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?
> 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 <12n4u6c4ununmcc@corp.supernews.com>,
> Wireless@MountainWirelessNOSPAN.com (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
On Sun, 3 Dec 2006 00:12:12 -0700, "Bill Radio"
<Wireless@MountainWirelessNOSPAN.com> 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.
On Mon, 4 Dec 2006 16:45:24 -0700, massello@newsguy.com (Neill Massello)
wrote in <1hptxiz.15axnr41d7hdfqN%massello@newsguy.com>:
>Bill Radio <Wireless@MountainWirelessNOSPAN.com> 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>
>>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 <spamfilter0@navasgroup.com> 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.
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" <jeffl@comix.santa-cruz.ca.us> wrote in message
news:kksen2p9daauck6hqlmdg8knhchrku6g4i@4ax.com...
> John Navas <spamfilter0@navasgroup.com> 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 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
>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.
Jeff,
You're sharpening the pencil to just as sharp a point as I.
"Jeff Liebermann" <jeffl@comix.santa-cruz.ca.us> wrote in message
news:0ddfn2pihch4ra7g1qvmojm681r3r95b2t@4ax.com...
>
> Well, if you're using the Windoze "show available networks" or
> Netstumbler, you're only seeing those access points set to broadcast
> their SSID.
No, I'm using the Netgear wizard which is wonderfully sensitive, including
systems with no SSID.
> Are you sure about channel 9? It's my understanding that 9 is NOT a
> default channel.
Yes, Qwest is currently delivering all of their "approved" Actiontec routers
defaulted to Channel 9. Their older units (b) were all supplied on Channel
6, but there are only a couple of those here.
> 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.
That explains one Linksys signal that jumps all over the band, but it mostly
uses 1,3,6 & 9, the ones that are used most in this area.
In one trip around the block I found:
ch 1 3
ch 3 2
ch 5 1
ch 6 8
ch 7 1
ch 9 10
ch 10 1
ch 11 4
> Think of the off channel neighbor breaking the CSMA/CA
> collision avoidance mechanism of 802.11. It works with an on channel
> jammer.
The hard part is determining at what threshold the weak off-channel signals
will actually cause a problem.
> 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 means if I choose a channel with a neighboring "b" user, my network
will slow down to their level. Then it makes a case for using the busier
but more compatible channel 6 where there are no "b" users.
> 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.
Under that scenerio, It can kill 75% of my packets and still get better than
5Mb, which is as good as a 1.5Mb DSL connection needs.
> The order of priorities is usually wiring
> first, wireless a poor second. This is why I like seperate routers
> and wireless access points.
I decided to leave the router in the upstairs office so that one computer
can be connected by ethernet cable...one less wireless point to be concerned
about.
> 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.
Yes, they may not have a "problem", but I would if I lived over there. But
these home systems just aren't that active, so problems may be minor.
> 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.
There's only one. Yes, for a time, Qwest ran out of regular DSL routers, so
they were sending out wireless routers to customers who did not know they
had wireless. I'm sorta glad to see they come WEP-enabled seeing as these
neighbors would be unknowing wi-fi points.
>>-.-. --.- -.-. .... .----
> C Q C H 1
Yes, it appears as though I will be sending a CQ to other CHannel 1 users
after I have determined that it isn't affected by our Uniden cordless
phones. Uniden claims that they use channels above, below & between
channels 1, 6 & 11. I bet they just scan for inactive channels starting at
the bottom of the band and would be happy to land wherever.
>You're sharpening the pencil to just as sharp a point as I.
4:30AM and I can't sleep. Might as well do something useful.
>No, I'm using the Netgear wizard which is wonderfully sensitive, including
>systems with no SSID.
Amazing. I never thought to check if it will detect AP's with SSID
broadcast turned off. I just happen to have a WG511v2 here I can try.
Nifty. It works. It shows a blank SSID but does display the MAC
address, channel number, encryption level, and mode. Thanks. That's
going to be handy.
>> Are you sure about channel 9? It's my understanding that 9 is NOT a
>> default channel.
>
>Yes, Qwest is currently delivering all of their "approved" Actiontec routers
>defaulted to Channel 9. Their older units (b) were all supplied on Channel
>6, but there are only a couple of those here.
Hmmmm.... very odd. I have a Quest Actiontec GT701-WG wireless router
in the office collecting dust. I could swear it was on Ch 1. I'll
check when I can. The URL says the default is Ch 1:
<http://www.avvanta.com/QDSLSupportFAQGT701Wireless>
>That explains one Linksys signal that jumps all over the band, but it mostly
>uses 1,3,6 & 9, the ones that are used most in this area.
