WCS - coverage hole alert

Unanswered Question
Apr 24th, 2007

Hi forum,

I am constantly getting the below coverage hole alert, the AP is used to cover a warehouse area, sometimes the users will carry their notebook out of the warehouse and back in. The radio frequency is set to auto. What could be the cause of this issue?

Thank you,

PN

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An Alert of Category Coverage Hole is generated with severity 3 by Radio KM3_Ship_AP01/1 .

The message of the alert is AP 'KM3_Ship_AP01', interface '802.11b/g' on Controller '10.16.14.3'. Coverage threshold of '12' violated. Total no of clients is '3' and no of failed clients is '2'. Worst signal quality experienced by client='00:19:7d:1e:41:f3' RSSI='-65' dBm SNR='-116', client='00:19:7d:18:12:d6' RSSI='-68' dBm SNR='-65'..

I have this problem too.
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rob.huffman Wed, 04/25/2007 - 06:26

Hi Paul,

If you have users walking out of the Warehouse I would expect you to see these alerts. Here is some info on what causes the "Coverage Hole" alerts;

Coverage Hole Detection and Correction

If clients on a lightweight access point are detected at signal-to-noise ratio (SNR) levels that are lower than the thresholds specified in the Auto RF configuration, the access point sends a "coverage hole" alert to the controller. The alert indicates the existence of an area where clients are continually experiencing poor signal coverage, without having a viable access point to which to roam.

The Coverage Hole Detection and Correction algorithm is primarily aimed at the determination of coverage holes based on the quality of client signal levels.

The algorithm determines if a coverage hole exists when the clients' SNR levels pass below a given SNR threshold. The SNR threshold is considered on an individual AP basis and is based primarily on each AP's transmit power level. The higher the AP power levels, the more noise the APs tolerate as compared to the client signal strength, which means a lower tolerated SNR value.

This SNR threshold varies based on two values: the AP transmit power and the controller Coverage profile value. The threshold is defined by the transmit power (represented in dBm) of each AP, minus the constant value of 17dBm, minus the user-configurable Coverage profile value. The Coverage value is defaulted to 12 dB. The client SNR threshold value is the absolute value, or the positive number, that results from this equation:

Coverage Hole SNR Threshold Equation:

Client SNR Cutoff Value (|dB|) = [AP Transmit Power (dBm) ? Constant (17 dBm) ? Coverage Profile (dB)]

Once the average SNR of a single client dips below this SNR threshold for at least 60 seconds, the AP transmit power of that client is increased to the appropriate level in order to mitigate the SNR violation, and correct the coverage hole. Only one client need violate this threshold in order to trigger a correction.

Each controller runs the Coverage Hole Detection and Correction algorithm for each radio on each of its APs every three minutes. The default value of 180 seconds can be changed, as described in the Monitor Intervals (60 to 3600 secs) section of this document.

From this doc;

http://www.cisco.com/en/US/tech/tk722/tk809/technologies_tech_note09186a008072c759.shtml#coverage_correction

The Coverage Hole SNR Threshold Equation:

Client SNR Cutoff Value (|dB|) = [AP Transmit Power (dBm) ? Constant (17 dBm) ? Coverage Profile (dB)]

Note: This equation represents a situation where a client experience signal issues in a poorly covered area of a floor.

In such a scenario, these items can be true:

A client has an SNR of 13dB.

The AP to which it is connected is configured to transmit at 11 dBm (power level 4).

This AP WLC has a Coverage profile threshold set to the default of 12 dB.

In order to determine if the client AP must be powered up, the numbers detailed are plugged into the Coverage Hole Threshold Equation. This is the result:

Client SNR cutoff = 11dBm (AP transmit power) ? 17dBm (constant value) ? 12dB (Coverage threshold) = |-18dB|.

Because a client SNR of 13dB is in violation of the present SNR cutoff of 18dB, the Coverage Hole Detection and Correction algorithm increase the AP transmit power to 17dBm.

When you use the Coverage Hole SNR Threshold Equation, it is evident that the new transmit power of 17dBm yields a Client SNR cutoff value of 12dB, which satisfies the client SNR level of 13 dBm.

This is the math for Step 3:

Client SNR cutoff = 17dBm (AP transmit power) ? 17dBm (constant value) ? 12dB (Coverage threshold) = |-12dB|.

http://www.cisco.com/en/US/tech/tk722/tk809/technologies_tech_note09186a008072c759.shtml#c_c_workflow

Hope this helps!

Rob

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