wifi technical enquiry

Yep.

Because it's the modulation that carries the information (data). No modulation, no data.

The modulation method varies with the speed of transmission. Slow speeds have greater range because they have a higher signal to noise ratio for a given signal level. Higher speeds have progressively lower signal to noise ratios for the same signal level. 802.11g will try to negotiate and establish a modulation method that produces the highest data rate, without having the signal to noise ratio become so low that all that is being received are errors and garbage. More simply, the different modulation methods trade transmission speed for error rate. If the errors increase for some reason, the access point will slow things down until the error rate returns to a reasonable value.

I guess you haven't looked at 802.11n, which as 77 modulation coding schemes.

The faster you go, the more complexicated the mess.

Thanks for the response.

Is this a way of looking at modulation.

The data has to be transmitted, so it is converted into a wave. That wave is the modulation.

So, at transmission point, the wave is created. At receiver point, it is demodulated, that is, converted back into data.

The problem with this model is that it doesn't explain how the coding schemes will vary, depending upon data rate and errors.

Thanks.

No.

No. Think of it as layers, where each layer "rides" on top of the previous lower layer.

At the bottom is the RF carrier. This is the electromagnetic signal that actually delivers the power from here to there.

The next layer is the modulation. It carries no information, but simply provides a means of modulation. It can be FM (frequency modulation), PM (phase modulation), AM (amplitude modulation), various pulse modulation methods, or combinations of these. If there is no input data to be sent, the modulation may still be present. There are also modulation methods (i.e. SSB) that cause the carrier power to disappear.

On top of that is the actual modulated data. That can be anything from voice to high speed data.

The whole mess is organized into protocols, which are agreements as to speed, waveform, bandwidth, error correction, etc.

Yeah, that's it.

I won't explain it either. It's too big a subject to explain in text on a usenet newsgroup. I suggest you obtain a book on basic communications theory, and read the basics. Warning... It's not easy.

Oh my.. A total lack of understanding of communications.

OK.

Let me draw some parallels here. A radio frequency carrier generator can only be used to convey information if it is modulated. So, for a parallel, your voice box (in your throat) is a carrier generator. Say ahhhhhhh. Not much information is carried there. Now move your mouth along with your lips and tongue. You can change the way you percieve the voice boxes generated carrier. Now you can talk, and convey information across the medium of air to a receiver (that would be an ear).

Get it?

What do they teach in school now-a-days?

Hmmm, What is your back? For a starter do you listen to AM radio? Or FM radio?

Hide quoted text -

No background in communications

Hide quoted text -

Have a look at this for starters:-

o?- Hide quoted text -

Will look at the pdf.

Thanks

Modulation:

Baseband information is converted into electrical information. That electrical information is then used to change a "carrier" in some way to convey the information.

So, if you were using morse code, with it's dits and dahs. You could simply turn the carrier on and off following the duration of the dit or the dah.

This is the first way we conveyed information. In essence, we varied the amplitude of the carrier. Once we wanted to carry sound, we converted it into an electrical signals and used it to directly affect the amplitude of a carrier wave signal. This was called Amplitude Modulation. (AM)

Another component of a carrier wave is frequency. We could also vary it in accordance with an electrically converted audio information. This is called Frequency Modulation. (FM)

Video signals, (in the USA) were impressed on to a carrier using AM. (Vestigal side band AM, but still AM). Later schemes allowed the color information to be modulated onto a 3.58 MHz subcarrier, using the combination of phase (something a carrier also has) and AM.

This is also how we have moved in data carriers. Using a phase of a carrier and the amplitude of the carrier we can set up a scheme that allows multiple bits to be conveyed with a single phase/amplitude change.

This is modulation. Pure and simple. Once we can convey information/data, we have other opportunities to set protocols for the transmission of data. Here is where error correction and validation come into play. In essence here we can control what a "packet" of data looks like and can use math to be sure the data is valid. Also, we can sequence the data with it.

Each thing is simple, put together very powerful.

Thanks for responding.

I have no background in the subject and am a bit confused.

'Baseband information is converted into electrical information. That electrical information is then used to change a "carrier" in some way to convey the information.'

What is 'Baseband information'.

How is it converted into electrical information?

How does the electrical information change a carrier. I assume that a carrier is just another signal. Since the electrical information is a signal in itself, why not just send that?

Excuse my ignorance, but if I don't ask, I 'll never learn.

Best wishes

Thanks for responding.

I have no background in the subject and am a bit confused.

'Baseband information is converted into electrical information. That electrical information is then used to change a "carrier" in some way to convey the information.'

What is 'Baseband information'.

How is it converted into electrical information?

How does the electrical information change a carrier. I assume that a carrier is just another signal. Since the electrical information is a signal in itself, why not just send that?

Excuse my ignorance, but if I don't ask, I 'll never learn.

Best wishes

Baseband information is the electrical representation of the information you wish to send (convey).

Let's take audio. When you speak you vibrate air molecules (generate an acoustic wave). Those vibrations transmit out across the air (medium) and cause another persons ear drum to vibrate in response to the acoustic wave. The ear then converts the wave into electrical impulses in nerve cells, again from the wave.

A man made device that does the same thing with an acoustic wave is a microphone. (Via several different methods.) One way it does the conversion is a conductive ribon suspended at two points vibrates with the acoustic wave. It is surrounded by magnets and thus in a magnetic field. When a conductor is moved in a magnetic field, a voltage is generated. Since the movement is caused by the acoustic wave, the resultant voltage is an electrical representation of the acoustic wave. This is baseband information (audio) that can be used to alter a RF carrier to convey information over a long distance.

At the receiving end, a demodulator is used to detect the information on the radio carrier. (With AM, the device that does this is called a detector, with FM it is called a discriminator.)

By the way, if you are interested in this, I suggest you take a few electronics classes. DC and AC theory, then RF theory. These are basic to understanding the technology you are using.

Oops, I did not answer all of your questions. Now that you know what base band signal is, you ask why don't we send it directly. We can do that through a wire and as long as there are no other signals using the same frequencies, we are OK.

Baseband audio is from 20 Hz through 20KHz. If you had another signal there, it would be like being in a room full of very loud people and trying to carry a conversation on with another person 100 ft away in the same room. Not a good way to do that.

However if in the room, we have everyone take turns, you can possibly talk to someone across the room, but would have to wait turns, and the conversation would take a very long time.

Here we have the need to carry all the conversations at once. Well, we have the ability to put the baseband information on to a much higher frequency and send it. Keep every conversation to their own frequency and now everyone can talk at the same time, as long the two involved in the specific conversation are on the same frequency. (This is called frequency division multiplexing in RF, and wave division multiplexing in fiber optic communications.)

If you listen to radio, or TV, you do this. Tune to the channel (frequency) you want to hear, and volia!

There is another type of multiplexing, called time division multiplexing. An example of that was the conversation room where we all took turns talking. You can do both time and frequency division multiplexing. Think of it as watching your favorite program on a TV channel, then when it is over, someone else's program comes on for the next half hour.