Choosing a Channel?

I have a simple 802.11g home network, but every home surrounding mine has
one too. Choosing an "open" 2.4 GHz channel for me is an exercize in logic
versus bandwidth. Every reference, including the manufacturer of my 2.4GHz
cordless phones, says choosing channels 1, 6 or 11 is optimum. Of course
those 3 are heavily used, with most units staying on their default channel
of 6 or 9. So, I feel one of the interval channels may be superior based on
the fact that I can have a unique center frequency with the greatest amount
of overlap only with the weakest neighbor transmitters.

However, I'm wondering how much concern I should have for users who are 1
vs. 2 channels removed? Most users near me use channel 6. Logic tells me
to avoid channels 5 and 7. Channel 2 removes me the farthest from the
meelee on channel 6, but it also places me closer to a single user on
channel 1. In striking a balance, am I better to distance myself from a
busy channel than one adjacent to just one user, or to split the difference
between occupied center frequencies? IOW, is it better to share 3/4ths of
your channel with a single signal, than 1/2 of your channel with a bunch?

Am I focusing on the wrong criteria? My weakest signal received from my
own network is 65% in the kitchen. Should I just ignore every signal below
that level and just concern myself only with the one or two signals that may
actually get that strong in my house?

-Bill Radio

In article <(E-Mail Removed)>,
(E-Mail Removed) (known to some as Bill Radio)
scribed...

> I have a simple 802.11g home network, but every home surrounding mine has
> one too. Choosing an "open" 2.4 GHz channel for me is an exercize in logic
> versus bandwidth.

<snippety>

You can avoid the whole mess, and get the same potential speed, by
using 802.11a (5GHz) equipment. It's fairly easy to find multi-standard
cards, and access points for the 11a standard seem to turn up on Greed-
bay pretty regularly.

Happy hunting.


--
Dr. Anton T. Squeegee, Director, Dutch Surrealist Plumbing Institute
(Known to some as Bruce Lane, KC7GR)
http://www.bluefeathertech.com -- kyrrin a/t bluefeathertech d-o=t calm
"Salvadore Dali's computer has surreal ports..."

Dr. Anton T. Squeegee wrote:
> In article <(E-Mail Removed)>,
> (E-Mail Removed) (known to some as Bill Radio)
> scribed...
>
> > I have a simple 802.11g home network, but every home surrounding mine has
> > one too. Choosing an "open" 2.4 GHz channel for me is an exercize in logic
> > versus bandwidth.

the 11-14 b/g chanels can be confusing as they overlap - 1 6 and 11 or
in japan only 14 are the non overlaping chanels check what your
neighbors are using.

or as dr anton says

> <snippety>
>
> You can avoid the whole mess, and get the same potential speed, by
> using 802.11a (5GHz) equipment. It's fairly easy to find multi-standard
> cards, and access points for the 11a standard seem to turn up on Greed-
> bay pretty regularly.
>
> Happy hunting.
>
>
> --
> Dr. Anton T. Squeegee, Director, Dutch Surrealist Plumbing Institute

Bill Radio <(E-Mail Removed)> wrote:

> I have a simple 802.11g home network, but every home surrounding mine has
> one too. Choosing an "open" 2.4 GHz channel for me is an exercize in logic
> versus bandwidth. Every reference, including the manufacturer of my 2.4GHz
> cordless phones, says choosing channels 1, 6 or 11 is optimum. Of course
> those 3 are heavily used, with most units staying on their default channel
> of 6 or 9. So, I feel one of the interval channels may be superior based on
> the fact that I can have a unique center frequency with the greatest amount
> of overlap only with the weakest neighbor transmitters.
>
> However, I'm wondering how much concern I should have for users who are 1
> vs. 2 channels removed? Most users near me use channel 6. Logic tells me
> to avoid channels 5 and 7. Channel 2 removes me the farthest from the
> meelee on channel 6, but it also places me closer to a single user on
> channel 1. In striking a balance, am I better to distance myself from a
> busy channel than one adjacent to just one user, or to split the difference
> between occupied center frequencies? IOW, is it better to share 3/4ths of
> your channel with a single signal, than 1/2 of your channel with a bunch?
>
> Am I focusing on the wrong criteria? My weakest signal received from my
> own network is 65% in the kitchen. Should I just ignore every signal below
> that level and just concern myself only with the one or two signals that may
> actually get that strong in my house?

