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AP Power Setting for 7925 Deployment

I have read the 7925 deployment guide, and I see recommendations for signal strength, overlap, SNR, jitter, delay, load, etc.  However, I don't see anything regarding the power level of the AP vs. the phone.  If the AP is stronger than the phone, you can end up with one way audio because the phone can't communicate back to the AP at its current range.  I know that if the overlap and SNR is appropriate, the phone will probably roam to an AP it can communicate with, thus avoiding the one way audio situation.  Do most of you that perform site surveys for VoIP with the 7925 on 5 GHz rely on U-APSD and DTPC to manage power settings on the phone?  If so, what power do most of you set the AP to (depending on survey conditions) - full, half, quarter?

Regards,
Scott

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4 REPLIES 4
Hall of Fame Community Legend

Re: AP Power Setting for 7925 Deployment

During site survey I'd set the AP to half.

Cisco Employee

Re: AP Power Setting for 7925 Deployment

i'm not a site survey expert, but in practice when system is in production, make sure DTPC is enabled and it will manage the rest.

as per one way audio due to rf range shortage from the phone; i wouldn't imagine you will reach that point if you have lower data rates disabled on AP in order to pushthe phone to a nearby AP.

Re: AP Power Setting for 7925 Deployment

ouch ... I drafted a long detailed email and I got an error when I hit send.. Let me try this again ..

1. I would use the phone when you survey in concert with the survey software you use. Client radios have different receive sensitivity. Having surveyed more then 300+ projects you learn real quick the difference between clients.

Example:Suppose your survey client shows a -60 dBm and your Cisco handset shows -65 dBm, you know you have a -5 dBm delta. In this case your phone always "wins". If you dont consider this and you survey with a client that hears differently from your phone your survey for voice may have issues. As you roll out the phones you will not get the same data as you surveyed. So, ALWAYS use the phone as PART of your assessment.

2. TX Power. Ive learned one VERY important rule the hard way! ALWAYS ALWAYS ALWAYS survey at the lowest client TX  power in mind. Example: I am going to pick on Vocera. Suppose your Vocera badge is TX at 25mW and you survey at 50 or 100mW. You badge can certainly hear your AP as its speaking "louder". However, the badge is whispering back, if you will.

Here is a link to my blog about just this issue: http://www.my80211.com/client-labs/2009/11/17/cisco-wlc-linktest-how-well-does-the-ap-hear-your-client.html

I get a kick when people look at the window bars and they show all GREEN . But you are only seeing one thing when you look at this .... Its how loud the AP is and how your client understands the signal. What you dont see is how loud the AP or controller hears you! You need to go into the AP or controller to grab this information.

3. Channel overlap is VERY important for roaming. You should read Keith Parsons The Fallacy of Channel Overlap @ http://wlaniconoclast.blogspot.com/2009/04/fallacy-of-channel-overlap.html

4. I never use the AUTO features during any of my assessments and here is WHY. I want to see the STATIC coverage without RRM, DTPC, etc ... I dont want anything "playing" with my survey data at all. I static my client at 25mW, AP 25mW and leave all AUTO OFF.

I hope this helps bud ... Please kindly rate helpful post!

"Satisfaction does not come from knowing the solution, it comes from knowing why." - Rosalind Franklin
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Hall of Fame Community Legend

Re: AP Power Setting for 7925 Deployment

Nice one George!  +5

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