How do WLC's work out coverage holes etc

Question

Hi all
with the cisco wlc and lwapp access points, how do the access points adjust there power settings to compensate, what are they looking for?
do they look for noise / interference from the nearby ap, then increase or backoff the power until there is roughly 50% overlap between access points, can anyone explain how it works here?
cheers
Carl

Answer 1

Hello Carl,
ok , to make it clear for you how WLC power and channel settings work will explain here how RRM ( radio resource management work).
RRM consist of 3 componenets , DCA ( dynamic channel assignment) , TPC ( transmit power control)  , CHD ( coverage hole detection).
1) DCA: once the WLC boot and AP's registered the WLC will force channel configuration to the AP to make sure the least possibile co-channel interference between the AP's.
so this algorithm will configure for each AP at will channel it need to operate.
the DCA can be configured from wireless ->>> 802.11 A/B/G->>> RRM ->>> DCA.
the DCA interval specify how frequent the WLC handle the channel updates on the AP's.
note after boot the WLC will require 6 DCA iterations to stabilize the RF.
2) TPC: this algorithm will configure how much power the AP transmit.
it has levels from 1-7 ( 1 is the maximum level)
note : this algorithm can downgrade the pwoer level if it find it is too high...
how the algorithm determine this is too high? and how much overlapping?
this is based on TPC configuration that can be configured from wireless ->>> 802.11 A/B/G->>> RRM ->>> TPC.Power Threshold (-80 to -50 dBm)
so if you configure here lets say -65 , if the AP hearing other neighbours with signal strongest than -65 (lets say -50)then there is possibility RF issues and the WLC will instruct corresponding  AP's to downgrade their power to meet the criteria ( - -65 or less from other neighbours)
this will minimize co-channel interference.
note for TPC/DCA to work fine each AP should be designed (places) to be heraed by 3 other neighbours. with signal no stonger than X ( that is configured as power threshold)
3) CHD: this will increase the power level of the AP in case it notices some clients has bad signal.
when it will instrcut to increase power?
if you go to  wireless ->>> 802.11 A/B/G->>> RRM ->>> coverage you can see two components (RSSI and Min Failed Client Count per AP (lets say X).
now if single AP hearing that number of X clients with signal strengh below the configured RSSI . then this is coverage hole and WLC instruc to increase its power to cover those clients with better signal.
for more details about WLC radio managment please refer to:
1)                 WLAN Radio Frequency Design Considerations
http://tools.cisco.com/squish/1Ea09
2) Deploying Cisco 440X Series Wireless LAN Controllers ( Radio Management and RF Domains section)
http://tools.cisco.com/squish/51a58
hope this helps.
Kind regards
Talal
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