A blog with tips, tricks and tutorials to help you prepare your CCIE Wireless lab exam.

Tuesday, January 19, 2010

Coverage hole algorithm thresholds

You know the coverage hole algorithm, triggered when some clients get below a certain SNR, and configured here:

 Now here is a question: I have wireless clients that do not react well when their AP signal gets too low... could you configure the Coverage Hole algorithm to be triggered if any client signal on AP1 gets below 19 dBm SNR? Oh, AP1 power level is statically set to 2  (and level 2, as you could see if you ran the show run-config command on my EUROPEAN controller, is 14 dBm).
Can you find what the threshold should be?

Ok, here is the solution. First, Coverage Minimum Exception Level is to be set to 1 (as the algorithm as to be triggered if "any client"...).
The harder part is the Coverage value. The client SNR cutoff triggering the coverage hole algorithm is calculated as follows:
Client SNR Cutoff Value (|dB|) = [AP Transmit Power (dBm) – Constant (17 dBm) – Coverage Profile (dB)]
My AP power level is set to 14 dBm, and as you can see on the above picture, the default Coverage Profile value is set to 12. So we have:
Client SNR Cutoff Value (|dB|) = 14 -17 -12 = -15
Although this number is negative, we are looking at an SNR cutoff of 15 dBm.
You want 19 instead (so result must be -19)?
The only value we can change is the Coverage Profile, setting it to 16 solves the problem...
Easy he? Yeah, I know...
In code 5.2, it gets a little better, you can also set RSSI values etc...


  1. What if the power of the AP isn't statically configured ?
    Depending on the AP Power, the Cutoff value will be dynamic ?

  2. Yes you are right Sjostke. The algorithm runs all the time, based on the AP current power value. If the power value changes due to RRM (that reduces the AP power if it is seen as too loud from the other APs), then next algorithm calculation might suddenly find that too many clients are below the threshold, and will increase the power back up.

  3. Hi Jerome,
    As in 5.2 version we can set RSSI value. But how will you calculte Client SNR Cutoff Value.
    Can it be possible to explain with example by taking using below values.

    Data RSSI -80
    Voice RSSI -80
    Min Failed Client Count per AP (1 to 75) 3
    Coverage exception level per AP (0 to 100 %) 25%

    Whenever my client compain about signal problem, I normally set Power Level Assignment to 1. Is there better way to deal with signal issues?

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