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Date: Sat, 29 May 2010 13:47:20 -0700
From: Sam Leffler <sam@freebsd.org>
Organization: FreeBSD Project
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Subject: Re: Atheros and how to convert RSSI to dBm
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On 5/26/10 3:25 PM, Miroslav Chlastak wrote:
> Hello,
>
> is there a way how to convert RSSI from output of "ifconfig ath0 list
> sta" to dBm?:
>
> [snip]
>
> ADDR AID CHAN RATE RSSI IDLE TXSEQ RXSEQ CAPS FLAG
> 00:4f:62:1d:d8:5d 14 9 11M 18.5 0 15932 22400 ES A
>
> [snip]
>
> I my test enviroment I use this function:
> noise floor + RSSI*2 = signal of client in dBm
>
> Then i get the same value as from "tcpdump -n -i ath0 -p -y
> IEEE802_11_RADIO":
>
> [snip]
>
> 10:41:04.746395 86402611933us tsft short preamble 58.5 Mb/s -61dB signal
> -98dB noise antenna 1 [0x00000012] IP 10.0.0.1.2049 > 192.168.100.2.123:
> NTPv3, Client, length 48
>
> [snip]
>
>
> Right? Or is this conversion "stupid"? Some better idea?
>
> Is there a way how to get noise floor via "ifconfig"? I get it from
> output of "athstats" :(
>
>

tcpdump calculates the signal value using rssi as you described.  I 
personally never cared much for this value because getting an accurate 
measure for noise floor is hard (especially when you consider 11n) and 
for ath at least the available noise floor dat is only approximate.

I can think of very few situations where you want signal and NOT rssi.

	Sam