a question about channels?

I have a ware house that has 6 AP's. They are broadcasting on several
different bands.... 1, 6 and 11 are all accounted for. Would it be
better to have them all on the same band?

No, you have to separate them so that AP's are on no-overlapping
channels.

For instance, an AP on channel 1 should only be able to see AP's on
channels 6 and 11. Likewise for an ap on channel 6, or channel 11.

If you're getting a lot of interference, I'm working with a company
that has built a wireless networking system that works on a single
channel, and has zero-latency between access points.

If interested, give me a call.

Chris

- - - - - - - - - - - - - - - - - - - -
Christopher M. Hutchison, CEO
NetSteady Communications, Ltd.

Phone: 614-255-5575
Mobile: 614-853-0091
Skype: wifi_chris

http://www.netsteady.cc


200mg wrote:
> I have a ware house that has 6 AP's. They are broadcasting on several
> different bands.... 1, 6 and 11 are all accounted for. Would it be
> better to have them all on the same band?

On 28 Nov 2006 07:08:24 -0800, "200mg" <(E-Mail Removed)> wrote:

>I have a ware house that has 6 AP's. They are broadcasting on several
>different bands.... 1, 6 and 11 are all accounted for. Would it be
>better to have them all on the same band?

You have a problem. Each channel number is 5MHz from the next
channel. However, the 802.11 signal occupies about 22Mhz or about 5
channels wide. Therefore, to prevent interference, the only
non-overlapping channels are 1, 6, and 11. If you pick a channel in
between these, you'll get interference from BOTH channels on either
side. The trick is to limit the radiation pattern from each xmitter
using fancy antenna patterns, allowing you to re-use channels in
different parts of the warehouse. Otherwise, just turn down the tx
power until the interference is reduced. It will never go away
completely, but lower power does help.

See the hexagonal channel pattern mentioned in the Intel Hotspot Guide
in section 5.2.3.1. The link on the Intel web pile went away and I
can't find the replacement. For now, here's my copy:
<http://802.11junk.com/jeffl/crud/Intel%20HotSpot%20Guide.pdf>
I suggest you skim the whole thing as there's quite a bit of really
useful info buried in it.


--
# 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

the situation I have now is that one AP is reporting 87% signal, then
if you walk literally 3 ft in one direction it drops to zero. The 2
closest AP's were xmitting on chan's 1 and 11, the ap in question was
also xmitting on 11, so i changed it to 6, with the same results. I
also tried 11 again with a lower power setting and still had the same
results.

There is apparently alot of power conduit and coming out the top of the
office I am trying to extend this signal to. I am also going top try
some antenna with a higher db, I just find it odd that it goes from
close to 90 to 0 in such a short distance.


Jeff Liebermann wrote:
> On 28 Nov 2006 07:08:24 -0800, "200mg" <(E-Mail Removed)> wrote:
>
> >I have a ware house that has 6 AP's. They are broadcasting on several
> >different bands.... 1, 6 and 11 are all accounted for. Would it be
> >better to have them all on the same band?
>
> You have a problem. Each channel number is 5MHz from the next
> channel. However, the 802.11 signal occupies about 22Mhz or about 5
> channels wide. Therefore, to prevent interference, the only
> non-overlapping channels are 1, 6, and 11. If you pick a channel in
> between these, you'll get interference from BOTH channels on either
> side. The trick is to limit the radiation pattern from each xmitter
> using fancy antenna patterns, allowing you to re-use channels in
> different parts of the warehouse. Otherwise, just turn down the tx
> power until the interference is reduced. It will never go away
> completely, but lower power does help.
>
> See the hexagonal channel pattern mentioned in the Intel Hotspot Guide
> in section 5.2.3.1. The link on the Intel web pile went away and I
> can't find the replacement. For now, here's my copy:
> <http://802.11junk.com/jeffl/crud/Intel%20HotSpot%20Guide.pdf>
> I suggest you skim the whole thing as there's quite a bit of really
> useful info buried in it.
>
>
> --
> # 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 30 Nov 2006 12:17:51 -0800, "200mg" <(E-Mail Removed)> wrote:

>the situation I have now is that one AP is reporting 87% signal, then
>if you walk literally 3 ft in one direction it drops to zero.

