2 cissp ® Official Study Guide Eighth Edition

Simplex

One-way communication

Half-Duplex

Two-way communication, but only one direction can send data at a time

Full-Duplex

Two-way communication, in which data can be sent in both directions 

simultaneously

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Chapter 11 
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Secure Network Architecture and Securing Network Components
Presentation Layer
The 
Presentation layer (layer 6)
is responsible for transforming data received from the 
Application layer into a format that any system following the OSI model can understand. 
It imposes common or standardized structure and formatting rules onto the data. The 
Presentation layer is also responsible for encryption and compression. Thus, it acts as 
an interface between the network and applications. This layer is what allows various 
applications to interact over a network, and it does so by ensuring that the data formats 
are supported by both systems. Most file or data formats operate within this layer. This 
includes formats for images, video, sound, documents, email, web pages, control sessions, 
and so on. The following list includes some of the format standards that exist within the 
Presentation layer:
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American Standard Code for Information Interchange (ASCII)
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Extended Binary-Coded Decimal Interchange Mode (EBCDICM)
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Tagged Image File Format (TIFF)
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Joint Photographic Experts Group (JPEG)
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Moving Picture Experts Group (MPEG)
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Musical Instrument Digital Interface (MIDI)
So many Protocols, So many layers
With seven layers and more than 50 protocols, it may seem daunting to remember
the layer in which each protocol resides. One way to learn this is to create flash cards. 
On the front of each card, write the name of the protocol; then on the back, write the 
layer name. After shuffling the cards, put the card for each protocol in a pile represent-
ing its supposed layer. Once you have placed all the protocols, check your work by 
viewing the backs of the cards. Repeat this process until you are able to place each
one correctly.