Wireless Access

Hi,

I've noticed that even though i've configured an 802.11a ARM profile with an ARM range of min 3 dBm and max 3 dBm, when I do a 'show ap active' command the AP is showing an EIRP of 4.5 dBm. I'm wondering why it's adding an extra 1.5 dBm to the transmit power and if this can be controled or configured? I beleive I've seen this behaviour with other WAPs as well and different WLC's. I'm using code 6.5.1.9 and an AP225. Below is an output of the ARM profile and the 'show ap active command':

#show rf arm-profile dot11A-ARM-Profile

Adaptive Radio Management (ARM) profile "dot11A-ARM-Profile"
---------------------------------------------------------
Parameter Value
--------- -----
Assignment single-band
Allowed bands for 40MHz channels None
80MHz support Disabled
160MHz-support None
Client Aware Enabled
Max Tx EIRP 3 dBm
Min Tx EIRP 3 dBm
Rogue AP Aware Disabled
Scan Interval 10 sec
Aggressive scanning true
Active Scan Disabled
ARM Over the Air Updates Enabled
Scanning Enabled
Multi Band Scan Enabled

# show ap active

Active AP Table
---------------
Name Group IP Address 11g Clients 11g Ch/EIRP/MaxEIRP 11a Clients 11a Ch/EIRP/MaxEIRP AP Type Flags Uptime Outer IP
---- ----- ---------- ----------- ------------------- ----------- ------------------- ------- ----- ------ --------
b4:5d:a0:56:54:a3 MHF 192.168.46.17 0 1 AP:VHT:161/4.5/22.5 OAW-AP225 Aa 16h:31m:37s N/A

Flags: 1 = 802.1x authenticated AP; 2 = Using IKE version 2;
A = Enet1 in active/standby mode; B = Battery Boost On; C = Cellular;
D = Disconn. Extra Calls On; E = Wired AP enabled; F = AP failed 802.1x authentication;
H = Hotspot Enabled; K = 802.11K Enabled; L = Client Balancing Enabled; M = Mesh;
N = 802.11b protection disabled; P = PPPOE; R = Remote AP;
S = AP connected as standby; X = Maintenance Mode;
a = Reduce ARP packets in the air; d = Drop Mcast/Bcast On; u = Custom-Cert RAP;
i = Provisioned as Indoor; o = Provisioned as Outdoor;
r = 802.11r Enabled
Q = DFS CAC timer running

Channel followed by "*" indicates channel selected due to unsupported configured channel.
"Spectrum" followed by "^" indicates Local Spectrum Override in effect.

Channel flags: +/- = 40 MHz, E = 80 MHz, S = 160 MHz, E+E = 80 + 80 MHz (i.e. 36E+149E)

Num APs:1

Thanks in advance

My understanding about EIRP in was that it's the measurment of the signal as it leaves the antenna (and that it calculates the loss between the radio and antenna as well as the gain from the antenna itself).

Looking at the AP225 datasheet again I can see it mentions that the internal loss between the radio and antenna is 3 dB in 5 GHz and that the max antenna gain is 4.5 dBi in 5 GHz, which ends up being a gain of 1.5 dBm which is what I'm seeing. 

So I guess this mean that the EIRP in the ARM profile is the dBm generated at the radio interface and we should take into account each WAP's properties to know what the final EIRP will be?

EIRP is total conducted power minus any cabling losses, plus antenna gain.

So for example on the AP-225, the max conducted power (at the internal antenna connectors) is 18dBm. Since there are 3 radio chains for each radio, the aggregate conducted power is three times that, or +4.8dB, for a total of 21.8dBm.

The internal cabling loss applies to the AP-224 (loss between the RF connectors on the radio cards and external connectors, which in this case also includes diplexing circuitry). For AP-225, all internal losses are already accounted for in the 18dBm number.

Adding antenna gain to the 21.8dBm number results in the max EIRP numbers for each of the two radios of the AP-225.

Note that antenna gain varies by direction; the number that's reported in the datasheet is the max gain in a particular direction, so actual EIRP will vary in the 3D space around the AP. In general it is so that the the higher the (max) gain, the more variation in gain there is as well (since antenna gain does not change the actual amount of power transmitted; it only focuses the energy).

The EIRP power numbers and settings in our software are based on the max gain number.

Finally, EIRP is typically limited by regulatory rules, and our software will often have to back off the conducted power level to ensure that we do not exceed the applicable EIRP limits.

Hope this helps.