Is the AP reporting the signal strength (i.e. using SNMP or some kind
of managment tools) or is the client you're using reporting the 87%
signal?

Also, I don't think my walking in any direction will have an effect on
your signal strength. However, if you do the walking, you might see a
change.

Wireless doesn't work like that. It will fade away, not die suddenly.
The only way I can think of creating such a fast drop off is to have
substantial interference in the area on the same or adjacent channels.
As you get closer to the interference source, the desired signal goes
away. Your system might also be switching to a different access
point, which appears as a loss of signal.

>The 2
>closest AP's were xmitting on chan's 1 and 11, the ap in question was
>also xmitting on 11, so i changed it to 6, with the same results. I
>also tried 11 again with a lower power setting and still had the same
>results.

Fine. Try turning OFF all but one access point and see how the client
signal strength behaves. I think you'll get a really good clue as to
how much interference you're dealing with by testing just one access
point at a time, with the others turn off (that's powered off, not the
ethernet cable unplugged).

>There is apparently alot of power conduit and coming out the top of the
>office I am trying to extend this signal to. I am also going top try
>some antenna with a higher db, I just find it odd that it goes from
>close to 90 to 0 in such a short distance.

I'll pass. You're not supplying any hardware or topology details and
concentrating on the channel layout. While this is important, I
suspect you have bigger problems. I can't suggest much beyond basic
troubleshooting (by elimination) unless I know the hardware, topology,
layout, and setup.

--
# 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 Fri, 01 Dec 2006 05:17:29 GMT, Jeff Liebermann
<(E-Mail Removed)> wrote in
<(E-Mail Removed)>:

>Wireless doesn't work like that. It will fade away, not die suddenly.

Perhaps we should add an item to "Top Ten Fixes to Wi-Fi Problems":

11. It's not what you think -- something else is going on.

>The only way I can think of creating such a fast drop off is to have
>substantial interference in the area on the same or adjacent channels.

Or moving from an open doorway down which the signal has been traveling
into a room with metal in the walls.

>As you get closer to the interference source, the desired signal goes
>away.

Multipath interference?

>... Try turning OFF all but one access point and see how the client
>signal strength behaves. I think you'll get a really good clue as to
>how much interference you're dealing with by testing just one access
>point at a time, with the others turn off (that's powered off, not the
>ethernet cable unplugged).

Never fails to amaze my how many people take the scattergun approach to
troubleshooting, instead of the far more productive step-by-step
approach.

>I'll pass. You're not supplying any hardware or topology details and
>concentrating on the channel layout. While this is important, I
>suspect you have bigger problems. I can't suggest much beyond basic
>troubleshooting (by elimination) unless I know the hardware, topology,
>layout, and setup.

What?! Not willing to waste time in wild guesswork?!

--
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>

Thanks for your replies.

About the topology:

There are 6 AP's spurring from 2 linksys unmanaged switches. The one
in question exists about 80 feet from the office I am trying to get
this signal to extend to.

I am using a symbol scanner with pocket pc2003 on it and some symbol
software on these scanners to test for signal.

Now in my warehouse in Cincy, the signal behaves just as you mention,
it fades away the further you walk from an AP. I can tell this by the
results given to me by the same means I am testing in my Baltimore
warehouse. In Baltimore, I'll be 10 feet in front of this office,
check for signal, it will say %85, i will take literally 3 steps toward
the office and it drops to zero.

It's not re-associating with another ap because while it's up there are
3 visible AP's, then when you get closer by approx. 3 feet it loses not
only the one it's connected to but the other two as well.

There's not necessarily anything different between the two warehouses.
I cant see anything on the ceiling that would interfere with the signal

