Interpretation RSSI measurements

[Nigrofasciatum]

Hello,

I've been observing with multiple access points and multiple wireless clients that RSSI signal received varies even 10dBm depending on whether the device is associated with the access point or just is listening to. When is associated is the maximum value.

Why this variation occurs?

What would be the correct value, the device gives me when I am associated or gives me when I'm not?

Thanks

[Scott Fella]

You always want to look at the RSSI on an associated client.... clients that are not associated will not tell you anything to be honest.

Thanks,

Scott

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[Nigrofasciatum]

I think it is also important the signal level that receive not associated devices, because to join need a minimum level of signal, at least enough to "listen" to the AP.

For example if I disable in AP data rates lower than 54Mbps, the client will need a signal level of at least -71 dBm to listen  the AP, according to this table:

We also know that mobile devices such as smartphones or tablets are more "deaf" and therefore it is important the signal level that can listen without being associated with the AP.

In any case my question is why there is this difference between the values ​​with or without associated associated device? has something to do with the antenna gain of the AP or client technologies like client link?

What would be the correct way to measure for a site survey?

[Scott Fella]

Well the issue is that the client is listening to all the beacons in the air. If you have tools like AirMagnet or Ekahau, those can run in passive mode to determine valid RSSI. If your looking at a Windows machine using the windows tool, I would say no. Use a tool or a free tool and I would say yes. Otherwise, association will really tell you the RSSI for that client be a use that client and ap has negotiated its modulation and will continue to negotiate. You will see a slight difference in RSSI from a passive and active client.

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[Nigrofasciatum]

Sometimes I use inSSIDer software, and with it the result can vary up to 10 dB being associated (active mode) to not be associated (passive mode). With other devices or software i see +-7dB difference. I think it's not a small difference, and so I wanted to know what is due to know which is the best way to make the site survey.

In the attached picture you can see the screenshot of how the signal varies from -50dBm (stable) being associated, to -60dBm about without being associated.

       

I do not understand why if the RSSI level is a parameter of radioelectrical power it varies being associated or not ... power emitted by the AP is the same, not it?

[Scott Fella]

Well in passive mode, the device is scanning various channels as when compared to an actual association. The expensive tools allows you to passively scan an ssid and not all SSID's like many of the free tools. Fluctuation is minimal with these tools. You might want to email the tools support or if they have a forum, ask the question there. They probably can answer your question.

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[Abhishek Abhishek]

RSSI is a Radio-Frequency (RF) term and stands for Received Signal  Strength Indicator. It is a measure of the power level that a RF device,  such as WiFi or 3G client, is receiving from the radio infrastructure  at a given location and time. For instance, the power level a laptop is  detecting from a nearby AP.

Usually, the higher the RSSI the better the quality and speed of the communication through the radio segment.

The closer the figure is to zero, the better. So in the figures you've given above, -75 is better than -100.

In my experience -75 provide a reasonable service. -85 provides an OK  service. -100 provides a barely usable service. -110 provides no service  at all.

This figure represents your signal strength to the cellular tower. The  closer you are to a tower, the closer to zero the number will be.

Note that although I mentioned above that -75 provides a reasonable  service, keep in mind that if the tower you're connected to is heavily  congested the service won't be all that great. It is similar to the way  in which ADSL works - even if you sync at 24mb/1mb, if the DSLAM is  heavily congested, you'll get poor throughput.