Last Post: June 23, 2004:
Just so's you know, I have been trying to find the answer in the FCC regs.....
In researching an answer to a customer question, I discovered that Cisco 802.11g radios transmit at a max power of 30mw. I looked at Proxim's and 3com's web sites and both seem reluctant to disclose what the max transmit power is for their products. If I infer correctly from 3com, I come up with appx. 40mw for 17dB output.
My question: is there a regulatory requirement regarding the power output of 802.11g radios? I presume Cisco makes every effort to comply with the regs. I just find it difficult to believe that a b radio is permitted to transmit at 100mw while a g radio is max'ed at something substantially lower. And as you know, customers don't take kindly to specululative answers ;)
Did not see the in the CCNA materials either, BTW.
Your assumption is correct. Since the 802.11g radios reside in the 2.4 GHz ISM band, the same rules apply to them that applies to 802.11 and 802.11b.
Check out 15.247
So, now the question is, "Why does Cisco only go up to 30mW when they could just as easily go up to 100mW just like they've done with their 802.11b radio?"
From what I'm told by Cisco, it's because the 802.11g radio, when used on channel 1, interferes too much with frequencies below 2.4000 GHz. Also, I hear that it has something to do with radio sensitivity, but I'm still looking into that one.
Never one to let these things go, I did get the following response from someone at one of the manufacturers:
"We don't have anything published but it is basically because the OFDM waveform has a peak to average power ratio that is larger than CCK. We specify the average power on our data sheet but the FCC for example specifies the peak, so we have to back off further."
I did check out FCC part 15, and there is indeed a LOT about peak power measurement. Not being a radio guy, I won't claim I understood much of it, but given the FCC regs and what all manufacturers appear to be doing - reducing the max power of their G radios - this makes more sense to me than any of the other spurious explanations provided by some vendors I have asked. ;)
Make sense to you folks?