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Signal Strength - dBm calculation?

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Signal strength values returned by the WSN nodes seem to range from 0 to 100.  How are these signal strength values calculated?  The output power and sensitivity ratings are listed as +17dBm and -102dBm respectively, yeilding a theoretical spread of 119dBm.  Can the signal strength values be correlated to dBm?

 

Thanks!

 

Garrett

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Hi Garrett,

 

The signal strength range of 0-100% is the RSSI (Received Signal Strength Indication) from the parent device to the node, with 100 being the best percentage you can get.

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Is that RSSI percentage value cut off at a certain point or does it span somewhat linearly from the entire range of dBm values (-102 to +17)?  I understand that the NI WSN nodes aren't quite the same as a Wireless NIC but many of them measure RSSI to a certain point, making values of dBm which are less than the maximum achievable also appear as 100%.  There is an article I found (http://www.wildpackets.com/elements/whitepapers/Converting_Signal_Strength.pdf) that explains it as follows (on page 8):

 

The effectiveness or reasonability of using dBm measurements obtained from a standard
wireless NIC is questionable when used as part of a real-world network troubleshooting
exercise. This is because most NICs only provide RSSI in a range that is below –10dBm,
and everything above that is mapped to RSSI_Max (or, 100% signal strength).

 

When using the NI WSN nodes then, if I was receiving a signal strength of -5 dbm, might that show up as ~80% {i.e. (-5 + 102) / (17 + 102)}, or does the WSN also have a similar cutoff that might make values of -5dBm, +10dBm, etc all appear as 100%, until they are less than a predetermined value?  And if so, what is that value?  We are using these values to roughly estimate the degree to which obstacles interfere with the signal, and knowing the minimum dBm that could result in a signal strength value of 100% would help make our estimates more accurate.

 

Thanks!

 

Garrett

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Solution
Accepted by topic author Garrett Cook

Hi Garrett,

 

The radio returns link quality from 0-255.  Currently, we have  -39 dB representing 100% link quality, and -97 dB representing 10% link quality.  Note that these values could change in the future. 

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I had a question regarding the link quality indicator in LabVIEW - is it more of an indication of the interference rather than the power of the incoming signal?

 

Also, does the link quality somehow relate to the amount of power consumed by the node when transmitting its signal?

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