the use of RRM ?

Answered Question
Feb 23rd, 2007

we did an site survey for a warehouse in witch we had 20 ap's (1050) foreseen, we made a channel map with power adjustments, once the installation was complete we enabled the RRM feature on the controller, this feature should fine tune the rf enviorement but in this case it put the ap's on full power (big overlap of co channels) an uses only 2 times channel 6 on 20 ap's.

do i need to change the default parrameters of the RRM to tweak arround this issue ?

are there default parameters for an Data, voice or location based enviorments ?

if somebody has experience with these isues can you please share...

thx.

I have this problem too.
0 votes
Correct Answer by rob.huffman about 9 years 7 months ago

Hi Joeri,

Have a look at what determines Dynamic Channel Assignment Algorithm and Transmit Power Control Algorithm changes in the LWAPP environment. It will explain why the WLC has applied the settings the way it has. These power and channel settings can be adjusted manually as can the RRM defaults if required;

Dynamic Channel Assignment Algorithm

The DCA algorithm, run at the RF Group Leader, is applied on a per-RF-Group basis in order to determine optimal AP channel settings for all the RF Group APs. Each set of APs that can hear the neighbor messages of each other, referred to as a logical RF sub-Group in this document, has its channel configuration performed independently of other logical RF sub-Groups. This is due to the fact that signals do not overlap. With regard to the DCA process, the leader considers a handful of AP-specific metrics during the determination of necessary channel changes.

Note: When all APs boot up for the first time (new, out-of-the-box APs), they transmit on the first non-overlapping channel in the band(s) supported (channel 1 for 11b/g and channel 36 for 11a). When APs power cycle, the APs use their previous channel settings. DCA adjustments subsequently occur as needed.

The values described are then used by the Group Leader in order to determine if another channel schema will result in at least a bettering of the worst performing AP by 5dB (SNR) or more. Weighting is given to APs on their operating channels such that channel adjustments are made locally, which dampens changes in order to prevent a domino effect, whereby a single change would trigger system-wide channel alterations. AP utilization information is derived from the load measurement report of each AP. Preference is given to APs based on the determined utilization, so that a less-used AP has a higher likelihood of a channel change compared to a heavily utilized neighbor if a change is needed.

Transmit Power Control Algorithm

The TPC algorithm, run at an un-modifiable ten-minute interval, is used by the RF Group Leader to determine APs RF proximities and adjust each band?s transmit power level downward, as necessary. this is performed in order to limit excessive cell overlap and co-channel interference.

Each AP reports an RSSI-ordered list of all neighboring APs. Provided an AP has three or more neighboring APs, the RF Group Leader applies the TPC algorithm on a per-band, per-AP basis in order to adjust AP power transmit levels downward such that the third loudest neighbor AP is heard at a signal level of -65dBm (default value) or lower.

Power changes are only made when the third loudest neighbor of a given AP is heard at a signal level higher than the default value of -65 dBm.

Note: When all APs boot up for the first time, the APs transmit at their maximum power levels. Also, if the APs do not see each other with a -65dBm (default value), they push the power level up to the maximum in order to achieve this. If you go to the Controller CLI and issue the show ap auto-rf 802.11b command, there is a Nearby APs section near the bottom of the command output. In this section, you find the dBm listing. If this dBm listing has values more than -65, then the AP transmit power is at the maximum. This is tweakable to an extent. You can issue the Config advanced 802.11b tx-power-control-thresh <-50 to -80> command.

From this excellent doc (Thanks Ben!);

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

Also,here is a really good thread that has alot of RRM "Hands on" details from NetPros John,Eric,Mark,Kevin and Aaron with some great info;

http://forum.cisco.com/eforum/servlet/NetProf?page=netprof&forum=Wireless%20-%20Mobility&topic=WLAN%20Radio%20Standards&CommCmd=MB%3Fcmd%3Ddisplay_location%26location%3D.1ddc5409

Hope this helps!

