How does this EIRP cli output correlate??

  • 1
  • Question
  • Updated 4 years ago
  • Doesn't Need an Answer
I have manually configured my B radio at 11dBm and my A radio at 12dBm. When I run the "sh int wifi0" and "sh int wifi1" command it gives the following ouput:

Freq(Chan)=2462Mhz(11); EIRP power=18.01dBm; Diversity=disabled;
Tx range=300m; Noise floor=-95dBm;

Freq(Chan)=5180Mhz(36); EIRP power=19.01dBm; Diversity=disabled;
Tx range=300m; Noise floor=-96dBm;

How does this EIRP cli output correlate to manual settings? Shouldn't they be the same?

Thank you,
Photo of jacob600

jacob600

  • 12 Posts
  • 0 Reply Likes

Posted 4 years ago

  • 1
Photo of Brian Powers

Brian Powers, Champ

  • 396 Posts
  • 92 Reply Likes
http://en.wikipedia.org/wiki/Equivale...

EIRP = Output power - any cable losses + antenna gain

18.01 = 11 dBm - ?? (antenna cable loss, if any) + ?? (what AP/antennas are in use)

19.01 = 12 dBm - ?? (antenna cable loss, if any) + ?? (what AP/antennas are in use)
Photo of jacob600

jacob600

  • 12 Posts
  • 0 Reply Likes
I'm just using the default antenna kit (4dbi omn) with 141 APs. I guess it's normal..???
Photo of Brian Powers

Brian Powers, Champ

  • 396 Posts
  • 92 Reply Likes
So if we ignore cable loss (even though there may be some here, we just cant define it), a quick search for a EIRP calculator (cause I'm lazy)!

http://www.csgnetwork.com/antennaecal...

Converting the 11 dBm to mW = ~12.5 mW and to W = ~.0125 W and using 4 dBi for the antenna gain, puts you at ~0.01915. Plugging that in the calculator from the link above yeilds 0.01915 W. Flopping that back to mW = ~19.15 mW.

Your output from the command gave you 18.01. There may be that much loss in the small antenna cable connecting the antenna to the circuit board. I'd say thats within reason and looks fine.

12 dBm is roughly 16 mW if you wish to do the math on the other (I think its around 24.5 mW).
Photo of alejandro cadarso

alejandro cadarso

  • 14 Posts
  • 0 Reply Likes
The output of the command is dBm not mW
Freq(Chan)=2437Mhz(6); EIRP power=20.01dBm; Diversity=disabled;
The previous output is obtained with radio configuration of 12 in the Power (dBm) setting in an AP-121 . I think It is calculated by adding 4 dBi x 2 because of the 2 antennas the model 121 has, I don't agree with this calculation ..... It must be 16 dBm


Additionally I don't understand how you can set 20 dBm in the setting resulting in the below output

Freq(Chan)=2462Mhz(11); EIRP power=28.01dBm; Diversity=disabled;
Because with ETSI regulation only is permitted a maximum EIRP of 20 dBm (100 mW). I will try to measure tomorrow or after tomorrow real EIRP in my lab.

BRegards
Photo of Crowdie

Crowdie, Champ

  • 972 Posts
  • 272 Reply Likes
Alejandro, I recommend you get a copy of the excellent CWNA Study Guide (http://www.cwnp.com/certifications/cwna) as this covers RF mathematics in some detail.

Photo of alejandro cadarso

alejandro cadarso

  • 14 Posts
  • 0 Reply Likes
Thanks,

Are you really telling me you can use 28 dBm EIRP in ETSI???? 

Maybe you don't know regulations
http://www.etsi.org/deliver/etsi_en/300300_300399/300328/01.08.01_60/en_300328v010801p.pdf
As you can read maximum is 20 dBm. 

You can't use it in FCC regulator domain either,  I think it is 23 or 24 dBm 

FYI, if you have a RF device using ISM band in ETSI domain, the maximum permitted power for a mimo device with two radios is 17 dBm from each of them considering the antenna Gain, but I don't fully agree to define this as a resulting combined 20 dBm EIRP power.

