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New Member

I looking for a documentation or a table equivalence between RSSI with dBm (or rate).

Thanks

5 REPLIES
Silver

I pulled the attached out of a Cisco powerpoint a while ago. Note that these are guidelines to use as a survey starting point, not carved in stone. Every client behaves slightly differently.

New Member

I found a wildpackets document called "Converting signal strength percentages to dBm Values". The table has RSSI positive values 0 to RSSI_Max.

I want to know why the Cisco IP Phone 7921G shows negative RSSI numbers (-82 to -29)in a Site Survey (-29 close to the antenna, -82 some meters away).

Silver

RSSI values are negative because that's the nature of the dBm scale. The unit "dBm" means Decibels (dB) relative to a standard of 1 milliwatt (m).

A decibel scale is logarithmic. Your reference unit is always 0 dB. 10 times the reference unit is 10 dB. 100 times the reference unit is 20 dB. 1000 times the reference unit is 30 dB, and so forth.

As signal goes down, your dB values become negative: 1/10th of the reference unit is -10 dB, 1/100th of the reference unit is -20 dB, 1/1000th of the reference unit is -30 dB.

So when we're dealing with RSSI, as you get farther and farther away, your numbers become more and more negative. The FCC limits broadcast power to 100 mW... that's 20 dBm. But a single receiver will never absorb more than a tiny fraction of that energy, and that amount decreases as you get further and further from the transmitter.

In the real world, your wireless device will almost never absorb more than about a thousandth of a milliwatt even when you're right on top of the AP... that's -30 dBm. As you get farther away, signal strength drops off and the dBm values become even more negative. Remember that each -10 dBm means that the wattage of the signal has been divided by 10. So, where -30 dBm means 0.001 mW, -40 dBm means 0.0001 mW, -50 dBm means 0.00001 mW, and so forth.

New Member

Sincerly speaking, I don't know why decibels are so difficult to be known.

Once again:

Signal is measured in dBm (or Watt, or milliwatt, or boiler horsepower)

dB are used to indicate ANY ratio:

x dB = 10 * Log a / b.

If you have doubled your incomes, you have increased them by 3dB. That's because Log 2 is very close to 0.3.

If you have halved them, they have decreased by 3dB (or increased by -3dB)

Log 1/2 = - Log 2 = - 0.3

Remember the rules concerning Logarithms.

This applies to EVERYTHING

If b is equal to 1milliwatt, you have obtained dBm.

Therefore:

x dBm + (or -) y dB = z dBm

x dB + (or -) y dB = z dB

Silver

it depends on what your looking for, RSSI is a measure of the received power as seen by the radio, which means that includes the gain of the antenna and such.

there is no standard correlation between RSSI and Signal Strength Percentages and you can't directly compare a transmit power of 20dbm to a -65dbm signal strength except that if you follow the standard logarithmic equation you can figure it out.

100mw = 20dbm

so to get to -64dbm is approximately 0.00000037 mw