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Distance between WiFi access points
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Vladimir Vassilevsky
Mar 26, 2013, 7:30:26 PM3/26/13
to
There are two WiFi access points (802.11b/g/n, 2.4GHz, SOHO type); one
operating on channel 1, the other on channel 12. Considering typical
selectivity and out of band emission, what should be minimum distance
between the access points so they won't jam each other?
Vladimir Vassilevsky
DSP and Mixed Signal Designs
www.abvolt.com
operating on channel 1, the other on channel 12. Considering typical
selectivity and out of band emission, what should be minimum distance
between the access points so they won't jam each other?
Vladimir Vassilevsky
DSP and Mixed Signal Designs
www.abvolt.com
Ecnerwal
Mar 26, 2013, 7:42:44 PM3/26/13
to
In article <MtWdnV00Xt8Mss_M...@giganews.com>,
I've had them sitting within a foot without problems (channel 12,
really? Mine only go 1 to 11 unless they are the ones with the higher
number 5 ghz channels.)
Unless there is a particular need to do that, I usually avoid it, but
that's engineering caution, not any actual "jamming" going on.
--
Cats, coffee, chocolate...vices to live by
Please don't feed the trolls. Killfile and ignore them so they will go away.
really? Mine only go 1 to 11 unless they are the ones with the higher
number 5 ghz channels.)
Unless there is a particular need to do that, I usually avoid it, but
that's engineering caution, not any actual "jamming" going on.
--
Cats, coffee, chocolate...vices to live by
Please don't feed the trolls. Killfile and ignore them so they will go away.
Jeff Liebermann
Mar 26, 2013, 8:58:36 PM3/26/13
to
On Tue, 26 Mar 2013 18:30:26 -0500, Vladimir Vassilevsky
<nos...@nowhere.com> wrote:
>There are two WiFi access points (802.11b/g/n, 2.4GHz, SOHO type); one
>operating on channel 1, the other on channel 12. Considering typical
>selectivity and out of band emission, what should be minimum distance
>between the access points so they won't jam each other?
Are you sure it's Ch 12 and not Ch 11? In the USA, there are only 11
<nos...@nowhere.com> wrote:
>There are two WiFi access points (802.11b/g/n, 2.4GHz, SOHO type); one
>operating on channel 1, the other on channel 12. Considering typical
>selectivity and out of band emission, what should be minimum distance
>between the access points so they won't jam each other?
channels.
For 802.11b/g, channels 1 and 12 are essentially non-overlapping so
any distance where the RF doesn't produce overload (blocking) of the
receiver is adequate. Even 40 Mhz 802.11n is ok, because that
channels again do not overlap. I've stacked routers with built in
antennas on top of each other and they still manage to function
normally. However, a safe distance would be about 3 ft using
horizontally spaced omnidirectional vertical antenna. Directional
antennas will need to be on a case by case basis. If you want me to
grind better numbers, please disclose the tx power output, chipset,
and antenna description.
However, I can offer a rough guess(tm) based on commodity 802.11g
hardware:
- Tx +15dBm into a 2dBi rubber omni antenna.
- Rx starts to overloads at about -20dBm anywhere in the 2.4GHz band
(because there's no RF front end channel filter).
- Rx antenna is another 2dBi rubber omni.
Grinding numbers:
Tx Tx ant Path loss Rx ant Rx
+15dbm + 2dB - XX dB +2dB + -20dBm (approx)
Plugging into:
<http://www.proxim.com/products/knowledge-center/calculations/calculations-system-operating-margin-som>
and guessing various values for the distance, until I get a -20dBm Rx
signal level, the distance is approximately 0.0005 miles or 2.6ft.
(Note that Rx sensitivity value in the calculator is irrelevant).
Note that overload (blocking) is NOT the same as co-channel or
adjacent channel interference. With blocking, the receiver front end
rectifies and amplifies the RF until something downstream saturates
and distorts the modulation sufficiently to make the receiver appear
to be comatose or deaf to desired signals. It need not be spread
spectrum and can be an unmodulated carrier. This blocking RF signal
can be anywhere in the receiver front end bandpass. Considering the
rather wide bandwidth of commodity wi-fi ceramic filters, anywhere
between 2.4000 and 2.4835 will suffice. If you have a 2.4GHz RF
source, you can simulate the problem by attaching an antenna to the
generator, and play jammer to a wireless link running some manner of
performance benchmarking software, such a iPerf or Jperf. As the
blocking signal level increases, you'll see a rather abrupt drop in
thruput.
