Question about TXPower (WLC9K8 and C9120AXI-E AP)

[nanu]

Hi,

We are facing some strange behaviuor regarding the txpower of C9120AXI-E power, i will show you some outputs (I sanitized the output for privacity reasons), this is the output of show ap dot11 24Ghz summary:

AP1 MACXXXX 0 Enabled Up 20 *1/7 (12 dBm) (11)* REAP
AP2 MACXXXX 0 Enabled Up 20 *1/7 (12 dBm) (1)* REAP
AP3 MACXXXX 0 Enabled Up 20 *3/7 (6 dBm) (11)* REAP
AP4 MACXXXX 0 Enabled Up 20 *2/7 (9 dBm) (6)* REAP

As far I known, the 1/7 means from 7 levels of power, 1 Is the maximum available, 2 will be 50%, 3 will be 25%... that's correct?

The Quesiton is, why at maxium 1/7 ths AP only transmit to 12dBm when per datahseet the has more power?

Thank you,

[pieterh]

it means in the current environment, present clients and surrounding AP's a low power level is sufficient.
"*1/7 " -> the asterix in front means power is dynamically assigned.
This also means power can be increased when the AP detects a client needs more signal.
but to limit power consumption and reduce interference, the current power level is at minimum.

[nanu]

Hi Pieter,

But, how can I force the AP to radiate to more than 12dBm? I mean, I understand the 1/7 and the * in front, but how this maximum 12dBm was selected? If you take a look to the datahseet there is a maximum of 23dBm and in Europe the maximum EIRP for 24Ghz is 20dBm and this AP supports up to 23dBm:

https://www.cisco.com/c/en/us/products/collateral/wireless/catalyst-9120ax-series-access-points/datasheet-c78-742115.html

How can I force the maximum txpower to 20dBm?

Thank you,

[pieterh]

there is a difference between EIRP and transmit (radio) power.

>>> in Europe the maximum EIRP for 24Ghz is 20dBm and this AP supports up to 23dBm:<<<

1) as this is an C9120AXI-E AP (-E- regulatory domain) the internal radio electrical circuit can give more power, but  the AP is hardware/firmware limited to never radiate more than 20dBm!

2) you need to take into account the (internal) antenna!

● 2.4 GHz, peak gain 4 dBi, internal antenna, omnidirectional in azimuth 
as the antenna adds 4dB gain the radio needs to be throttled down to keep the EIRP (= radiated power by the antenna !) at 20dBm . 20-4=16 -> steps are 11, 14,17 -> still some mismatch with 12 (17 would result in 17+4=21 dBm !)

3) if you enabled multiple countries on your wireless controller, you end up with the lowest common settings for power and channels.

[nanu]

Hi Pieterh,

How you get the steps 11, 14, 17?

Then, following this explanation, 17 overpass the 20 limit regulation, so ins't a available.

Then why the WLC isn't chossing 14 at maximum power and chosing 12 instead? Where this 12 is calculated?

Thank you!

[pieterh]

yes, I too cannot fully explain the "12dBm" it does not even show in the table,

there may be some other parameter that needs to be included in the calculation

steps 11,14 and 17 dBm are from the 9120AX datasheet,  

Available transmit power settings

2.4 GHz ●  23 dBm (200 mW) ●  20 dBm (100 mW) ●  17 dBm (50 mW) ●  14 dBm (25 mW) ●  11 dBm (12.5 mW) ●  8 dBm (6.25 mW) ●  5 dBm (3.13 mW) ●  2 dBm (1.56 mW) ●  -1dBm (0.79mW) ●  -4dBm(0.39mW)

5 GHz ●  26 dBm (400 mW) ●  23 dBm (200 mW) ●  20 dBm (100 mW) ●  17 dBm (50 mW) ●  14 dBm (25 mW) ●  11 dBm (12.5 mW) ●  8 dBm (6.25 mW) ●  5 dBm (3.13 mW) ●  2 dBm (1.56 mW) ●  -1dBm (0.79mW)

[JPavonM]

According to Cisco c9120 data sheet:

2.4 GHz

●  23 dBm (200 mW)

●  20 dBm (100 mW)

●  17 dBm (50 mW)

●  14 dBm (25 mW)

●  11 dBm (12.5 mW)

●  8 dBm (6.25 mW)

●  5 dBm (3.13 mW)

●  2 dBm (1.56 mW)

●  -1dBm (0.79mW)

●  -4dBm(0.39mW)

Also 5GHz summary appears the same way, 8 out of 10 power levels only (using channel 36 should show up to 23 dBm, so 9 levels) and AP currently set to Pwr Lvl 2 (14 dBm) what appears to be wrong either.

