What is RSSI value in 802.11 packet

Question

I see two values of SSI signal in 802.11 packet when viewed in wireshark. I would like tot know that which one value is the correct RSSI value

Information from wireshark:

SSI Signal: -40 dBm
SSI Noise: -100 dBm
Signal Quality: 64
Antenna: 0
SSI Signal: 60 dB

Also note that SSI signal(second time) is the ((SSI signal) - (SSI Noise))

I am just confused which one is correct. Also the wikipedia entry tells that these implementation can be vendor dependent. I am totally confused about which is the correct value.

Answer 1

Take my answer with a pinch of salt- this is what makes sense to me, need not be correct..if it makes sense to you use it.

The first SSI signal is measurement of the the Rx Signal Strength at the/after the Rx antenna( its doing this calculation at the ADC stage)

The SSI noise is the noise at the ADC stage (probably measured noise ).

The 2nd SSI signal is the SNR, which would be original SSI Siganl - SSI Noise = 60 dB -this difference would be 60 dB not dBm- the way you get that is by converting both values to dB before subtraction. Now, you need not do the dB conversion I mentioned before subtraction , you'll still get the same magnitude, just ensure to use dB as the units.

Movin on, to answer your specific question, neither of them are actually RSSI as per IEEE definitions- RSSI is defined to be a number between two values. It does not have a dBm unit, although a lot of popular apps now give it a dBm value which has led to significant confusion. Cisco uses values between 0-100, atheros 0 to 127 etc. So going by that logic the RSSI in this case would probably be Signal Quality -64.

Answer 2

Take my answer with a pinch of salt- this is what makes sense to me, need not be correct..if it makes sense to you use it.

The first SSI signal is measurement of the the Rx Signal Strength at the/after the Rx antenna( its doing this calculation at the ADC stage)

The SSI noise is the noise at the ADC stage (probably measured noise ).

The 2nd SSI signal is the SNR, which would be original SSI Siganl - SSI Noise = 60 dB -this difference would be 60 dB not dBm- the way you get that is by converting both values to dB before subtraction. You neednt do it but youll still get the same magnitude just ensure to use dB as the units.

Neither of them are actually RSSI as per IEEE definitions- RSSI is defined to be a number between two values. It does not have a dBm unit, although a lot of popular apps now give it a dBm value which has led to significant confusion. Cisco uses values between 0-100, atheros 0 to 127 etc. So going by that logic the RSSI in this case would probably be Signal Quality -64.

Answer 3

As per the link (on 06/10/2016) http://www.radiotap.org/suggested-fields/RSSI

RSSI is still a "suggested" field only usable with OpenBSD OS.

(I was trying to get the same info with an AirPcap and a Windows machine)

Answer 4

I see two values of SSI signal in 802.11 packet when viewed in wireshark

It sounds as if the driver for the 802.11 adapter used to capture the packet is being weird and supplying both antenna signal strength in dBm and antenna strength in dB. What type of adapter was that, and what operating system is the machine on which the machine that did the capture running?

"dBm", as the link above indicates, decibels from 1 milliwatt of power; "dB", as the other link above indicates, is decibels from some unspecified arbitrary point. dBm tells you the actual signal power at the antenna; dB doesn't - you can only use dB values to compare with other dB values.

Neither of those are "RSSI" as defined by 802.11; that RSSI value is also arbitrary, but it's even more arbitrary - 802.11 doesn't even say what it measures, just that larger values correspond to stronger signals, and those values are vendor-dependent.

Also note that SSI signal(second time) is the ((SSI signal) - (SSI Noise))

The writer of the driver for your adapter might not have properly read the Radiotap page about the "dB antenna signal" value (linked above), and might have thought it was supposed to be a signal-to-noise ratio, and calculated that by subtracting the noise value from the signal value (decibels are a logarithmic scale, and the quotient of two values is the difference between the logarithms of those values). I would ignore that value, and use "SSI signal" as an indication of signal strength in milliwatts (-40 dBm = 100 nanowatts, at least as per the table in the Wikipedia article on dBm).