--
Jeff Liebermann je...@cruzio.com
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558
Vladimir Vassilevsky
Mar 26, 2013, 10:42:47 PM3/26/13
to
On 3/26/2013 7:58 PM, Jeff Liebermann wrote:
> On Tue, 26 Mar 2013 18:30:26 -0500, Vladimir Vassilevsky
> <nos...@nowhere.com> wrote:
>
>> There are two WiFi access points (802.11b/g/n, 2.4GHz, SOHO type); one
>> operating on channel 1, the other on channel 12. Considering typical
>> selectivity and out of band emission, what should be minimum distance
>> between the access points so they won't jam each other?
>
> On Tue, 26 Mar 2013 18:30:26 -0500, Vladimir Vassilevsky
> <nos...@nowhere.com> wrote:
>
>> There are two WiFi access points (802.11b/g/n, 2.4GHz, SOHO type); one
>> operating on channel 1, the other on channel 12. Considering typical
>> selectivity and out of band emission, what should be minimum distance
>> between the access points so they won't jam each other?
>
> For 802.11b/g, channels 1 and 12 are essentially non-overlapping so
> any distance where the RF doesn't produce overload (blocking) of the
> receiver is adequate.
Transmit channel produces substantial spill into other cnannels, as well
> any distance where the RF doesn't produce overload (blocking) of the
> receiver is adequate.
as wideband white noise. That feeds directly into receiver and could
degrade noise figure. The selectiveness of the receiver is limited also.
I don't have the numbers however I guess the receiver attenuation
between 1 and 11 is 20dB or so.
> Even 40 Mhz 802.11n is ok, because that
> channels again do not overlap. I've stacked routers with built in
> antennas on top of each other and they still manage to function
> normally.
reduction in throughput.
> However, a safe distance would be about 3 ft using
> horizontally spaced omnidirectional vertical antenna.
noticeable?
WiFi networks in the area.
VLV
Jeff Liebermann
Mar 27, 2013, 3:00:05 AM3/27/13
to
On Tue, 26 Mar 2013 21:42:47 -0500, Vladimir Vassilevsky
11 (but are allowed lower power on ch 12 and 13).
>Transmit channel produces substantial spill into other channels, as well
sin(x)/(x) waveform on a spectrum analyzer. It does on forever and
never really hits zero. What makes it hit zero are ceramic bandpass
filters found in some (not all) consumer wireless devices, and
requirements that the transmitter conform to the FCC spectral mask:
<http://www.rfcafe.com/references/electrical/wlan-masks.htm>
Ch 1 is centered on 2412 MHz. Ch 12 is centered on 2467 MHz.
The difference in frequencies is 55MHz. Half that is 22.5MHz.
According to the 802.11g spectral mask, the noise should be about
-22dB down. By the time it gets to the center of the other channel,
the noise should be -40dB down.
One of the nasty problem with direct sequence spread spectrum is that
it doesn't take much of a signal to degrade the BER (bit error rate)
and thruput. My guess(tm) is that anything strong than about -6dB
below the desired signal is going to appear as some form of
degradation, either in BER, association speed, or thruput. Using my
-6dB guess, that means that the noise from the other xmitter can be
-34dB down before causing problems. Using the same equipment,
calculations, and guesses as in the previous example calculation, and
throwing in a receiver sensitivity of -68dBm at 54Mbits/sec from:
<http://wireless.navas.us/index.php?title=Wi-Fi#Link_Calculations>
I get a distance of 0.0025 miles or 13ft, at a fade margin of about
34dB.
With 802.11n, if your radios have adaptive beamforming, you can pickup
some additional isolation. The radio will recognize the emissions
from the other radio as noise, and insert a null in the antenna
pattern in that direction. Effectiveness of such a scheme varies
significantly with reflections and implementation.
<http://www.theruckusroom.net/2010/12/beamforming-in-all-its-glory.html>
>> Even 40 Mhz 802.11n is ok, because that
>> channels again do not overlap. I've stacked routers with built in
>> antennas on top of each other and they still manage to function
>> normally.
>
>They seem to work in ideal tabletop situation; with substantial
>reduction in throughput.
It depends on what you're trying to accomplish by stacking routers. In
general, it's a bad idea. However, the typical indoor wireless
environment is so bad, that most wireless routers spend their life
operating at the low end of the speed range due to perceived
interference, where any further reduction is unlikely.
