Channels Overlap

LoneStar · 05-08-2002

I had been told that with 802.11b channels 1, 6 and 11 did not overlap with each other. I did some research and found out that the bandwidth of 802.11b channels are 30 MHz. So I did a little calculating. I learned that channel 6 overlaps with all of the 802.11b channels. So in theory this would be the most undesirable channel from an interference standpoint. Now I did this late at night so I'm sure I must have made a mistake. Someone tell me where I've gone wrong.

Channel Frequency (Center)

1 2.412
2 2.417
3 2.422
4 2.427
5 2.432
6 2.437
7 2.442
8 2.447
9 2.452
10 2.457
11 2.462

CH-1
2397-2427
CH-2
2402-2432
CH-3
2407-2437
CH-4
2412-2442
CH-5
2417-2447
CH-6
2422-2452
CH-7
2427-2457
CH-8
2432-2462
CH-9
2437-2467
CH-10
2442-2472
CH-11
2447-2477

Channels that do not interfere with each other
Ch-1: Ch-7, 8, 9, 10, 11
Ch-2: Ch-8, 9, 10, 11
Ch-3: Ch-9, 10, 11
Ch-4: Ch-10, 11
Ch-5: Ch-11
Ch-6: All channels overlap channel 6
Ch-7: Ch-1
Ch-8: Ch-1, 2
Ch-9: Ch-1, 2, 3
Ch-10: Ch-1, 2, 3, 4
Ch-11: Ch-1, 2, 3, 4, 5

tulse_luper · 05-08-2002

yep. they overlap. in graphical form (assuming your bandwidth and center freq values are correct)

Thorn · 05-08-2002

I thought the channels were 20MHz wide with 5 MHz seperation. That would add up to a total of 30MHz, but not actually overlapping on 1, 6, & 11. The diagram below shows the way that it was originally explained to me.

Cheers,

carbolic · 05-08-2002

The channels do overlap. But I think the peak is much steeper than the diagram shows. Which means you do have some separation, although at lower power (or distance) you will lose the signal earlier.

In any case, one can argue that all RF overlaps because the sidebands go out almost to infinity.

It's just a matter of getting your signal to stand out above the noise (or other channels.)

:)

tulse_luper · 05-08-2002

Here's the 20 MHz version. Once again, assuming center positions are correct and all peaks have the same peak power and bandwidth (see below)

I assumed a Gaussian frequency envelope, so unless the frequency content is Lorentzian or something else, the peak heights and slopes are correct. The horizontal black line represents the full-width half max, which should be used to judge the relative separation of the channels.

it is known that not all the channels operate at the same power (the WAP11 hack), so in reality, the relative peak heights will be a bit different. Carbolic is right tho: getting your signal above the noise threshold is the most important thing even if there is overlap

tulse

LoneStar · 05-08-2002

Thorn, I think you are right about the channels being 20 MHz wide and the total bandwidth being 30 MHz. I was getting my numbers from an article on 80211planet.com and I think the author had his numbers jumbled.

I also found another source that says the channels are 22 MHz wide. tulse_luper, you wanna make another graph?

In my town it appears that most of the APs are on channels 1, 6, and 11. I guess that many of the manufactures set one of these channels as the default (linksys 6). I think if I were setting up a wireless LAN that choosing a channel other than 1, 6, or 11 would be best for keeping the potential for interference at a minimum.

Networker · 05-09-2002

Quote:

Originally posted by LoneStar
Thorn, I think you are right about the channels being 20 MHz wide and the total bandwidth being 30 MHz. I was getting my numbers from an article on 80211planet.com and I think the author had his numbers jumbled.

I also found another source that says the channels are 22 MHz wide. tulse_luper, you wanna make another graph?

In my town it appears that most of the APs are on channels 1, 6, and 11. I guess that many of the manufactures set one of these channels as the default (linksys 6). I think if I were setting up a wireless LAN that choosing a channel other than 1, 6, or 11 would be best for keeping the potential for interference at a minimum.

Indeed, I found Ch 3. best for my home setup.

wanman · 05-10-2002

Each channel ia a contiguous band of frequencies 22MHz wide, and 1MHz carrier frequencies are used. Channel 1, for instance, operates from 2.401 GHz to 2.423 GHz (2.412 GHz +- 11MHz); Channel 2 operates from 2.406 to 2.429 GHz (2.417 +- 11MHz) and so forth

Channels 1,6 and 11 do not overlap

I have a graph I can email to show this

This comes from my 802.11 wireless manual

wanman

Thorn · 05-10-2002

wanman,
Yeah, post it. I'd like to see it.
Cheers,

edweird · 05-10-2002

Quote:

Originally posted by LoneStar
I think if I were setting up a wireless LAN that choosing a channel other than 1, 6, or 11 would be best for keeping the potential for interference at a minimum.

For a WLAN using a single AP, choosing a channel free of overlap and interference would make the most sense.

When you are designing a WLAN that will be using multiple AP's to provide coverage over a large area, or with overlapping coverage areas to increase capacity and/or for fault tolerance, using multiple channels that don't overlap becomes a key issue.

Channels 1, 6, and 11 are the only US combination that allows for three AP's in a common area to have some overlap in their cells, with the least amount of channel RF overlap.

wanman · 05-13-2002

Thorn

The pics are too big to post, so I have sent them on to the administrator to be forwarded to you.

