| An Introduction To Network Concepts |
Introduction
At this point you should
know how to do computer controlled measurements. However, the computer you use
is probably connected to a network, and that connection allows for some
interesting possibilities. In particular, you can take measurements and do
control remotely. However, there are a few topics you should be conversant with
before you try that.
Why learn about Basic Network Concepts?
Using computer measurement
and control across a network allows for possibilities of operation in remote or
otherwise inaccessible locations, and it allows for measurement and control of
multiple locations from a single location. To take advantage of those
possibilities, you need to have a basic familiarity with networked computers.
You need to learn about basic concepts of network addressing and how to
determine addresses. When you are finished with this unit you should be able to
use a program (LabVIEW) to perform measurements and control across a network and
you will learn about URLs, and IP addresses, and how to determine them. In
addition, you will learn about some basic network concepts (servers, etc.)
Some Basic Network Concepts
Let's start with what
happens when you "go to" www.SomeCompany.com to get information about their
products. Actually, you don't go anywhere, but you do send some information
across the network. The information you send does the following.
First, the
URL (www.SomeCompany.com)
gets sent over the network to a special computer - a
name server - that
translates this URL into an address of the form below. This form (all
numbers) is the numeric IP address.
-
Next, computers on the
network (routers,
etc.) try to send the message along so that it will get to the right
computer - i.e. the one that has the IP address you are sending to.
(Routers route messages along the network, that's why they are called
routers.
-
When the message gets
to the correct computer - the server,
wherever in the world it might be - the server sends the file - often
written in HTML - back to your computer - the
client.
This is an over-simplified
picture of what goes on, but it contains all the basic ideas about what
happens. There are several points that you should note in this sequence of
events.
The message you - the client
- send to the server has to have the server's address, otherwise the message
will not get to the correct server. The message you send to the
server must also contain the address of your computer, otherwise the
information that the server sends out on the network will not make it back
to you. The message you send to
the server will also include a command. The command to get an HTML file is
GET. When you are
in a browser and you send a request for a file, you send a GET command along
with the name of the file you want to GET. The message you send to the
server may not go out as a single message. It may be broken into packets,
and each packet needs to contain enough information that the complete
information request can be reassembled by the server. The information sent back by
the server may not arrive as a single message. It will probably be broken
into packets, and each packet needs to contain enough information to permit
your computer to reassemble the complete file/set of information sent by the
server. In the above process when
packets are sent over the network, there are no guarantees that they will
arrive in the correct order, and computers on either end - both the client
and the server - have to have the capability of reassembling all of the
information. In the case of the client, you will often want that
information displayed as a web page.
That is a short summary of what takes place
in a typical client-server situation.
Domain Name Servers
When you send a request for a web page to
a URL www.SomeCompany.com,
for example) that information gets translated into an IP address (www.xxx.yyy.zzz)
by a Domain Name Server
(DNS). The DNS system has a vast database that contains all of the URL-IP
pairs. It changes constantly, and it is probably the most highly accessed
database on the planet.
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