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Subject

Author

Date

11 Channels

MEOW

07-25-2006

Walter Roberson

Mike

Bill Kearney

Jeff Liebermann

John Navas

Jeff Liebermann

Derek Broughton

Posted by MEOW on July 25, 2006, 11:07 am

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Is it true that one can only use 3 channels in 802.11b even though the
advertisement said you have 11 (FCC)? How would you like to get only 2
toppings with your Pizza even though you ordered 3? Oh and you pay for 3
as well. ;)

Have a nice day!

Posted by Walter Roberson on July 25, 2006, 1:16 pm

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>Is it true that one can only use 3 channels in 802.11b even though the

>advertisement said you have 11 (FCC)?

Pretty much by definition, wireless is not a Local Area Network, so
comp.dcom.lans.ethernet is not really the appropriate newsgroup
for this matter. alt.internet.wireless would be a better newsgroup;
I have added that newsgroup into the header.

In any case, the answer is NO, that is not true.

What is true is that each channel transmits on a frequency range
centered around a base frequency, and that the frequency drop-off
around the defined band is not required to be sharp. There is a
power drop off in the channel on either side, more of a drop off
for the channels beside those, more yet for the ones beside those.

This leak of signal into the next channel does not make those
other channels unusable: it adds noise to those other channels,
making them -less- usable than if there were no overlap.

The drop-off is defined such that by the third channel over, the
interference is negligable. This situation leads to the following
typical allocations, in increasing frequency:

[band 1] - [detectable leak of channel 1]
[band 2] - [noticable leak of channel 1]
band 3 - [peak of channel 1]
[band 4] - [noticable leak of channel 1]
[band 5] - [detectable leak of channel 1]
[band 6] - [detectable leak of channel 6]
[band 7] - [noticable leak of channel 6]
band 8 - [peak of channel 6]
[band 9] - [noticable leak of channel 6]
[band 10] - [detectable leak of channel 6]
[band 11] - [detectable leak of channel 11]
[band 12] - [noticable leak of channel 11]
band 13 - [peak of channel 11]
[band 14] - [noticable leak of channel 11]
[band 15] - [detectable leak of channel 11]

Someone did a study and found that this allocation was not -necessary-
in order to get very good performance. Instead, they found that 4
transmitting channels could be used:

[band 1] - [detectable leak of channel 1]
[band 2] - [noticable leak of channel 1]
band 3 - [peak of channel 1]
[band 4] - [noticable leak of channel 1] + [detectable leak of channel 4]
[band 5] - [detectable leak of channel 1] + [noticable leak of channel 4]
band 6 - [peak of channel 4]
[band 7] - [noticable leak of channel 4]
[band 8] - [detectable leak of channel 8]
[band 9] - [noticable leak of channel 8]
band 10 - [peak of channel 8]
[band 11] - [noticable leak of channel 8] + [detectable leak of channel 11]
[band 12] - [detectable leak of channel 8] + [noticable leak of channel 11]
band 13 - [peak of channel 11]
[band 14] - [noticable leak of channel 11]
[band 15] - [detectable leak of channel 11]

You could shift the channel 8 activity down to channel 7 and shift the
channel 11 activity down to channel 9, and still get very good performance,
but there isn't any reason to do this unless you can transmit on
channel 12 -- which *is* allowed in some areas of the world. In those
areas, you can fit 5 useful channels in 12 allocated channels.

> How would you like to get only 2

> toppings with your Pizza even though you ordered 3? Oh and you pay for 3

> as well. ;)

It's more like, "You've asked for your tomato to be put on only
1/11th of the pizza, but we can't slice our tomato that small: we can
put the bulk of it on 1/11th of the pizza, but some of it is going to
end up on the next 1/11th, and a bit of it will even end up on the
part after that. If you insist that none of your toppings overlap,
then we can fit in 3 toppings, but if you are comfortable with a tiny
bit of topping overlap, hardly even enough to taste, then we can fit in
4 toppings. And if you don't mind topping overlap, we can give you up
to 11 different toppings.

Posted by Mike on July 25, 2006, 1:26 pm

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Walter Roberson wrote:

<snip>

>> How would you like to get only 2

>> toppings with your Pizza even though you ordered 3? Oh and you pay for 3

>> as well. ;)

> It's more like, "You've asked for your tomato to be put on only

> 1/11th of the pizza, but we can't slice our tomato that small: we can

> put the bulk of it on 1/11th of the pizza, but some of it is going to

> end up on the next 1/11th, and a bit of it will even end up on the

> part after that. If you insist that none of your toppings overlap,

> then we can fit in 3 toppings, but if you are comfortable with a tiny

> bit of topping overlap, hardly even enough to taste, then we can fit in

> 4 toppings. And if you don't mind topping overlap, we can give you up

> to 11 different toppings.

I have 13 channels on my linksys WRT54GS (after a firmware update) ,
does this mean I will get 2 free toppings when I order pizza via
domino's website?

Mike

Posted by Bill Kearney on July 25, 2006, 1:55 pm

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> I have 13 channels on my linksys WRT54GS (after a firmware update) ,

> does this mean I will get 2 free toppings when I order pizza via

> domino's website?

Yeah, but they'd only be from the countries that let you use those
frequencies...

Posted by Jeff Liebermann on July 25, 2006, 2:28 pm

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roberson@hushmail.com (Walter Roberson) hath wroth:

>>Is it true that one can only use 3 channels in 802.11b even though the

>>advertisement said you have 11 (FCC)?

>Pretty much by definition, wireless is not a Local Area Network, so

>comp.dcom.lans.ethernet is not really the appropriate newsgroup

>for this matter. alt.internet.wireless would be a better newsgroup;

>I have added that newsgroup into the header.

>In any case, the answer is NO, that is not true.

He left off one important word. There are only 3 _non-overlapping_
channels. These are 1, 6, and 11. Channels in between tend to
overlap these channels. DSSS (direct sequence spread spectrum)
spectra is about 22Mhz wide. Each FCC channel is 5MHz wide, which
means that wi-fi spectra occupies approximately 5 channels.

Intel Wireless Hotspot Deployment Guide
ftp://download.intel.com/business/bss/infrastructure/wireless/deployment/hotspot.pdf
See Fig 6 and Fig 7 on Page 40.

--
Jeff Liebermann jeffl@comix.santa-cruz.ca.us
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558

other useful resources:
The Federal Communications Commission (FCC)
Telecommunications Industry Association
Electronic and Software Security Products and Services
International Telecommunication Union

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