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I just started in the IT department of a small company (200 users) and it's clear that one of the main problems that is driving everyone crazy is the spotty nature of the wireless connectivity throughout the office, particularly in certain conference rooms. This is a huge problem because the connection often drops during important presentations to clients.

I was hired to help ease the load on the existing IT admin, who has done a great job, but is overloaded with many other tasks to deal with. So I would like to try to help out with this wireless issue. I am looking for advice on the best way to solve this problem--a realistic troubleshooting methodology that does not require me to spend any money. So far, I've experimented with Ekahau Heat Mapper, which is free and helps create a site survey. But I'm not exactly sure what I'm looking for or if there are other programs/tools/methods I should try as well. Any advice would be greatly appreciated.

[Some background: The wireless setup consists of an HP ProCurve Mobility MSM (710?) controller that controls 10 access points throughout the building. There are three virtual wireless networks configured on the controller: one seems to be a default that cannot be changed, one is for internal employees and authenticates via Active Directory, and the third is a guest network for visitors. When I use HeatMapper, these show up as three different SSIDs, with different MAC addresses, all on the same channel. At first I thought maybe this would cause interference, but this seems to be the way the controller works;apparently, it automatically configures the channels to avoid interference from the other APs on the network.]

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Is it spotty as in one moment the signal is strong and the next it's gone, or spotty as in it works fine at this end of the table but you move ten feet and there's no signal? Also does that controller have any logging/SNMP/web interface, anything that you can look at to see if it's logging errors? And have you verified that the network connection from the switch to the AP is good, using a cable tester, maybe check that something isn't wonky on the switch? –  Bart Silverstrim May 3 '10 at 18:35
    
It's spotty as in, the signal is plenty strong throughout the conference room for both the employee and guest networks. But the wireless connection will just drop out while you're sitting there. My crude site survey indicates that there my NIC can see the closest AP (channel 2) with a SNR of 60, and it is also seeing more distant APs with the same SSID on Channels 1 (SNR 30) and 3 (SNR 28). Obviously these are some of our other APs on the same SSID, but could they be causing interference because their channels (1 and 3) are so close to the Channel 2 AP that is right near the conf room? Thanks. –  johnnyb10 May 3 '10 at 19:23
    
There is no "right answer" here without some more data. How big is the site? What kind of obstructions are you facing? –  joeybaker May 4 '10 at 16:16
    
It's a fairly open Office layout with no obstructions in the most problematic areas. I think the problem may have something to do with nearby AP signals (on the same SSID) being too strong and the channels being too close. –  johnnyb10 May 7 '10 at 19:45

4 Answers 4

up vote 2 down vote accepted

If you are using 2.4Ghz WLAN protocols then Channels 1,2 & 3 are too close to be used in a cell type layout like this. You want to use Channels 1,6 & 11 - Cisco have a good explanation of the issues in this article on the 2.4Ghz channel overlap problem and cell coverage strategies.

The primary effect of having channel overlap is poor throughput but the secondary effect will be that the various WLAN auto connect\zero-config services on your client PC's will be much more likely to drop an active connection and attempt to connect to an alternate. This behavior has been significantly improved in recent OS's but anything older than Windows 7 or recent OSX is likely to require some work tuning the client NIC settings to work optimally in a large distributed office WLAN like this if the channels are not widely separated. One other common source of trouble that I've come across were incompatibilities between the access point firmware and the client WLAN NIC settings - things like power management (aka CAM, Continuously Aware Mode) caused issues between early Intel Centrino WLAN adaptors and Cisco infrastructure that caused similar issues to those you describe. That issue was resolved years ago but there have been others and it's worth checking that there are no known issues with your specific client WiFi adapters and your AP's.

Things are much better in terms of the number of channels and the separation between channels in the 5Ghz spectrum - if your infrastructure supports 5Ghz 802.11n\a and the clients support it then set 5Ghz 802.11n as the preferred option for clients (or 802.11a which is only 5Ghz).

One other thing that is worth remembering is that there is a lot of junk in the 2.4Ghz spectrum. Try to get a basic spectrum analyser in to check that there isn't some other ambient noise source leaking into your environment, that can cause the sort of symptoms you describe too.

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I suspected that channel overlap is an issue, but during a call with an HP rep, he said that would be an issue with wireless b, but not g. Now, thanks to the Cisco article you linked to, I see that the channel problem is likely WORSE with 802.11g. We have an HP-certified engineer (who seems to be on the ball) coming in next week so I think that will help answer some of these questions. I'm betting he winds up manually separating those channels instead of letting the controller automatically choose three adjacent channels for nearby APs. Thanks very much. –  johnnyb10 May 7 '10 at 19:44

It sounds to me like you just need more access points, and to make sure the ones you have are positioned correctly. There is almost nothing that doesn't degrade the signal. The more wall the signal goes through, the worse the signal.

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Thanks, I have a feeling that we do need more access points in general; I guess I'm just wondering why I can be sitting in the conference room in which the AP is located and still get dropped. I would think that the signal is not an issue. –  johnnyb10 May 3 '10 at 19:52
    
I would agree, but I am not there. Does your client software show signal strength? Are your AP's automagically picking channels? If so I would change that. I would only re-assign channels if needed, and the AP's that shared channels would be as distant from one another as possible. –  dbasnett May 4 '10 at 13:07

Without spending any money, chances are your only real option is to move APs to attempt to get better signal. One possible option is to consider areas where you don't need wireless coverage and move APs from there to a more important area.

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Can you position your laptop with a direct line of sight/very close to the AP in question and test if it drops with 80%+ signal strength?

If it still drops with great signal strength/LOS, it seems logical to rule out out that signal attenuation is causing the drop. You could then:

  • Physically switch that AP with another and run the same test.
  • If still dropped: try manually changing the channels. (dbasnett)
  • Still fail: you could switch that AP's cable location with a working AP's port on the switch
  • Still dropping: could be some equipment causing interference, especially if your company does some kind of manufacturing or employs the use of old microwaves/wireless phones. Can investigate using wispy (starts @ $99 USD - http://www.metageek.net/products/wi-spy-24i)
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