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.
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.)
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.
- www.xxx.yyy.zzz
- 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.
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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