I'm not sure, but I don't think that any of the Linksys routers have
that feature. I know Dlink and Buffalo have it. Checking:
<http://www.linksysdata.com/ui/>
Y'er right. WRT54GX4 has automagic channel selection. Looks like
it's also on by default. Aaaagh. Time for another minor crusade
(along with my "secure by default" crusade).
>> Think of the off channel neighbor breaking the CSMA/CA
>> collision avoidance mechanism of 802.11. It works with an on channel
>> jammer.
>
>The hard part is determining at what threshold the weak off-channel signals
>will actually cause a problem.
Time to enable the technobabble option flag. The basic problem is
what the receive declares to be decodeable data and what it considers
to be noise. If the receiver happens to be listening for 802.11g
data, anything that arrives smelling like 802.11b is going to be
treated as noise. Same with the other way around. Same with
everything that's off frequency. Only when the "802.11b
compatibility" sampling window arrives, is it considered data. Same
with Afterburner, SpeedBooster, SuperSpeed, TurboG 125mbps, HSP125,
G+, SuperG 108mbps,
<http://www.dd-wrt.com/wiki/index.php/Afterburner>
I can never remember which ones are 802.11g compatible. Most of the
time, I just turn them all off. Anyway, the one's that are not, are
treated as noise.
One of the really nice things about spread spectrum is processing
gain. For FCC Part 15 DSSS, that's 10dB required minimum. That gives
802.11 a 10dB S/N ratio advantage over any form of inband noise or CW
carrier, which is what makes SS so immune to most forms of jamming. In
theory, it should be possible to have the jamming signal 10dB
*STRONGER* than the desired signal, and still have a functional
system. Unfortunately, the typical demodulators are not quite that
good. I settled for a 0dB signal to jamming ratio because I was lazy,
but also because it's a fairly close approximation. I do have some
signal to jamming ratio test results for some old 802.11 (1 and
2mbits/sec only) cards. I'll see if I can dig them out.
Bingo:
<http://www.ladyada.net/techproj/freshair/doc/stahlberg%20cell%20jamming.pdf>
Section 4 is on 802.11 jamming. Ugh, I gotta chew on this one. Looks
like C/J ratio of +2dB at 11Mbits/sec and the author uses the same
10dB processing gain assumption that I mentioned.
>> 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 means if I choose a channel with a neighboring "b" user, my network
>will slow down to their level. Then it makes a case for using the busier
>but more compatible channel 6 where there are no "b" users.
Pretty much that's what happens. You don't even need to have an
802.11b neighbor to be slowed down. Just turning on the 802.11b
compatibility mode in the AP will slow down the *maximum* 802.11g
thruput from about 25Mbits/sec to about 15Mbits/sec.
<http://wireless.wikia.com/wiki/Wi-Fi#Performance_and_Speed>
Realizing that this is a problem, the chipset manufacturers have added
some intelligence. The sampling period is shortened if it doesn't
hear anything on 802.11b to the point where it's barely noticeable
when running 802.11g thruput benchmarks. However, when the AP does
hear something that smells like 802.11b, it reverts to about a 25-30%
sampling window resulting in slothish thruput.
Also, note that 802.11b sends *ALL* it's management packets at
1Mbit/sec. That's a huge airtime burn. 802.11g packets will simply
have to wait until the 802.11b access points shut up.
>Under that scenerio, It can kill 75% of my packets and still get better than
>5Mb, which is as good as a 1.5Mb DSL connection needs.
Well sorta. If the 75% lost packets were randomly distributed, you
would still get 1.5Mbit/sec thruput. However, interference effects
tend to be synchronous with whatever both systems use for timing,
resulting in long periods of continuous collisions, followed by long
periods of passible transmissions. Since there are two retransmission
mechanisms running (802.11 and IP layer) this will create considerable
exessive traffic, which actually increases the probability of
collisions. The relatively long outages is also what causes the
disconnects that are common in the presence of wireless interference.
I vaguely recall seeing a Matlab 802.11 simulation that was able to
predict the packet loss that could be tolerated to obtain a given
thruput. I'll see what I can excavate.
>> The order of priorities is usually wiring
>> first, wireless a poor second. This is why I like seperate routers
>> and wireless access points.
>
>I decided to leave the router in the upstairs office so that one computer
>can be connected by ethernet cable...one less wireless point to be concerned
>about.
Well, until you mentioned it, I've never considered intentionally
burying the access point in order to reduce interference. The usual
problem is lack of coverage in the house, which I usually solve by
positioning of the wireless access point or with custom antennas.
Somehow, burying the AP in the basement isn't very compatible with
solving the coverage problem. Also, we don't have many basements on
the left coast.
