Configure Transmit Power Control

This is one of the features of RRM on WLC and in this post we will see and learn the option under TPC.

This algorithm is responsible for reducing the power level on the APs to reduce excessive cell overlap and co-channel interference. TPC uses the RSSI calculations for the neighbor APs, and it determines effective changes only if there are more than three neighbor APs.

The TPC algorithm runs every 10 minutes (600Secs). The RF group leader runs TPC on a per-radio, per-AP basis. Therefore, a power adjustment on 802.11b/g has no bearing on the 802.11a power level settings for the same AP.

The minimum requirement for TPC is that a single AP needs to be heard by at least three other APs at -70 dBm or greater. Therefore, we must have at least four APs total. The logic behind the lowering of the power levels is that the third loudest neighbor is heard at -70 dBm or lower after the change.

The final purpose of the algorithm is to make sure that the third-loudest neighbor AP is heard at a signal level lower than the configured threshold (by default its –70 dBm).

***Note: The TPC algorithm is only responsible for turning power levels down.

TCP goes through these stages which decide if a transmit power change is necessary:

  1. Find out if there is a third neighbor, and if that third neighbor is above the transmit power control threshold (-70dBm).
  2. Determine the transmit power using this equation:

Tx_Max for given AP + (Tx power control thresh – RSSI of 3rd highest neighbor above the threshold).

  1. Compare the calculation from step two with the current Tx power level and verify if it exceeds the TPC hysteresis.
  • If Tx power needs to be turned down: TPC hysteresis of at least 6dBm must be met. OR
  • If Tx power needs to be increased: TPC hysteresis of 3dBm must be met.

***Note: When a brand new APs boot up for the first time, it transmit at their maximum power level (its 1). When AP is power cycled, it uses their previous power settings.

***Note: It is important to remember that decreases in AP radio power levels are gradual, whereas increases can take place immediately. Therefore, if we change the RRM configuration settings, do not expect to start seeing the APs changing channels and adjusting their power as soon as we click Apply.

Now we will see the configuration steps@TPC

Via GUI:

Go to Wireless -> 802.11a/n or 802.11b/g/n -> RRM ->TPC

On this screen we have these options:

Power Level Assignment Method: There are 3 ways to configure TPC algorithm:

  • Automatic: This is the default configuration and the TPC algorithm runs every ten minutes (600 seconds).
  • On Demand: The algorithm can be manually triggered if we click the Invoke Channel Update Now
  • Fixed

Min/Max Power: Maximum and minimum power level assignment and we can choose between -10 to 30dBm.

Power Threshold: Default value for this parameter is –70 dBm but can be changed when access points are transmitting at higher (or lower) than desired power levels.

Power Neighbor Count: The minimum number of neighbors an AP must have for the TPC algorithm to run.

Power Assignment Leader: This field displays the IP address of the WLC that is currently the RF Group Leader. Because RF Grouping is performed per-AP, per-radio, this value can be different for the 802.11a & 802.11b/g networks.

Last Power Level Assignment: The TPC algorithm runs every 600 seconds (10 minutes). This field only indicates the time (in seconds) since the algorithm last.

Verification:

(WLAN1) >show advanced 802.11a txpower
 Automatic Transmit Power Assignment
 Transmit Power Assignment Mode................. OFF
 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............... WLAN1 (10.35.80.1)
 Last Run....................................... 98 seconds ago
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2 Comments

  1. Hi,
    In an enterprise wlan , what are the uses cases of tpc set to be ‘fixed’. Did you ever come across a situation like that ? . Another thing i noticed one of my AP running channel 11 and i put another ap in the same channel (non cisco ) next to it , i did not find anything like cisco changing channel to another non overlapping channel . Is there any relation between this and that ?

  2. 1. With fixed power levels there is no degree of freedom for WLC to address channel re-use interference by neighbor AP´s or Rogue AP´s. I recommend to use TPCv1 Auto power which gives WLC the ability to optimize each AP´s power Level of the own Network to compromise coverage, interference and maximize it for total network throughput. This is advantageous especially in dense WiFi Environments. We did excessive testing in closed cabin Environments with high AP density and found that WLC Auto power worked quite reliable.

    2. Putting a rogue AP next to a Cisco AP doesn´t mean that the Cisco AP will change immediately the channel. In general WLC (or the DCA function respectively) assess the RF environment and implements a better channel scheme (swap channels) only if the results gives an improvement of the total performance (e.g. throughput or SNR). If e.g. the channel utilization of the rogue AP is very low, there is no reason to change the channel, especially if own neighbor AP´s utilize the adjacent channels in g-band (because there are only 3 non-overlapping channels available).

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