STANDARD DATA FLOW ARCHITECTURE
The OSI model presents a standard data flow architecture, with
protocols specified so that layer N at the destination station
receives exactly the same object that was sent by layer N at the
source station.
- The sending process passes data to the application layer
which attaches an application header (AH).
- The presentation layer may transform the data in various
ways, and possibly add a header to the front, before giving
the result to the session layer. The presentation layer aware
of which portion of the data given to it by the application
layer is AH, if any, and which portion is actual user data.
Nor does it need to know.
- This process is repeated until the frame reaches the data
link layer, where, in addition to a header, a trailer (DT)
is added to aid in frame synchronization. The frame is then
passed down to the physical layer, where it is actually
transmitted to the receiving station.
- On the receiving station, the various headers (and DT)
are stripped off one by one as the message is passed up
the layers until it finally reaches the receiving process.
Remember that although the actual data transmission is vertical,
each layer is programmed as though it were really horizontal. When
the sending transport layer, for example, gets a message from the
session layer, it attaches a transport header and sends it to the
receiving transport layer. From its point of view, the fact that it
must first hand the message to the network layer on its own machine is
technically unimportant.
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