Researchers at Cisco found that, because of the way data is transmitted
in 802.11b/g, it is actually better to use one of the non-overlapping
channels (1, 6 or 11), even if it's in use by another network. See
<http://www.cisco.com/en/US/products/...d_technical_re
ference09186a00802846a2.html>.

I suggest trying the non-overlapping channel (1, 6, or 11) with the
weakest signal from other networks before you try any of the other
channels.

On Sun, 3 Dec 2006 00:12:12 -0700, "Bill Radio"
<(E-Mail Removed)> wrote:

>I have a simple 802.11g home network, but every home surrounding mine has
>one too.

Move to a new location.
Switch to 802.11a (5.8GHz).

>Choosing an "open" 2.4 GHz channel for me is an exercize in logic
>versus bandwidth.

Ummm.... It's not that easy.

>Every reference, including the manufacturer of my 2.4GHz
>cordless phones, says choosing channels 1, 6 or 11 is optimum.

Correct. Those are the non-overlapping channels. Each channel number
is 5MHz wide. However, the 802.11b/g signal is 22MHz wide. If you
grind the numbers, that leaves you 1, 6, and 11 as the only
non-overlapping channels. Incidentally, I've noticed that most 2.4GHz
cordless phones seems to prefer the lower end of the band (i.e.
Channel 1) and move up the band depending on interference. I can see
them clearly with my spectrum analyzer all cluttering the lower end of
the band. Not all cordless phones work this way, just the ones I can
see.

>Of course
>those 3 are heavily used, with most units staying on their default channel
>of 6 or 9. So, I feel one of the interval channels may be superior based on
>the fact that I can have a unique center frequency with the greatest amount
>of overlap only with the weakest neighbor transmitters.

Wrong(tm). The problem has to do with how your wireless receiver
perceives these other channels. If you land on a channel with an
existing user, your receiver will decode their data as valid data and
your xmitter will wait the requisite time before attempting to xmit.
The idea is to reduce collisions (or partial collisions). Things slow
down, but do not stop.

However, if you have someone adjacent to the your channel, their off
frequency transmissions will be decoded as noise rather than valid
data. Your xmitter will not wait and simply transmit on top of them.
The net result is a continuous series of collision where nobody moves
data.

In addition, when you wedge yourself between two heavily used
channels, you get the interference from BOTH of these channels,
instead of just the one.

It's better to stay on 1, 6, and 11 than the others.

>However, I'm wondering how much concern I should have for users who are 1
>vs. 2 channels removed? Most users near me use channel 6. Logic tells me
>to avoid channels 5 and 7. Channel 2 removes me the farthest from the
>meelee on channel 6, but it also places me closer to a single user on
>channel 1. In striking a balance, am I better to distance myself from a
>busy channel than one adjacent to just one user, or to split the difference
>between occupied center frequencies? IOW, is it better to share 3/4ths of
>your channel with a single signal, than 1/2 of your channel with a bunch?

See my previous explanation. Stay on 1, 6, and 11.

I once troubleshot a system in high rise office building. Netstumbler
showed some huge number of wireless access points, mostly on channel
6, but others scattered on other channels. Someone had installed
their access points on channels 3 and 8 using your logic. It worked
fairly well late at night, but was effectively useless during the day.
I moved the access points to channel 1 and 11, and things started to
work much better. However, I wasn't satisfied with the result so I
spent the rest of the day installing panel antennas and relocating the
access points away from windows with a view of the city. I had some
problems with users that had a window office, but the users in the
core of the building did quite well. My guess(tm) is that the
antennas and repositioning had the biggest effect, but the channel
change was also somewhat of a help.