Rob

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Correct Answer
rob.huffman Fri, 02/23/2007 - 06:05

Hi Joeri,

Have a look at what determines Dynamic Channel Assignment Algorithm and Transmit Power Control Algorithm changes in the LWAPP environment. It will explain why the WLC has applied the settings the way it has. These power and channel settings can be adjusted manually as can the RRM defaults if required;

Dynamic Channel Assignment Algorithm

The DCA algorithm, run at the RF Group Leader, is applied on a per-RF-Group basis in order to determine optimal AP channel settings for all the RF Group APs. Each set of APs that can hear the neighbor messages of each other, referred to as a logical RF sub-Group in this document, has its channel configuration performed independently of other logical RF sub-Groups. This is due to the fact that signals do not overlap. With regard to the DCA process, the leader considers a handful of AP-specific metrics during the determination of necessary channel changes.

Note: When all APs boot up for the first time (new, out-of-the-box APs), they transmit on the first non-overlapping channel in the band(s) supported (channel 1 for 11b/g and channel 36 for 11a). When APs power cycle, the APs use their previous channel settings. DCA adjustments subsequently occur as needed.

The values described are then used by the Group Leader in order to determine if another channel schema will result in at least a bettering of the worst performing AP by 5dB (SNR) or more. Weighting is given to APs on their operating channels such that channel adjustments are made locally, which dampens changes in order to prevent a domino effect, whereby a single change would trigger system-wide channel alterations. AP utilization information is derived from the load measurement report of each AP. Preference is given to APs based on the determined utilization, so that a less-used AP has a higher likelihood of a channel change compared to a heavily utilized neighbor if a change is needed.

Transmit Power Control Algorithm

The TPC algorithm, run at an un-modifiable ten-minute interval, is used by the RF Group Leader to determine APs RF proximities and adjust each band?s transmit power level downward, as necessary. this is performed in order to limit excessive cell overlap and co-channel interference.

Each AP reports an RSSI-ordered list of all neighboring APs. Provided an AP has three or more neighboring APs, the RF Group Leader applies the TPC algorithm on a per-band, per-AP basis in order to adjust AP power transmit levels downward such that the third loudest neighbor AP is heard at a signal level of -65dBm (default value) or lower.

Power changes are only made when the third loudest neighbor of a given AP is heard at a signal level higher than the default value of -65 dBm.

Note: When all APs boot up for the first time, the APs transmit at their maximum power levels. Also, if the APs do not see each other with a -65dBm (default value), they push the power level up to the maximum in order to achieve this. If you go to the Controller CLI and issue the show ap auto-rf 802.11b command, there is a Nearby APs section near the bottom of the command output. In this section, you find the dBm listing. If this dBm listing has values more than -65, then the AP transmit power is at the maximum. This is tweakable to an extent. You can issue the Config advanced 802.11b tx-power-control-thresh <-50 to -80> command.

From this excellent doc (Thanks Ben!);

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

Also,here is a really good thread that has alot of RRM "Hands on" details from NetPros John,Eric,Mark,Kevin and Aaron with some great info;

http://forum.cisco.com/eforum/servlet/NetProf?page=netprof&forum=Wireless%20-%20Mobility&topic=WLAN%20Radio%20Standards&CommCmd=MB%3Fcmd%3Ddisplay_location%26location%3D.1ddc5409

Hope this helps!

Rob

I have had mixed results with the automatic channel selection portion of the algorithm.

Sometimes it works very well (single floor, spread out coverage-only design). At other times, (higher AP density, multifloor buildings, etc.), it is not unusual to find that the manual settings you select can significantly enhance your overall coverage.

A simple way to do this is to turn off automatic channel selection, force the channels to the desired settings, and then re-enable automatic channel selection.

The RRM channel selection algorithm will typically leave the channel you have manually selected, unless it needs to adjust to events like significant foreign interference.

Note: if you have a WCS, you can use Access Point Templates to set the channel settings as well as return them back to automatic selection in bulk and save yourself a great deal of time.

It is my understanding that more than a few engineers have called for some sort of "preferred channel" setting to "prime" the RRM channel selection algorithm.

- John

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