Basic concept of Networks

Introduction

Networking computersAt 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. 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.

Now, you can examine a simulator that shows how a web page is loaded. Click here to get to the simulator.

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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