>Am I focusing on the wrong criteria? My weakest signal received from my
>own network is 65% in the kitchen. Should I just ignore every signal below
>that level and just concern myself only with the one or two signals that may
>actually get that strong in my house?
>
>-Bill Radio

That reminds me.... I should get back to doing my (late) billing.


--
# Jeff Liebermann 150 Felker St #D Santa Cruz CA 95060
# 831-336-2558 (E-Mail Removed)
# http://802.11junk.com (E-Mail Removed)
# http://www.LearnByDestroying.com AE6KS

On Mon, 4 Dec 2006 16:45:24 -0700, (E-Mail Removed) (Neill Massello)
wrote in <1hptxiz.15axnr41d7hdfqN%(E-Mail Removed)>:

>Bill Radio <(E-Mail Removed)> wrote:
>
>> I have a simple 802.11g home network, but every home surrounding mine has
>> one too. Choosing an "open" 2.4 GHz channel for me is an exercize in logic
>> versus bandwidth. Every reference, including the manufacturer of my 2.4GHz
>> cordless phones, says choosing channels 1, 6 or 11 is optimum. Of course
>> those 3 are heavily used, with most units staying on their default channel
>> of 6 or 9.

9???

>> So, I feel one of the interval channels may be superior based on
>> the fact that I can have a unique center frequency with the greatest amount
>> of overlap only with the weakest neighbor transmitters.

There's no way to know without actual extensive testing.

>> However, I'm wondering how much concern I should have for users who are 1
>> vs. 2 channels removed? Most users near me use channel 6. Logic tells me
>> to avoid channels 5 and 7. Channel 2 removes me the farthest from the
>> meelee on channel 6, but it also places me closer to a single user on
>> channel 1. In striking a balance, am I better to distance myself from a
>> busy channel than one adjacent to just one user, or to split the difference
>> between occupied center frequencies? IOW, is it better to share 3/4ths of
>> your channel with a single signal, than 1/2 of your channel with a bunch?

There's no way to know without actual extensive testing. Radio issues
often seem illogical simply because they are so complex.

>> Am I focusing on the wrong criteria? My weakest signal received from my
>> own network is 65% in the kitchen. Should I just ignore every signal below
>> that level and just concern myself only with the one or two signals that may
>> actually get that strong in my house?

You should just test, particularly to find out if your 2.4 GHz phone is
enough of a problem to warrant replacement (e.g., with 900 MHz).

>Researchers at Cisco found that, because of the way data is transmitted
>in 802.11b/g, it is actually better to use one of the non-overlapping
>channels (1, 6 or 11), even if it's in use by another network. See
><http://www.cisco.com/en/US/products/...d_technical_re
>ference09186a00802846a2.html>.

"All generalizations are false," and that's not exactly what that
article says -- it's mostly responding to suggestions to use 4 channels
(e.g., 1, 4, 8, 11) instead of 3 channels (1, 6, 11) in a single
multi-access point network. That study doesn't necessarily extend to
separate interfering networks.

>I suggest trying the non-overlapping channel (1, 6, or 11) with the
>weakest signal from other networks before you try any of the other
>channels.

Sure, but I have found some cases where one of the other channels (e.g.,
4) does work better -- so it's worth trying 3, 4, 8, and 9 if you can't
get good operation on 1, 6, or 11. (Note that even 1 and 6, and 6 and
11, overlap to some degree.)

--
Best regards, FAQ for Wireless Internet: <http://Wireless.wikia.com>
John Navas FAQ for Wi-Fi: <http://wireless.wikia.com/wiki/Wi-Fi>
Wi-Fi How To: <http://wireless.wikia.com/wiki/Wi-Fi_HowTo>
Fixes to Wi-Fi Problems: <http://wireless.wikia.com/wiki/Wi-Fi_Fixes>

On Tue, 05 Dec 2006 00:32:48 GMT, Jeff Liebermann
<(E-Mail Removed)> wrote in
<(E-Mail Removed)>:

>On Sun, 3 Dec 2006 00:12:12 -0700, "Bill Radio"
><(E-Mail Removed)> wrote:

>>Every reference, including the manufacturer of my 2.4GHz
>>cordless phones, says choosing channels 1, 6 or 11 is optimum.
>
>Correct. Those are the non-overlapping channels. Each channel number
>is 5MHz wide. However, the 802.11b/g signal is 22MHz wide. If you
>grind the numbers, that leaves you 1, 6, and 11 as the only
>non-overlapping channels.

