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macsrwe
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Automatic power limits are wrong (confirmed)

Thu May 10, 2012 11:25 am

I have an AP driven by a RB711 feeding a single-chain 15dBi omni.

USA FCC rules (assuming I'm not making some stupid mistake in interpreting them) say I should be able to use up to 21dBm on the radio. Reference: http://www.wlanantennas.com/faq.php

If I set the interface to "regulatory domain, United States, antenna gain 15," RouterOS sets the transmit power to 12dBm. Same if I change "United States" to "no country set".

Why does it do this?
Last edited by macsrwe on Fri Jun 22, 2012 10:46 am, edited 1 time in total.
 
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macsrwe
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Re: Automatic power limits are wrong?

Sat May 19, 2012 10:22 am

Nobody knows this? This would seem to be a pretty basic problem with this feature, surely others have come across it...?

On my CPE side, RB711-2Hn, interface is set to station, regulatory domain, USA, 14dBi antenna, and radio power gets automatically set to 13dBm, when regulatory limit should be 22dBm (and actually could be higher if the Mikrotik had any way of knowing that the antenna is directional).

What's up here??
 
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macsrwe
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Re: Automatic power limits are wrong (confirmed)

Thu Jun 21, 2012 9:51 pm

Since I wasn't getting any answers here, I asked support directly. As I read their responses (reproduced below), the bottom line is that the feature currently cannot be relied on for robust work in the USA.

Me: If I set the interface to "regulatory domain, United States, antenna gain 15," RouterOS sets the transmit power to 12dBm. Same if I change "United States" to "no country set".

Why does it do this?


MikroTik: RouterOS wireless settings country="united states" = country="no country set"

In RouterOS we don't keep the track if it is an point to point or point to multipoint link so the recommendation is to follow your guidelines from that website and apply them using manual-tx-power option.


Me: I take your somewhat cryptic first line to mean, "the setting for United States is the same to the software as the setting for No Country Set." This seems to me to be extremely counter-intuitive, but I will take your word for it.

You say that "In RouterOS we don't keep the track if it is an point to point or point to multipoint link," and I certainly understand this. Yet the power setting that RouterOS chooses is wildly inaccurate for EITHER case.

For example, in the case originally quoted, the proper limit for PTMP radio output is 22dBm, and the proper limit for PTP is even higher, depending on antenna gain. Yet RouterOS sets the radio to 13dBm, which is extremely underpowered for either use.

Why does RouterOS choose this limit? If RouterOS is going to both ignore the country "united states" and then choose the wrong limit, why have this feature? Why does the documentation not simply warn users away from using this feature if the results cannot be trusted?


MikroTik: currently the united states country max eirp in the RouterOS is 27. The wireless radio has built in values in the eeprom called band edge limit which are different depending on the frequency that you choose and also depending on the band/rate. You can't change those limits on the board. For you the best and the correct way would be to calculated the EIRP and tx-power manually and then adjust that power by using frequency-mode=manual-txpower and by specifying the output power using tx-power-mode=card-rates and tx-power value.

Me: "currently the united states country max eirp in the RouterOS is 27."

I believe this statement reaches the heart of the problem.

My understanding is that the US max EIRP is 36. See http://www.wlanantennas.com/faq.php#fcc . Please let me know if I am wrong; otherwise, let me suggest that the limit in RouterOS should be corrected.


MikroTik: currently we will not change this implementation as it requires very complex calculations. So you will need to use that table and do the calculation by yourself and then adjust the tx-power values like i recommended in the last email.

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