about noise level, signal level and quality

Hairong Sun hairong
Mon Jan 13 15:30:07 PST 2003


I set up a testing environment and tried to measure the noise,signal and
quality of each channel.

I have two WLANs in lab (same room, close to each other):
WLAN 1: one linksys AP and one STA associated with the linksys AP,
set channel # =8, and download a long file.
WLAN 2: one hostap and one STA associated with the hostap
I ran the site survey and confirmed there is no other WLAN nearby.
I printed out the noise, signal and quality of the STA associated with the
hostap (by inserting printk somewhere in source code, and using dmesg to

What I got is kind of confusing:

1. When I set the channel # of WLAN 2 the same as WLAN 1 (i.,e, channel 8
), I printed out the noise level as around 157~159. When I set the channel
# of WLAN 2 to be 1, the noise level is surprisingly still 157~159. I shut
down WLAN 1, and the noise level is still ~158. NOTE: I restarted hostap
and reassociated STA each time I measured the noise level in order to
ensure the noise level reading are updated. I repeated the testing
a couple of times, and still got the same results (noise level around
157~159). I am puzzled by the results. WLAN1 should have interference to
WLAN 2 if the two channels are close, esp. choosing the same channel.
The noise level should decrease if WLAN2 is shut down or chooses channel 1
instead of channel 8.

2. When "signal" - "noise"<35, the quality would be larger than 200 (say
240). It is explanable according to the definition of "quality"
("signal"-"noise"-35, being u8), though "quality" is defined between 0~92.
Is it a bug need to be fixed?

Since "signal" and "noise" are converted by (1x100+v*100/255-100) and
"quality" is converted by (signal-noise-35) in "prism2_ap_get_sta_qual()" from their
original values (last_rx_signal and last_rx_silence) in the firmware, my
questions are

1. what do "signal","noise", last_rx_signal, and last_rx_silence
really mean in the driver AND firmware?
2. how to explain the noise level reading in my experiments (i.e.,noise
level changes little with the interference from neighboring AP)?



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