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  • Hi,

    I'm doing my first ever Airmagnet planner project and I've hit a stumbling block in my understanding of power settings.

    From what I've been reading, it appears to be good practice to have your APs power set to approximately half that of your weakest client device - i.e. for something like smartphones or tablets, that would be about 11dBm, therefore the APs ought to be set to roughly 8dBm, which equates to about 6mW.  In the planner 'Advisor Criteria' window, you specify the power settings of your APs as well as antenna gain, but it's not clear whether this power setting is IR or EIRP.

    In other words, if you set the power to, say, 6mW and specify the antenna gain (which on the APs we are using is 5dBi), then you end up with an EIRP of about 13dBm ?  (6mW <8dBm> + 5dBi gain = 13dBm).   - OR - is it like this; If I'm using a 5dBi gain antenna and I want an EIRP of 8dBm then I need to set the power setting to just 2mW ?  (2mW <3dBm> + 5dBi gain = 8dBm) ??

    That seems crazy - to install an AP with the capability of running at 100mW and have it run at just 2mW !

    One other thing I'm puzzled about is after you use the planner advisor and it sticks the APs where it thinks is best, if you then edit each AP so that the model reflects what you are actually going to use, the heatmap changes completely - so... what's the point of using a planner advisor that doesn't take into account what you are actually putting up on the walls ?

    I'd be very grateful for any help !! - I've been scratching my head over this all day, and Fluke's "Gold Support" line just goes straight to voicemail !

    Cheers,

    Kev.

  • By MikeG - edited: June 5, 2015

    I'll take a shot at this.

    I would say to not include the antenna gain - after all it helps on the receive side as much as on the transmit side.  What if you were using, for example, a big sector on the AP?  You could easily end up with AP transmit power settings well below 0 dBm  if you included antenna gain!

    Remember why you are keeping the power level on the APs down: First you want to keep APs on the same channel from 'hearing' each other (APs hear each other better than clients do - they are up on the ceiling where there are fewer obstructions, and they usually have antennas with gain in the horizontal plane), Second, you can control hidden node problems by keeping the cell size smaller, so the clients anywhere in the cell can hear each other.

    Don't get too occupied with keeping the power levels down, there is nothing inherently evil about a link that is a little unbalanced, you just get higher data rates in one direction (AP -> Client, usually)  than in the other.

    Now, maybe somebody will disagree and we can get a good discussion going :-).

    -Mike

  • By Howard - edited: June 6, 2015

    I would say that much of the decision for AP power levels should be based on the density of your AP's and more importantly, the power and sensitivity in your client devices.  Also, their output power alone may already be less than what you think it is.  Not all client devices advertise their true levels, only their maximums. 

    Yes, a higher density of AP's will allow/require you to lower the power levels, but at the same time you don't want your AP's to be able to hear the clients, and the clients unable to hear the AP's.   Due to the relative costs of components, a good AP radio will always have better receive sensitivity than its clients.  I have seen this be as much as  2 to 6 dBm better for devices of the same vintage. 

    I spend the majority of my time testing client radios, and I have found that, by far, the most important factor regarding rate/range performance, between multiple copies of the same client device, is receive sensitivity.   Between two devices, it can vary by 2.5 dB even on the same channel.   The same for power levels, but by a lower amount. Client radios also diverge on power levels between channels more so than AP's - much more than manufacturers would want you to believe.   These factors will complicate any predictions you make.

    I prefer having balanced power levels, where the power of the AP's is set to the same power level as the lowest powered client - certainly not half of that.  

    It would be convenient if we could always count on all client devices having a certain power level, but that is not going to happen.

  • Lots of reasons to lower AP power for surveys/deployments etc.  Some that haven't been mentioned include user density (That 100mW might work great for two guys in a warehouse but would provide far too large a cell for an auditorium full of smart phones) as well as ensuring that should an AP fail, surrounding APs still have plenty of room to increase their power and fill the coverage gaps.

    Also a word to Airmagnet's Planner, it's definitely one of the better predictive programs I've seen, but having done validation surveys on Planner designs, I can say I've always had to make changes.  If possible always do an active survey onsite ahead of any deployments.  The planner can be a nice starting point, but once you move one or two APs the entire design is basically useless.

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