Understanding the power used by the Access Point 2702i

[aleopoldie]

Hello all,

I was checking the meaning of the transmit power of an AP 2702i.

I have 1 in our lab, connected to a WLC 5508.

On the datasheet of the AP2702, here is what we have : 

Available transmit power settings :

2.4 GHz :

●  22 dBm (160 mW)

●  19 dBm (80 mW)

●  16 dBm (40 mW)

5 GHz :

●  23 dBm (200 mW)

●  20 dBm (100 mW)

●  17 dBm (50 mW)

About the gain : 

●  2.4 GHz, gain 4 dBi, internal omni, horizontal beamwidth 360°

●  5 GHz, gain 4 dBi, internal omni, horizontal beamwidth 360°

we also have the maximum transmit power :

2,4GHz :

●  802.11b -> 22 dBm, 3 antennas

●  802.11g -> 22 dBm, 3 antennas

●  802.11n (HT20) -> dBm, 3 antennas

5GHz : 

5 GHz

●  802.11a -> 23 dBm, 4 antennas

●  802.11n (HT20) -> 23 dBm, 4 antennas

●  802.11n (HT40) -> 23 dBm, 4 antennas

The WLC and the AP are based in Luxembourg, where the EIRP allowed for 2,4 GHz is 100 mW.

But when I run the command show ap config 802.11a / 802.11b over SSH, I get strange informations.

For 2,4GHz, I see that I have 16 dBm (Tx power level 1) which is not the maximum allowed in Luxembourg. (See screenshot in attached)

Why this is not 20 dBm ? Should I consider 16dBm + 4 dBi because of the antennas = 20 dBm ?

Thanks by advance,

AL

[Ric Beeching]

You're spot on with your assumption already, your regulatory domain allows 20dBm EIRP which is the final output of the AP:

EIRP = Transmit Power - Cable Loss + Antenna Gain 

So for you it is 16dBm - 0dBm + 4dBi = 20dBm or 100 mW.

This article is handy for explaining some of the more common terms/equations:

https://supportforums.cisco.com/document/49506/snr-rssi-eirp-and-free-space-path-loss#EIRP_Effective_Isotropic_Radiated_Power

Ric

[aleopoldie]

Hi Ric,

Thank you, that point is clear.

Nevertheless, I made 2 tests and I'm confusing something, I guess. I used antennas omnidirectionnal 2568.

• With 2.4 GHz : 1 with an AP 1572 connected to my WLC, with no antennas connected, with power level fixed to 1. The antenna gain was by default 0 x 0.5 dBi. (See the screenshot "No_Antennas_2.4GHz")      I saw on the WLC that the AP was transmitting at 14 dBm (see screenshot "No_Antennas_2.4GHz_CLI"). That test was ok.

Then I added the 4 omnidirectional antennas, and I made a 2nd test.

I didnt touch to the gain, it was still 0 x 0.5 dBi.

And I got the same result -> 14 dBm.

Then I added the 4 antennas and set 12 x 0.5 sBi on the gain (see screenshot "With_Antennas_2.4GHz") and i still got 14 dBm output (see screenshot "With_Antennas_2.4GHZ_CLI")

With 5 GHz :

With power level 1 /  With the 4 antennas and 0 x 0.5 dBi -> I got 23 dBm on channel 116.

then i set 16 x 0.5 dBi, and I have 21 dBm on channel 116

But how should i consider these values ?  Because I should go up to 30 dBm, but I do not get the results I expect...

AL