It's a pretty safe assumption that TPC raises power levels in addition to lowering them, otherwise why would there be a hysteresis for it?
But you have the general idea correct. Power levels are changed until you get within the hysteresis range. Here is another way of writing the formula (in my opinion). Max AP power - final AP power is approximately equal to 3rd loudest neighbor - TPC threshold The only real variables that we have any say over would be the final AP power and the TPC threshold. Max AP power and 3rd loudest neighbor are theoretically static. The only thing that we can configure is the TPC threshold. So when it comes to an exam, you should be able to assume that you will be either be told the threshold and asked what power level it would settle into, or given the desired power level and asked what to set the threshold to. If you are told the desired power level, just configure the threshold accordingly. If given the threshold, figure out what power will get the equation closest to 0. Assuming you are starting at a higher power and working down towards the desired power, you may want to choose the final AP power that yields a result between 0 and 2. Again, this is just my 2 cents based on less than clear documentation and my own thought process. Really wish they would put out an updated document. Jeff Rensink - CCIE #24834 (Wireless, R&S) Senior Technical Instructor - IPexpert On Thu, Sep 12, 2013 at 8:47 AM, Jeen Sern Chew <[email protected]> wrote: > Thanks Jason and Jeff, > > It is really confusing, especially with conflicting information out there, > like with the hysteresis of being 3 dBm both ways or 3 dBm one and 6dBm the > other. > > Also after calculating, how would you know to increase or decrease the > power? > > The following is how I understand it after some thinking. Some of the > figures are just base on real world experience. Say you do a voice survey > and place your APs to provide coverage of -67 dBm, 20% overlap and power > level of 6-25 mW. The following are the assumptions: > - Cisco formula as mentioned above is correct > - TPC only decreases power > - Hysteresis of 3 dBm > - There's another APs see each other rather than by transmitting neighbour > message at full power > > Starts off at level 1 / 100mW. I chose the 3rd highest RSSI to be -50 dBm > as that would happen in a voice/high density survey. > > 20 - (-67 - (-50)) = 20 - (-17) = 37 Hysteresis met, decrease > from level 1 / 20dB to level 2 / 17dB > 20 - (-67 - (-60)) = 20 - (-7) = 27 Hysteresis met, decrease > from level 2 / 17dB to level 3 / 14dB > 20 - (-67 - (-70)) = 20 - 3 = 17 Hysteresis met, decrease > from level 3 / 14dB to level 4 / 11dB > 20 - (-67 - (-80)) = 20 - 13 = 7 Hysteresis met, decrease > from level 4 / 11dB to level 5 / 9dB > 20 - (-67 - (-85)) = 20 - 18 = 2 Hysteresis not met. Stay > at level 5 / 9dB > > At level 5, it is close to the figures of some deployments out there. > > Does this calculation make sense? Did I miss anything? If it correct, > great! Just need to confirm the assumptions above then. > > Thanks > J Chew > > > On 12 September 2013 01:39, Jeff Rensink <[email protected]> wrote: > >> 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
