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Cha p te r

User 2 F o u r User 1
Message Confirmation
7 7 Application
layer
6 6 Presentation
Peer layer
raltionships
5 5 Session
layer
4 4 Transport
layer
3 3 Network
layer
2 2 Data Link
layer
1 1 Physical
layer
Medium

FIGURE 4.9 Message movement within a computer network.

framework. Figure 4.9 illustrates how two users might employ a
computer network. As illustrated, the transfer of information takes
place from User 1 to User 2. Each message passes through a series of
layers that are associated with message processing:

Sequence of Layers—User 1 Sequence of Layers—User 2
(PhL)1 Physical layer (PhL)1 Physical layer
(DL)2 Data link layer (DL)2 Data link layer
(NL)3 Network layer (NL)3 Network layer
(TL)4 Transport layer (TL)4 Transport layer
(SL)5 Session layer (SL)5 Session layer
(PL)6 Presentation layer (PL)6 Presentation layer
(AL)7 Application layer (AL)7 Application layer

A message is sent from User 1 to User 2, with message acknowl-
edgment sent back from User 2 to User 1. The objective of this com-
munication system is to transfer a message from User 1 to User 2 and
to confirm message receipt. The message is developed at application