Available power levels depend on the RF domain or configured channels (in the case of 5 GHz band).

There should be any reason why C9120 is not using proper configured power level. Maybe a bug. I have a TAC case open because of this.

HTH
-Jesus
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[nanu]

Hi Jesús,

Regarding 5Ghz Band:

AP1 MAC_X 1 Enabled Up 40 *1/8 (23 dBm) (136,132)* REAP --> This looks correct.

But another AP:

AP3 MAC_Y 1 Enabled Up 40 *1/8 (20 dBm) (136,132)*---Here for some reason maximum txpower is 20dBM.

I think we are misunderstanding something.. I don't think this can be another bug with this combination of AP and WLC.. :(!

Thank you ,

[Jurgens L]

Can you run the following command on the RF profile you currently have applied to the AP's?
C9800# show ap rf-profile name <your_24ghz_rf_profile> detail

[nanu]

Hi Jurgens;

WLC9800#show ap rf-profile name RF-P3-24gh detail

Description :
RF Profile Name : RF-P3-24gh
Band : 2.4 GHz
Transmit Power Threshold v1 : -70 dBm
Min Transmit Power : -10 dBm
Max Transmit Power : 30 dBm
Operational Rates
802.11b 1M Rate : Unsupported
802.11b 2M Rate : Unsupported
802.11b 5.5M Rate : Unsupported
802.11b 11M Rate : Mandatory
802.11b 6M Rate : Unsupported
802.11b 9M Rate : Unsupported
802.11b 12M Rate : Supported
802.11b 18M Rate : Supported
802.11b 24M Rate : Supported
802.11b 36M Rate : Supported
802.11b 48M Rate : Supported
802.11b 54M Rate : Supported
Max Clients : 200
Trap Threshold
Clients : 12 clients
Interference : 10%
Noise : -70 dBm
Utilization : 80%
Multicast Data Rate : auto
Rx SOP Threshold : auto
Band Select
Probe Response : Disabled
Cycle Count : 2 cycles
Cycle Threshold : 200 milliseconds
Expire Suppression : 20 seconds
Expire Dual Band : 60 seconds
Client RSSI : -80 dBm
Client Mid RSSI : -80 dBm
High Speed Roam
hsr mode : Disabled
Load Balancing
Window : 5 clients
Denial : 3 count
Coverage Data
Data : -80 dBm
Voice : -80 dBm
Minimum Client Level : 3 clients
Exception Level : 25%
DCA Channel List : 1,6,11
Unused Channel List : 2,3,4,5,7,8,9,10,12,13
DCA Foreign AP Contribution : Enabled
802.11n MCS Rates
MCS 0 : Enabled
MCS 1 : Enabled
MCS 2 : Enabled
MCS 3 : Enabled
MCS 4 : Enabled
MCS 5 : Enabled
MCS 6 : Enabled
MCS 7 : Enabled
MCS 8 : Enabled
MCS 9 : Enabled
MCS 10 : Enabled
MCS 11 : Enabled
MCS 12 : Enabled
MCS 13 : Enabled
MCS 14 : Enabled
MCS 15 : Enabled
MCS 16 : Enabled
MCS 17 : Enabled
MCS 18 : Enabled
MCS 19 : Enabled
MCS 20 : Enabled
MCS 21 : Enabled
MCS 22 : Enabled
MCS 23 : Enabled
MCS 24 : Enabled
MCS 25 : Enabled
MCS 26 : Enabled
MCS 27 : Enabled
MCS 28 : Enabled
MCS 29 : Enabled
MCS 30 : Enabled
MCS 31 : Enabled
State : Up
Client Aware FRA : Disabled
Client Network Preference : default
Airtime Fairness Mode : Disable
Airtime Stealing : N/A
Mesh Client Access Airtime Alloc : Disabled

Thank you team for your help!

