Syngress - Eleventh Hour Network+ Exam N10-004 Study Guide (11 ...

CHAPTER 5
The OSI Model and
Networking Protocols
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Exam objectives in this chapter
■ The OSI Model
■ The Department of Defense Networking Model
■ Networking Protocols
THE OSI MODEL
In 1977, the International Organization for Standardization formed a subcommittee
called the Open Systems Interconnection (OSI) from which a seven-layered
framework was established and is still used as the model for distributed
communications. The OSI model is shown in Figure 5.1.
The first two layers of the OSI model involve both hardware and software. In
the five upper layers (Layers 3 to 7), the OSI model typically is implemented via
software only.
Crunch Time
The OSI model is represented as a stack because data
that is sent across the network has to move through each
layer at both the sending and receiving ends. The sending
computer generally initiates the process at the application
layer and the data is sent down the stack to the
physical layer and across the network to the receiving
computer. On the receiving end, the data is received at
the physical layer and the data packet is sent up the stack
to the application layer.
The application layer starts the process. Small pieces of
information relative to the transmission of information are
added to the data at each layer; this is called encapsulation.
The process is then reversed on the receiving side
to get back to just the data.