>> However, a safe distance would be about 3 ft using
>> horizontally spaced omnidirectional vertical antenna.
>
>What would be the distance that the degradation of NF wouldn't be
>noticeable?
According to my above guesswork, about 13ft for 20MHz modulation
between channels 1 and 12. Seems a bit high. From my experience,
between ch 1 and 11, about 8-10 ft. It should be easy enough to test.
Start a file transfer between one access point and a laptop and watch
the transfer speed. Throw another access point and client laptop into
the mix, and move the access points together until something changes.
+20dBm (100 milliwatts) is not very powerful by todays standards and
is not a problem. The common AT&T branded 2wire 2701HG-B wireless
router belches +26dBm (400mw). There are illegal devices available
online that produce +30dBm (1 watt) and more. The problem is that
these devices produce an "alligator" which is an animal with a big
mouth and small ears. A +30dBm (1 watt) access point can be heard
over a much further distance than the +15dBm laptop with which it's
communicating. The laptop can easily hear the access point, but the
access point cannot hear the laptop. Beyond the range of the laptop,
there's a rather large area where nobody can reliably connect using a
+15dBm transmitter, where the +30dBm access point is polluting the
area preventing other users, of other access points, from using their
systems.
Also, if you're dealing with an overload (blocking or desensitization)
problem caused by the proximity to a high power wi-fi xmitter, such
excessive power levels will require additional separation.
<nos...@nowhere.com> wrote:
>On 3/26/2013 7:58 PM, Jeff Liebermann wrote:
>> On Tue, 26 Mar 2013 18:30:26 -0500, Vladimir Vassilevsky
>> <nos...@nowhere.com> wrote:
>>
>>> There are two WiFi access points (802.11b/g/n, 2.4GHz, SOHO type); one
>>> operating on channel 1, the other on channel 12. Considering typical
>>> selectivity and out of band emission, what should be minimum distance
>>> between the access points so they won't jam each other?
>>
>> For 802.11b/g, channels 1 and 12 are essentially non-overlapping so
>> any distance where the RF doesn't produce overload (blocking) of the
>> receiver is adequate.
Are you sure it's Channel 12? In the US, we only have Channels 1 thru
>On 3/26/2013 7:58 PM, Jeff Liebermann wrote:
>> On Tue, 26 Mar 2013 18:30:26 -0500, Vladimir Vassilevsky
>> <nos...@nowhere.com> wrote:
>>
>>> There are two WiFi access points (802.11b/g/n, 2.4GHz, SOHO type); one
>>> operating on channel 1, the other on channel 12. Considering typical
>>> selectivity and out of band emission, what should be minimum distance
>>> between the access points so they won't jam each other?
>>
>> For 802.11b/g, channels 1 and 12 are essentially non-overlapping so
>> any distance where the RF doesn't produce overload (blocking) of the
>> receiver is adequate.
11 (but are allowed lower power on ch 12 and 13).
>Transmit channel produces substantial spill into other channels, as well
>as wideband white noise. That feeds directly into receiver and could
>degrade noise figure. The selectiveness of the receiver is limited also.
>I don't have the numbers however I guess the receiver attenuation
>between 1 and 11 is 20dB or so.
The xmit spectra for direct sequence spread spectrum looks like a
>degrade noise figure. The selectiveness of the receiver is limited also.
>I don't have the numbers however I guess the receiver attenuation
>between 1 and 11 is 20dB or so.
sin(x)/(x) waveform on a spectrum analyzer. It does on forever and
never really hits zero. What makes it hit zero are ceramic bandpass
filters found in some (not all) consumer wireless devices, and
requirements that the transmitter conform to the FCC spectral mask:
<http://www.rfcafe.com/references/electrical/wlan-masks.htm>
Ch 1 is centered on 2412 MHz. Ch 12 is centered on 2467 MHz.
The difference in frequencies is 55MHz. Half that is 22.5MHz.
According to the 802.11g spectral mask, the noise should be about
-22dB down. By the time it gets to the center of the other channel,
the noise should be -40dB down.
