10.1.1. The OSI Model
A protocol is, simply put, a set of rules for communication.
In order to get data over the network, for instance an E-mail
from your computer to some computer at the other end of the world,
lots of different hard- and software needs to work together.
All these pieces of hardware and the different software programs
speak different languages. Imagine your E-mail program: it is able
to talk to the computer operating system, through a specific
protocol, but it is not able to talk to the computer hardware. We
need a special program in the operating system that performs this
function. In turn, the computer needs to be able to communicate
with the telephone line or other Internet hookup method. And behind
the scenes, network connection hardware needs to be able to
communicate in order to pass your E-mail from one appliance to the
other, all the way to the destination computer.
All these different types of communication protocols are
classified in 7 layers, which are known as the Open Systems
Interconnection Reference Model, the OSI Model for
short. For easy understanding, this model is reduced to a 4-layer
protocol description, as described in the table below:
Table 10-1. The simplified OSI Model
| Layer name |
Layer Protocols |
| Application layer |
HTTP, DNS, SMTP, POP, ... |
| Transport layer |
TCP, UDP |
| Network layer |
IP, IPv6 |
| Network access layer |
PPP, PPPoE, Ethernet |
Each layer can only use the functionality of the layer below;
each layer can only export functionality to the layer above. In
other words: layers communicate only with adjacent layers. Let's
take the example of your E-mail message again: you enter it through
the application layer. In your computer, it travels down the
transport and network layer. Your computer puts it on the network
through the network access layer. That is also the layer that will
move the message around the world. At the destination, the
receiving computer will accept the message through it's own network
layer, and will display it to the recepient using the transport and
application layer.
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It's really much more
complicated |
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The above and following sections are included because you will
come across some networking terms sooner or later; they will give
you some starting points, should you want to find out about the
details.
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