Actually minimally overlapping channels -- there's no sharp cutoff at 22
MHz -- significant energy goes beyond those boundaries. Worse, many
wireless products now use various forms of multiple-channel transmission
that pollutes much more than a normal channel.

>Incidentally, I've noticed that most 2.4GHz
>cordless phones seems to prefer the lower end of the band (i.e.
>Channel 1) and move up the band depending on interference. I can see
>them clearly with my spectrum analyzer all cluttering the lower end of
>the band. Not all cordless phones work this way, just the ones I can
>see.

I've seen that behavior with Panasonic Gigarange phones. Others were
all over the place.

>>Of course
>>those 3 are heavily used, with most units staying on their default channel
>>of 6 or 9. So, I feel one of the interval channels may be superior based on
>>the fact that I can have a unique center frequency with the greatest amount
>>of overlap only with the weakest neighbor transmitters.
>
>Wrong(tm). The problem has to do with how your wireless receiver
>perceives these other channels. If you land on a channel with an
>existing user, your receiver will decode their data as valid data and
>your xmitter will wait the requisite time before attempting to xmit.
>The idea is to reduce collisions (or partial collisions). Things slow
>down, but do not stop.
>
>However, if you have someone adjacent to the your channel, their off
>frequency transmissions will be decoded as noise rather than valid
>data. Your xmitter will not wait and simply transmit on top of them.
>The net result is a continuous series of collision where nobody moves
>data.
>
>In addition, when you wedge yourself between two heavily used
>channels, you get the interference from BOTH of these channels,
>instead of just the one.
>
>It's better to stay on 1, 6, and 11 than the others.

With all due respect, I think that's a bit simplistic, exaggerated, and
misleading.

True, Wi-Fi devices are designed to avoid each other, but there's no
free lunch, and the total can be much less than the sum of the parts,
sometimes much less. A common problem is where remote units on network
A cannot clearly hear remote units on network B, and vice versa, so they
merrily belch away at the same time, wreaking havoc for other units on
both networks that can hear both of them.

On the other hand, just as Wi-Fi is designed to share a channel, it's
also designed to deal with interference, principally by falling back in
speed. In at least some cases this will work better on overlapping
channels than having both networks on the same channel, particularly
where the interference is less severe and/or when units are throttled
back to lower speed. I'll often throttle "g" networks to as low as 11
Mbps when I know the client has no need for higher speed (a tip I picked
up, as I recall, from you.

>See my previous explanation. Stay on 1, 6, and 11.

I'd say try 1, 6, and 11 first, but if results aren't satisfactory, also
try 3, 4, 8, and 9.

>I once troubleshot a system in high rise office building. Netstumbler
>showed some huge number of wireless access points, mostly on channel
>6, but others scattered on other channels. Someone had installed
>their access points on channels 3 and 8 using your logic. It worked
>fairly well late at night, but was effectively useless during the day.
>I moved the access points to channel 1 and 11, and things started to
>work much better. However, I wasn't satisfied with the result so I
>spent the rest of the day installing panel antennas and relocating the
>access points away from windows with a view of the city. I had some
>problems with users that had a window office, but the users in the
>core of the building did quite well. My guess(tm) is that the
>antennas and repositioning had the biggest effect, but the channel
>change was also somewhat of a help.

My speculation(sm) is that the antenna change and repositioning would
have been enough without a channel change.