[Jurgens L]

Can you confirm if the AP is connected to PoE+ switches? It seems that the AP's are not getting enough power and therefore it reduces the dBm for the same power level.
Example of 9120 getting 25.5W:
C9120-01 xxxx.xxxx.xxxx 0 Enabled Up 20 7/8 (5 dBm) (11)*

Example of 9120 getting 15.4W:
C9120-01 xxxx.xxxx.xxxx 0 Enabled Up 20 7/8 (2 dBm) (11)*

[nanu]

Hi Jurgens:

Please, see the attached below, as you can see are saying "Configured" where is this value configured? I cannot see in my configuration, as per power looks like is using 30W.

Radio Info Summary:

=======================

Radio: 2.4GHz

Carrier Set: (-E)  ( ES )

Base radio MAC: 5C:E1:76:18:61:A0

Supported Channels:

1 2 3 4 5 6 7 8 9 10 11 12 13

Configured Frequency: (Slot 0) 2412MHz  Channel:1  20MHz

Radio Mode: FlexConnect

Radio SubType: Main

FRA Capable: False

Serving Frequency: (Slot 0) 2412Mhz Channel: 1 20MHz

Configured TX Antenna Selection: [ a b c d         ]

Configured RX Antenna Selection: [ a b c d         ]

Number of Transmit Antennas: 4

Configured Antenna Gain(dBi): 5

Legal Antenna Gain in use(dBi): 4

Configured Level: 1

Configured TxPower(dBm): 12

Total Supported Power Levels:7

Allowed total powers(dBm):

12 9 6 3 0 -3 -4

Allowed per-path powers(dBm):

6 3 0 -3 -6 -9 -10

Current Band              : 2.4GHz

Channel Width            : 20

***** show cdp inline_power *****

Power_Requested(mW) Power_Available(mW) Power_request-ID Power_management-ID

              30000               30000            59167                   4

Thank you,

[pieterh]

>>> like is using 30W. <<<

this is not fully correct it means it is REQUESTING 30W (max.). 
the switch needs to RESERVE 30W for this AP when determining total amount of power needed for all connected PoE devices.

>>>
Allowed total powers(dBm): 12 9 6 3 0 -3 -4
Allowed per-path powers(dBm): 6 3 0 -3 -6 -9 -10
<<<

Now this is interesting this suggests that also (MU-)MIMO contributes to maximum power !

 it looks like the earlier calculated 16dBm EIRP must be calculated for the whole AP , hence all transmitting radios, as there are 4 radios/antenna present in the AP where all radios active on a stream must be added to calculate EIRP, this could explain the 12dBm for a single radio!

this is new to me, never came across this before in power calculation!
but this led me to this acticle Directional Gain and EIRP Calculations for Transmitters with Multiple Outputs

[Jurgens L]

Do a show power inline on the switch the AP is connected and that should indicate the amount of power the AP consume. You want to see the AP consume 25.5W to be on full power.
I've done some tests in my lab after seeing this post and to my surprise, I saw the dBm values decrease by 3dBm when decreasing the power inline of the switch port to 15.4W while the WLC power level (7/8) still stayed the same, reason I'm curious to see if your AP's are getting the required power they need.
The 9120's are much more efficient compare to the 2800 series were the 2800s would normally shut it's radios when it on gets only 15.4W. Cisco market the 9120 very clearly that it can operate with fewer radios of course when on 15.4W, but nothing mentioned about reduced dBm. Which I assume could lead to MIMO as you only operate at 1x1 antennas when on 15.4W

[pieterh]

>>>  It seems that the AP's are not getting enough power and therefore it reduces the dBm for the same power level. <<<

I disagree on this!

if the AP gets insufficient power it will shut down the radio (only the 5GHz or even both), not reduce the power level
your different power level on 2.4GHz has the same reason as the original question : Radio Resource Management