I'm 45 years old and I studied RF mathematics 25 years ago, many years before that book was written, anyway I think 20 dBm is equivalent to 100 mW, don't you agree?

Regards

(Edited)
Photo of Crowdie

Crowdie, Champ

  • 972 Posts
  • 272 Reply Likes

When we write answers to questions we aim them at both the person who originally asked and anybody else who is reading as this is a public forum.  We don't know the technical level or experience of people so we generally try to provide answers that everybody can understand.  So when I recommended the book, which is an excellent book by the way, it was aimed at anybody who does not understand RF mathematics.  If I had known of your knowledge and experience level I would have phrased the response something like "For anybody who would like to know how RF mathematics works I recommend....."
Photo of alejandro cadarso

alejandro cadarso

  • 14 Posts
  • 0 Reply Likes
I completely agree, it's a great book. I read it when it was published. I think that if anybody reads my post must notice I know about dBm, dBi, mW, EIRP, etc

But there is something wrong with the output of that command that I think somebody must answer. Again, it is impossible that the AP can be radiating with 28 or 30 dBm of EIRP power

As I indicated in mi first answer I hope I can measure the signal with a spectrum analyzer during this week with an Aerohive AP in my lab

BRegards


Photo of Crowdie

Crowdie, Champ

  • 972 Posts
  • 272 Reply Likes
I will be interested to see what results you get from your lab.

The point I was trying to make with my response was that I wasn't trying to cause you offense I just don't know your level of knowledge on the subject.  I am wondering whether the CLI output is not allowing for regulatory requirements.  As I have said on other posts a number of vendors, including Cisco, log when a radio has reduced its transmit power due to regulatory requirements and I find this really useful - hint hint Aerohive.
Photo of Andrew Garcia

Andrew Garcia, Official Rep

  • 368 Posts
  • 120 Reply Likes
I'm pretty sure the EIRP value reported by the CLI includes both the antenna gain and the gain from multiple chains.  I believe Jacob's AP in question is a 141, and the standard antenna has 4 dBi gain in both 2.4 and 5 GHz.  Add to that the 3dB gain from the 2 chains, so ~7 dB difference.  

(Edited)
Photo of alejandro cadarso

alejandro cadarso

  • 14 Posts
  • 0 Reply Likes
"Add to that the 3 dB gain from the 2 chains"  . Does this means that Aerohive 141 has some kind of beamforming?. 

Additionally do you know the answer to my question?

in the bellow output from an AP-121 in version 6.1r2 with 20 dBm configured as Power setting
Freq(Chan)=2462Mhz(11); EIRP power=28.01dBm; Diversity=disabled;
How is this calculated  28 dBm calculated?
If this is true, it is not a permitted Tx Power in ETSI regulatory domain ....

BRegards 

Photo of Andrew Garcia

Andrew Garcia, Official Rep

  • 368 Posts
  • 120 Reply Likes
No beamforming in the AP121.  It's something to do with antenna diversity.  2 chains transmitting the same data at the same time effectively doubles the power.  Or that's what the book tells me:)

As for your particular situation, I guess my first question is whether your AP is configured for the right Country Code.  What does a 'show boot-param' tell you for region and country code?