--
Best regards, FAQ for Wireless Internet: <http://Wireless.wikia.com>
John Navas FAQ for Wi-Fi: <http://wireless.wikia.com/wiki/Wi-Fi>
Wi-Fi How To: <http://wireless.wikia.com/wiki/Wi-Fi_HowTo>
Fixes to Wi-Fi Problems: <http://wireless.wikia.com/wiki/Wi-Fi_Fixes>

John Navas <(E-Mail Removed)> hath wroth:

>>Correct. Those are the non-overlapping channels. Each channel number
>>is 5MHz wide. However, the 802.11b/g signal is 22MHz wide. If you
>>grind the numbers, that leaves you 1, 6, and 11 as the only
>>non-overlapping channels.

>Actually minimally overlapping channels -- there's no sharp cutoff at 22
>MHz -- significant energy goes beyond those boundaries. Worse, many
>wireless products now use various forms of multiple-channel transmission
>that pollutes much more than a normal channel.

See the Cisco article:
<http://www.cisco.com/en/US/products/hw/wireless/ps430/prod_technical_reference09186a00802846a2.html#wp13 4354>
at Fig 7, which shows the spectral mask for 802.11g. Note that the
signal level is -20dB down at about 10Mhz (2 channels) away. If I
assume that the garbage is a constant -20dB down at any channel that's
more than 10MHz away, I can assume 20dB of isolation.

So, how far away must two radios be isolated before the 20dB isolation
becomes a problem? Let's make it really easy (because I'm lazy) and
say the antennas have +2dB gain, are prefectly aligned with each
other, the xmitter belches +15dBm and the receiver has a sensitivity
of about -85dBm. The required path isolation for the interfering
signal to be equal to a minimal receive signal is:
15dBm + 2dB + 2dB - -85dBm = 104dB isolation
The noise level is -20dB below the signal so we only need:
104 - 20 = 84dB of path isolation
Plugging into:
<http://www.terabeam.com/support/calculations/som.php>
I find that the for 84dB of free space loss, the antennas are 0.1 mile
or 520ft apart.

So, if your neighbor and your access points are stareing at each other
(i.e. line of sight), and you're using the stock rubber ducky
antennas, and the neighbor is on Ch 3 while you're on Ch 6, you can be
528ft apart before there will be any signifigant interference.
Actually, it might a considerably closer because the receive reference
level did not include any fade margin.

Just for fun, the numbers for Ch 1 to Ch 6 show about -30dB isolation.
Using the same assumptions as before, the required path isolation is
74dB, which works out to 0.03 miles or 158ft. Again, if you are
furthur than 158 from the source, there won't be much interference.

>I've seen that behavior with Panasonic Gigarange phones. Others were
>all over the place.

I took my Wi-Spy spectrum analyzer to Office Max and tested a few
phones (before they threw me out). You're right. They do vary. A
few even hogged the entire 2.4GHz band. Unless there's a predominance
of Panasonic phones in the neighborhood, everything I can see from my
house seems to favor the bottom of the band.

>I'd say try 1, 6, and 11 first, but if results aren't satisfactory, also
>try 3, 4, 8, and 9.

Sure, try other channels.

>My speculation(sm) is that the antenna change and repositioning would
>have been enough without a channel change.

Possibly. However, I could have left the customer with a working
system simply by changing channels. Frankly, I was rather suprised at
how well it mostly fixed the problem. However, my ping tests showed
that there was still some interference so I decided to pump up my
exhorbitant charges and try to eliminate the interference completely.
Besides, I didn't want to return the antennas and pigtails I had
bought. So, the new antennas and new location finished the job. I
probably could have put the system back on the original channels, but
didn't see any reason to bother.

--
Jeff Liebermann (E-Mail Removed)
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558

I appreciate the detailed answers! I decided to drive around the
neighborhood to see what was being used, and find the default channels of 6
and 9 are used even more than i thought. With so many on those 2 channels,
use of channels 6 and 11 would both be subject to interference. If I agree
with the Cisco figures, it leaves me just one choice: Channel 1.
Unfortunately, one of the strongest signals received at the client is on
Channel 1, although not that strong.

