DOCUMENT:Q103886  30-JUL-2001  [crossnet]
TITLE   :OSI Model: Standard Data Flow Architecture
PRODUCT :Windows for Workgroups and Windows NT Networking Issues
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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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