Wanman

Rich G · 12-26-2002

Also remember that some of the AP channels fall in the Amateur Radio 2.4 Ghz band. Amateurs are allowed to run much more power on these frequencies than others and can cause interference issues since 802.11 is not the only modulation type allowed for hams. I think Amateur Operators have the primary allocation for these frequencies.

05-05-2005

When multiple access points are in proximity and operating on the same or overlapping channel how is the contention resolved? Does one AP act the point co-ordinator? I would think that the number of collisions go up significantly but things still work ..

G8tK33per · 05-05-2005

Quote:

Originally Posted by adonis

When multiple access points are in proximity and operating on the same or overlapping channel how is the contention resolved? Does one AP act the point co-ordinator? I would think that the number of collisions go up significantly but things still work ..

You have grown to be quite a pain in the ass, this post is 2+ fucking years old.

Banned for a week.

05-08-2002	#1 (permalink)
LoneStar Registered Member Join Date: May 2002 Location: Texas Posts: 38	Channels Overlap I had been told that with 802.11b channels 1, 6 and 11 did not overlap with each other. I did some research and found out that the bandwidth of 802.11b channels are 30 MHz. So I did a little calculating. I learned that channel 6 overlaps with all of the 802.11b channels. So in theory this would be the most undesirable channel from an interference standpoint. Now I did this late at night so I'm sure I must have made a mistake. Someone tell me where I've gone wrong. Channel Frequency (Center) 1 2.412 2 2.417 3 2.422 4 2.427 5 2.432 6 2.437 7 2.442 8 2.447 9 2.452 10 2.457 11 2.462 CH-1 2397-2427 CH-2 2402-2432 CH-3 2407-2437 CH-4 2412-2442 CH-5 2417-2447 CH-6 2422-2452 CH-7 2427-2457 CH-8 2432-2462 CH-9 2437-2467 CH-10 2442-2472 CH-11 2447-2477 Channels that do not interfere with each other Ch-1: Ch-7, 8, 9, 10, 11 Ch-2: Ch-8, 9, 10, 11 Ch-3: Ch-9, 10, 11 Ch-4: Ch-10, 11 Ch-5: Ch-11 Ch-6: All channels overlap channel 6 Ch-7: Ch-1 Ch-8: Ch-1, 2 Ch-9: Ch-1, 2, 3 Ch-10: Ch-1, 2, 3, 4 Ch-11: Ch-1, 2, 3, 4, 5

05-08-2002	#4 (permalink)
carbolic Move Zig Join Date: Apr 2002 Location: Los Angeles Posts: 107	Overlap The channels do overlap. But I think the peak is much steeper than the diagram shows. Which means you do have some separation, although at lower power (or distance) you will lose the signal earlier. In any case, one can argue that all RF overlaps because the sidebands go out almost to infinity. It's just a matter of getting your signal to stand out above the noise (or other channels.) :) __________________ www.SoCalWUG.org

05-10-2002	#8 (permalink)
wanman UK Stumbler Join Date: Apr 2002 Location: UK Posts: 26	This Should Help Each channel ia a contiguous band of frequencies 22MHz wide, and 1MHz carrier frequencies are used. Channel 1, for instance, operates from 2.401 GHz to 2.423 GHz (2.412 GHz +- 11MHz); Channel 2 operates from 2.406 to 2.429 GHz (2.417 +- 11MHz) and so forth Channels 1,6 and 11 do not overlap I have a graph I can email to show this This comes from my 802.11 wireless manual wanman

05-10-2002	#9 (permalink)
Thorn Did you do the math? Join Date: Apr 2002 Location: Villa Straylight Posts: 10,429	wanman, Yeah, post it. I'd like to see it. Cheers, __________________ Thorn "Read Altas Shrugged. Compare it to today. Repeat as necessary"

05-13-2002	#11 (permalink)
wanman UK Stumbler Join Date: Apr 2002 Location: UK Posts: 26	On Its Way Thorn The pics are too big to post, so I have sent them on to the administrator to be forwarded to you. Wanman

05-08-2002	#6 (permalink)
LoneStar Registered Member Join Date: May 2002 Location: Texas Posts: 38	Thorn, I think you are right about the channels being 20 MHz wide and the total bandwidth being 30 MHz. I was getting my numbers from an article on 80211planet.com and I think the author had his numbers jumbled. I also found another source that says the channels are 22 MHz wide. tulse_luper, you wanna make another graph? In my town it appears that most of the APs are on channels 1, 6, and 11. I guess that many of the manufactures set one of these channels as the default (linksys 6). I think if I were setting up a wireless LAN that choosing a channel other than 1, 6, or 11 would be best for keeping the potential for interference at a minimum.

12-26-2002	#12 (permalink)
Rich G Registered Member Join Date: Dec 2002 Posts: 1	Also remember that some of the AP channels fall in the Amateur Radio 2.4 Ghz band. Amateurs are allowed to run much more power on these frequencies than others and can cause interference issues since 802.11 is not the only modulation type allowed for hams. I think Amateur Operators have the primary allocation for these frequencies.

05-05-2005	#13 (permalink)
adonis Posts: n/a	When multiple access points are in proximity and operating on the same or overlapping channel how is the contention resolved? Does one AP act the point co-ordinator? I would think that the number of collisions go up significantly but things still work ..

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