Photo of alejandro cadarso

alejandro cadarso

  • 14 Posts
  • 0 Reply Likes
AP121-LaCcava#show boot-param 
boot parameters:
Device IP: 0.0.0.0
Netmask: 0.0.0.0
TFTP Server IP: 0.0.0.0
Gateway IP: 0.0.0.0
VLAN ID: 0
Native-VLAN ID: 0
Image Download: Enabled
Netboot: Disabled
Boot File:
Netdump: Disabled
Netdump File: 12113080605715.netdump
Region Code: World
Country Code: 724
AC=access category; be=best-effort; bg=background; vi=video; vo=voice;
AIFS=Arbitration Inter-Frame Space; Txoplimit=transmission opportunity limit;
IDP=Intrusion detection and prevention; BGSCAN=background scan; PS=Power save;
HT=High throughput; A-MPDU=Aggregate MAC protocol data unit;
DFS=Dynamc Frequency Selection; CU=Channel interference;
Summary state=High collision;
Mode=access; Radio disabled=no;
Admin state=enabled; Operational state=up;
MAC addr=4018:b19c:fed4; MTU=1500;
Freq(Chan)=2462Mhz(11); EIRP power=28.01dBm; Diversity=disabled;

I confirm you that the Country Code is correct (724) Spain as you can see,  it is equivalent to (826) United Kingdom. Maximum regulatory EIRP is 20 dBm

I don't know which book tell you that, but mine tells that when we have diversity only one tx or rx antenna is used then it doesn't double the power. Additionally as you can see, the command tells "Diversity=disabled"

Anyway in a 100 AP when MIMO is used, 2 transmit chains are used simultaneously when transmitting parallel data streams on the same channel. And this doesn't double de power, are two different chains and you can tx EIRP power of 20 dBm each chain. 

I guess the output of the command is not correct .......

Any explanation/confirmation?
 
Photo of Andrew Garcia

Andrew Garcia, Official Rep

  • 368 Posts
  • 120 Reply Likes
That's what I get for trying to do this from memory, spitting out half correct answers.  Array gain for a 2x2 MIMO system is ~3dBm.

According to Matthew Gast's blog,
"MIMO works by using an antenna array that performs better than a single antenna. (The array gain is related to the number of antennas in the array; with two antennas, the array gain is 10 log (2) = 3.01 dB; with three antennas, the antenna array gain is 10 log (3) = 4.77 dB.)   - See more at: http://blogs.aerohive.com/blog/the-wi-fi-security-blog/did-the-fcc-really-limit-80211ac-beamforming#..."

I am not too familiar with ETSI regulations, so I can't say I know whether array gain is factored into their EIRP limits.  
Photo of alejandro cadarso

alejandro cadarso

  • 14 Posts
  • 0 Reply Likes
that reinforces what I don't understand !!!!!

Without beamforming (Aerohive case MIMO, not Beamforming) no change 0 dB. Then How can be a 28 dBm EIRP with a 4 dBi antenna , this must be 24 dBm, and Aerohive OS shows 28 dBm !!!

And following this,  in ETSI I confirm you that the maximum permitted EIRP is 20 dBm in 2,4 band as you can see in the below link

http://www.etsi.org/deliver/etsi_en/300300_300399/300328/01.08.01_60/en_300328v010801p.pdf

Photo of Andrew Garcia

Andrew Garcia, Official Rep

  • 368 Posts
  • 120 Reply Likes
There seem to be three main questions in play here:
1) How do we derive the EIRP value.  I have provided that answer:
We report the maximum EIRP given your transmit power + antenna gain + array gain.  This answer comes from our engineering team.

2) Why does your AP report 28.01 dBm for EIRP?
Given the above calculation and your transmit power, I would expect the AP to report 27.01 dBm.  There is a 1 dB discrepancy between your AP report and expectation.  I don't have a good answer for why that 1 dB discrepancy is there.

3) Why is your AP reporting an EIRP above the ETSI requirements for your country code, and if the EIRP is accurate, why isn't this accounted for in the max transmit power? 
Unfortunately, i don't have an answer for this.

I suggest you contact your support representatives to engage in a deeper conversation about #2 and #3.


Photo of alejandro cadarso

alejandro cadarso

  • 14 Posts
  • 0 Reply Likes
thanks for your answers Andrew

For answer #1 of course I believe it comes from your engineering, but I completely disagree with it. I state that is not EIRP as per definition of it

For questions #2 and #3 I will open a support case but I am pretty confident that the value shown in the output of the command is not the one really tx

BRegards 



Photo of Roberto Casula

Roberto Casula, Champ

  • 231 Posts
  • 111 Reply Likes
This is a very interesting topic and I'd like some clarity myself.

