Channel Numbers and AP locations

Unanswered Question
Apr 23rd, 2007

I've just been brought into a problem that is happening at one of our warehouses. We have 802.11b on our forklifts to get 'pick' information. A few months back, we installed Cisco Aironets throughout the warehouse as a pilot. Since then, all devices that attach through those devices are dropping their telnet sessions 'randomly' (RST). In the packet trace it looks like there's a retransmission right before the RST which kills their session and the unix admin has to killed their session before they can reconnect.

In looking at the configuration and maps of the building I noticed that the channels on the aironet AP's alternate throughout the building. It could be possible that one host would get the same channel from two different APs if they are equal distance between the two.

Is it a good idea to duplicate channels on APs in the same area?

I have this problem too.
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srosenthal Mon, 04/23/2007 - 11:23

You want to alternate you channel assignments to avoid overlapping the same channel. The ideal is to have about 10-15% overlap with non-overlapping channel's 1, 6 and 11.

If in your location you are finding that the same channel's are overlapping each other, you may need to adjust power output. Using the proper antenna can also help to control the signal propogation from each access point.

Same channel interference will cause a lot of retransmitting of packets as can be seen on the access points interfaces.

john.preves Mon, 04/23/2007 - 21:10

Just to add a few notes to the previous post you mentioned that your clients drop when you associate to 'those'devices. Does this suggest that you have other AP's as well?

It almost sounds like you are roaming between subnets.

I hate to ask, but you do have all your AP's on the same subnet, right?

I apologize in advance but given what I have seen over the years, that unfortunately, is a legitimate question.

When you did the site survey to determine where the AP's would go, what antennas you would use, and what power and channel settings would be, what was the threshold for the datarate? I got popped once because I wasn't aware that older Symbol scanners transmit Data at 1 and 2 Mbps. I was locking the AP at 11Mbps to control my cell sizes.

That became quite apparent rather quickly. Multipath is another issue you need to consider as well.

I would consider a walkthrough with some Airmagnet to see if you are having any of these issues and possibly schedule a re-survey using same vendor gear.

sjohnson1969 Wed, 04/25/2007 - 06:44

Good question, but yes, they are all on the same subnet.

And yes, we are roaming in the warehouse. THere's a total of 8 AP's (all cisco's).

I set the datarates to 1 mandatory and 2-11 enabled and the rest disabled. I can see the AP's as using 1 and 2mb only. I do not see other cards locking at any other rates.

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