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New Member

Signal strength oscillating up to 20dbs


I have been receiving complaints for a period of time from one of our sites concerning the connection stability. There are 18 c1130 series access points on site with both 11g and 11a turned on. The switches we are using at the site are ce500s with POE.

I've noticed using Netstumbler and inSSIDer that the signal strength (dB) is yoyoing up and down for all of the access points.

See also the attached graph for illustration. The time index is along the bottom and the signal strength is along the side. This graph only shows three of the APs.

I can't imagine that this is normal and I'm thinking, especially for those users who are near the outside range of the APs, that this is the reason they are loosing their connections.

This is the first time I've noticed the APs acting like that. We've 300+ sites with cisco APs and i've been on the team for 2 years.

Any ideas as to what the reason is?

If I had to guess I'd say it was power related. But I don't see the APs complaining in sho log.

Other than that, this is a downtown location. There are 18 APs on site and another friendly 20 (estimate) neighbouring APs. It's not pretty for 802.11g.

But we have 802.11a enabled as well and those radios show the same yoyoing signal.

Thanks for any advice,


Hall of Fame Super Gold

Re: Signal strength oscillating up to 20dbs

300+ sites? Do you have a WLC?

New Member

Re: Signal strength oscillating up to 20dbs

No, no Wireless LAN Controller for that site; they are autonomous APs.

Re: Signal strength oscillating up to 20dbs

Please tell me a little about your operating environment. What type of building? What type walls? Room number and sizes. What are your local neighbors? Microwaves present? 2.4ghz phones or headsets? I am looking to verify the layer 1 aspect of your network. You could be seeing periodic periods of time where high levels of noise or interference are hurting your SNR or in very rare cases this might be multipath. Multipath to this size is very very rare however. You might also look at PoE issues around your switches. Try turning off CDP on the ports that APs are attached to. Even try an AP on an exteranl PoE brick. The symetry of your signal strength in the attachment makes me think physical power to the AP is the issue.

New Member

Re: Signal strength oscillating up to 20dbs

It's an old building with bad bad walls. Big 30x20ft open rooms with center corridors. One interior wall + 20 feet from AP gives -70 dB for example.

There are microwaves. 2.4ghz phones are possible as well. Blue tooth devices such as writing tablets. We are surrounded by neighbours with their home 802.11g. The spectrum analyzer shows a messy mess.

The building has three floors and three wings off the main building. It's a thin coverage installation... so many users don't have a choice but to connect to the 11g radios.

The site tech knows all this of course; it's a terrible environment for 11g. And I normally would not feel the need for further investigation. There is a specific user area where the reports are repeated; and even though they have been given denser coverage to compensate the reports persist.

But it's really those signals going up and down that have me curious. Since I had never noticed that before I was wondering if there is a problem.


During my test which produced the attachment I had the spectrum analyser going as well. The building was empty except for janitorial staff and myself.

Although I did not see anything in the spectrum analysis that looked like a microwave or 2.4ghz phone (and I'd never be able to pick out blue tooth in that mess), still the signal yoyoing is consistent to every 5 seconds... I found the number of blips is 12 blips per minute with regular interval in the test that produced the attachement.

With the thin coverage and bad walls, those signal blips drops to below -85dB for those users I would normally consider in range.

... I hope it's not just normal dead air when the AP doesn't have anything to say... :)

Re: Signal strength oscillating up to 20dbs

I really think it is power related. DO you have the ability to call in the states? You can email me at"> I will then email you back my contact info. I am really intrigued by this graph myself. I work closely with Cisco's BU and this is the first time I have seen this. I have an idea that it is a bad poe power supply in the CE500 or possibly a ac power problem on the electrical circuit the switch is using for power. Possibly a reversed ground or power on the neutral.

New Member

Re: Signal strength oscillating up to 20dbs


Was this ever resolved? We have an AR1200, and the signal is performing the same, up and down. And clients get periodically disconnected. I am going to swap AC adapters today.

New Member

Re: Signal strength oscillating up to 20dbs

No, never officially. It's one of those work items that just got pushed to the back burner, then off the stove completely.

New Member

Re: Signal strength oscillating up to 20dbs

This from the onsite tech:

"Nope she is having no problems at all. I fixed a few things, tried another driver, and she hasn't had a problem since."

This suggests the comlaint and the observation may not be related.

I'll go back onsite (probably in a couple of days) and check to see if the graph I get is the same... I think I'll bring several models of laptops... as I recall I got inconsistent results with InSSIDer depending on which model I ran it on.

New Member

Re: Signal strength oscillating up to 20dbs

I don't think our issue is driver related. I can watch the signal fluctiate on my IBM notebook running WirelessMon. And can watch the signal strength go up and down on various other notebooks, and PDA's connect to this AP.

New Member

Re: Signal strength oscillating up to 20dbs

Exactly, even though I noticed the graph while I was troubleshooting for this one user, this users problem may have been something else.

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