Controllers use the TPC (Tx Power Control) algorithm to determine whether the power of an AP needs to be adjusted down. Reducing the power of an AP helps mitigate co-channel interference with another AP on same channel in close proximity.

The TPC algorithm run in every 10 min (600s). Like DCA, the RF group leader runs TPC on per-radio, per AP basis. The more APs hear each other at -70dBm or greater, the better the TPC algorithm functions. Minimum requirement for TPC is that a single AP needs to be heard by at least 3 other APs at -70dBm or greater. Therefore you must have at least 4 APs total.

TPC use following criteria to determine if a power level change is necessary.

Step 1. Are thre neighbors at -70 dBm or greater ?
Step 2. If step 1 satisfied, determine the transmit power using following equation.

Tx_Max for given AP + (Tx Power Control threshold – RSSI of third highest neighbor)

Step 3. If the value from step 2 with current Tx power level ? Verify whether it exceeds the TPC hysteresis.

If the value is 6 dB or greater, the controller lower the power of the AP by one power level.

If the value is 3 dB , the AP increases its power. A power increase is almost always the result of not having 3rd neighbor.

Here is the TPC configuration page (Wireless -> 802.11a/n or 802.11b/g/n -> RRM ->TPC).

Like DCA, we can select power assignment method for either TPC “Automatic”, “On Demand” or “Fixed” options. Default is “Automatic”.

Min & Max power level you can set. Value range is -10dBm to 30 dBm & default min is -10 & default max is 30.

Power Threshold is the cutoff signal level used by RRM to determine whether to reduce the power of an AP. Default threshold is -70dBm. Range is from -50 to -80. Increase this value (ie between -70 -50) causes the AP to operate at higher transmit power rates.Decreasing the value has opposite effect.

TPC page also shows few non-configurable parameter settings.

1 Power Neighbor Count : The minimum number of neighbors an AP must have for TPC algorithm to run.
2.Power Assignment Leader: The IP Address of RF group leader, who is responsible for power level assignment.
3.Last Power Level Assignmnet: The last time RRM evaluated current TPC assignment.

You can view the same information via CLI by “show advanced {802.11a|802.11b}  txpower” command. Show below is for output of that command. Configurable settings are highlighted.

(WLC1) >show advanced 802.11a txpower 
Automatic Transmit Power Assignment
  Transmit Power Assignment Mode................. AUTO
  Transmit Power Update Interval................. 600 seconds
  Transmit Power Threshold....................... -70 dBm
  Transmit Power Neighbor Count.................. 3 APs
  Min Transmit Power............................. -10 dBm
  Max Transmit Power............................. 30 dBm
  Transmit Power Update Contribution............. SNI..
  Transmit Power Assignment Leader............... WLC1 (10.10.111.10)
  Last Run....................................... 111 seconds ago

For example if you want to change TPC threshod to -75dBm via CLI you can use ” config advanced 802.11a tx-power-control-thresh -75”

In certain deployments where you have power-save clients you sometime needs to defer RRM’s normal off-channel scanning to avoid missing critical information from low volume clients ( eg medical devices that use power-save mode & send periodically telemetry info). You can configure off-channel scanning defer for a WLAN under advanced settings.

By default scan differ priority selected as “4,5,6” which means a AP has a packet UP value of 4 , 5 or 6 in its queue (to send to wireless client) it will defer the off-channel scanning by 100ms. You can change these setting via CLI as well.

config wlan disable 4
config wlan channel-scan defer-priority 6 enable 4 <- WLAN ID 4 for example
config wlan channel-scan defer-priority 4 enable 4
config wlan channel-scan defer-priority 5 disable  4
config wlan channel-scan defer-time 200 4
config wlan enable 4
!
# to verify you can type following show commands 
(WLC1) >show wlan  4                                       
WLAN Identifier.................................. 4
Network Name (SSID).............................. guest
.
.
Quality of Service............................... Silver (best effort)
Scan Defer Priority.............................. 4,6
Scan Defer Time.................................. 200 milliseconds

When configuring this remember following UP values marking for downstream traffic

Bronze marks all downstream traffic to UP=1
Silver marks all downstream traffic to UP=0
Gold marks all downstream traffic to UP=4
Platinum marks all downstream traffic to UP=6

Related Posts

1. RRM Basics
2. Configuring RRM
3. Configuring DCA
4. Configuring CHD
5. Configuring ClientLink
6. Override RRM
7.
8. Rogue Access Point Detection