Wireless Access


APs output power

Hello i decided to open a tread about this interesting topic

Fallowing @jhoward post   which i quote 




The issue with running your indoor APs at such a high power is that, while it makes the coverage 'look' good (since there's RF blasted everywhere and so all the client devices report super great signal), the clients themselves do not have the same power and antenna gain to support 'reach back' at the same level. So take an AP at 23dBm and a client at 17dBm (which is actually higher than most devices like phones and tablets), that is at least a 6dB difference in the link budget. That means the AP has at least 4 times the power output of the client.

What occurs next is the client says 'hey I think I have great signal, I am going to stay with this AP and ask to negotiate a high data rate'. Except when this happens, it doesn't actually work that way. The client may try to run at MCS15 (as an extreme example), but in reality, the client is too far away such that the smaller power output of the client can't really 'push' MCS15 back to the AP. The result is the transmit fails and the client falls back on slower rates until it gets an ACK back from the AP. Carrying it further, if a client is a long distance away from the AP, while the 23dB power output may be enough to get the frames to the client, the 17dB client is not powerful enough to send a frame back. A such, the client keeps re-transmitting without ever getting an ACK, and may chose not to want to roam because it thinks it has great signal from it's associated AP.


In short, everything works best indoors when the AP power output (in general) matches the client power output you are supporting. 25-50mW is pretty much the range of client device power output for most consumer WiFi devices (small phones to laptops, respectively). If all your laptops have a 100mW radio with a 3dB antenna then I would say you can run your APs wide open."



Fallowing that order of ideas you were mentioning

How would work if i got an AP with high gain omnidirectional antenas, maybe to  have more range,  what would happen to a antenna of 17db of a client? how good it will work? will it have lot of retransmitions?

If this is correct why would i would want to have this kind of antennas? it will just work if i know that my clients got more than a 17db antenna on it that can handle it? but for a normal situation with normal wifi devices it would not work as expected???


Now this just make me think that a correct design migh means more APS  as far i see  you should not be using the APS at max power, in a normal situation? which of course the client will not like, and then you have to explain them why your design has a way more APS than the others quotes of the other companies...

This just make me think a lot... this just make it harder when you have to sell it( because your design is more expensive, and at the same time you must convince the client that you are telling them the truth and you are just not trying to sell them more APS, or that the other companies just dont have better aps or that your aps does not match otheres in range when its not true....) sigh...


Any comments



Product Manager - Aruba Networks
Alternetworks Corp

Re: APs output power

To address your last question first, it's not a vendor-specific problem. If someone wants full-speed, best performance designs for low powered devices like phones and tablets, then they should be looking at a more dense coverage model with APs at lower power. That's true with Cisco, Aruba, HP, Aerohive, Ruckus, etc. That said, you CAN make lower AP density models work with lower powered devices, but it will require more tuning and adjustments to work 100% without issue. Most of that tuning is to add or remove MCS rates, adjust PRT, CSR, Client-Match tuning, etc (in Aruba-speak). 


Far too often, you have partners and customers just wanting to do a 1:1 replacement of a legacy 2.4Ghz system (designed around much lower data rates with 2.4Ghz coverage models that supported higher powered laptops in the past), with a 5Ghz VoiP/Video support model for high density client environments filled with low powered phones and tablets. That doesn't work well, and slowly but surely people are figuring that out. It's a physics problem, not a vendor problem (and TxBF is not the cure-all for that issue either as some vendor's proprietary TxBF solutions introduce other problems as well). 




To your first point, the same issues still applies, as the link budget still is not symmetrical. However, since you are increasing or magnifying the RF with the directional antenna on the transmit (Tx) side, you are also increasing the Receive (Rx) 'sensitiviy (or gain) on the AP so that it can hear clients farther away. We have gone up to a km distance with 14dB directional antennas to laptops, about 600m to tablets/phones with clear line of sight (which is a pretty long way) and got reliable connections. Ultimately there is a limit with how far most consumer devices can go that has more to do with PHY rate limits than RF limits. But the same premise is true on the asymetrical link budget between a client and AP with high gain directional antenna.


Here's an example. We will use dB since the math is easier. Positive numbers are gain, negative numbers are loss, and these are just values to make a point, actuals would need to be calculated based on a set distance, frequencies used, etc and assumes similar receive sensitivities on each side (AP and Client).


AP to client:

(+20dB radio)+(14dBi antenna)+(-100dB approx free space path loss over 1km)+(2dBi client antenna) = Client's receive SNR of around -64dB


Client to AP:

(+14dB radio)+(2dBi antenna)+(-100dB approx free space path loss over 1km)+(14dBi AP antenna) = AP's receive SNR of around of -70dB


So in the above example, which is JUST a numbers excercise, the actual SNR at 1km would be much lower due to other factors (interference, noise floor, any MRC or lack therof, etc), the AP hears the client at 4x less 'power', or with a 6dB SNR deficiency. Also don't forget that noise floor, interference, etc. 


Again, if your client devices are mostly high powered devices and can use external antennas, then you have far more options and can spread out the APs like in the good ol days. But the general concept (again due to physics, not vendor) is that the smaller and more low-powered devices you plan to support, the smaller your AP cell sizes should be to accomodate. 


That said, it doesn't mean run your APs at the *exact* power as your lowest powered client devices. But in the OP's example, if the AP's are all wide open at 23dB, and they are having issues with phones and tablets which may max out around 14-15dB, that is a *significiant* link budget difference that can cause some drastic performance issues in regards to roaming, client 'stickyness', throughput, etc.


Hope that helps. 



Jerrod Howard
Distinguished Technologist, TME

Re: APs output power


Really good explanation thank you very much for your time in explaning us this topic!!!

You guys should put this on the Knowledge base this explanation... its really important and im sure many of us does not know this or are not too sure about this.


Thanks again!!!! :)




Product Manager - Aruba Networks
Alternetworks Corp

Re: APs output power



Great great explanation! This is an old post but it is worth reviving since all of us should know this as Carlos said.

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