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Cisco AP signal quality value

RSSI (receive signal strength indicator) is sometimes on a scale of

0-255, 0-100, or 0-60 depending on the chipset. The original conversion method is in the 802.11-1999 document (I'm too lazy to find it). All the manufacturers have their own idea of how the conversion should be done. See:

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details. There's a Cisco conversion table on Page 10.

"The IEEE 802.11 standard defines a mechanism by which RF energy is to be measured by the circuitry on a wireless NIC. This numeric value is an integer with an allowable range of 0-255 (a 1-byte value) called the Receive Signal Strength Indicator (RSSI). No vendors have chosen to actually measure 256 different signal levels, and so each vendor?s

802.11 NIC will have a specific maximum RSSI value (RSSI_Max). For example, Cisco chooses to measure 101 separate values for RF energy, and their RSSI_Max is 100. Symbol uses an RSSI_Max value of 31. The Atheros chipset uses an RSSI_Max value of 60. Therefore, it can be seen that the RF energy level reported by a particular vendor?s NIC will range between 0 and RSSI_Max."
Reply to
Jeff Liebermann
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Does anyone have any insight or mathematical idea how Cisco calculates its "signal quality" on a client connected to a Cisco AP (i.e. 1230)? It is in the form of "%" and the documentation says it is some relationship with the signal level (expressed in dBm). However, I would interested in determining how it is calculated so I can use it in comparison to other non Cisco products on my network.

Thanks, Bob

Reply to
Robert G. Becnel

Jeff,

Thank you very much. Good information.

Sincerely, Bob Becnel

Reply to
Robert G. Becnel

Jeff,

I gave this some more thought and want to make sure we are discussing apples to apples. What I am calling the signal quality is that value found on the web interface of a AP under "Association" just below the "signal strength" value. I have not seen Cisco refer to RSSI for this value, but are you saying that RSSI=signal quality?

S> >

Reply to
Robert G. Becnel

Ye'r right. We have a muddle. my fault. RSSI is NOT always equal to signal quality, just signal strength. I checked some (printed) literature for various SNMP MIBS used by Cisco and Breezcom (Alvarion) and found that some access points don't measure the Noise value (similar to RSSI) necessary to calculate a proper signal to noise ratio or signal quality. Therefore, they use the RSSI signal strength value as some kind of quality measurement. That's acceptable if you assume that the Noise RSSI to be negligible. I don't have easy access to a Cisco 1200 to see what's available, but as I recall (or guess) from a 340 series bridge, Cisco does measure the noise level and calculates using the RSSI values either: S/N * 100% or S/(S+N) * 100% for signal quality.

Signal quality is the ratio of signal strength to noise level. I've seen several ways to present it which vary by chipset manufactory.

  1. 10 * log(S/(S+N)). This is real signal to noise ratio calculated from the RSSI values of signal and noise.
  2. Signal minus noise in dB. This is not exactly correct, but close enough. The RSSI and Noise values are converted to to -dBm and subtracted.
  3. Signal minus noise. This is just plain wrong, but is a good indication of relative quality to see if you're making an improvement. The RSSI and noise level values are simply subtracted.
  4. S/N * 100% or S/(S+N) * 100% Ugly but functional. This is just the RSSI divided by the Noise values times 100%. The actual numbers are meaningless, but for the fairly small range of useable values found in wireless, it's more than adequate. My guess is this is what Cisco displays.
Reply to
Jeff Liebermann

Do you what scale does Linksys use?

I am testing two WRT54GS boxes in a WDS bridge and I get about -80 as RSSI value in the WRT54GS web interface: is that dBm or other?

Thanks.

Reply to
meATprivacyDOTnet

Thanks again for the help, Jeff.

Bob

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>> for details. There's a Cisco conversion table on Page 10.

Reply to
Robert G. Becnel

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