Wireless Access


AMs and off channel monitoring

Hello i was wondering if that when you deploy in an enviroment AMs do you guys turn off the off channel scanning for the normal APs?(its on by default)

I ask this because i read in a white paper that this has impact in the troughput of the wireless network as the AP serving to clients has to spent time on other channel


This is what the white paper says

"Some performance impact is unavoidable with off-channel scanning. Multi-vendor lab testing recently found that when using scanning APs for both client service and off-channel monitoring, a throughput drop of up to 16% was possible when APs were required to spend significant time off-channel."


Anyways i got a deployment in which we deploying also Air monitors

I also read that its recommended to deploy 1 AM for every 4 APs, can someone confirm this? or at least how many APS should i put as air monitor?

Product Manager - Aruba Networks
Alternetworks Corp

Re: AMs and off channel monitoring

I always leave it ON/ENABLED/DEFAULT.


Yes there is performance impact IFF (if and only iff) the WLAN is 'busy'.  In a typical WLAN the utilization of a given channel is low/medium and scanning really isn't going to impact things to the worst case degree.


Also keep in mind scanning is not an all-or-nothing configuration... you can feel free to leave scanning enabled BUT make the time between scans longer... by default its 10 seconds... make it 20, 30, 60 to reduce any impact on performance and yet still maintain scanning.


My own philosophy is that I want as many 'eyes and ears' around the deployment as possible... hence I leave scanning on.


Caveat:  If you are doing a bake-off for pedal-to-the-metal throughput, sure go ahead and disable scanning (as the other vendors/competitors will do that too...) to get your best result.... but in practical WLANs I would say leave it on and use Airwave to tell you what utilization levels are going on within the WLAN... likely manageable even with scanning on.


To the second question on the 4:1 ratio of APs to AMs... sure that's about right for deployments as a rule of thumb... and is based on the fact that the AMs are trying to listen to (IDS mode)  and leverage (IPS mode) the 802.11 management frames that are transmitted at 1 or 2 Mbps within typical WLAN networks.   Since these particular frames are only using these (really...) slow data rates, the frames can be detected  and understood at greater distances than the high speed data frames that the AP placement is designed to provide.   Thus you need fewer AMs to do the job than you do APs.   



Re: AMs and off channel monitoring

Interesting jfernyc

Thanks for giving me your point of view :)



Product Manager - Aruba Networks
Alternetworks Corp
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