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Chapter 1

Internetworking

Advantages of using the OSI layered model include, but are not limited to,

the following:

Allows multiple-vendor development through standardization of
network components

Allows various types of network hardware and software to communicate

Prevents changes in one layer from affecting other layers, so it does not
hamper development

The OSI Reference Model

he OSI reference model was created in the late 1970s to help facilitate

data transfer between network nodes. One of the greatest functions of the
OSI specifications is to assist in data transfer between disparate hosts, mean-
ing, they enable us to transfer data between a Unix host and a PC or a Mac.

The OSI isn't a physical model, though. Rather, it's a set of guidelines that

application developers can use to create and implement applications that run
on a network. It also provides a framework for creating and implementing
networking standards, devices, and internetworking schemes.

The main reason the International Organization for Standardization

(ISO) released the OSI model was so different vendor networks could work
with each other.

The OSI has seven different layers, divided into two groups. The top three

layers define how the applications within the end stations will communicate
with each other and with users. The bottom four layers define how data is
transmitted end-to-end. Figure 1.2 shows the three upper layers and their
functions, and Figure 1.3 shows the four lower layers and their functions.

When you study Figure 1.2, you can see that the user interfaces with the

computer at the Application layer, and also that the upper layers are respon-
sible for applications communicating between hosts. Remember that none of
the upper layers know anything about networking or network addresses.
That's the responsibility of the four bottom layers.

In Figure 1.3 you can see that it's the four bottom layers that define how

data is transferred through a physical wire or through switches and routers.
These bottom layers also determine how to rebuild a data stream from a
transmitting host to a destination host's application.

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