TIA

I'm using a symbol scanner that checks
John Navas wrote:
> On Fri, 01 Dec 2006 05:17:29 GMT, Jeff Liebermann
> <(E-Mail Removed)> wrote in
> <(E-Mail Removed)>:
>
> >Wireless doesn't work like that. It will fade away, not die suddenly.
>
> Perhaps we should add an item to "Top Ten Fixes to Wi-Fi Problems":
>
> 11. It's not what you think -- something else is going on.
>
> >The only way I can think of creating such a fast drop off is to have
> >substantial interference in the area on the same or adjacent channels.
>
> Or moving from an open doorway down which the signal has been traveling
> into a room with metal in the walls.
>
> >As you get closer to the interference source, the desired signal goes
> >away.
>
> Multipath interference?
>
> >... Try turning OFF all but one access point and see how the client
> >signal strength behaves. I think you'll get a really good clue as to
> >how much interference you're dealing with by testing just one access
> >point at a time, with the others turn off (that's powered off, not the
> >ethernet cable unplugged).
>
> Never fails to amaze my how many people take the scattergun approach to
> troubleshooting, instead of the far more productive step-by-step
> approach.
>
> >I'll pass. You're not supplying any hardware or topology details and
> >concentrating on the channel layout. While this is important, I
> >suspect you have bigger problems. I can't suggest much beyond basic
> >troubleshooting (by elimination) unless I know the hardware, topology,
> >layout, and setup.
>
> What?! Not willing to waste time in wild guesswork?!
>
> --
> 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>

"200mg" <(E-Mail Removed)> hath wroth:

>There are 6 AP's spurring from 2 linksys unmanaged switches. The one
>in question exists about 80 feet from the office I am trying to get
>this signal to extend to.
>
>I am using a symbol scanner with pocket pc2003 on it and some symbol
>software on these scanners to test for signal.

I kinda wanted the maker and model number of the devices (access
points, antennas, clients, hand-helds, etc). The reason is that
different hardware and devices behave differently. For example, a
common way to do a warehouse is to use leaky coax. I know how that
behaves. Another common method is a very directional antenna shooting
down the aisles of the warehouse. That will cause a rapid fade as you
turn a corner or move behind a stack of absorbent or reflective
merchandise.

Basically, you cannot "flood" a warehouse with RF any more than you
can do the same with light. If you put overhead lights on the
ceiling, and then block it with merchandise, you're going to have dark
zones. Same with 2.4Ghz but worse as you will have diffraction,
refraction, reflection, and absorption effects.

What I was hinting is that the channel juggling is important but that
something else is apparently happening. Wireless does not magically
fade away that quickly. I've done troubleshooting jobs like this
before including one big produce storage warehouse from hell. Their
problem was similar to yours. They had too many access points (thanks
to the salesman). Whenever there was a coverage problem, the sales
guy just sold them another access point. 200,000 sq ft and they had
something like 20 access points when I arrived. Nothing worked
because of mutual interference. I reduced it down to about 11 access
points, with very directional antennas to allow me to re-use channels
and to get great coverage of the aisles near the ground level. It
didn't work too well when the warehouse was empty or near the roof,
but that eventually turned into a useless requirement. Management
didn't want workers punching buttons on the radios while 20ft in the
air balanced on the edge of a fork lift. Again, note that the antenna
selection was more important than the interference reduction. When I
was done, only one of the AP's had omni antennas.

In order to get it right, I had to map the coverage area of each
access point individually. It was a tedious pain and I hate doing it.
I had a large sketch of the warehouse floor space. I walked around
with a laptop running Netstumbler, picked a relatively strong signal
level, and drew the approximate contours. Accuracy was awful, but I
got the general coverage area. By combining the plots for each access
point, I was able to demonstrate that some were redundant, some were
useless, and some were mis-located. Management was impressed,
especially when I told them that they could use the AP's I ripped out
in the office area. There are commercial products that do this and/or
attempt to predict coverage.
http://www.ekahau.com

>Now in my warehouse in Cincy, the signal behaves just as you mention,
>it fades away the further you walk from an AP. I can tell this by the
>results given to me by the same means I am testing in my Baltimore
>warehouse. In Baltimore, I'll be 10 feet in front of this office,
>check for signal, it will say %85, i will take literally 3 steps toward
>the office and it drops to zero.

Ok, so you've seen how it's suppose to function. The question is
"what's the difference". I don't think it's channel selection,
although it's possible.

The only way to troubleshoot a complex system is to rip it apart and
build it up from scratch. As I suggested, turn OFF (as in power off)
all but one access point and see how it plays. My guess is that it
will work just fine in the immediate area. Try to get a feel for how
much RELIABLE coverage it has with just one AP. Then start turning on
the others, one at a time. Start with one that's on the same channel
which should give the worst case results. I usually monitor the
traffic with an SNMP managed switch but you indicated that the Linksys
switch was not managed. Oh well. Use file transfer throughput as an
indication of reliability, not signal strength. A highly reflective
environment will give a great signal strength, but crappy thruput.

