2 cissp ® Official Study Guide Eighth Edition

Encapsulation/Deencapsulation
Protocols based on the OSI model employ a mechanism called encapsulation. 
Encapsulation
is the addition of a header, and possibly a footer, to the data received by 
each layer from the layer above before it’s handed off to the layer below. As the message is 
encapsulated at each layer, the previous layer’s header and payload combine to become the 
payload of the current layer. Encapsulation occurs as the data moves down through the OSI 
model layers from Application to Physical. The inverse action occurring as data moves up 
through the OSI model layers from Physical to Application is known as 
deencapsulation
. 
The encapsulation/deencapsulation process is as follows:
1.
The Application layer creates a message.
2.
The Application layer passes the message to the Presentation layer.
3.
The Presentation layer encapsulates the message by adding information to it. Informa-
tion is usually added only at the beginning of the message (called a header); however, 
some layers also add material at the end of the message (called a footer), as shown in 
Figure 11.2.
F I g u r e 11. 2
Representation of OSI model encapsulation
Application
Presentation
Session
Transport
Network
Data Link
Physical
DATA
Footer
Header
DATA
DATA
DATA
DATA
DATA
DATA

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OSI Model 
443
4.
The process of passing the message down and adding layer-specific information contin-
ues until the message reaches the Physical layer.
5.
At the Physical layer, the message is converted into electrical impulses that represent 
bits and is transmitted over the physical connection.