowledgement of my acknowledgment of your ability
to receive information at 22300 bps.
Then the transport layer segment is passed to the network layer. The network layer adds
the source and destination ip addresses (logical addresses) plus some other stuff (we will
look at later). Then the new “packet” is sent to the data link layer. There the data link
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