AP Power & Channel Assignments

[Terence Lockette]

Hello all,

Does AP Tx power levels matter when nearby access points are using different channels?  I'm trying to determine if access points are interfering with each other on our wireless network.  I'm noticing in WCS that I have quite a bit of interference threshold notifications and would like to know if power is a likely candidate of some of our issues.  Thanks!

Terence

[Scott Fella]

The non overlapping channel in the 2.4ghz is 1, 6, and 11.  So if a neighbor AP is on a different channel, it isn't interfering. If you have AP's that can hear each other on the same channel, well yes, that would raise the noise floor for that channel and cause the SNR to decrease and might send out an alert due to the threshold you have configured on the WLC.  What you have to look at is AP's that can hear other AP's on different floors or even by an outer window in which you might be able to hear other AP's on the same channel from multiple floors or buildings.

Thanks,

Scott

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[Terence Lockette]

Understood.  Now if I'm using TPC on my controller, shouldn't it tune the tx power level accordingly to avoid any interference from an AP that may be using the same channel?

[Scott Fella]

Its really hard to say without knowing the environment.  So, do you have issues at all or is the alerts bothering you.  If so, then just raise the threshold higher.  Tweaking the TX is fine, but you have to understand what your doing and how it can impact your environment.  RRM does work well and only needs to be tweaked when really you have a high density design.  Interference that you have no control over, is hard to stop and you just have to work around that if the interference isn't something you can prevent.  Like neighboring access points or interference from another tenant in the building, etc.

Thanks,

Scott

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[Terence Lockette]

I honestly believe the interference is coming from sources not on our network (ie. APs from other businesses).  I could raise the threshold but that won't do anything for the interference.  Right now, one of our WLANs is operating slowly (Web pages open slowly and email from cell phones won't send/receive randomly).  The newest issue is that some of our Cisco 7925s are getting dropped during normal business hours.  I'm currently working to get them in their own VLAN & WLAN to see if that helps.

[Scott Fella]

Well you can't control interference unless its something your generating. Move your phones to the 5ghz if possible.

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[George Stefanick]

General rule of thumb, aps on same channel should be at 20 dB difference. This will reduce the cca busy.

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[Terence Lockette]

George Stefanick,

Can you explain a little further by what you mean that aps on the same channel should be at 20dB difference?

[Scott Fella]

Take a look at this link as it will explain the separation required.

http://www.cisco.com/en/US/docs/solutions/Enterprise/Mobility/emob41dg/ch9_Voic.html#wp1074030

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[George Stefanick]

I highly recommend reading the link Scott posted.

But in short I will explain what I mean.This is VERY important and most folks dont understand what is going on under the covers.

If two access points share the same frequency (channel) they will interfere with each other. Think of walkie talkies all on the same channel. Everyone can hears everyone else, right.

WiFi uses CSMA/CA, which uses NAV and CCA to manage the medium. Clear Channel Assessment (CCA) uses ED (energy detect) and decodes 802.11 transmissions from preambles to dectect IF the medium is free.

ED is specific to NON 802.11 interference like jammers and microwave ovens. If loud energy (non 802.11) is close to the client (or ap), the client (or ap) will not transmit. So you can see same channel AP interference is not in play for ED.

Where same channel interfernce is in play is when demodulated frames come from same channel access points in close range. This is also true for NAV.

If a client hears an AP or client within 20dB that client will back off.

Example -

AP#1 - Channel 1

AP#2 - Channel 1

You stand next to channel AP#1 and your client shows a RSSI of -50 for AP#1. Your next AP, AP#2 should be 20dBm away. For example RSSI -70.

If you are closer, say -60 then you are competeing for the medium from both cells.

Make sense ?

__________________________________________________________________________________________
"Satisfaction does not come from knowing the solution, it comes from knowing why." - Rosalind Franklin
__________________________________________________________________________________________
‎"I'm in a serious relationship with my Wi-Fi. You could say we have a connection."

[Terence Lockette]

Ok that explanation helped a lot and makes a lot of sense.  Thanks to you and Scott for helping me explain this.  I'll check into this and will provide feedback on my findings.  Thanks again!

Terence

[George Stefanick]

No worries.. if you find any of this helpful feel free to rate post !

Thanks!

__________________________________________________________________________________________
"Satisfaction does not come from knowing the solution, it comes from knowing why." - Rosalind Franklin
__________________________________________________________________________________________
‎"I'm in a serious relationship with my Wi-Fi. You could say we have a connection."