>It's not re-associating with another ap because while it's up there are
>3 visible AP's, then when you get closer by approx. 3 feet it loses not
>only the one it's connected to but the other two as well.

Ok, that eliminates AP hopping as a problem. Symbol AP's have a
feature called "automatic channel selection". Hopefully, you're NOT
using this feature as it has caused me nothing but grief when I tried
it. If things seem to work just fine one day, but truly awful the
next, look for this "feature".

Also, I have the habit of disabling automatic speed selection. 802.11g
will tend to go for the fastest connection. It will continuously try
to go as fast as it can possibly go. This is great for bloated
applications moving massive files around, but a real PITA when dealing
with small bursts of data. I usually fix the RF speed at 12Mbits/sec
which is more than enough for most interactive apps. A side benefit
is that the usable range tends to improve.

>There's not necessarily anything different between the two warehouses.
>I cant see anything on the ceiling that would interfere with the signal
>
>TIA
>
>I'm using a symbol scanner that checks

Keep going? Which Symbol scanner? 802.11b or 802.11g? Omni antennas
on the AP's? Lots of ways of causing problems.

Incidentally, don't ignore the possible sources of interference. The
same warehouse I previously mentioned had a 2.4GHz cordless phone base
located a few feet from one of the AP's. There was plenty of
interference when the phone was active. Nobody connected the
interaction because different people used the phone and the scanners.
<http://wireless.wikia.com/wiki/Wi-Fi#Interference>

--
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

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

> 11. It's not what you think -- something else is going on.

That which is most obviously correct, beyond all need of checking, is
usually the problem.

Assumption, the mother of all screwups.

Yesterday afternoon, a customer calls to announce that their server is
dead. No screen, no boot, but the drives spin. I arrive and
determine that it's dead. I drag the box (about 50lbs) to my palatial
office, plug it in, and it works. Tinker for an hour and don't find
anything wrong. Drag it back, plug it in, and it's dead. I
eventually find a shorted keyboard. Well, at least I got some good
exercise.

>>The only way I can think of creating such a fast drop off is to have
>>substantial interference in the area on the same or adjacent channels.
>
>Or moving from an open doorway down which the signal has been traveling
>into a room with metal in the walls.

It's not that abrupt, even in a doorway. 2.4Ghz tends to refract from
apertures. However, if there's an interference source through the
doorway, that's very possible.

>>As you get closer to the interference source, the desired signal goes
>>away.
>
>Multipath interference?

No. Frequency selective interference comes and goes as one moves
around. Also, the diversity reception in the access point does a good
job of reducing any such effects.

>Never fails to amaze my how many people take the scattergun approach to
>troubleshooting, instead of the far more productive step-by-step
>approach.

Add me to the list. I tend to do what easiest first, even if I know
it's well down the list of probable culprits. I like to reduce the
laundry list by eliminating the easy stuff first. To the casual
observer, this resembles a disorganized effort. Troubleshooting is
not something the come naturally and certainly not without a good
understanding of how such wireless systems operate. I would not
expect an intelligent beginner to be very good at it. That's why I
often suggest troubleshooting by replacement, which both easy to
accomplish and understand. When we get into complex wireless systems
that include everything from propagation to topology, everything
starts to interact with everything else. That's why I suggested
ripping it apart and build it up in pieces. Problems are far more
obvious when the systems are simplified.

>>I'll pass. You're not supplying any hardware or topology details and
>>concentrating on the channel layout. While this is important, I
>>suspect you have bigger problems. I can't suggest much beyond basic
>>troubleshooting (by elimination) unless I know the hardware, topology,
>>layout, and setup.
>
>What?! Not willing to waste time in wild guesswork?!

Knowing when to give up is the secret of engineering. I guess it also
applies to troubleshooting wireless networks. Juggling channels
might have an effect, but my sense of smell tells me that something
else is happening.

--
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

Jeff Liebermann <(E-Mail Removed)> hath wroth:

>There are commercial products that do this and/or
>attempt to predict coverage.
> http://www.ekahau.com

One more:
http://www.airmagnet.com/products/survey.htm

--
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