>There's only one. Yes, for a time, Qwest ran out of regular DSL routers, so
>they were sending out wireless routers to customers who did not know they
>had wireless. I'm sorta glad to see they come WEP-enabled seeing as these
>neighbors would be unknowing wi-fi points.
Well, 2wire is one of the few manufacturers that ships their routers
secure by default. Unfortunately, even if their router has WPA and
WPA2 encryption available, it's shipped with only WEP enabled.
>>>-.-. --.- -.-. .... .----
>> C Q C H 1
You don't wanna hear what I think of CW operation.
>Yes, it appears as though I will be sending a CQ to other CHannel 1 users
>after I have determined that it isn't affected by our Uniden cordless
>phones. Uniden claims that they use channels above, below & between
>channels 1, 6 & 11. I bet they just scan for inactive channels starting at
>the bottom of the band and would be happy to land wherever.
Well, the other down side of using Ch 1 is also that it overlaps the
satellite portion of the 13cm ham band. No clue on the Uniden
phone. Watch out for those that use 2.4Ghz in one direction, and
900Mhz or 5.8Ghz in the other. Crossband duplex is cheaper to
impliment than inband.
>I wonder: many WAPs have the possibility to choose the channel
>automatically. My previous 3COM OfficeConnect had that feature and
>my current HP Procurve has that option.
>
>Does it work in choosing the right (best) channel for me?
Hell no. It drove me nuts. The most common problem was random
disconnects of the client. The AP is smart, but the clients are
really stupid. Let's say the AP decides to change channel for some
reason. The clients are suppose to be smart and follow the change.
Nope. All they know is that the access point has "disappeared".
There's no 802.11 command available from the AP that tells all the
clients to simultaneously change channel and continue doing whatever
they were doing. So, the client eventually times out, scans for a new
connection, and reconnects. If the client is REALLY stupid and is set
to connect to any available access point, it may even connect to some
other AP. I've seen it happen. The manufacturers answer to the
problem is to simply raise the requirements for having the AP change
channel. In other words, it switches less often, which effectively
defeats its purpose. When I played with a DLink DI-624 to see how
autochannel select worked, it was so conservative that I literally
couldn't make it switch. My guess is that it requires substantial
packet loss before it will decide to switch channels. I guess this is
a suitable compromise.
Anyway, unless you have absolute control over the client, how it
operates, and how it's configured, automagic channel juggling is an
invitation to have the phone ring at odd hours.
Hmmm.... 5:30AM. Maybe I should get some sleep tonite.
Jeff Liebermann <jeffl@comix.santa-cruz.ca.us> wrote:
>
> [snip very interesting discussion]
>
I wonder: many WAPs have the possibility to choose the channel
automatically. My previous 3COM OfficeConnect had that feature and
my current HP Procurve has that option.
Does it work in choosing the right (best) channel for me?
On Wed, 06 Dec 2006 23:21:39 -0800, Jeff Liebermann
<jeffl@comix.santa-cruz.ca.us> wrote in
<0ddfn2pihch4ra7g1qvmojm681r3r95b2t@4ax.com>:
>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.
Some perhaps, but not all, as shown by data I've previously posted here.
Better g devices seem to be unaffected unless at least one b device is
actually active.
>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.
The drawback, of course, can be excessive hunting and retries. Some
products are truly horrible in this regard, much too aggressive in
cranking speed back up.
>>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.
And on VoIP.
>>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.
But that's still a problem, and the bigger issue is the gain pattern of
typical antennas, which assume horizontal paths, and thus suck in the
upward direction.
--
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>
Jeff Liebermann <jeffl@comix.santa-cruz.ca.us> wrote:
> nobody@nowhere.invalid (Peter Boosten) hath wroth:
>
>>I wonder: many WAPs have the possibility to choose the channel
>>automatically. My previous 3COM OfficeConnect had that feature and
>>my current HP Procurve has that option.
>>
>>Does it work in choosing the right (best) channel for me?
>
> Hell no. It drove me nuts. The most common problem was random
> disconnects of the client. The AP is smart, but the clients are
> really stupid. Let's say the AP decides to change channel for some
> reason. The clients are suppose to be smart and follow the change.
> Nope. All they know is that the access point has "disappeared".
> There's no 802.11 command available from the AP that tells all the
> clients to simultaneously change channel and continue doing whatever
> they were doing. So, the client eventually times out, scans for a new
> connection, and reconnects. If the client is REALLY stupid and is set
> to connect to any available access point, it may even connect to some
> other AP. I've seen it happen. The manufacturers answer to the
> problem is to simply raise the requirements for having the AP change
> channel. In other words, it switches less often, which effectively
> defeats its purpose. When I played with a DLink DI-624 to see how
> autochannel select worked, it was so conservative that I literally
> couldn't make it switch. My guess is that it requires substantial
> packet loss before it will decide to switch channels. I guess this is
> a suitable compromise.
