• In reading the CWDP study guide on page 133 it says,  "Testing with these features on and off has often shown that it’s best to disable the feature that allows an AP to power up to fill in a coverage hole".    Could some explain why this would be true?  Perhaps I have a Cisco centric view on this where you start with a higher power level and work your way down. How does this vary with supplier?  I could see if power adjustments were made so APs around the hole failed AP were a LOT stronger than the other APs in the area that would a problem.  I would be inclined to tweak the power adjustment algorithm but not turn it off.

  • By Sathwik - edited: April 18, 2014

    Coverage hole recovery means when the neighboring AP (AP2) goes down the current AP(AP1) will increase the power to cover the area which is initially covered by the AP2. There are many issue with this kind of nature,

    1) As mentioned already in the book, as AP increases the power, the clients always dont necessarily increase power with the AP. The client will receive the good signal from AP1 because of increase in power. However the Client(which is connected to AP2) may not be able to reach the AP properly since Client always has the inferior radio and antennas than AP.

    2) Most of the deployment will have omni directional. Lets say AP2 which goes down is at the right side of the AP1. When AP1 increases the power because of coverage hole recovery, it not only extends power on the right side, but the coverage hole of AP1 increases in all direction which causes issues for other APs as well.

  • Not really a coverage hole issue, but AP's with auto adjusting power levels that are installed far off the floor, as in a warehouse, have been known to lower their power levels too far - they each see the other AP(s) too well.

  • Thanks for the comments.  That helps.  For a power mismatch between AP and client it is possible to adjust the AP maximum power to keep there from being too much of a mismatch.  (See attached image.)  Your point number 2 is interesting.  The power threshold setting may help, but yes the omnidirectional nature of most APs could make it more complicated. 

  • The difference between the low and High Power levels seems awfully large- they are both set to the extremes.   

    Are these the default levels?.

    Was there a site survey done before the AP's were installed ?

  • I should add I am not here to report a problem, just studying for the CWDP.  These are the default settings on the controller.  Given a common client transmit power of 30mW that translates to 10*log(30mw) = 14.8 dBm for the client.

    When you look at the bottom of you see the maximum transmit power of an 1140 AP is  17 dBm.  In practice the difference is between 14.8 dBm for the client and 17dBm for a 1140 AP.  You are right, for other APs the present controller settings could allow a large mismatch between client and AP transmit power.  Fortunately for the present APs it is close.  Good point, need to check this before getting other APs.

  • Use of any “auto RF” feature from any manufacturer needs to be part of the design process, typically either all static or some combination of auto-channel and auto –power, right?

    I usually take the contrarian position and like everything static, designing and engineering a WLAN to use some manufacturers “auto-rf” features just has always seemed to reek of nepotism and at the same time requires the engineering team to buy in to a vendor’s kool-aid. WLAN’s need to be designed around a minimum RSSI for the worst client, if this is a mission-critical WLAN then don’t you already have redundant coverage from neighboring AP’s, isn’t that how the WLAN was designed to operate, try and tell a hospital CIO that a given AP just reset the radio and is momentarily unavailable.

     Auto-RF features?


    CK31 anyone???


  • These are good perspectives to get.  Certainly you can't just slap something in and expect automatic power and channel settings to compensate for a bad AP layout.  (My pet peeve is APs that have a nice view of each other that are arranged along a nice line in a corridor.)  That sounds like something from a glossy sales brochure. 

    It does seem like there are some legitimate uses for automatic power and channel changes.  If there was a lot of interference on a channel, it would be good to have the system be able to change channels.  If one AP failed it would be good to have the APs in the area increase their power by a certain amount.  Yes, I know, you do not want to increase the power a lot because you risk a mismatch between the AP and client and therefore a one-way situation between client and AP.   The maximum power a AP can adjust to can be configured however. You can also configure the system to not change channels when certain important things are going on like voice calls. 

    My conclusion from this discussion would be automatic power and channel changes can not compensate for a bad layout, but it does have some uses.  Another lesson would be do not passively accept the power/channel change parameters.  There are a variety of parameters can be adjusted to keep the system from overacting.

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