Using the same channel as a neighbor, and avoiding channels that are
separated by as much as 10 MHz, is counter-intuitive to those of us in the
RF world. Most of the wireless AP's are from the local DSL supplier and are
probably running speeds as slow at 1.5 Mb and maybe slower, which would
reduce the likelihood of full bandwidth use of the "g" spectrum. However,
the tests showed the lower-bandwidth "b" systems also benefitted from the
greater channel separation.

Every report I can find on the 'net claims the channel used is normally not
of great importance. As Jeff points out, if a system has a problem,
changing channels could do a lot to rectify the situation. But if I am
getting the same throughput on the wireless network as I am on the
ethernet-connected computer, I don't have a problem.

I am a bit surprised how few homeowners consider mounting their AP in their
basement which would isolate it almost entirely from neighbors. The client
would see the same interference, but the AP would see little, if any. Also,
on my walkabout I found some houses with a much greater problem with
neighboring systems than mine. But some of us will always try to optimize
our system...it's what we do.

-.-. --.- -.-. .... .----

Bill,
NAqNA


"Jeff Liebermann" <(E-Mail Removed)> wrote in message
news:(E-Mail Removed)...
> John Navas <(E-Mail Removed)> hath wroth:
>
>>>Correct. Those are the non-overlapping channels. Each channel number
>>>is 5MHz wide. However, the 802.11b/g signal is 22MHz wide. If you
>>>grind the numbers, that leaves you 1, 6, and 11 as the only
>>>non-overlapping channels.
>
>>Actually minimally overlapping channels -- there's no sharp cutoff at 22
>>MHz -- significant energy goes beyond those boundaries. Worse, many
>>wireless products now use various forms of multiple-channel transmission
>>that pollutes much more than a normal channel.
>
> See the Cisco article:
> <http://www.cisco.com/en/US/products/hw/wireless/ps430/prod_technical_reference09186a00802846a2.html#wp13 4354>
> at Fig 7, which shows the spectral mask for 802.11g. Note that the
> signal level is -20dB down at about 10Mhz (2 channels) away. If I
> assume that the garbage is a constant -20dB down at any channel that's
> more than 10MHz away, I can assume 20dB of isolation.
>
> So, how far away must two radios be isolated before the 20dB isolation
> becomes a problem? Let's make it really easy (because I'm lazy) and
> say the antennas have +2dB gain, are prefectly aligned with each
> other, the xmitter belches +15dBm and the receiver has a sensitivity
> of about -85dBm. The required path isolation for the interfering
> signal to be equal to a minimal receive signal is:
> 15dBm + 2dB + 2dB - -85dBm = 104dB isolation
> The noise level is -20dB below the signal so we only need:
> 104 - 20 = 84dB of path isolation
> Plugging into:
> <http://www.terabeam.com/support/calculations/som.php>
> I find that the for 84dB of free space loss, the antennas are 0.1 mile
> or 520ft apart.
>
> So, if your neighbor and your access points are stareing at each other
> (i.e. line of sight), and you're using the stock rubber ducky
> antennas, and the neighbor is on Ch 3 while you're on Ch 6, you can be
> 528ft apart before there will be any signifigant interference.
> Actually, it might a considerably closer because the receive reference
> level did not include any fade margin.
>
> Just for fun, the numbers for Ch 1 to Ch 6 show about -30dB isolation.
> Using the same assumptions as before, the required path isolation is
> 74dB, which works out to 0.03 miles or 158ft. Again, if you are
> furthur than 158 from the source, there won't be much interference.
>
>>I've seen that behavior with Panasonic Gigarange phones. Others were
>>all over the place.
>
> I took my Wi-Spy spectrum analyzer to Office Max and tested a few
> phones (before they threw me out). You're right. They do vary. A
> few even hogged the entire 2.4GHz band. Unless there's a predominance
> of Panasonic phones in the neighborhood, everything I can see from my
> house seems to favor the bottom of the band.
>
>>I'd say try 1, 6, and 11 first, but if results aren't satisfactory, also
>>try 3, 4, 8, and 9.
>
> Sure, try other channels.
>
>>My speculation(sm) is that the antenna change and repositioning would
>>have been enough without a channel change.
>
> Possibly. However, I could have left the customer with a working
> system simply by changing channels. Frankly, I was rather suprised at
> how well it mostly fixed the problem. However, my ping tests showed
> that there was still some interference so I decided to pump up my
> exhorbitant charges and try to eliminate the interference completely.
> Besides, I didn't want to return the antennas and pigtails I had
> bought. So, the new antennas and new location finished the job. I
> probably could have put the system back on the original channels, but
> didn't see any reason to bother.
>
> --
> Jeff Liebermann (E-Mail Removed)
> 150 Felker St #D http://www.LearnByDestroying.com
> Santa Cruz CA 95060 http://802.11junk.com
> Skype: JeffLiebermann AE6KS 831-336-2558

