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Posted by:eldanielo
Subject:Signal strength inconsistent
Date:22:25:52 01/01/2013

> > Hello,
> >
> > at the moment, I am trying to measure the signal strength (RSSI) of APs in the vicinity with a OpenWRT device to implement a positioning algorithm.
> >
> > The setup: TL-WA901NDv2 with r34812 in monitoring mode in the 2.4 GHz network with kismet-drone opkg package installed.
> > The hardware is based on the ar71xx architecture and uses the Atheros AR5416 (onboard)wireless adaptor and uses the ath9k driver.
> >
> > I discovered that there are basically two different methods to measure the received signal strength which yield inconsistent results:
> >
> > 1. Active Scanning
> > e.g. using "iwinfo wlan0 scan"
> >
> > 2. Passive with RSSI per captured packet
> > e.g. using a Kismet drone
> >
> >
> > However, it is very strange that method 1. and 2. yield totally different results:
> > Using iwinfo the RSSI value for the closest AP is between -20dBm and -25 dBm but the values displayed using Kismet are between -50dBm and -60dBm.
> >
> > How comes there is such a discrepancy?
> >
>
> Values in rfmon are usually very weird if not totally bogus; ultimately all Kismet can do is what the card reports.
>
> Rfmon signal levels will also fluctuate between packets due to different packet speeds leading to different signal fidelities (beacons are slower and will be 'louder', for example)
>
> I don't know if the card interprets different levels internally in monitor mode, or if it can fake some signal knowledge from retry rates in normal mode, but it's definitely common for monitor mode signal levels to be weird or totally bogus.


Thank you for the reply. After doing some more tests I can confirm that in rf monitor mode the values are really unrealistic.

Do you know what is the difference when using scan commands such as "iwlist wlan0 scan"? When I execute these commands the results are more reasonable e.g. avg -25 dBm when standing next to the AP compared to values of avg. 50 dBm reported in Kismet.

Cheers


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