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From: species8350 on 7 Jan 2010 06:58
I know that the wifi signal is modulated.

Firstly, why does the signal need to be modulated.

Secondly regarding g: why are ther so many coding systems used.

Thanks
From: Jeff Liebermann on 7 Jan 2010 12:19
On Thu, 7 Jan 2010 03:58:50 -0800 (PST), species8350
<not_here.5.species8350(a)xoxy.net> wrote:

>I know that the wifi signal is modulated.

Yep.

>Firstly, why does the signal need to be modulated.

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

>Secondly regarding g: why are ther so many coding systems used.

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.
<http://www.airmagnet.com/assets/whitepaper/WP-802.11nPrimer.pdf>
The faster you go, the more complexicated the mess.

--
Jeff Liebermann jeffl(a)cruzio.com
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558
From: species8350 on 8 Jan 2010 07:04
On Jan 7, 5:19 pm, Jeff Liebermann <je...(a)cruzio.com> wrote:
> On Thu, 7 Jan 2010 03:58:50 -0800 (PST), species8350
>
> <not_here.5.species8...(a)xoxy.net> wrote:
> >I know that the wifi signal is modulated.
>
> Yep.
>
> >Firstly, why does the signal need to be modulated.
>
> Because it's the modulation that carries the information (data).  No
> modulation, no data.
>
> >Secondly regarding g: why are ther so many coding systems used.
>
> 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.
> <http://www.airmagnet.com/assets/whitepaper/WP-802.11nPrimer.pdf>
> The faster you go, the more complexicated the mess.
>
> --
> Jeff Liebermann     je...(a)cruzio.com
> 150 Felker St #D    http://www.LearnByDestroying.com
> Santa Cruz CA 95060http://802.11junk.com
> Skype: JeffLiebermann     AE6KS    831-336-2558

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.
From: Jeff Liebermann on 8 Jan 2010 12:33
On Fri, 8 Jan 2010 04:04:18 -0800 (PST), species8350
<not_here.5.species8350(a)xoxy.net> wrote:

>Is this a way of looking at modulation.

No.

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

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.

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

Yeah, that's it.

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

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.

--
Jeff Liebermann jeffl(a)cruzio.com
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558
From: Rich Johnson on 9 Jan 2010 00:16
On 1/7/2010 3:58 AM, species8350 wrote:
> I know that the wifi signal is modulated.
>
> Firstly, why does the signal need to be modulated.
>
> Secondly regarding g: why are ther so many coding systems used.
>
> Thanks
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?
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