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Information flow: The flow of information is from the top layer (usually application layer) down to the physical layer at information source node; and from the physical layer up to the application later at the destination node as illustrated in the figure below.
The
information exchange process occurs between peer OSI layers. Each layer in the
source system adds
control information to data and each layer in the destination system analyzes
and removes
the control information from that
data.
If
System A has data from a software application to send to System B, the data is
passed to the application layer. The application layer in System a then
communicates any control information required by the application layer in System
B by prepending a header to the data. The resulting information unit (a header
and the data) is passed to the presentation layer, which pretends its own header
containing control information intended for the presentation layer in System B.
The information unit grows in size as each layer prepends its own header (and in
some cases a trailer) that
contains
control information to be used by its peer layer in System B. At the physical
layer, the entire information unit is placed onto the network medium.
The
physical layer in System B receives the information unit and passes it to the
data-link
layer. The data
link layer in System B then reads the control information contained in
the header prepended by the data
link layer in System A. The header is then removed, and the remainder of
the information unit is passed to the network layer. Each layer performs the
same actions: The
layer reads the header from its peer layer, strips it off, and passes the
remaining information unit to the next highest layer. After the application
layer performs these actions, the data is passed to the recipient software
application in System B, in exactly the form in which it was transmitted by the
application in System A.
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