One of the nasty problem with direct sequence spread spectrum is that
it doesn't take much of a signal to degrade the BER (bit error rate)
and thruput. My guess(tm) is that anything strong than about -6dB
below the desired signal is going to appear as some form of
degradation, either in BER, association speed, or thruput. Using my
-6dB guess, that means that the noise from the other xmitter can be
-34dB down before causing problems. Using the same equipment,
calculations, and guesses as in the previous example calculation, and
throwing in a receiver sensitivity of -68dBm at 54Mbits/sec from:
<http://wireless.navas.us/index.php?title=Wi-Fi#Link_Calculations>
I get a distance of 0.0025 miles or 13ft, at a fade margin of about
34dB.
With 802.11n, if your radios have adaptive beamforming, you can pickup
some additional isolation. The radio will recognize the emissions
from the other radio as noise, and insert a null in the antenna
pattern in that direction. Effectiveness of such a scheme varies
significantly with reflections and implementation.
<http://www.theruckusroom.net/2010/12/beamforming-in-all-its-glory.html>
>> Even 40 Mhz 802.11n is ok, because that
>> channels again do not overlap. I've stacked routers with built in
>> antennas on top of each other and they still manage to function
>> normally.
>
>They seem to work in ideal tabletop situation; with substantial
>reduction in throughput.
general, it's a bad idea. However, the typical indoor wireless
environment is so bad, that most wireless routers spend their life
operating at the low end of the speed range due to perceived
interference, where any further reduction is unlikely.
>> However, a safe distance would be about 3 ft using
>> horizontally spaced omnidirectional vertical antenna.
>
>What would be the distance that the degradation of NF wouldn't be
>noticeable?
between channels 1 and 12. Seems a bit high. From my experience,
between ch 1 and 11, about 8-10 ft. It should be easy enough to test.
Start a file transfer between one access point and a laptop and watch
the transfer speed. Throw another access point and client laptop into
the mix, and move the access points together until something changes.
is not a problem. The common AT&T branded 2wire 2701HG-B wireless
router belches +26dBm (400mw). There are illegal devices available
online that produce +30dBm (1 watt) and more. The problem is that
these devices produce an "alligator" which is an animal with a big
mouth and small ears. A +30dBm (1 watt) access point can be heard
over a much further distance than the +15dBm laptop with which it's
communicating. The laptop can easily hear the access point, but the
access point cannot hear the laptop. Beyond the range of the laptop,
there's a rather large area where nobody can reliably connect using a
+15dBm transmitter, where the +30dBm access point is polluting the
area preventing other users, of other access points, from using their
systems.
Also, if you're dealing with an overload (blocking or desensitization)
problem caused by the proximity to a high power wi-fi xmitter, such
excessive power levels will require additional separation.
Jeff Liebermann
Mar 27, 2013, 3:07:07 AM3/27/13
to
On Wed, 27 Mar 2013 00:00:05 -0700, Jeff Liebermann <je...@cruzio.com>
wrote:
><http://www.rfcafe.com/references/electrical/wlan-masks.htm>
>Ch 1 is centered on 2412 MHz. Ch 12 is centered on 2467 MHz.
>The difference in frequencies is 55MHz. Half that is 22.5MHz.
That should be 27.5MHz.
>According to the 802.11g spectral mask, the noise should be about
>-22dB down.
and that should be -30dB down.
>By the time it gets to the center of the other channel,
>the noise should be -40dB down.
I never could do math in my head while talking on the phone and
filling out the online warranty return forms for yet another failed
hard disk drive.
wrote:
><http://www.rfcafe.com/references/electrical/wlan-masks.htm>
>Ch 1 is centered on 2412 MHz. Ch 12 is centered on 2467 MHz.
>The difference in frequencies is 55MHz. Half that is 22.5MHz.
>According to the 802.11g spectral mask, the noise should be about
>-22dB down.
>By the time it gets to the center of the other channel,
>the noise should be -40dB down.
filling out the online warranty return forms for yet another failed
hard disk drive.
Martin Brown
Mar 27, 2013, 3:51:06 AM3/27/13
to
signals much at all you can operate them stacked one on top of the other
subject to overheating. Try the experiment to be certain but I would
guess that even 1m apart would be enough that they didn't diminish
effective throughput to a measureable extent. You will obviously have to
load both networks hard and simultaneously.
http://en.wikipedia.org/wiki/File:NonOverlappingChannels2.4GHzWLAN-en.svg
As a rule of thumb you need to be at least 5 channels apart from any
other strong WiFi signals which means that since round here almost
everything BT defaults to channel 1 that 6 or 11 are good choices.
--
Regards,
Martin Brown
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