First of all, to clean up some confusion...

What ETSI limit is the "RF Output Power" which is the gain-adjusted RMS of measured power levels at the equipment's transmit port over the sample period of a "transmission burst". The calculation has to take the RMS of the "worst case" transmission burst and then add an adjustment both for antenna gain and an "array gain" to accommodate "smart antenna" systems, which covers systems that use beamforming and other array techniques.

It's easy to see why this is necessary. The measurements are taken "raw" at the port - we have to take into account the involvement of the antennae in the RF transmission process. This used to be a fairly simple matter of adding the stated antenna gain, but multiple transmit chains now complicate matters, especially with "smart antennae" where the different transmit chains do not transmit identically.

With multiple transmit chains, the simultaneous "raw" measurements taken at each transmit port are summed PRIOR to the RMS calculation being performed. In the case of beamforming and other similar techniques, this therefore isn't simply a "doubling" of the instantaneous power levels as the phasing of the output signals is variable. ETSI therefore take the measurements as they are but adjust the final RF Output Power figure by an additional "gain" due to beamforming or other array technique (they refer to it as "beamforming gain" but "array gain" is a more general term).

In the case of the Aerohive APs with beamforming disabled, there should be no "array gain" to factor in.

The final result of this calculation of RF Output Power must not exceed the 20dBm limit.

It's also interesting to note the specifics of the test criteria for 802.11n vs prior versions of the standard (see Annex D of ETSI EN 300 328 v1.8.1). The PHY may (does) use very different transmission characteristcs depending on the MCS being used. The ETSI testing requires the manufacturer to test the equipment using the "worst case" MCS.

Now I am not certain exactly what it is that Aerohive reports as "EIRP power" (I'll ignore the fact that the P in EIRP already stands for power!). One possibility is it refers to peak or unmodulated EIRP; this will obviously be higher than the RMS value across a "transmission burst" (remember for the simple case of a sine wave, peak = rms / 0.707; that's an 8.3dB difference). It may even be the average peak EIRP across all possible MCS choices.

An interesting fact is that if you change the radio profile phymode on an AP121 from 11ng to 11b/g - the "EIRP power" reported changes from 28.01dBm to 25dBm for a 20dBm tx power setting...

What I find even more interesting is the difference in that value between an AP120 and an AP121 with the same tx power setting; both are 2x2:2 with 4dBi 2.4GHz antennae yet the "show interface wifi0" for a tx power setting of 20dBm gives an "EIRP power" of 23.01dBm on the AP120 and 28.01dBm on the AP121. Whence the 5dBm difference???

One of the key stated differences between the AP120 and AP121 was an "increased rate over range" - maybe this is the reason for the difference, i.e. that there is a bigger variation in power depending on the MCS so that the average of the peak EIRPs is significantly different.

Just my musings...I'd be very interested in an official response from Aerohive that "makes sense" (I'm afraid the responses so far really don't, as Alejandro points out).
Photo of alejandro cadarso

alejandro cadarso

  • 14 Posts
  • 0 Reply Likes
Hi Roberto,

I didn't know the difference with the 120 !!!, I can tell that AP170 is showing 28 dBm as the AP121

I had a support case related with TX powers 3 years ago with AP340 in 5Ghz, and they never gave me a good response .....

I tried to measure with wi-spy the power of the AP121 in my lab , but it is very difficult to guess the multipath losses because of the office environment.
What I can tell is that I tested with an AP of another vendor (which I am completely sure it is tx with 28 EIRP including its antenna) simultaneously with the AP121 in other channel, and I can see consistently that the signal is always lower for the 121, between 5 and 12 dBm

I also would like an official response from Aerohive too, with your technical level and a clear explanation, not the one they give us :-)