Difficulty with APs in Jet Hangar

Answered Question
Oct 21st, 2008

My environment is a jet hanger - two actually. Each is about 300 ft wide by 100 ft deep. And they are perhaps 30-40 feet high. A predecessor mounted two Cisco APs in each hanger - mounted them high. They use TKIP for auth. The problem is that users are constantly getting their connections dropped. I verified the channels in use are sufficiently spread apart and they are (say 1 then 11). The site manager wants to throw more APs at the situation but I'm not sure that's the underlying issue. I'm thinking a problem with the metal walls of the hangars, or interference from the plane bodies or perhaps interference from walkie talkies which about there. Any recommendations on how to approach this complaint-ridden Cisco wireless environment? Here's a snippet of some recent logging. The constant Reassociated Key Mgt seems odd.

1

Apr 25 12:45:12.318 UTC

Information

Interface Dot11Radio0, Station 0014.d0d0.03be Reassociated KEY_MGMT[WPA PSK]

3

Apr 25 12:42:25.737 UTC

Information

Interface Dot11Radio0, Station 0014.d0d0.03be Reassociated KEY_MGMT[WPA PSK]

5

Apr 25 12:40:17.899 UTC

Information

Interface Dot11Radio0, Station 0014.d0d0.03be Reassociated KEY_MGMT[WPA PSK]

7

Apr 25 12:38:13.722 UTC

Information

Interface Dot11Radio0, Station 0014.d0d0.03be Reassociated KEY_MGMT[WPA PSK]

9

Apr 25 12:36:06.011 UTC

Information

Interface Dot11Radio0, Station 0014.d0d0.03be Reassociated KEY_MGMT[WPA PSK]

11

Apr 25 12:35:03.041 UTC

Information

Interface Dot11Radio0, Station 0014.d0d0.03be Reassociated KEY_MGMT[WPA PSK]

13

Apr 25 12:32:45.606 UTC

Information

Interface Dot11Radio0, Station 0014.d0d0.03be Reassociated KEY_MGMT[WPA PSK]

15

Apr 25 12:30:41.444 UTC

Information

Interface Dot11Radio0, Station 0014.d0d0.03be Reassociated KEY_MGMT[WPA PSK]

17

Apr 25 12:26:32.620 UTC

Information

Interface Dot11Radio0, Station 0014.d0d0.03be Reassociated KEY_MGMT[WPA PSK]

19

Apr 25 12:25:32.411 UTC

Information

Interface Dot11Radio0, Station 0014.d0d0.03be Reassociated KEY_MGMT[WPA PSK]

I have this problem too.
0 votes
Correct Answer by scottmac about 8 years 1 month ago

Interesting situation, but really, not much different than a warehouse with tall shelves & moving fork lift trucks.

The short version is "creative use of antenna patterns and power adjustments."

This is likely (no way to tell without survey data) a case where you'd use some tight sector antennas, and or "microcell" layouts (low-power, footprint limited, lots of APs).

Reflective spaces tend to be a PITA from a multipath perspective ... so you start thinking "stealth;" i.e., direct the antenna fields such that reflections from major surfaces (the walls, the floor, the ceiling, partitions) do not bounce back into the service area ... or are so weak that they are easily overcome by the primary waves.

If they the customer is looking at throwing whatever money it takes to make it right (send me their names, I'll handle it ... j/k ;-) ) then look at something like moving to 802.11a, which provides many more non-overlapping channels, much less interference, etc ... or something that uses MIMO ... a scheme that takes advantage of multipath.

Constant drop/associate/drop/associate cycles tend to indicate poor coverage |weak signals |interference ... no way to tell without survey data. The only way to fix that is to 1) better antenna coverage 2) adjust power 3) (usually) add more APs in conjunction with 1 & 2.

A good survey tells the tale. Start there.

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Overall Rating: 5 (4 ratings)
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Correct Answer
scottmac Wed, 10/22/2008 - 11:36

Interesting situation, but really, not much different than a warehouse with tall shelves & moving fork lift trucks.

The short version is "creative use of antenna patterns and power adjustments."

This is likely (no way to tell without survey data) a case where you'd use some tight sector antennas, and or "microcell" layouts (low-power, footprint limited, lots of APs).

Reflective spaces tend to be a PITA from a multipath perspective ... so you start thinking "stealth;" i.e., direct the antenna fields such that reflections from major surfaces (the walls, the floor, the ceiling, partitions) do not bounce back into the service area ... or are so weak that they are easily overcome by the primary waves.

If they the customer is looking at throwing whatever money it takes to make it right (send me their names, I'll handle it ... j/k ;-) ) then look at something like moving to 802.11a, which provides many more non-overlapping channels, much less interference, etc ... or something that uses MIMO ... a scheme that takes advantage of multipath.

Constant drop/associate/drop/associate cycles tend to indicate poor coverage |weak signals |interference ... no way to tell without survey data. The only way to fix that is to 1) better antenna coverage 2) adjust power 3) (usually) add more APs in conjunction with 1 & 2.

A good survey tells the tale. Start there.

mmedwid Wed, 10/22/2008 - 21:33

So I went up the hangar for the first time. What I found we very strong singal throughout both 300x100 hangar. 5 bars, 24 to 48Mbps according to Windows wireless widget. So...I pulled up NetStumbler and I could quickly see that the radio channels were spaced too close to each other. South hanger had channel 1 on AP1 and channel 2 on AP2 just 50 feet apart and 40 feet up nothing in between. The other hanger the AP 3 and 4 were programmed for channels 3 & 4 respectively. So...I spread them apart as widely as possible and performance appears to have improved dramatically. It will take experience over the next few days to declare victory. But they have persistent connectivity they haven't seen before.

scottmac Thu, 10/23/2008 - 05:17

Thanks for posting the answer, it help everyone to see how these things ultimately resolve themselves.

Good Luck

Scott

rob.huffman Thu, 10/23/2008 - 05:48

Hey Scott,

Excellent tips (as usual!) +5 points for your always wonderful answers (plus you find a way to add some humor as well ;-)

Cheers!

Rob

dennischolmes Sun, 10/26/2008 - 08:25

Make sure to put those APs on channels 1,6, and 11 only. Doing otherwise will cause you problems down the road. A good linear deployment in this case would be 1,6, 11, 1. This places 2 cells between the APs on channel 1.

mmedwid Sun, 10/26/2008 - 18:42

Thanks for the note Dennis. This is just what I did. The performance for the entire site improved dramatically - extending even beyond the hangars. There had also been problems with roaming from area to area. Spreading out the four APs spectrum-wise appears to have solved those issues as well.

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