Information Technology Reference
In-Depth Information
Foundation Topics
The OSI Reference Model
The discussion of the Open Systems Interconnection (OSI) model is always a mandatory topic
in any internetworking topic. The CCIE candidate must understand the OSI model, and in
which OSI layer different networking protocols reside. The OSI model was developed by the
International Standards Organization (ISO) in 1984 to describe the flow of data on a network.
The model describes seven layers that start with the physical connection and end with the
application. As shown in Figure 2-1, the seven layers are as follows: physical, data-link,
network, transport, session, presentation, and application.
Seven-Layer OSI Model
Figure 2-1
Layer
Number
OSI Layer
Name
7
Application
6
Presentation
5
Session
4
Transport
3
Network
2
Data Link
1
Physical
The OSI model divides the tasks involved in moving data into seven smaller, more manageable
layers. Each layer is more or less self-contained so that each layer can be implemented
independently. You can run an application over the IP (Layer 3), Ethernet (Layer 2), Frame
Relay (Layer 2), or Gigabit Ethernet (Layer 2). As the packets route through the Internet, the
Layer-2 media changes independently from the upper-layer protocols. The OSI model helps
standardize the design and construction of the networks for developers and hardware
manufacturers, and also helps network engineers and analysts with a framework for the
understanding of internetworking.
Layered implementations of internetworking technologies do not necessarily map directly to
the OSI model. The TCP/IP architecture model only describes four layers, with the upper-layer
mapping to the three upper layers of the OSI model (application, presentation, and session). The
development of IP predates the OSI model. For a more thorough discussion of the TCP/IP
model, see Chapter 6, “Internet Protocols.”
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