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Cha p te r
F o u r
Message
User 1 Information
added to message
as it moves from
layer 7 to layer 1
Extended
message
User 2
Information
removed from message
as it moves from
layer 1 to layer 7
Message
FIGURE 4.10 Open system interconnect (OSI) layered model.
The two users see only the message that is originated and deliv-
ered; they do not see all of the intermediate steps. This is analogous
to the steps involved in making a telephone call or sending a letter, in
which the two users know only that they have communicated with
one another, but do not have any specific knowledge of the details
involved in passing a message from one location to another. This
orderly and structured approach to communications is useful because
it separates the various tasks that must take place. It provides a means
for assuring that the methods for processing and addressing mes-
sages are always the same at every node. Whether a message is being
sent or is being received, a sequential processing activity always takes
place.
