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 add 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.