Here's what I found in the kernel source's iwl3945-base.c file:
/* Convert linear signal-to-noise ratio into dB */
static u8 ratio2dB[100] = {
/* 0 1 2 3 4 5 6 7 8 9 */
0, 0, 6, 10, 12, 14, 16, 17, 18, 19, /* 00 - 09 */
20, 21, 22, 22, 23, 23, 24, 25, 26, 26, /* 10 - 19 */
26, 26, 26, 27, 27, 28, 28, 28, 29, 29, /* 20 - 29 */
29, 30, 30, 30, 31, 31, 31, 31, 32, 32, /* 30 - 39 */
32, 32, 32, 33, 33, 33, 33, 33, 34, 34, /* 40 - 49 */
34, 34, 34, 34, 35, 35, 35, 35, 35, 35, /* 50 - 59 */
36, 36, 36, 36, 36, 36, 36, 37, 37, 37, /* 60 - 69 */
37, 37, 37, 37, 37, 38, 38, 38, 38, 38, /* 70 - 79 */
38, 38, 38, 38, 38, 39, 39, 39, 39, 39, /* 80 - 89 */
39, 39, 39, 39, 39, 40, 40, 40, 40, 40 /* 90 - 99 */
};
/* Calculates a relative dB value from a ratio of linear
* (i.e. not dB) signal levels.
* Conversion assumes that levels are voltages (20*log), not powers (10*log). */
int iwl3945_calc_db_from_ratio(int sig_ratio)
{
/* 1000:1 or higher just report as 60 dB */
if (sig_ratio >= 1000)
return 60;
/* 100:1 or higher, divide by 10 and use table,
* add 20 dB to make up for divide by 10 */
if (sig_ratio >= 100)
return (20 + (int)ratio2dB[sig_ratio/10]);
/* We shouldn't see this */
if (sig_ratio < 1)
return 0;
/* Use table for ratios 1:1 - 99:1 */
return (int)ratio2dB[sig_ratio];
}
#define PERFECT_RSSI (-20) /* dBm */
#define WORST_RSSI (-95) /* dBm */
#define RSSI_RANGE (PERFECT_RSSI - WORST_RSSI)
/* Calculate an indication of rx signal quality (a percentage, not dBm!).
* See http://www.ces.clemson.edu/linux/signal_quality.shtml for info
* about formulas used below. */
int iwl3945_calc_sig_qual(int rssi_dbm, int noise_dbm)
{
int sig_qual;
int degradation = PERFECT_RSSI - rssi_dbm;
/* If we get a noise measurement, use signal-to-noise ratio (SNR)
* as indicator; formula is (signal dbm - noise dbm).
* SNR at or above 40 is a great signal (100%).
* Below that, scale to fit SNR of 0 - 40 dB within 0 - 100% indicator.
* Weakest usable signal is usually 10 - 15 dB SNR. */
if (noise_dbm) {
if (rssi_dbm - noise_dbm >= 40)
return 100;
else if (rssi_dbm < noise_dbm)
return 0;
sig_qual = ((rssi_dbm - noise_dbm) * 5) / 2;
/* Else use just the signal level.
* This formula is a least squares fit of data points collected and
* compared with a reference system that had a percentage (%) display
* for signal quality. */
} else
sig_qual = (100 * (RSSI_RANGE * RSSI_RANGE) - degradation *
(15 * RSSI_RANGE + 62 * degradation)) /
(RSSI_RANGE * RSSI_RANGE);
if (sig_qual > 100)
sig_qual = 100;
else if (sig_qual < 1)
sig_qual = 0;
return sig_qual;
}
--
iwl3945 sometimes does not detect ESSIDs, whereas ipw3945 works perfectly
https://bugs.launchpad.net/bugs/177624
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