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