>
> Anyway, unless you have absolute control over the client, how it
> operates, and how it's configured, automagic channel juggling is an
> invitation to have the phone ring at odd hours.
>
> Hmmm.... 5:30AM. Maybe I should get some sleep tonite.
>
Thanks for your answer, Jeff.
I'll keep it better fixed then. My WAP makes it kinda easy for me
with the builtin 'Rogue AP detection' feature (it detects even the ones
without SSID).
>Thanks for your answer, Jeff.
>I'll keep it better fixed then. My WAP makes it kinda easy for me
>with the builtin 'Rogue AP detection' feature (it detects even the ones
>without SSID).
I left out one more reason why automagic channel selection doesn't
fly. Not every type of client can scan channels to follow a channel
changing access point.
I have several "wireless bridge" or "ethernet client" (or whatever)
devices on my neighborhood WLAN/LAN. They're mostly DLink DWL-900AP+
boxes I picked up cheap over the years. Good enough for the tightwad
neighbors so I don't have to rip into their computahs, game machines,
or install USB drivers. These devices require that the connection
details of the wireless access point must be setup in advance and
stored in their NVRAM. That includes SSID, MAC address of the AP,
encryption details, and (insert drum roll), the channel number. None
of these can change automagically. More specifically, the only time
such wireless client bridge devices scan for a connection is when one
manually selects the "site survey" feature. If the AP changes to a
different channel, the connection is simply lost.
John Navas <spamfilter0@navasgroup.com> hath wroth:
Warning. Technobabble flag is still on.
>The drawback, of course, can be excessive hunting and retries. Some
>products are truly horrible in this regard, much too aggressive in
>cranking speed back up.
Yep. I keep wanting to dig into the MIT Roofnet (mesh network)
summary which covers the tradeoff between speed and probability of
packet delivery.
<http://pdos.csail.mit.edu/roofnet/doku.php?id=interesting>
There was one report that had some really interesting things to say
about the tradeoff between speed and reliability.
The issue is simple but the calculations are complex. Slowing the
network down to relatively slow speeds increases the S/N ratio, which
improves the probability of delivering a packet. However, it also
reduces the available bandwidth by the same amount. Overly
simplified, a 1Mbit/sec data packet might have the same probability of
delivery as a 54Mbits/sec connection that fails to deliver the same
packet 53 times, but succeeds on the 54th try. (This isn't
numerically true because the 54Mbit/sec connection has a higher
overhead of management packets, and inter-symbol timing).
In the presense of noise and interference, things change again. A
long 1Mbit/sec packet makes a very big target and has a high
probability the noise and junk are going to trash the packet. At
54Mbits/sec, the packets are much shorter (in time) and therefore have
a lower probability of getting trashed. However, the required S/N
ratio at 54Mbits/sec is MUCH higher than at 1Mbit/sec, making it
easier to trash packets at 54Mbits/sec than a 1Mbit/sec. I think you
can see where I'm heading. Everything affects everything and the math
is messy.
Just to make life complicated, there are two retransmission mechanisms
working. One is at the wireless MAC layer, where the 802.11 protocol
detects delivery failures and retries sending the packet. The other
is at the IP layer, where TCP detects errors and asks for an instant
replay. If you ever see packet loss at the IP layer (using netstat),
you can be sure that the packet loss at the MAC layer is truely
horrible.
The wireless chip manufactories have made their decision with
automatic speed selection that favors the higher speeds. This is a
vote for "damn the retries, full speed ahead". In theory, there's a
highly intelligent processor, running a patented algorithm for
selecting the correct speed. From my observations, it's a crap shoot.
So, we see lots of retransmissions and the sale of lots of add on
antennas to improve the S/N ratio. Wanna greatly improve the range of
your wireless system? Easy, just slow it down.
>And on VoIP.
Yep. Any streaming media hates packet loss because there are no
retransmissions. The current technology is optimized for TCP, where
packet loss can be tolerated to some degree by retransmissions. With
UDP, if a packet is lost, it's lost forever. So, the access points
now have a "wireless multi-media mode". This is a form of QoS that
roughly give priority to UDP packets over TCP. I couldn't find
anything that connects WMM to wireless speed control, but suspect that
there may be some connection in the actual implimentation. It makes
sense to slow things down to improve reliability.
<http://en.wikipedia.org/wiki/Wireless_Multimedia_Extensions>
<http://www.wi-fi.org/files/uploaded_files/wp_1_WMM%20QoS%20In%20Wi-Fi_9-1-04.pdf>
Choosing a Channel? 
Linear Mode