"Bill Radio" <(E-Mail Removed)> hath wroth:

>I decided to drive around the
>neighborhood to see what was being used, and find the default channels of 6
>and 9 are used even more than i thought.

Well, if you're using the Windoze "show available networks" or
Netstumbler, you're only seeing those access points set to broadcast
their SSID. You'll see more if you use a passive sniffer, such as
Kismet under Linux. No need to reformat your hard disk to use Kismet.
Boot a LiveCD with Linux and you have all the tools available.
<http://www.remote-exploit.org/index.php/BackTrack>
<http://www.remote-exploit.org/index.php/Auditor>
First, make sure your wireless card will work:
<http://www.remote-exploit.org/index.php/Auditor_dev_list1>

Are you sure about channel 9? It's my understanding that 9 is NOT a
default channel. Ch 6 is the most common.

Incidentally, I did a very crude site survey of a local small town
this morning. Looking at the results from WiFiFoFum (active probe
similar to Netstumbler) on my cell phone, I saw 18 access points with:
Ch Number
1 1
6 14
10 1
11 2
However, when I fired up Kismet (passive sniffer) on my laptop, and
let it run while we were at lunch, I found 25 access points:
1 2
3 1
6 16
7 1
10 1
11 4
This is fairly typical is what I see in predominantly residential
areas, where access points tend to be installed with the defaults
largely intact. That fact that the overwhelmingly large number of
access points tend to be on Ch 6 and appear to coexist with each other
seems to indicate that either:
1. a large number of access points can peacefully coexist on the same
channel.
2. or most users can't tell when they're getting interference and
have simply gotten used to the crappy and unreliable performance.

Also, I've seen some of the SSID's change channel over time. This is
a feature of some access points where it searches for an unoccupied
channel. It seems like a good idea, but I've seen nothing but
problems when it is used.

Also, you might not be seeing interference from the new MIMO systems
that monopolize more than 22Mhz of bandwidth. They usually show up on
Ch 6 but are much wider than the typical 802.11b/g signals. There are
multiple types of MIMO. Some are "good neighbors". Others are no
better than jammers. The only way to identify these (at this time) is
with a spectrum analyzer.

>With so many on those 2 channels,
>use of channels 6 and 11 would both be subject to interference. If I agree
>with the Cisco figures, it leaves me just one choice: Channel 1.
>Unfortunately, one of the strongest signals received at the client is on
>Channel 1, although not that strong.

It will take a considerable number of weak signals to equal the effect
of one strong signal. What weak signals do is just raise the overall
baseline noise level. It's more difficult to work reliably at long
range and with weak signals, but these weak signals do not materially
affect the comparatively strong signals used by your local WLAN.
However, a strong signal on the same channel will require sharing of
the available airtime with the neighbor and will slow you down. As I
previously mentioned, and is confirmed in the Cisco article, the
collision avoidance mechanism is more effective with a co-channel
interfering user, than with an off channel noise source. However,
note that the Cisco article implies that the comparison is based on an
equal signal strength comparison between the two systems.

