Non-Overlapping channels bleed over

Unanswered Question
Oct 26th, 2007

I have multiple AP's in an arena enviornment. Now without violating the rule where an AP is assigned a non-overlapping channel, what is best practice if there are neighboring AP's within close proximity of each other and they are assigned the same channel?

For example, in our press level I have 2 AP's both on channel 11, the suite level below has a few more ap's where the signal bleeds over to the press level and even if some were on channel 1, 6, & 11 there would still be conflicting channels. I hope this questions makes sense.

I have this problem too.
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scottmac Fri, 10/26/2007 - 10:44

Your only real choice, given that there are really only three non-overlapping channels, is to manipulate the power levels of the APs, and (not "or") creative use of different antennas.

This is where understanding "those damn Smith charts" pays off. The Smith charts are the ones that show the radiation pattern from the top view and the side view.

It also helps to understand the types of antennas and the general patterns they usually throw ... a 90 sector will cover 90 degrees laterally, but how far out from the corner so you have to be before you get "into the donut" of the radiation pattern?

This is not a problem you wll likely be able to solve in one fell swoop; expect that you will first decide what kind of antennas to generally fill the critical areas, do a survey, adjust your power levels/cell sizes, then start filling in the blank spots as necessary.

Each step requires a survey to determine adequate coverage. Then adjust the antenna position and / or AP power level.

The LWAPP / WLCs will do automatic power levels & stuff, but they're are better (IMO) to maintain, rather than decide initially, what the power levels should be.

If you get serious overlap, move or change the antenna, reduce the AP power ... add more APs (running at lower power = smaller cell size).

(all assuming you can't switch to 802.11a, which has many more non-overlapping channels)

This is the kind of situation where the big-ticket analyzers pay for themselves.

Good Luck


rochoa8aeg Sat, 10/27/2007 - 08:29

Being that there are 11 channels I could theoretically use would it be possible to place some of the AP's on the remaining channels so as to utilize all the channels, will this minimize interference or will I encounter a whole new set of issues?

scottmac Sat, 10/27/2007 - 12:55

For all other channels (other than 1, 6, 11), they will interfere with the two adjacent (so-called) non-overlapping channels (i.e., channels 2,3,4, and 5 will interfere, and be interfered, by/with channels 1 and 6 ... 7,8,9,10 will do the same with channels 6 & 11).

You really get three channels (1, 6, 11) that don't interfere with each other, each channel is defined in 5MHz increments, but actually occupies 25Mhz of bandspace (i.e., 5 channels wide).

The issues you will see id degraded performance across the board as evey transmission interferes with every other transmission in the same bandspace.

Smaller cells, lower power, clever use of the right kind of antennas; that's all you have to work with.

Good Luck


rochoa8aeg Sun, 10/28/2007 - 12:03

Thank you for your response, brief to the point and best of all it makes total sense. I guess its back to the drawing board for me.

Just one more question though, so does this mean that if I have several AP's in my guest entrance which belong to Ticketmaster for scanning tickets and several of my AP's in the same lobby, I will encounter interference even if they are both broadcasting different SSID's?

rob.huffman Sun, 10/28/2007 - 07:03

Hi Scott,

Nice to see such an excellent answer here! 5 points for understanding "those Damn Smith charts" My eyes tend to glaze over whenever I try to determine these patterns. Good on you :)

For Rene,

How many AP's below in the Suite Level are we talking about here? There should be some way to stagger the Channels and tweak the power as Scott mentioned to mitigate this problem. Maybe in the Press Box use 1 and 11 on the closest one below use 6 etc. etc. I know this is overly simplified but you have to start somewhere. Having the two in the Press Box using the same channels is just starting off with a design that won't work :(

Just my two cents,


rochoa8aeg Sun, 10/28/2007 - 12:15

We have 3 suite levels each with 10 AP's that circulate around the entire corridor of the arena. On the top suite level C our AP's have the rubber dipole antennas, suite level B and A below have the AIR ANT-3213 antennas.

All AP's are controlled by a WLC4404 and have the power level set to 1. I did attempt to stagger them on Friday but I was only taking into account the horizontal neighbors and not the vertical neighbors above and below. THe 2 in the pressbox I did change already to how you had suggested, as a matter of fact this is where I noticed the first symptoms of a problem because members of the press were able to see the strong signal strength but were having issues associating. I think because of the interference from the AP's below.

scottmac Sun, 10/28/2007 - 15:09

Thanks for the points Rob! Things have been pretty busy the last couple months.

Planning RF fields is a three-dimensional thing, more or less dependent on the construction materials laterally and vertically.

Given that there are only the three channels (for 802.11g), you have no option other than stacking and staggering, like:

level3: 1--6--11--1--6--11--1--6--11

Level2: 6--11--1--6--11--1--6--11--1

Level1: 11--1--6--11--1--6--11--1--6

With the only real variation being the type of antennas you choose, the topology of the spaces, and the power levels of the APs. There's really no other way to do it (using only 802.11g).

NOW, what I'd recommend that you recommend is to add the additional capability for 802.11a. Many/most computer systems have dual band capability, most (Cisco) APs are rigged for dual band.

So light up that 802.11a channel and encourage the folks that can to move up there. It's an easy sell: more bandwidth ("sorta" ...but we're marketing here so outright lies are generally permitted), less interference, and better security (from the aspect of fewer hackers work this band because there's not enough "interesting" stuff on it, etc.).

The benefit to you is that you would have 40 non-overlapping channels ... much easier to plan.

I understand that you have some equipment and users that must remain on 802.11g (try to forbit B if you can, it's making the system slower and reducing your availability), but usually if you offer / encourage those with the capability to move to the better band, they'll take it.

Otherwise, start looking for places where you could say " You know, if I had an antenna with *this* kind of pattern, I could ..." because there probably is an antenna, somewhere, with that radiation pattern.

What you can't get in pattern, you can make up for with barriers, reflectors, etc.

Good Luck


dennischolmes Tue, 10/30/2007 - 05:42

Not a huge sales pitch here but if you moved to the Cisco Unified Wireless network with a Wireless LAN Controller, the controller would configure the power and channel assignments of each AP automatically and you wouldn't need to mess with that.

scottmac Tue, 10/30/2007 - 07:53

IMO, It's usually better to manually configure the system and let the "automatic" systems maintain the operation.

Starting and running it in "Auto" means you letting some programming weenie make your choices for you ... and you know how THOSE PEOPLE are ..... ;-}




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