How does this work?

Posted Sep 26, 2010 7:19 UTC (Sun) by johill (subscriber, #25196)
In reply to: How does this work? by gutschke
Parent article: Broadcom firmware and regulatory compliance

Err, no, regulatory restrictions aren't in place to stop wifi devices from interfering with each other (after all, they use carrier sensing and all play nice with each other) -- they are in place to avoid interfering with, to take a recent example, weather radars.

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How does this work?

Posted Sep 27, 2010 5:55 UTC (Mon) by jzbiciak (guest, #5246) [Link] (2 responses)

Right, but if there are no other WiFi devices around, presumably the card would fall back into a much more passive mode that's likely to be in the intersection of all domain's requirements. ie. I doubt it'd come out with guns-a-blazin', pumping out 10W until it got a friendly tap on the shoulder that told it "Hey, you're only allowed 100mW peak around these parts."

Rather, if the supported regulatory domains had maximum transmit powers of, say, 120mW, 100mW and 80mW, in the absence of any guidance to the contrary, I'd imagine it'd stick to under-80mW until it got a whiff of what regulatory environment it was in.

After all, when you drive on a new local road you've never driven, do you floor it and go 65MPH until you see a speed limit sign telling you it's 30MPH? (100km/h and 50km/h if you prefer.) Somehow I doubt it. You probably look at the other cars and see what range of speeds they're going, and if there's no other traffic, you probably make an educated guess what an appropriate speed would be.

How does this work?

Posted Sep 27, 2010 5:57 UTC (Mon) by dlang (guest, #313) [Link] (1 responses)

the biggest problem isn't the power level (as you point out it's easy to do something that's safe everywhere)

the problem is what frequencies are you allowed to transmit on. guess wrong and you will never talk on the same frequency as the access point, or you could start interfering with other things and get into more trouble.

How does this work?

Posted Sep 27, 2010 6:29 UTC (Mon) by jzbiciak (guest, #5246) [Link]

Fair enough and a good point I hadn't considered.

On further reflection, it seems like a computer operating in something resembling a infrastructure mode client would have little reason to transmit until it saw something that gave it an idea of what frequencies and powers were appropriate.

And, since I'm actually allergic to too much idle speculation[*] without a good dose of raw facts, I actually had a go at looking up the relevant standard. Indeed, the actual default is "passive scanning" (highlighted below):

9.9.1 Operation upon entering a regulatory domain

A STA that is enabled for operation across regulatory domains shall default to passive scanning when it has lost connectivity with its ESS. Passive scanning is performed using only the receive capabilities of the station and is, thus, compatible with regulatory requirements. The timeout for determining the loss of connectivity is system dependent and beyond the scope of this standard.

When a STA enters a regulatory domain, it shall passively scan to learn at least one valid channel, i.e., a channel upon which it detects IEEE Std 802.11 frames. The Beacon frame contains information on the country code, the maximum allowable transmit power, and the channels to be used for the regulatory domain. Optionally, the Beacon frame may also include, on a periodic basis, the regulatory information that would be returned in a Probe Response frame. Once the STA has acquired the information so that it is able to meet the transmit requirements of the regulatory domain, it shall transmit a Probe Request to an AP to gain the additional regulatory domain information contained in the Probe Response frame, unless the information was previously received in a Beacon fame. The STA then has sufficient information available to configure its PHY for operation in the regulatory domain.

(Quoted from here.)

So, I guess that's the ultimate way to not break any transmit laws... transmit nothing until you hear a valid local transmission.

[*] That's not to say I haven't been guilty of such in the past, nor does it say I won't be guilty of it in the future. ;-)