This is a frustrating topic indeed. TPC will do both power decreases and increases as the environment changes. But the formula that the document references has a flaw that I haven't totally reconciled.
Tx_Max for given AP + (Tx power control thresh – RSSI of 3rd highest neighbor above the threshold) Tx_Max for a given AP should be a static value, correct? It's supposedly the max possible power that the radio can transmit at for its chosen channel. The Tx power control threshold is also a static value that is configurable. So the assumption is that the RSSI of the 3rd loudest neighbor changes along with the current AP power level. If I understand things correctly, the RSSI of the 3rd loudest neighbor is determined by AP neighbor messages. According to documentation (and I believe I've tested this myself), neighbor messages are always sent out at the highest power level and lowest data rate. If that is indeed true, then the RSSI of the third loudest neighbor is yet another static value (assuming the environment isn't changing). So if none of the variables in the equation are actually variable, the results of the equation are always the same. That would mean either the power level would always settle into the max power or the minimum power. That's not true, so something seems off in the equation or the description of the variables in the equation. My guess is the formula is closer to (Max_AP_Power - Current_AP_Power) - (3rd_loudest_neighbor - TPC_Threshold) A positive result would result in a power increase. A negative result would result in a power decrease based on the 3 dBm hysteresis. According to Jerome Henry, it's 3 dBm in both directions in the 7.0.116.0 code. This assumes that the 3rd loudest neighbor measurement is pretty much always the same since neighbor messages are always sent as max power. So let's say we start at full power on the 2.4 GHz band with a 3rd loudest neighbor of -60 and threshold of -67. (20-20)-(-60-(-67)) = 0-7 = -7 -7 is lower than -3, so we drop the power down a level (20-17)-(-60-(-67)) = 3 - 7 = -4 -4 is lower than -3, so we drop the power once more. (20-14)-(-60-(-67)) = 6 - 7 = -1 -1 is not lower than -3, so it doesn't change. This is a total guess, but it makes more sense to me than the formula in the documentation. But in the real world, the higher the TPC threshold, the higher the power levels will be. For instance, a threshold of -60 would result in higher power levels than a threshold of -70. So if you are seeing your APs in a particular location settle in at too high or too low of power levels on average, tweak the threshold as needed. Every increment of 3 should result in powers going up/down one level on average. Jeff Rensink - CCIE #24834 (Wireless, R&S) Senior Technical Instructor - IPexpert On Wed, Sep 11, 2013 at 9:14 AM, Jason Boyers <[email protected]> wrote: > This is a confusing algorithm. Basically, think of it this way: > > 1) Three neighbors (at least) hear an APs radio at or stronger than the > TPC Tx mac value configured under the WLC TPC settings for that PHY > (802.11a/n or 802.11/b/g/n) > 2) If #1 is true, then that formula provided is used. > - The "Tx_Max for given AP" is the maximum transmit power for that channel > on that particular radio. So, for a 2.4GHz radio, that would be 100mW (20 > dBm), since that is the maximum for 802.11b. 5GHz is more confusing, as it > various based on channel and model of AP. > - The "Tx power control thresh" is the value that is entered in the WLC > - Note that there is an "hysteresis" value of 3dB for decreasing and 6dB > (or it was at one point - someone correct me for the current values) for > increasing the power levels. In other words, the value that is given at > the end of the formula has to be at least that dB difference from the > current value in order for a change to occur. > 3) The AP's radio that is heard by the other three APs is the one that is > decreased (or it can be increased) if the hysteresis value is exceeded. > 4) When the formula is run again, it is presumed that the three neighbors > that are hearing the APs signal are hearing a weaker signal. > 5) Keep in mind that the algorithm is always from the perspective of an AP > being heard by 3 or more neighbors, not an AP hearing 3 or more neighbors. > > Hope this helps. > > Jason Boyers, CCIE #26024 (Wireless) > Blog: netboyers.wordpress.com > > > On Wed, Sep 11, 2013 at 9:36 AM, Jeen Sern Chew <[email protected]>wrote: > >> Hey guys, >> >> I have a TPC question. >> >> There are many conflicting information from different Cisco docs. Some >> say TPC increase and decrease power, some say TPC only decreases power >> while CHD increases power. Does TPC do both or only increase? >> >> Also Cisco gives the TPC formula of: >> Tx_Max for given AP + (Tx power control thresh – RSSI of 3rd highest >> neighbor above the threshold). >> >> Is the Tx_Max is the Tx_Max of the specific AP? or Is it the Tx_Max >> configured on under TPC in WLC? >> >> Also, when the calculation is done, I am assuming the power >> increase/decrease occurs on the neighbour with the third highest RSSI? >> >> Thanks >> >> Regards, >> J Chew >> >> _______________________________________________ >> For more information regarding industry leading CCIE Lab training, please >> visit www.ipexpert.com >> >> Are you a CCNP or CCIE and looking for a job? Check out >> www.PlatinumPlacement.com >> >> > > _______________________________________________ > For more information regarding industry leading CCIE Lab training, please > visit www.ipexpert.com > > Are you a CCNP or CCIE and looking for a job? Check out > www.PlatinumPlacement.com > >
_______________________________________________ For more information regarding industry leading CCIE Lab training, please visit www.ipexpert.com Are you a CCNP or CCIE and looking for a job? Check out www.PlatinumPlacement.com
