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Two RF networks running simultaneously

Unanswered Question
Oct 19th, 2006
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I'm being tasked with upgrading one of our warehouses from autonomous to LWAPP AP's. (both Cisco). The challenge is that I'm being asked to install new equipment and keep both environments active during a two week UAT period.

I'll be using unique SSID's but I'm concerned about RF contention.

Should I be concerned? Has anyone else ever run into this?

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ethiel Thu, 10/19/2006 - 09:53
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Well, you will definitely have contention for the 3 B/G channels. A will be fine unless you have a LOT of APs (12 non-overlapping channels). If everyone is on only the new or only the old network it will lessen the interference and will probably be bearable, but if you have heavy usage on both you might have slow performance on both because of the interference.


-Eric


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RDowey Thu, 10/19/2006 - 11:32
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Thanks Eric. The new environment (LWAPP) will consist of nine (9) B/G AP's. (No 'A') I believe the existing environment has both A and B.


Are you saying that the possible contention is related to the amount of traffic? I don't understand how traffic factors in. Wouldn't contention of RF signal be more closely related to the number,density and proximity of both the old and new AP's?


My understanding is that the 4402 WLC's will try and space RF signal frequencies out so that there's as little contention as possible in the LWAPP environment by alternating those AP's on 1,6, and 11 as well as dynamically adjusting signal strength.


However, the existing autonomous AP's have no such mechanism. Therefore if LWAPP1 is located next to Autonomous AP1 and both are trying using RF freq 6 - won't that be an issue regardless of traffic?


TIA

ethiel Thu, 10/19/2006 - 12:42
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The amount of traffic will impact how much interference there is. If an AP is on channel one, it sends out a regular beacon on that channel. If no clients are on, that is basically the only traffic that is on that frequency, so there is not a lot of interference. The AP sitting next to it on channel 1 will barely be impacted. If a user is transferring a 100MB file on AP1, then that client is sending out a LOT of traffic on that frequency, and there will be many more corrupted packets for clients of AP2.


As for auto-channel selection and interference avoidance on the controller, here is how that would work. If you have 2 autonomous APs on channel 1 and 6, and one LWAPP AP, the controller would pick 11 for it. If you have 3 autonomous APs on 1, 6, and 11, the one LWAPP ap would get whichever channel it hears least. In a complex environment with intermingled LWAPP and autonomous APs, the controller will do what it can. However, if you have full coverage with overlap currently, it is very likely that the LWAPP APs won't be able to avoid the interference entirely.


That is when the amount of traffic on each will play into how badly they interfere with each other. If both are in use, they will each be sending traffic out on all 3 frequencys, and all 3 will have significant interference from the other network. If one is only sending out the beacons occasionally, you should only have minimal interference.


-Eric


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