Make TPC work, is it possible? Part 2, from the WLC perspective

My recent blogpost was a theoretical approach using Ekahau ESS to find out if its possible to use Cisco WLCs TPC algorithm to set Tx-power on my access point according my predictive design in ESS

My design was based on Cisco 2802i access points, primary/secondary coverage -67dBm/-75dBm, Tx-power at 25mW/14dBm and 5GHz only

My conclusion from the theoretical approach was that the WLC would have problems with a consistent Tx-power setting. But I would give it a try

The access points was installed at theirs correct place and everything was default on the controller.

Ciscos RRM white paper uses the WLC CLI command “show advanced 802.11a summary” to show both Rx_neighbors and Tx_neighbors. On my WLC that command had a shorter output

So I used these two CLI-commands on the WLC

  • show advanced 802.1a txpower
    • That command gave me allowed power levels and it showed that 17dBm was Tx_max
  • show ap auto-rf 802.11a <ap name>
    • That command gave me nearby APs. How other AP hear us = Rx_neighbor.
    • Since each AP was represented with it base BSSID with hex-value “f” at last digits it became some work with excel to find corresponding 3´strongest Tx_neighbor

I will not show that excel-spreadsheet, but I will summarise it

Next table show the RSSI_3´strongest and Tx_ideal from my predictive approach and measured values on the WLC
Remarks: I used Tx_max =20dBm in my last blog. The value of Tx_max is adjusted to 17dBm

RSSI_3´strongest and Tx_ideal from predictive and WLC-output datapoints
Tx_max = 17dBm RSSI_threshold= -70dBm
Predictive WLC output
3´strongest AP Tx_ideal (dBm) 3´strongest AP Tx_ideal (dBm)
(calculated)
AP1 -91 38 -88 35
AP2 -80 27 -86 33
AP3 -82 29 -82 29
AP4 -78 25 -84 31
AP5 -72 19 -81 28
AP6 -74 21 -85 32
AP7 -74 21 -72 19
AP8 -73 20 -84 31

As we can see AP1, AP2, AP3, AP4 and AP7 is fairly close between predictive and what he AP has measured (less than 6dB difference), but for AP5, AP6 and AAP8 the difference is huge (9-11dB).
Since all APs had calculated Tx_ideal above Tx_max every AP used power level 1 (17dB)

Next was adjusting the RSSI_threshold in the WLC TPC algorithm from its default value at -70dBm to -80dBm.
Next table compare the predictive approach and the calculated values on Tx_ideal and corresponding Cisco power levels based on measured values

TX_ideal and power levels from predictive and WLC-measured datapoints
Tx_max = 17dBm RSSI_threshold= -80dBm
Predictive Measured
Tx_ideal(dBm) Power level Tx_ideal (dBm) Power level
AP1 28 1 25 1
AP2 17 1 23 1
AP3 19 1 19 1
AP4 15 1 21 1
AP5 9 ´3-4 18 1
AP6 11 3 22 1
AP7 11 3 9 3-4
AP8 10 3 21 1

As we can se from the table on measured values the only AP that should change its Tx_power is AP7, even if we adjust the RSSI_threshold to its absolute minimum. AP7 has its calculated Tx_ideal based on measured values at 9dBm and the WLC set it on power level 3 (11dBm)

This leads to that 7 of my APs transmit at max power level (17 dB) and only one of the APs in the middle of the floor reduces its power level. This it not a good wifi-network.

During my research for this blogarticle a find five youtube-videos from Jerome Henry where he goes a RMM deep dive, check references

Two key element he says is

  • The TPC algorithm kick in when you have 3 AP-neighbors at -70dBm or higher (or your configured RSSI_threshold or higher)
  • The TPC algorithm is only used to automatically lower your Tx_power. The Coverage Hole Detection  is used to increase your Tx_level

My design with -67dBm/-75dBm and Tx= 25mw is no way near the TPC-goal to have three AP-neighbor at RSSI_threshold or higher, especially those in the outer edge of the floor

Conclusion
To have APs with consistent Tx-power its two choice

  1. Go for static Tx-power on the APs
  2. Limit the TX_min/Tx_max in your RF-profile to the values you want

 

References

Cisco RRM white paper, 2018
Jerome Henry youtube-videoseries, 5 videos 
mrn-cciew rrm-blogposts

Part 3 in this blog series will be on DCA

I´ll be back

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