Page 740 - (ISC)² CISSP Certified Information Systems Security Professional Official Study Guide

radio signals through the air). Physical devices establish channels
through which electronic signals can pass from one computer to

another. These physical device channels are only one type of the seven
logical communication types defined by the OSI model. Each layer of
the OSI model communicates via a logical channel with its peer layer
on another computer. This enables protocols based on the OSI model
to support a type of authentication by being able to identify the remote
communication entity as well as authenticate the source of the
received data.

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. Information 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.

4. The process of passing the message down and adding layer-specific
information continues 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.