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OSI Layers and Their Functions

Today, because most people happen to be much more familiar with TCP/IP functions than
with OSI functions, one of the best ways to learn about the function of different OSI layers
is to think about the functions in the TCP/IP model and to correlate those with the OSI
model. For the purposes of learning, you can think of five of the OSI layers as doing the
same kinds of things as the matching five layers in the TCP/IP model. For example, the
application layer of each model defines protocols to be used directly by the applications,
and the physical layer of each defines the electro-mechanical details of communicating over
physical connections. Table 1-4 briefly describes each OSI layer.

Table 1-4 OSI Reference Model Layer Descriptions
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Table 1-5 lists a sampling of the devices and protocols and their comparable OSI layers.
Note that many network devices must actually understand the protocols at multiple OSI
layers, so the layer listed in Table 1-5 actually refers to the highest layer that the device
normally thinks about when performing its core work. For example, routers need to think
about Layer 3 concepts, but they must also support features at both Layers 1 and 2.

Table 1-5 OSI Reference Model: Device and Protocol Examples
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Besides remembering the basics of the features of each OSI layer (as in Table 1-4), and
some protocol and device example at each layer (as in Table 1-5), you should also memorize
the names of the layers. You can simply memorize them, but some people like to use
a mnemonic phrase to make memorization easier. In the following three phrases, the first letter of each word is the same as the first letter of an OSI layer name, in the order specified
in parentheses:
■ All People Seem To Need Data Processing (Layers 7 to 1)
■ Please Do Not Take Sausage Pizzas Away (Layers 1 to 7)
■ Pew! Dead Ninja Turtles Smell Particularly Awful (Layers 1 to 7)

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