>Using the same channel as a neighbor, and avoiding channels that are
>separated by as much as 10 MHz, is counter-intuitive to those of us in the
>RF world.

Not really. Think of the off channel neighbor breaking the CSMA/CA
collision avoidance mechanism of 802.11. It works with an on channel
jammer.

>Most of the wireless AP's are from the local DSL supplier and are
>probably running speeds as slow at 1.5 Mb and maybe slower, which would
>reduce the likelihood of full bandwidth use of the "g" spectrum. However,
>the tests showed the lower-bandwidth "b" systems also benefitted from the
>greater channel separation.

Sorta. The problem is that 802.11b and 802.11g are really quite
incompatible. The only reason they coexist is that 802.11g optionally
includes an "802.11b compatible" feature which really means it time
slices and listens for 802.11b clients. When it hears one, it
switches temporarily to 802.11b mode. That's why 802.11g benchmark
speeds are much higher when 802.11b compatibility mode is turned off.

The difference also shows up as how they share airtime. It's more
efficient to run at a much higher speed than the DSL backhaul. For
example, the typical 1.5Mbit/sec DSL line would theoretically not
benifit from any wireless speeds faster than perhaps 5.5Mbits/sec
(because the transfer speed is about half the connection speed).
However, the air time used to move the same 1.5Mbits/sec data at
5.5Mbits/sec is much more than the same system running at 54Mbits/sec.
This leaves more air time for other users. This is why 802.11g tends
to constantly try to run at the fastest possible wireless speed.

>Every report I can find on the 'net claims the channel used is normally not
>of great importance.

It really isn't too important. For example, lets pretend you have a
neighboring system that trashes every other packet for a 50% reduction
in thruput. That's fairly bad, but you won't notice it if your
wireless is running at perhaps 24Mbits/sec while your DSL is at
1.5Mbit/sec. It will only show up on connection reliability and local
(LAN to WLAN) benchmarks.

>As Jeff points out, if a system has a problem,
>changing channels could do a lot to rectify the situation. But if I am
>getting the same throughput on the wireless network as I am on the
>ethernet-connected computer, I don't have a problem.

Sorta. If the wireless reliability were stable and didn't change, you
could say you don't have a problem. WISP and wireless bridge systems
might do this because both ends of the link are fixed in a fairly
stable environment. However, your indoor performance is infested with
reflections, multipath, and a changing environment. Line of sight is
usually a dream. So, indoor WLAN's are just not stable. What works
fine today, may not work tomorrow. Add some interference into this
mix, and you will see highly variable performance and reliability.

>I am a bit surprised how few homeowners consider mounting their AP in their
>basement which would isolate it almost entirely from neighbors. The client
>would see the same interference, but the AP would see little, if any.

I've done something like that in office buildings, but not in
residential installs. The problem with home routers is that they tend
to be all in one conglomerations where the location is largely
dictated by the location of where all the CAT5 wires, phone lines,
CATV, and such come together. It might end up in a closet, dungeon,
or sometimes in an attic. The order of priorities is usually wiring
first, wireless a poor second. This is why I like seperate routers
and wireless access points.

>Also,
>on my walkabout I found some houses with a much greater problem with
>neighboring systems than mine. But some of us will always try to optimize
>our system...it's what we do.

Are you sure they have a problem? Like I mumbled, if all those users
can co-infest ch 6, having everyone on one channel is either workable,
or the users standards of performance is minimal.

Actually, there may be another explanation. The SSID's that look like
"2WIRExxx" (where XXX is the last few digits of the MAC address) are
locally "home networking" systems sold by PBI/SBC/at&t. These come
with wireless enabled (but encrypted with a WEP key by default). Many
of these systems don't have any wireless users or devices other than
the access point. It may look like a crowded Ch 6, but many of those
systems show no traffic other than broadcasts.

>-.-. --.- -.-. .... .----
C Q C H 1

Huh?

--
Jeff Liebermann (E-Mail Removed)
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558